Positive and negative ion formation in deep-core excited molecules: S 1s excitation in dimethyl sulfoxide L. H. Coutinho, D. J. Gardenghi, A. S. Schlachter, G. G. B. de Souza, and W. C. Stolte Citation: The Journal of Chemical Physics 140, 024314 (2014); doi: 10.1063/1.4861050 View online: http://dx.doi.org/10.1063/1.4861050 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/140/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Vibrational and electronic excitations in fluorinated ethene cations from the ground up J. Chem. Phys. 138, 124301 (2013); 10.1063/1.4795428 Core-level positive-ion and negative-ion fragmentation of gaseous and condensed HCCl3 using synchrotron radiation J. Chem. Phys. 135, 044303 (2011); 10.1063/1.3615626 Specific formation of negative ions from leucine and isoleucine molecules J. Chem. Phys. 132, 014301 (2010); 10.1063/1.3270154 Hydrogen bonds in 1,4-dioxane/ammonia binary clusters J. Chem. Phys. 120, 8453 (2004); 10.1063/1.1689291 Dissociation dynamics of thiolactic acid at 193 nm: Detection of the nascent OH product by laser-induced fluorescence J. Chem. Phys. 120, 6964 (2004); 10.1063/1.1667878
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 137.149.200.5 On: Sun, 30 Nov 2014 08:28:49
THE JOURNAL OF CHEMICAL PHYSICS 140, 024314 (2014)
Positive and negative ion formation in deep-core excited molecules: S 1s excitation in dimethyl sulfoxide L. H. Coutinho,1,a) D. J. Gardenghi,2 A. S. Schlachter,3 G. G. B. de Souza,4 and W. C. Stolte2,3 1
Physics Institute, Federal University of Rio de Janeiro, Rio de Janeiro, RJ 21941-972, Brazil Department of Chemistry, University of Nevada, Las Vegas, Nevada 89154-4003, USA 3 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA 4 Chemistry Institute, Federal University of Rio de Janeiro, Rio de Janeiro, RJ 21949-900, Brazil 2
(Received 12 September 2013; accepted 17 December 2013; published online 13 January 2014) The photo-fragmentation of the dimethyl sulfoxide (DMSO) molecule was studied using synchrotron radiation and a magnetic mass spectrometer. The total cationic yield spectrum was recorded in the photon energy region around the sulfur K edge. The sulfur composition of the highest occupied molecular orbital’s and lowest unoccupied molecular orbital’s in the DMSO molecule has been obtained using both ab initio and density functional theory methods. Partial cation and anion-yield measurements were obtained in the same energy range. An intense resonance is observed at 2475.4 eV. Sulfur atomic ions present a richer structure around this resonant feature, as compared to other fragment ions. The yield curves are similar for most of the other ionic species, which we interpret as due to cascade Auger processes leading to multiply charged species which then undergo Coulomb explosion. The anions S− , C− , and O− are observed for the first time in deep-core-level excitation of DMSO. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4861050] I. INTRODUCTION
The excitation of core electrons in molecules usually leads to strong resonances in the photoabsorption spectrum, observed below and above the ionization threshold. In the former case, the observed resonances are either associated with virtual, molecular antibonding states or with diffuse Rydberg states.1 Above the edge, the observed prominent features are usually classified as shape resonances.2 Very close to the ionization edge, doubly (or multiply) excited states are also observed. All these electronically excited states have a short lifetime (from one to tens of femtoseconds) and thus decay very rapidly. Excited states located at energies below the edge decay mainly through resonant Auger (participator or spectator) processes. In the first case, the excited electron participates in the Auger-type process and the final state is a molecular ion, while in the second case, the excited electron remains as a spectator during the Auger process, and the final state is an excited molecular ion (two holes-one electron). It has been experimentally observed that molecular antibonding states decay preferentially through participator Auger processes, while Rydberg states usually decay by spectator Auger processes.3 Excited final ionic states reached after spectator decay are in general highly dissociative and as a result excitation to Rydberg state results in an intensification of the fragmentation of the molecule. Above the ionization edge, normal Auger processes become very important and Auger decay is associated with double ionization. Following deep core excitation, an Auger cascade can follow, leading to multiply charged states. Doubly or multiply ionized states are usually dissociative, due to the Coulomb repulsion and, as a consequence, an increasing degree of fragmentation of the molecule is observed. a) Author to whom correspondence should be addressed. [email protected].
0021-9606/2014/140(2)/024314/8/$30.00
Although most of the experimental information gathered about the decay of core-excited molecules has been related to the detection of positive ions, the decay of core excited molecules into anions, albeit associated with much smaller cross sections, is also relevant, and has been the focus of several important pieces of work.4–6 It has been shown, for instance, that negative ions can be formed in association with the decay of resonances energetically located below and above the ionization edge.7 Although in small molecules the production of negative ions is concentrated in the decay of resonances below the edge, for larger molecules a significant proportion of negative ions is formed above the edge, due to the polar dissociation of triply charged cations. Very close to the edge, post-collision-interaction (PCI) effects are considered to play an important role in the formation of anions.8–12 Up to the present and to the best of our knowledge, studies of negative ion formation following core excitation have been dedicated to the excitation and ionization of shallow core levels (B 1s, C 1s, O 1s, S 2p, Cl 2p) which decay mostly through core-valence-valence (KVV or LVV) states. In this case, depletion of the valence shell leads to the ionic dissociation of the molecule. With the excitation and ionization of deep core levels, with much shorter lifetimes, cascading Auger processes may also contribute to the formation of multiply charged ions and to an enhancement in the degree of fragmentation of the molecule. Recently, a study on the Cl 1s edge of the chloroform molecule showed the formation of H− , C− , and Cl− .5 In the present work, positive and negative-ion-yield spectroscopy of the dimethyl sulfoxide (DMSO) molecule has been studied around the Sulfur K-edge. DMSO, (CH3 )2 SO, is a colorless liquid with a large number of applications, both in laboratory and industry, being considered one of the safest
140, 024314-1
© 2014 AIP Publishing LLC
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 137.149.200.5 On: Sun, 30 Nov 2014 08:28:49
024314-2
Coutinho et al.
J. Chem. Phys. 140, 024314 (2014)
solvents available. In the ocean sulfur cycle, a large fraction of dimethyl sulfide (DMS) is converted into the less volatile DMSO by bacteria.13 It also acts as a rinsing agent in the electronic industry and as a cryoprotectant to preserve organs and tissues, and in long-term storage of embryonic stem cells.14 The photoabsorption spectrum of DMSO in the sulfur K edge region was recorded previously by Sze et al.15 using synchrotron radiation from the LURE ACO storage ring. They assigned five features around the S 1s ionization threshold, related to transitions to unoccupied virtual valence levels and Rydberg states. Electron-energy-loss spectra in the valence shell and the S 2s, S 2p, O 1s, and C 1s inner shells were also presented. The ultraviolet photoelectron spectrum for this molecule was obtained by Bock and Solouki.16 The first valence-shell ionization potential occurs at 9.01 eV. The sulfur KLL Auger and 1s and 2p photoionization of DMSO were studied by Sodhi and Cavell,17 who determined the S 1s edge occurs at 2480.90 eV. More recently, Drage et al.18 reported measurements of the high-resolution VUV photoabsorption spectrum in the 3.7–10.8 eV energy range.
II. EXPERIMENTAL TECHNIQUE
The experimental arrangement has been described before.19 Briefly, the sample is inserted into the vacuumchamber collision area as a 2 mm effusive gas jet, which intersects the photon beam in a plane perpendicular to the photon beam. The resulting ions are accelerated and focused onto the entrance of a 180◦ magnetic mass analyzer, and are subsequently detected by a channel electron multiplier. The spectrometer is able to measure mass/charge ratios up to 60 with resolution of approximately 1/65. The polarity of the lenses and the magnetic field may be switched to allow measurement of either positive or negative ions produced in the ionization region. The vacuum chamber base pressure was approximately 10−7 Torr, while during the present experiment it was kept around 1 × 10−5 Torr. The measurements were performed on bending-magnet beamline 9.3.1 at the Advanced Light Source, Lawrence Berkeley National Laboratory. This beamline provides photons in the 2200–5500 keV energy region, with a resolving power of approximately 6800. Calibration at the S 1s-edge was performed by comparison to Reynaud et al.20 and Ferrett et al.21 using the S (1s)−1 → 6tu transition in SF6 at 2486.0 eV . We estimated the flux of the incident radiation near the S K-edge to be approximately 4×1010 photons/s, with a
(1a )2 S 1s
(2a )2 O 1s
(1a )2 (3a )2 C 1s
resolution of 0.4 eV by comparison to measurements near the S (1s)−1 → 6a1 3b2∗ transition in H2 S.22 The DMSO sample was purchased from Sigma-Aldrich (part number D8418-50ML) with purity of 99.9%. No further purification was used except for degassing the liquid sample by multiple freeze-pump-thaw cycles before admitting the vapor into the experimental chamber through a mechanical leak valve. The presented spectra for each fragment were obtained by the sum of several measurements, normalized separately by sample pressure and photon flux.
III. THEORETICAL METHOD
All calculations were performed using Gaussian09 Revision C.0123 starting from the crystal structure.24 The crystal structure was optimized in the gas phase using a high-level correlated wave function-based ab initio method, quadratic configuration interaction with single and double excitation levels (QCISD)25 with the Aldrich’s triple-ζ valence basis set with polarization functions used for atoms (TZVP).26 The electron density was calculated from the optimized geometry using two methods: Density Functional Theory (DFT) with the hybrid functional B3LYP5,27, 28 and a wave functionbased ab initio method, QCISD, both with TZVP. X-ray absorption spectroscopy (XAS) has been used to obtain quantitative ground state information about the orbital composition of the lower lying unoccupied frontier molecular orbitals.29 Furthermore, theoretical calculations have been successful at reproducing the experimental orbital composition.30–37 The theoretical orbital compositions were obtained from Bader Atoms in Molecules (AIM)38, 39 using AIMAll program40 and Weinhold natural population analysis (NPA)41 as implemented in Gaussian09. The NPA valance orbital sets for the sulfur were 3s3p. The XAS spectra were simulated with the ORCA computational package42 that is based on a one electron theoretical approach using time-dependent density functional theory (TD-DFT) that has been demonstrated to successful simulate XAS spectra.43–45 IV. RESULTS AND DISCUSSION
The DMSO molecule presents a Cs symmetry with the mirror plane passing through the formal sulfur-oxygen double bond. The optimized geometry produced a similar structure to those previously determined for both the ground and excited states.46 The molecular configuration of the ground state was obtained using B3LYP/TZVP and is as follows:
(4a )2 S 2s
(5a )2 (2a )2 (6a )2 S 2p
(7a )2 (8a )2 (3a )2 (9a )2 (10a )2 (11a )2 (4a )2 (5a )2 (6a )2 (12a )2 (13a )2 (7a )2 (14a )2 , valence orbitals
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 137.149.200.5 On: Sun, 30 Nov 2014 08:28:49
024314-3
Coutinho et al.
J. Chem. Phys. 140, 024314 (2014) TABLE I. Experimental energies and term values (TV) of major transitions of DMSO following S 1s excitation. The quoted ionization threshold was measured by Sodhi and Cavell.17 All errors in the table are standard deviations, total errors include a calibration error of 0.1 eV from using the S (1s)−1 → 6tu transition in SF6 at 2486.0 eV,17, 20, 21 when possible comparison is made to Sze et al.15 Present experiment
Configuration 8a + 16a 15a + 17a 18a + 19a + 9a 10a +11a 12a + 20a IP17 3d 3d
FIG. 1. Total-ion-yield (TIY) and partial ion yields in the photon energy region 2470–2485 eV for S+ , CH3 + , CH3 SO2 + , C2 H5 + , SO2 + , C+ , H+ , and (S2 + + O+ ) resulting from photoexcitation of DMSO near the S 1s ionization threshold. Dashed lines correspond to transitions summarized in Table I. The ionization threshold shown is from Ref.17.
with (8a )0 , (15a )0 , (16a )0 , (17a )0 , and (18a )0 being the five lowest unoccupied molecular orbitals. These results are nearly identical to those obtained using QCISD/TZVP and the previous work by Sze et al.15 As can be seen by the blue curve in the top panel of Fig. 1, the total ion yield (TIY) is similar to the photoabsorption spectrum previously measured by Sze et al.15 Differences include a more asymmetric primary resonance, followed by two clearly visible transitions prior to the ionization threshold, versus a more symmetric primary resonance and one clearly visible transition. Both appear identical in the region above the ionization threshold. It is possible that the differences are due to an improved energy resolution of the photon beam, but the estimated 0.35 eV bandpass quoted by Sze et al.15 is similar to the estimated 0.36 eV at 2500 eV for this measurement. The energies of these transitions, term values, calculated assignments, and a comparison to the previous measurement by Sze et al.,15 are given in Table I. Figure 2 is a comparison of the total ion yield and the calculated X-ray absorption spectra (see Table II). As expected, a large shift of 55.15 eV is observed for values obtained by B3LYP/TZVP calculations with respect to the experimental ones. The shift arises from failure to properly describe the core hole in the final excited state, and limitations of the basis set. The calculated spectrum aligns well with the experimental spectrum for the first two major peaks, but the higher states are shifted higher in energy than the observed transitions. This difference is likely a result of the higher-energy
Energy (eV) 2475.4(1) 2476.6(1) 2477.6 (2) 2478.4(1) 2479.0(3) 2480.4 2481.3(3) 2488.1(3)
Previous experiment15 TV (eV) 5 3.8 2.8 2 1.4
Energy (eV) 2475.8
TV (eV) 4.6
2479.0 2480.0
1.4 0.4
− 0.9 − 7.7
2484.8 2488.7
states not being correctly modeled at this level of theory. The inclusion of more excited states into the calculations could result in a lowering of the higher energy states. Also, this computational approach does not take into account electronic relaxation. Therefore, the three observed transitions between 2477 and 2480 eV are assigned to (18a + 19a + 9a ), (10a + 11a ), and (12a + 20a ) with the calculated positions being shifted by 0.5, 0.9, and 1.5 eV, respectively. Tables III and IV give the calculated sulfur composition of the five highest occupied molecular orbitals (HOMOs) and the five lowest unoccupied molecular orbitals (LUMOs) obtained with the two different methods (B3LYP in Table III and QCISD in Table IV). The two population analyses show minor differences. The ab initio and DFT methods generally agree on the sulfur composition of the molecular orbitals, but differ on the oxygen composition. The DFT method shows the HOMO being mostly oxygen-based and the LUMO being sulfur-based, while, the ab initio method shows less oxygen character and more sulfur character in the HOMO. The QCISD method shows more configuration interaction mixing between the HOMOs and LUMOs and thus spreading the electron density and making the sulfur slightly more nucleophilic than the oxygen. This suggests a π -bond is more relevant to for the smaller bond distance as suggested by crystal structure study.47 However, an AIM analysis point to a more electrostatic interaction. AIM theory postulates that molecular properties can be determined through topology of the electron charge density.38, 39 These properties are derived from the examination of the gradient of the electron charge density. The maximums and minimums of the gradient are known as critical points, where bond critical points are the minimum density between a pair of nuclei. The Laplacian of the electron charge density shows the relative depletion and concentration of electron charge across the atoms with values being positive and negative, respectively.38, 39 According to Bader,39 the depletion and concentration of charge at bond critical points determines the bonding character, ionic or covalent. Figure 3 shows the Laplacian of the electron charge density for DMSO in a molecular plane. As seen here, the bond critical point for
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 137.149.200.5 On: Sun, 30 Nov 2014 08:28:49
024314-4
Coutinho et al.
J. Chem. Phys. 140, 024314 (2014)
FIG. 2. Total ion yield spectrum (black) compared with calculated B3LYP/TZVP TDDFT spectrum (blue) where the maximum of both spectra was normalized to one. The calculated spectrum was energy shifted by 55.15 eV.
sulfur-oxygen bond has a positive Laplacian of the density (∼0.777 a.u.) and thus corresponds to an ionic bond. Therefore, the bond has a large electrostatic component as shown previously;48 however, small part is likely due to the π -bond as seen from the molecular orbital compositions. From the molecular orbital composition, the 8a orbital has partly carbon-sulfur σ ∗ character and partly sulfuroxygen π * character based on the QCISD; however, the DFT shows mostly carbon-sulfur σ ∗ character with a small amount of sulfur-oxygen π * character. From the QCISD, the 15a orbital decreases in carbon-sulfur σ ∗ character and
TABLE II. The calculated energies shifted 55.15 eV to match experiment and oscillator strengths for transitions obtained from ORCA at the B3LYP/TZVP level. Energy (eV) 2475.32 2475.36 2476.40 2476.72 2477.84 2477.99 2478.09 2478.85 2479.34 2480.48 2480.63 2481.43 2481.63 2481.87 2482.47
Transition 1a−1 → 8a (93%) + 9a (5%) 16a (97%) 15a (82%) + 17a (16%) 17a (76%) + 15a (18%) 18a (96%) + 17a (2%) 19a (95%) + 17a (4%) 9a (93%) + 8a (6%) 10a (97%) + 11a (2%) 11a (95%) + 10a (2%) 12a (98%) + 11a (1%) 20a (92%) + 22a (7%) 22a (88%) + 20a (7%) 13a (99%) 21a (94%) + 22a (3%) 23a (96%) + 21a (2%)
Oscillator strength (a.u.2 ) 5.413 × 10−3 4.365 × 10−3 1.204 × 10−4 1.673 × 10−3 2.643 × 10−5 1.189 × 10−4 7.412 × 10−4 1.148 × 10−4 5.432 × 10−4 6.377 × 10−4 1.399 × 10−3 3.123 × 10−3 5.696 × 10−6 4.523 × 10−4 3.780 × 10−4
increases in sulfur-oxygen π * character, while DFT suggests only carbon-hydrogen σ ∗ character. The 16a orbital has mostly sulfur-oxygen σ ∗ character with some contribution from the carbon-sulfur σ ∗ . The 17a has mainly carbon-sulfur σ ∗ character and carbon-hydrogen σ ∗ with a small amount of sulfur-oxygen σ ∗ . The spectrum is dominated by a large peak at 2475.4 eV, with two small shoulders on the high-energy side, at around 2476.6 eV and 2477.6 eV. We have assigned these features to transitions to the unoccupied virtual valence levels (8a + 16a ), (15a + 17a ), and (18a + 19a + 9a ). Next, there are two small peaks at 2478.4 and 2479.0 eV, corresponding, respectively, to excitations to (10a + 11a ) and (12a + 20a ), which were previously assigned as 5p and 6p Rydberg levels.15 We cannot directly measure the ionization threshold from this ion yield data, neither the TIY or any of the partial ion yields show any appreciable change in the region. Therefore, after carefully checking the calibration procedure, we have chosen to use the photoemission measurements of Sodhi and Cavell17 for this energy location. Two broad features located above the ionization threshold (see Fig. 4), centered at 2481.3 and 2488.1 eV, are also observed in the TIY spectrum and are assigned to S 3d continuum resonances.15 Similar to our measurements on ethylene sulfide49 and chloroform,6 we have detected a wealth of singly charged ions within this energy region: H+ , C+ , CH+ , CH2 + , CH3 + , C2 H5 + , O+ , and S+ , shown in Fig. 1. The spectra for CH+ and CH2 + are not shown since they look identical to that of CH3 + and the TIY. Interestingly, they do not show the sequence corresponding to subsequent fragmentation process similar to that seen for the CHx series in ethylene sulfide. Also similar to ethylene sulfide, the most intense partial ion
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 137.149.200.5 On: Sun, 30 Nov 2014 08:28:49
024314-5
Coutinho et al.
J. Chem. Phys. 140, 024314 (2014)
TABLE III. Comparison of the population analysis methods for the ground state of DMSO for first five HOMOs and the 11 LUMOs obtained from the B3LYP /TZVP level calculations. The percentage is the total character for a given element. The first LUMO is 8a orbital. Sulfur MO 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
Symmetry 12a 6a 13a 7a 14a 8a 15a 16a 17a 18a 19a 9a 10a 11a 12a 20a
AIM (%) 19 12 33 6 27 40 4 51 22 10 11 11 4 6 16 10
Oxygen NPA (%) 19 11 42 4 26 45 0 61 25 10 10 8 3 7 6 1
AIM (%) 49 5 46 75 58 9 3 20 9 3 6 3 2 4 1 6
yields are for the atomic cations, H+ , C+ , and S+ , closely followed by the molecular CH+ 3 fragment ion. This result points to a complete fragmentation of the DMSO molecule when excited in the core level of its central atom, sulfur. Additionally, we do not see a significant drop in the H+ signal as we scan across the 8a + 16a transition, unlike the observation by Hansen et al.,50 where a dramatic 40% drop in H+ signal was observed across the 6σ ∗ resonance, with a corresponding increase in signal for Cl3 + and Cl4 + . These intensity changes can be used as one possible fingerprint for an ultrafast dissociation pathway and the creation of neutral hydrogen. Unlike HCl or H2 S, the hydrogen is not bound to the absorbing atom; therefore, our measured H+ signal is due to an indirect, or is a secondary decay product of the main transition. This indicates ultrafast dissociation
Carbon NPA (%) 48 3 42 81 61 5 1 14 8 3 4 1 1 1 1 2
AIM (%) 19 44 9 16 11 22 18 14 29 20 20 24 19 21 33 31
Hydrogen NPA (%) 22 52 2 22 8 34 34 16 46 16 20 24 20 26 88 84
AIM (%) 13 39 13 4 4 29 74 15 40 68 64 62 75 68 51 53
NPA (%) 8 34 14 −2 2 16 68 12 24 74 70 72 80 72 12 14
only can occur when the hydrogen is bound to the absorbing atom. For doubly charged species, the following ions have been observed: O+ /S2 + (m/z 16), SO2 + (m/z 24), and CH3 SO2 + (m/z 31.5), see Fig 1. The most intense of these is a combination of the doubly charged S2 + and singly charged O+ , which have the same mass to charge ratio and cannot be distinguished with a single energy scan. Additional measurements similar to our measurements on SO2 , using the 34 S isotope would reveal the difference.51 Nevertheless, considering the similarity with the S+ ion yield, we may infer a higher prevalence of the S2 + cation in the mixed spectrum, with the O+ ion being similar to that of C+ and the majority of other single charged ions. Observation of a stable doubly charged ion with six atoms in this photon energy range (CH3 SO2 + ) points to
TABLE IV. Comparison of the population analysis methods for the ground state of DMSO for first five HOMOs and the 11 LUMOs obtained from the QCISD/TZVP level calculations. The percentage is the total character for a given element. The first LUMO is 8a orbital. Sulfur MO 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
Symmetry 5a 6a 13a 7a 14a 8a 15a 16a 17a 9a 18a 10a 19a 11a 20a 12a
AIM (%) 2 13 23 21 55 42 48 35 12 18 5 1 5 3 19 20
Oxygen NPA (%) −3 17 32 17 68 52 57 51 8 12 6 1 4 3 19 20
AIM (%) 0 39 59 31 22 25 28 41 12 14 2 0 7 6 27 24
Carbon NPA (%) 0 38 53 32 18 15 21 31 11 11 0 0 7 6 34 29
AIM (%) 48 24 12 41 18 29 21 21 40 35 50 54 44 46 31 32
Hydrogen NPA (%) 58 22 14 56 10 34 22 16 34 32 40 38 42 40 32 32
AIM (%) 50 24 5 7 5 4 3 4 35 34 43 45 43 45 23 25
NPA (%) 46 24 0 −2 4 0 0 2 50 48 56 60 48 52 18 22
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 137.149.200.5 On: Sun, 30 Nov 2014 08:28:49
024314-6
Coutinho et al.
FIG. 3. The Laplacian of the electron density contour map, in the molecular plane obtained with the AIMALL program, for DMSO, and calculated at the B3LYP/TZVP level. The contours begin at zero and decrease (blue solid lines) and increase (red dashed lines) in steps of ±0.02, ± 0.04, ± 0.08, ± 0.2, ± 0.4, ± 0.8, ± 2.0, ± 4.0, and ±8.0. The black solid lines represent the bond path between the nuclei and the bond critical points are denoted by the green spheres.
the high stability of this ionic species. The SO2 + ion presents a low intensity, comparable with the intensity of the observed anions. A triply charge ion, S3 + , has also been observed, but we have not measured the corresponding ion-yield curve, due to its extremely low count rate.
FIG. 4. Partial ion yields in the photon energy region 2465–2535 eV for S+ , CH3 + , CH3 SO2 + , C2 H5 + , SO2 + , C+ , H+ , and (S2 + + O+ ) resulting from photoexcitation of DMSO near the S 1s ionization threshold.
J. Chem. Phys. 140, 024314 (2014)
The ionic fragment with m/z 29 may be associated with two different ions, COH+ or C2 H5 + , both of which require an extremely fast rearrangement reaction. Studies on the photodissociation dynamics of DMSO on the VUV energy range points to a very stable CH3 SO intermediate fragment,52 which would favor the COH+ formation. However, a previous study using threshold photoelectron photoion (TPEPICO) mass spectrometry53 reported the detection of the m/z 29 fragment, assigning it to the C2 H5 + ion, which would result directly from the parent ion. The singly charged fragments C2 H3 + , H3 S+ , and CH3 S+ , observed in the VUV energy region19 were not detected in this work. Measurement of the partial yields in the same photon energy range for a selected ionic fragment allows to emphasize spectral features which would be otherwise hidden by electronic transitions with high absorption cross sections,54 as can be seen in this instance by the S+ partial-yield curve. The S+ spectrum shows a peculiar behavior in comparison to the TIY, with a very good definition of the transitions at 2476.6 eV and 2478.4 eV. This could perhaps be explained considering a smaller relative contribution of the strong resonance at 2475.5 eV to the formation of this ion. In this case, the hidden transitions at 2476.6 eV and 2478.4 eV would become more visible. All other cation yields (see Figs. 1 and 4) are quite similar. This is a rather general observation, already reported in some of our previous studies of molecules excited around a deep edge, such as the Cl K-edge (HCl, Cl2 , CHCl3 , and SOCl2 ).10, 19, 55, 56 Following photoexcitation or photoionization from shallower edges, the subsequent nonradiative decay is mainly core-valence-valence, with the creation of singly or doubly charged ions. Following deep core excitation or ionization, cascade Auger processes occur, with the fast creation of multiply charged species, which then undergo Coulomb explosion, and produce several ionic species at the same time with equal probability.
FIG. 5. Total-ion-yield (TIY) and partial anion yields in the photon energy region 2465–2535 eV for C− , S− , and O− following photoexcitation of DMSO near the S 1s ionization threshold.
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 137.149.200.5 On: Sun, 30 Nov 2014 08:28:49
024314-7
Coutinho et al.
The following anions have been observed: H− , C− , O− , and S− . H− had a very low intensity and will not be considered in the following discussion. The ion yields for the other three anions are shown in Figure 5. Cross sections for negative photoionization are usually several orders of magnitude smaller as compared to positive photoionization. As we have previously shown,7 anion-yield spectroscopy using x rays is a selective probe of molecular core-level processes, and can provide unique experimental verification of shape resonances. In this instance, we observe that all three anions are formed on the primary resonance features below ionization edge, with an enhancement of signal on the (12a + 20a ) resonance for the oxygen anion. This experimentally indicates a higher probability of breaking the S–O bond, which was not indicated by our ground state calculations. Post-collision interaction effects are clearly observed at the ionization threshold for the sulfur and carbon anions.8 This is indicated by the gradual exponential decaying shape of their intensity curves with increasing photon energy for the first 3–5 eV above the ionization threshold. All of the anions show a similar above threshold behavior, gradually decaying with no indication of a resonance. This indicates that the two above ionization threshold features observed in the TIY and cation partial yields are due to shape resonances, with no or minimal doubly excited or shake up states being observed.7 V. CONCLUSIONS
Negative and positive ionic fragmentation of the DMSO molecule has been observed around the S 1s edge. An extremely complex array of dissociation reactions is evidenced, as all detected atomic and molecular fragments are observed both below and above the ionization edge. An ionic fragment associated with a very fast rearrangement reaction has been observed, C2 H5 + . This reaction must occur in the time scale of the deep core ionization and relaxation, that is, a few femtoseconds. The similarity of most of the cation yield curves is explained by photoexcitation or photoionization followed by cascade Auger decay which leads to multiply charges species which then undergo Coulomb explosion. ACKNOWLEDGMENTS
The authors thank the staff of the ALS for their excellent support. We would also like to thank Dr. Maria Novella Piancastelli for all of the suggestions and help towards the preparation of this paper. L.H.C. and G.G.B.S. would like to acknowledge financial support from the Brazilian agencies CNPq and FAPERJ. W.C.S. and D.J.G. would like to acknowledge support from the National Science Foundation (NSF) under NSF Grant No. PHY-09-70125. This work was performed at the Advanced Light Source, which is supported by U.S. Department of Energy (DOE) (DE-AC0376SF00098). 1 R. Feifel and M. N. Piancastelli, J. Electron Spectrosc. Relat. Phenom. 183,
10 (2011) and references therein. N. Piancastelli, J. Electron Spectrosc. Relat. Phenom. 100, 167 (1999) and references therein.
2 M.
J. Chem. Phys. 140, 024314 (2014) c N. Piancastelli, R. Guillemin, W. C. Stolte, D. CÃolin, and D. W. Lindle, J. Electron Spectrosc. Relat. Phenom. 155, 86 (2007). 4 W. C. Stolte, M. M. Sant’Anna, G. Öhrwall, I. Dominguez Lopez, M. N. Piancastelli, and D. W. Lindle, Phys. Rev. A 68, 022701 (2003). 5 D. Céolin, M. N. Piancastelli, R. Guillemin, W. C. Stolte, S.-W. Yu, O. Hemmers, and D. W. Lindle, J. Chem. Phys. 126, 084309 (2007) and references therein. 6 A. F. Lago, R. L. Cavasso-Filho, G. G. B. de Souza, A. C. F. Santos, W. C. Stolte, and A. S. Schlachter, Chem. Phys. Lett. 543, 34 (2012). 7 W. C. Stolte, D. L. Hansen, M. N. Piancastelli, I. Dominguez Lopez, A. Rizvi, O. Hemmers, H. Wang, A. S. Schlachter, M. S. Lubell, and D. W. Lindle, Phys. Rev. Lett. 86, 4504 (2001). 8 D. L. Hansen, W. C. Stolte, O. Hemmers, R. Guillemin, and D. W. Lindle, J. Phys. B 35, L381 (2002). 9 D. L. Hansen, G. B. Armen, M. E. Arrasate, J. Cotter, G. R. Fisher, K. T. Leung, J. C. Levin, R. Martin, P. Neill, R. C. C. Perera, I. A. Sellin, M. Simon, Y. Uehara, B. Vanderford, S. B. Whitfield, and D. W. Lindle, Phys. Rev. A 57, R4090 (1998). 10 W. C. Stolte, R. Guillemin, I. N. Demchenko, G. Öhrwall, S.-W. Yu, J. A. Young, M. Taupin, O. Hemmers, M. N. Piancastelli, and D. W. Lindle, J. Phys. B 43, 155202 (2010). 11 R. Guillemin, S. Sheinerman, C. Bomme, T. Marin, L. Journel, T. Marchenko, R. K. Kushawaha, N. Trcera, M. N. Piancastelli, and M. Simon, Phys. Rev. Lett. 109, 013001 (2012) and references therein. 12 C. Bomme, R. Guillemin, S. Sheinerman, T. Marin, L. Journel, T. Marchenko, R. K. Kushawaha, N. Trcera, M. N. Picancastelli, and M. Simon, J. Phys. B 46, 215101 (2013). 13 R. Boden, J. C. Murrell, and H. Schafer, FEMS Microbiol. Lett. 322, 188 (2011). 14 J. Farrant, “General observations on cell preservation,” in Low-temperature Preservation in Medicine and Biology, edited by M. J. Ashwood-Smith (Pitman Medical Limited, Kent, England, 1980). 15 K. H. Sze, C. E. Brion, M. Tronc, S. Bodeur, and A. P. Hitchcock, Chem. Phys. 121, 279 (1988). 16 H. Bock and B. Solouki, Angew. Chem., Int. Ed. 11, 436 (1972). 17 R. N. S. Sodhi and R. G. Cavell, J. Electron Spectrosc. Relat. Phenom. 41, 1 (1986); 43, 215 (1987). 18 E. A. Drage, P. Cahillane, S. V. Hoffman, N. J. Mason, and P. Limão-Vieira, Chem. Phys. 331, 447 (2007). 19 W. C. Stolte, R. Guillemin, S.-W. Yu, and D. W. Lindle, J. Phys. B 41, 145102 (2008). 20 C. Reynaud, S. Bodeur, J. L. Maréchal, D. Bazin, P. Millié, I. Nenner, U. Rockland, and H. Baumgätel, Chem. Phys. 166, 411 (1992). 21 T. A. Ferrett, D. W. Lindle, P. A. Heimann, H. G. Kerkhoff, U. E. Becker, and D. A. Shirley, Phys. Rev. A 34, 1916 (1986). 22 S. Nemsak and C. S. Fadley, private communication (2012). 23 M. J. Frisch, G. W. Trucks, H. B. Schlegelet al., Gaussian 09, Revision C.1, Gaussian, Inc., Wallingford, CT, 2009. 24 V. Typke, J. Mol. Struct. 384, 35 (1996). 25 J. A. Pople, M. Head-Gordon, and K. Raghavachari, J. Chem. Phys. 87, 5968 (1987). 26 A. Schäfer, C. Huber, and R. Ahlrichs, J. Chem. Phys. 100, 5829 (1994). 27 C. Lee, W. Yang, and R. G. Parr, Phys. Rev. B 37, 785 (1988). 28 A. D. Becke, J. Chem. Phys. 98, 5648 (1993). 29 E. I. Solomon, B. Hedman, K. O. Hodgson, A. Dey, and R. K. Szilagyi, Coord. Chem. Rev. 249, 97 (2005). 30 S. V. Didziulis, S. L. Cohen, A. A. Gewirth, and E. I. Solomon, J. Am. Chem. Soc. 110, 250 (1988). 31 A. A. Gewirth, S. L. Cohen, H. J. Schugar, and E. I. Solomon, Inorg. Chem. 26, 1133 (1987). 32 D. Rokhsana, A. E. Howells, D. M. Dooley, and R. K. Szilagyi, Inorg. Chem. 51, 3513 (2012). 33 D. Rokhsana, D. M. Dooley, and R. K. Szilagyi, J. Biol. Inorg. Chem. 13, 371 (2008). 34 E. I. Solomon, S. I. Gorelsky, and A. J. Dey, J. Comput. Chem. 27, 1415 (2006). 35 E. I. Solomon, Inorg. Chem. 45, 8012 (2006). 36 R. K. Szilagyi and M. A. Winslow, J. Comput. Chem. 27, 1385 (2006). 37 R. Sarangi and S. Gorelsky, J. Am. Chem. Soc. 130, 3866 (2008). 38 R. F. W. Bader, Atoms in Molecules: A Quantum Theory (Oxford University Press, Oxford, 1990). 39 R. F. W. Bader, Chem. Rev. 91, 893 (1991). 40 T. A. Keith, AIMALL, version 13.05.06, TK Gristmill Software, Overland Park, KS, USA, 2013. 3 M.
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 137.149.200.5 On: Sun, 30 Nov 2014 08:28:49
024314-8 41 J.
Coutinho et al.
P. Foster and F. Weinhold, J. Am. Chem. Soc. 102, 7211 (1980). Neese, Interdiscip. Rev. Comput. Mol. Sci. 2, 73 (2012). 43 G. S. DeBeer, T. Petrenko, and F. Neese, J. Phys. Chem. A 112, 12936 (2008). 44 G. S. DeBeer, T. Petrenko, and F. Neese, Inorg. Chim. Acta 361, 965 (2008). 45 G. S. DeBeer and F. Neese, Inorg. Chem. 49, 1849 (2010). 46 J. W. Cubbage and W. S. Jenks, J. Phys. Chem. A 105, 10588 (2001). 47 R. Thomas, C. B. Shoemaker, and K. Eriks, Acta Crystallogr. 21, 12 (1966). 48 J. A. Dobado, H. Martnez-Garca, and M. R. Sundberg, J. Am. Chem. Soc. 121, 3156 (1999). 49 W. C. Stolte and G. Öhrwall, J. Chem. Phys. 133, 014306 (2010). 50 D. L. Hansen, M. E. Arrasate, J. Cotter, G. R. Fisher, K. T. Leung, J. C. Levin, R. Martin, P. Neill, R. C. C. Perera, I. A. Sellin, M. Simon, Y. 42 F.
J. Chem. Phys. 140, 024314 (2014) Uehara, B. Vanderford, S. B. Whitfield, and D. W. Lindle, Phys. Rev. A 57, 2608 (1998). 51 W. C. Stolte, R. Guillemin, A. Wolska, R. Feng, I. C. Tran, and D. W. Lindle, J. Electron Spectrosc. Relat. Phenom. 144–147, 157 (2005). 52 H.-Q. Zhao, Y.-S. Cheung, D. P. Heck, C. Y. Ng, T. Tetzlaff, and W. S. Jenks, J. Chem. Phys. 106, 86 (1997). 53 Q. Zha, T. Nihimura, and G. G. Meisels, Int. J. Mass Spectrom. Ion Process. 83, 1 (1988). 54 W. C. Stolte, I. Dimitriu, S.-W. Yu, G. Öhrwall, M. N. Piancastelli, and D. W. Lindle, J. Chem. Phys. 131, 174306 (2009). 55 K. P. Bowen, W. C. Stolte, A. F. Lago, J. Z. Dávalos, M. N. Picancastelli, and D. W. Lindle, J. Chem. Phys. 137, 204313 (2012). 56 A. F. Lago, A. C. F. Santos, W. C. Stolte, A. S. Schlachter, and G. G. B. de Souza, J. Electron Spectrosc. Relat. Phenom. 144–147, 161 (2005).
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 137.149.200.5 On: Sun, 30 Nov 2014 08:28:49
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 137.149.200.5 On: Sun, 30 Nov 2014 08:28:49
THE JOURNAL OF CHEMICAL PHYSICS 140, 024314 (2014)
Positive and negative ion formation in deep-core excited molecules: S 1s excitation in dimethyl sulfoxide L. H. Coutinho,1,a) D. J. Gardenghi,2 A. S. Schlachter,3 G. G. B. de Souza,4 and W. C. Stolte2,3 1
Physics Institute, Federal University of Rio de Janeiro, Rio de Janeiro, RJ 21941-972, Brazil Department of Chemistry, University of Nevada, Las Vegas, Nevada 89154-4003, USA 3 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA 4 Chemistry Institute, Federal University of Rio de Janeiro, Rio de Janeiro, RJ 21949-900, Brazil 2
(Received 12 September 2013; accepted 17 December 2013; published online 13 January 2014) The photo-fragmentation of the dimethyl sulfoxide (DMSO) molecule was studied using synchrotron radiation and a magnetic mass spectrometer. The total cationic yield spectrum was recorded in the photon energy region around the sulfur K edge. The sulfur composition of the highest occupied molecular orbital’s and lowest unoccupied molecular orbital’s in the DMSO molecule has been obtained using both ab initio and density functional theory methods. Partial cation and anion-yield measurements were obtained in the same energy range. An intense resonance is observed at 2475.4 eV. Sulfur atomic ions present a richer structure around this resonant feature, as compared to other fragment ions. The yield curves are similar for most of the other ionic species, which we interpret as due to cascade Auger processes leading to multiply charged species which then undergo Coulomb explosion. The anions S− , C− , and O− are observed for the first time in deep-core-level excitation of DMSO. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4861050] I. INTRODUCTION
The excitation of core electrons in molecules usually leads to strong resonances in the photoabsorption spectrum, observed below and above the ionization threshold. In the former case, the observed resonances are either associated with virtual, molecular antibonding states or with diffuse Rydberg states.1 Above the edge, the observed prominent features are usually classified as shape resonances.2 Very close to the ionization edge, doubly (or multiply) excited states are also observed. All these electronically excited states have a short lifetime (from one to tens of femtoseconds) and thus decay very rapidly. Excited states located at energies below the edge decay mainly through resonant Auger (participator or spectator) processes. In the first case, the excited electron participates in the Auger-type process and the final state is a molecular ion, while in the second case, the excited electron remains as a spectator during the Auger process, and the final state is an excited molecular ion (two holes-one electron). It has been experimentally observed that molecular antibonding states decay preferentially through participator Auger processes, while Rydberg states usually decay by spectator Auger processes.3 Excited final ionic states reached after spectator decay are in general highly dissociative and as a result excitation to Rydberg state results in an intensification of the fragmentation of the molecule. Above the ionization edge, normal Auger processes become very important and Auger decay is associated with double ionization. Following deep core excitation, an Auger cascade can follow, leading to multiply charged states. Doubly or multiply ionized states are usually dissociative, due to the Coulomb repulsion and, as a consequence, an increasing degree of fragmentation of the molecule is observed. a) Author to whom correspondence should be addressed. [email protected].
0021-9606/2014/140(2)/024314/8/$30.00
Although most of the experimental information gathered about the decay of core-excited molecules has been related to the detection of positive ions, the decay of core excited molecules into anions, albeit associated with much smaller cross sections, is also relevant, and has been the focus of several important pieces of work.4–6 It has been shown, for instance, that negative ions can be formed in association with the decay of resonances energetically located below and above the ionization edge.7 Although in small molecules the production of negative ions is concentrated in the decay of resonances below the edge, for larger molecules a significant proportion of negative ions is formed above the edge, due to the polar dissociation of triply charged cations. Very close to the edge, post-collision-interaction (PCI) effects are considered to play an important role in the formation of anions.8–12 Up to the present and to the best of our knowledge, studies of negative ion formation following core excitation have been dedicated to the excitation and ionization of shallow core levels (B 1s, C 1s, O 1s, S 2p, Cl 2p) which decay mostly through core-valence-valence (KVV or LVV) states. In this case, depletion of the valence shell leads to the ionic dissociation of the molecule. With the excitation and ionization of deep core levels, with much shorter lifetimes, cascading Auger processes may also contribute to the formation of multiply charged ions and to an enhancement in the degree of fragmentation of the molecule. Recently, a study on the Cl 1s edge of the chloroform molecule showed the formation of H− , C− , and Cl− .5 In the present work, positive and negative-ion-yield spectroscopy of the dimethyl sulfoxide (DMSO) molecule has been studied around the Sulfur K-edge. DMSO, (CH3 )2 SO, is a colorless liquid with a large number of applications, both in laboratory and industry, being considered one of the safest
140, 024314-1
© 2014 AIP Publishing LLC
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 137.149.200.5 On: Sun, 30 Nov 2014 08:28:49
024314-2
Coutinho et al.
J. Chem. Phys. 140, 024314 (2014)
solvents available. In the ocean sulfur cycle, a large fraction of dimethyl sulfide (DMS) is converted into the less volatile DMSO by bacteria.13 It also acts as a rinsing agent in the electronic industry and as a cryoprotectant to preserve organs and tissues, and in long-term storage of embryonic stem cells.14 The photoabsorption spectrum of DMSO in the sulfur K edge region was recorded previously by Sze et al.15 using synchrotron radiation from the LURE ACO storage ring. They assigned five features around the S 1s ionization threshold, related to transitions to unoccupied virtual valence levels and Rydberg states. Electron-energy-loss spectra in the valence shell and the S 2s, S 2p, O 1s, and C 1s inner shells were also presented. The ultraviolet photoelectron spectrum for this molecule was obtained by Bock and Solouki.16 The first valence-shell ionization potential occurs at 9.01 eV. The sulfur KLL Auger and 1s and 2p photoionization of DMSO were studied by Sodhi and Cavell,17 who determined the S 1s edge occurs at 2480.90 eV. More recently, Drage et al.18 reported measurements of the high-resolution VUV photoabsorption spectrum in the 3.7–10.8 eV energy range.
II. EXPERIMENTAL TECHNIQUE
The experimental arrangement has been described before.19 Briefly, the sample is inserted into the vacuumchamber collision area as a 2 mm effusive gas jet, which intersects the photon beam in a plane perpendicular to the photon beam. The resulting ions are accelerated and focused onto the entrance of a 180◦ magnetic mass analyzer, and are subsequently detected by a channel electron multiplier. The spectrometer is able to measure mass/charge ratios up to 60 with resolution of approximately 1/65. The polarity of the lenses and the magnetic field may be switched to allow measurement of either positive or negative ions produced in the ionization region. The vacuum chamber base pressure was approximately 10−7 Torr, while during the present experiment it was kept around 1 × 10−5 Torr. The measurements were performed on bending-magnet beamline 9.3.1 at the Advanced Light Source, Lawrence Berkeley National Laboratory. This beamline provides photons in the 2200–5500 keV energy region, with a resolving power of approximately 6800. Calibration at the S 1s-edge was performed by comparison to Reynaud et al.20 and Ferrett et al.21 using the S (1s)−1 → 6tu transition in SF6 at 2486.0 eV . We estimated the flux of the incident radiation near the S K-edge to be approximately 4×1010 photons/s, with a
(1a )2 S 1s
(2a )2 O 1s
(1a )2 (3a )2 C 1s
resolution of 0.4 eV by comparison to measurements near the S (1s)−1 → 6a1 3b2∗ transition in H2 S.22 The DMSO sample was purchased from Sigma-Aldrich (part number D8418-50ML) with purity of 99.9%. No further purification was used except for degassing the liquid sample by multiple freeze-pump-thaw cycles before admitting the vapor into the experimental chamber through a mechanical leak valve. The presented spectra for each fragment were obtained by the sum of several measurements, normalized separately by sample pressure and photon flux.
III. THEORETICAL METHOD
All calculations were performed using Gaussian09 Revision C.0123 starting from the crystal structure.24 The crystal structure was optimized in the gas phase using a high-level correlated wave function-based ab initio method, quadratic configuration interaction with single and double excitation levels (QCISD)25 with the Aldrich’s triple-ζ valence basis set with polarization functions used for atoms (TZVP).26 The electron density was calculated from the optimized geometry using two methods: Density Functional Theory (DFT) with the hybrid functional B3LYP5,27, 28 and a wave functionbased ab initio method, QCISD, both with TZVP. X-ray absorption spectroscopy (XAS) has been used to obtain quantitative ground state information about the orbital composition of the lower lying unoccupied frontier molecular orbitals.29 Furthermore, theoretical calculations have been successful at reproducing the experimental orbital composition.30–37 The theoretical orbital compositions were obtained from Bader Atoms in Molecules (AIM)38, 39 using AIMAll program40 and Weinhold natural population analysis (NPA)41 as implemented in Gaussian09. The NPA valance orbital sets for the sulfur were 3s3p. The XAS spectra were simulated with the ORCA computational package42 that is based on a one electron theoretical approach using time-dependent density functional theory (TD-DFT) that has been demonstrated to successful simulate XAS spectra.43–45 IV. RESULTS AND DISCUSSION
The DMSO molecule presents a Cs symmetry with the mirror plane passing through the formal sulfur-oxygen double bond. The optimized geometry produced a similar structure to those previously determined for both the ground and excited states.46 The molecular configuration of the ground state was obtained using B3LYP/TZVP and is as follows:
(4a )2 S 2s
(5a )2 (2a )2 (6a )2 S 2p
(7a )2 (8a )2 (3a )2 (9a )2 (10a )2 (11a )2 (4a )2 (5a )2 (6a )2 (12a )2 (13a )2 (7a )2 (14a )2 , valence orbitals
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 137.149.200.5 On: Sun, 30 Nov 2014 08:28:49
024314-3
Coutinho et al.
J. Chem. Phys. 140, 024314 (2014) TABLE I. Experimental energies and term values (TV) of major transitions of DMSO following S 1s excitation. The quoted ionization threshold was measured by Sodhi and Cavell.17 All errors in the table are standard deviations, total errors include a calibration error of 0.1 eV from using the S (1s)−1 → 6tu transition in SF6 at 2486.0 eV,17, 20, 21 when possible comparison is made to Sze et al.15 Present experiment
Configuration 8a + 16a 15a + 17a 18a + 19a + 9a 10a +11a 12a + 20a IP17 3d 3d
FIG. 1. Total-ion-yield (TIY) and partial ion yields in the photon energy region 2470–2485 eV for S+ , CH3 + , CH3 SO2 + , C2 H5 + , SO2 + , C+ , H+ , and (S2 + + O+ ) resulting from photoexcitation of DMSO near the S 1s ionization threshold. Dashed lines correspond to transitions summarized in Table I. The ionization threshold shown is from Ref.17.
with (8a )0 , (15a )0 , (16a )0 , (17a )0 , and (18a )0 being the five lowest unoccupied molecular orbitals. These results are nearly identical to those obtained using QCISD/TZVP and the previous work by Sze et al.15 As can be seen by the blue curve in the top panel of Fig. 1, the total ion yield (TIY) is similar to the photoabsorption spectrum previously measured by Sze et al.15 Differences include a more asymmetric primary resonance, followed by two clearly visible transitions prior to the ionization threshold, versus a more symmetric primary resonance and one clearly visible transition. Both appear identical in the region above the ionization threshold. It is possible that the differences are due to an improved energy resolution of the photon beam, but the estimated 0.35 eV bandpass quoted by Sze et al.15 is similar to the estimated 0.36 eV at 2500 eV for this measurement. The energies of these transitions, term values, calculated assignments, and a comparison to the previous measurement by Sze et al.,15 are given in Table I. Figure 2 is a comparison of the total ion yield and the calculated X-ray absorption spectra (see Table II). As expected, a large shift of 55.15 eV is observed for values obtained by B3LYP/TZVP calculations with respect to the experimental ones. The shift arises from failure to properly describe the core hole in the final excited state, and limitations of the basis set. The calculated spectrum aligns well with the experimental spectrum for the first two major peaks, but the higher states are shifted higher in energy than the observed transitions. This difference is likely a result of the higher-energy
Energy (eV) 2475.4(1) 2476.6(1) 2477.6 (2) 2478.4(1) 2479.0(3) 2480.4 2481.3(3) 2488.1(3)
Previous experiment15 TV (eV) 5 3.8 2.8 2 1.4
Energy (eV) 2475.8
TV (eV) 4.6
2479.0 2480.0
1.4 0.4
− 0.9 − 7.7
2484.8 2488.7
states not being correctly modeled at this level of theory. The inclusion of more excited states into the calculations could result in a lowering of the higher energy states. Also, this computational approach does not take into account electronic relaxation. Therefore, the three observed transitions between 2477 and 2480 eV are assigned to (18a + 19a + 9a ), (10a + 11a ), and (12a + 20a ) with the calculated positions being shifted by 0.5, 0.9, and 1.5 eV, respectively. Tables III and IV give the calculated sulfur composition of the five highest occupied molecular orbitals (HOMOs) and the five lowest unoccupied molecular orbitals (LUMOs) obtained with the two different methods (B3LYP in Table III and QCISD in Table IV). The two population analyses show minor differences. The ab initio and DFT methods generally agree on the sulfur composition of the molecular orbitals, but differ on the oxygen composition. The DFT method shows the HOMO being mostly oxygen-based and the LUMO being sulfur-based, while, the ab initio method shows less oxygen character and more sulfur character in the HOMO. The QCISD method shows more configuration interaction mixing between the HOMOs and LUMOs and thus spreading the electron density and making the sulfur slightly more nucleophilic than the oxygen. This suggests a π -bond is more relevant to for the smaller bond distance as suggested by crystal structure study.47 However, an AIM analysis point to a more electrostatic interaction. AIM theory postulates that molecular properties can be determined through topology of the electron charge density.38, 39 These properties are derived from the examination of the gradient of the electron charge density. The maximums and minimums of the gradient are known as critical points, where bond critical points are the minimum density between a pair of nuclei. The Laplacian of the electron charge density shows the relative depletion and concentration of electron charge across the atoms with values being positive and negative, respectively.38, 39 According to Bader,39 the depletion and concentration of charge at bond critical points determines the bonding character, ionic or covalent. Figure 3 shows the Laplacian of the electron charge density for DMSO in a molecular plane. As seen here, the bond critical point for
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 137.149.200.5 On: Sun, 30 Nov 2014 08:28:49
024314-4
Coutinho et al.
J. Chem. Phys. 140, 024314 (2014)
FIG. 2. Total ion yield spectrum (black) compared with calculated B3LYP/TZVP TDDFT spectrum (blue) where the maximum of both spectra was normalized to one. The calculated spectrum was energy shifted by 55.15 eV.
sulfur-oxygen bond has a positive Laplacian of the density (∼0.777 a.u.) and thus corresponds to an ionic bond. Therefore, the bond has a large electrostatic component as shown previously;48 however, small part is likely due to the π -bond as seen from the molecular orbital compositions. From the molecular orbital composition, the 8a orbital has partly carbon-sulfur σ ∗ character and partly sulfuroxygen π * character based on the QCISD; however, the DFT shows mostly carbon-sulfur σ ∗ character with a small amount of sulfur-oxygen π * character. From the QCISD, the 15a orbital decreases in carbon-sulfur σ ∗ character and
TABLE II. The calculated energies shifted 55.15 eV to match experiment and oscillator strengths for transitions obtained from ORCA at the B3LYP/TZVP level. Energy (eV) 2475.32 2475.36 2476.40 2476.72 2477.84 2477.99 2478.09 2478.85 2479.34 2480.48 2480.63 2481.43 2481.63 2481.87 2482.47
Transition 1a−1 → 8a (93%) + 9a (5%) 16a (97%) 15a (82%) + 17a (16%) 17a (76%) + 15a (18%) 18a (96%) + 17a (2%) 19a (95%) + 17a (4%) 9a (93%) + 8a (6%) 10a (97%) + 11a (2%) 11a (95%) + 10a (2%) 12a (98%) + 11a (1%) 20a (92%) + 22a (7%) 22a (88%) + 20a (7%) 13a (99%) 21a (94%) + 22a (3%) 23a (96%) + 21a (2%)
Oscillator strength (a.u.2 ) 5.413 × 10−3 4.365 × 10−3 1.204 × 10−4 1.673 × 10−3 2.643 × 10−5 1.189 × 10−4 7.412 × 10−4 1.148 × 10−4 5.432 × 10−4 6.377 × 10−4 1.399 × 10−3 3.123 × 10−3 5.696 × 10−6 4.523 × 10−4 3.780 × 10−4
increases in sulfur-oxygen π * character, while DFT suggests only carbon-hydrogen σ ∗ character. The 16a orbital has mostly sulfur-oxygen σ ∗ character with some contribution from the carbon-sulfur σ ∗ . The 17a has mainly carbon-sulfur σ ∗ character and carbon-hydrogen σ ∗ with a small amount of sulfur-oxygen σ ∗ . The spectrum is dominated by a large peak at 2475.4 eV, with two small shoulders on the high-energy side, at around 2476.6 eV and 2477.6 eV. We have assigned these features to transitions to the unoccupied virtual valence levels (8a + 16a ), (15a + 17a ), and (18a + 19a + 9a ). Next, there are two small peaks at 2478.4 and 2479.0 eV, corresponding, respectively, to excitations to (10a + 11a ) and (12a + 20a ), which were previously assigned as 5p and 6p Rydberg levels.15 We cannot directly measure the ionization threshold from this ion yield data, neither the TIY or any of the partial ion yields show any appreciable change in the region. Therefore, after carefully checking the calibration procedure, we have chosen to use the photoemission measurements of Sodhi and Cavell17 for this energy location. Two broad features located above the ionization threshold (see Fig. 4), centered at 2481.3 and 2488.1 eV, are also observed in the TIY spectrum and are assigned to S 3d continuum resonances.15 Similar to our measurements on ethylene sulfide49 and chloroform,6 we have detected a wealth of singly charged ions within this energy region: H+ , C+ , CH+ , CH2 + , CH3 + , C2 H5 + , O+ , and S+ , shown in Fig. 1. The spectra for CH+ and CH2 + are not shown since they look identical to that of CH3 + and the TIY. Interestingly, they do not show the sequence corresponding to subsequent fragmentation process similar to that seen for the CHx series in ethylene sulfide. Also similar to ethylene sulfide, the most intense partial ion
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 137.149.200.5 On: Sun, 30 Nov 2014 08:28:49
024314-5
Coutinho et al.
J. Chem. Phys. 140, 024314 (2014)
TABLE III. Comparison of the population analysis methods for the ground state of DMSO for first five HOMOs and the 11 LUMOs obtained from the B3LYP /TZVP level calculations. The percentage is the total character for a given element. The first LUMO is 8a orbital. Sulfur MO 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
Symmetry 12a 6a 13a 7a 14a 8a 15a 16a 17a 18a 19a 9a 10a 11a 12a 20a
AIM (%) 19 12 33 6 27 40 4 51 22 10 11 11 4 6 16 10
Oxygen NPA (%) 19 11 42 4 26 45 0 61 25 10 10 8 3 7 6 1
AIM (%) 49 5 46 75 58 9 3 20 9 3 6 3 2 4 1 6
yields are for the atomic cations, H+ , C+ , and S+ , closely followed by the molecular CH+ 3 fragment ion. This result points to a complete fragmentation of the DMSO molecule when excited in the core level of its central atom, sulfur. Additionally, we do not see a significant drop in the H+ signal as we scan across the 8a + 16a transition, unlike the observation by Hansen et al.,50 where a dramatic 40% drop in H+ signal was observed across the 6σ ∗ resonance, with a corresponding increase in signal for Cl3 + and Cl4 + . These intensity changes can be used as one possible fingerprint for an ultrafast dissociation pathway and the creation of neutral hydrogen. Unlike HCl or H2 S, the hydrogen is not bound to the absorbing atom; therefore, our measured H+ signal is due to an indirect, or is a secondary decay product of the main transition. This indicates ultrafast dissociation
Carbon NPA (%) 48 3 42 81 61 5 1 14 8 3 4 1 1 1 1 2
AIM (%) 19 44 9 16 11 22 18 14 29 20 20 24 19 21 33 31
Hydrogen NPA (%) 22 52 2 22 8 34 34 16 46 16 20 24 20 26 88 84
AIM (%) 13 39 13 4 4 29 74 15 40 68 64 62 75 68 51 53
NPA (%) 8 34 14 −2 2 16 68 12 24 74 70 72 80 72 12 14
only can occur when the hydrogen is bound to the absorbing atom. For doubly charged species, the following ions have been observed: O+ /S2 + (m/z 16), SO2 + (m/z 24), and CH3 SO2 + (m/z 31.5), see Fig 1. The most intense of these is a combination of the doubly charged S2 + and singly charged O+ , which have the same mass to charge ratio and cannot be distinguished with a single energy scan. Additional measurements similar to our measurements on SO2 , using the 34 S isotope would reveal the difference.51 Nevertheless, considering the similarity with the S+ ion yield, we may infer a higher prevalence of the S2 + cation in the mixed spectrum, with the O+ ion being similar to that of C+ and the majority of other single charged ions. Observation of a stable doubly charged ion with six atoms in this photon energy range (CH3 SO2 + ) points to
TABLE IV. Comparison of the population analysis methods for the ground state of DMSO for first five HOMOs and the 11 LUMOs obtained from the QCISD/TZVP level calculations. The percentage is the total character for a given element. The first LUMO is 8a orbital. Sulfur MO 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
Symmetry 5a 6a 13a 7a 14a 8a 15a 16a 17a 9a 18a 10a 19a 11a 20a 12a
AIM (%) 2 13 23 21 55 42 48 35 12 18 5 1 5 3 19 20
Oxygen NPA (%) −3 17 32 17 68 52 57 51 8 12 6 1 4 3 19 20
AIM (%) 0 39 59 31 22 25 28 41 12 14 2 0 7 6 27 24
Carbon NPA (%) 0 38 53 32 18 15 21 31 11 11 0 0 7 6 34 29
AIM (%) 48 24 12 41 18 29 21 21 40 35 50 54 44 46 31 32
Hydrogen NPA (%) 58 22 14 56 10 34 22 16 34 32 40 38 42 40 32 32
AIM (%) 50 24 5 7 5 4 3 4 35 34 43 45 43 45 23 25
NPA (%) 46 24 0 −2 4 0 0 2 50 48 56 60 48 52 18 22
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 137.149.200.5 On: Sun, 30 Nov 2014 08:28:49
024314-6
Coutinho et al.
FIG. 3. The Laplacian of the electron density contour map, in the molecular plane obtained with the AIMALL program, for DMSO, and calculated at the B3LYP/TZVP level. The contours begin at zero and decrease (blue solid lines) and increase (red dashed lines) in steps of ±0.02, ± 0.04, ± 0.08, ± 0.2, ± 0.4, ± 0.8, ± 2.0, ± 4.0, and ±8.0. The black solid lines represent the bond path between the nuclei and the bond critical points are denoted by the green spheres.
the high stability of this ionic species. The SO2 + ion presents a low intensity, comparable with the intensity of the observed anions. A triply charge ion, S3 + , has also been observed, but we have not measured the corresponding ion-yield curve, due to its extremely low count rate.
FIG. 4. Partial ion yields in the photon energy region 2465–2535 eV for S+ , CH3 + , CH3 SO2 + , C2 H5 + , SO2 + , C+ , H+ , and (S2 + + O+ ) resulting from photoexcitation of DMSO near the S 1s ionization threshold.
J. Chem. Phys. 140, 024314 (2014)
The ionic fragment with m/z 29 may be associated with two different ions, COH+ or C2 H5 + , both of which require an extremely fast rearrangement reaction. Studies on the photodissociation dynamics of DMSO on the VUV energy range points to a very stable CH3 SO intermediate fragment,52 which would favor the COH+ formation. However, a previous study using threshold photoelectron photoion (TPEPICO) mass spectrometry53 reported the detection of the m/z 29 fragment, assigning it to the C2 H5 + ion, which would result directly from the parent ion. The singly charged fragments C2 H3 + , H3 S+ , and CH3 S+ , observed in the VUV energy region19 were not detected in this work. Measurement of the partial yields in the same photon energy range for a selected ionic fragment allows to emphasize spectral features which would be otherwise hidden by electronic transitions with high absorption cross sections,54 as can be seen in this instance by the S+ partial-yield curve. The S+ spectrum shows a peculiar behavior in comparison to the TIY, with a very good definition of the transitions at 2476.6 eV and 2478.4 eV. This could perhaps be explained considering a smaller relative contribution of the strong resonance at 2475.5 eV to the formation of this ion. In this case, the hidden transitions at 2476.6 eV and 2478.4 eV would become more visible. All other cation yields (see Figs. 1 and 4) are quite similar. This is a rather general observation, already reported in some of our previous studies of molecules excited around a deep edge, such as the Cl K-edge (HCl, Cl2 , CHCl3 , and SOCl2 ).10, 19, 55, 56 Following photoexcitation or photoionization from shallower edges, the subsequent nonradiative decay is mainly core-valence-valence, with the creation of singly or doubly charged ions. Following deep core excitation or ionization, cascade Auger processes occur, with the fast creation of multiply charged species, which then undergo Coulomb explosion, and produce several ionic species at the same time with equal probability.
FIG. 5. Total-ion-yield (TIY) and partial anion yields in the photon energy region 2465–2535 eV for C− , S− , and O− following photoexcitation of DMSO near the S 1s ionization threshold.
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 137.149.200.5 On: Sun, 30 Nov 2014 08:28:49
024314-7
Coutinho et al.
The following anions have been observed: H− , C− , O− , and S− . H− had a very low intensity and will not be considered in the following discussion. The ion yields for the other three anions are shown in Figure 5. Cross sections for negative photoionization are usually several orders of magnitude smaller as compared to positive photoionization. As we have previously shown,7 anion-yield spectroscopy using x rays is a selective probe of molecular core-level processes, and can provide unique experimental verification of shape resonances. In this instance, we observe that all three anions are formed on the primary resonance features below ionization edge, with an enhancement of signal on the (12a + 20a ) resonance for the oxygen anion. This experimentally indicates a higher probability of breaking the S–O bond, which was not indicated by our ground state calculations. Post-collision interaction effects are clearly observed at the ionization threshold for the sulfur and carbon anions.8 This is indicated by the gradual exponential decaying shape of their intensity curves with increasing photon energy for the first 3–5 eV above the ionization threshold. All of the anions show a similar above threshold behavior, gradually decaying with no indication of a resonance. This indicates that the two above ionization threshold features observed in the TIY and cation partial yields are due to shape resonances, with no or minimal doubly excited or shake up states being observed.7 V. CONCLUSIONS
Negative and positive ionic fragmentation of the DMSO molecule has been observed around the S 1s edge. An extremely complex array of dissociation reactions is evidenced, as all detected atomic and molecular fragments are observed both below and above the ionization edge. An ionic fragment associated with a very fast rearrangement reaction has been observed, C2 H5 + . This reaction must occur in the time scale of the deep core ionization and relaxation, that is, a few femtoseconds. The similarity of most of the cation yield curves is explained by photoexcitation or photoionization followed by cascade Auger decay which leads to multiply charges species which then undergo Coulomb explosion. ACKNOWLEDGMENTS
The authors thank the staff of the ALS for their excellent support. We would also like to thank Dr. Maria Novella Piancastelli for all of the suggestions and help towards the preparation of this paper. L.H.C. and G.G.B.S. would like to acknowledge financial support from the Brazilian agencies CNPq and FAPERJ. W.C.S. and D.J.G. would like to acknowledge support from the National Science Foundation (NSF) under NSF Grant No. PHY-09-70125. This work was performed at the Advanced Light Source, which is supported by U.S. Department of Energy (DOE) (DE-AC0376SF00098). 1 R. Feifel and M. N. Piancastelli, J. Electron Spectrosc. Relat. Phenom. 183,
10 (2011) and references therein. N. Piancastelli, J. Electron Spectrosc. Relat. Phenom. 100, 167 (1999) and references therein.
2 M.
J. Chem. Phys. 140, 024314 (2014) c N. Piancastelli, R. Guillemin, W. C. Stolte, D. CÃolin, and D. W. Lindle, J. Electron Spectrosc. Relat. Phenom. 155, 86 (2007). 4 W. C. Stolte, M. M. Sant’Anna, G. Öhrwall, I. Dominguez Lopez, M. N. Piancastelli, and D. W. Lindle, Phys. Rev. A 68, 022701 (2003). 5 D. Céolin, M. N. Piancastelli, R. Guillemin, W. C. Stolte, S.-W. Yu, O. Hemmers, and D. W. Lindle, J. Chem. Phys. 126, 084309 (2007) and references therein. 6 A. F. Lago, R. L. Cavasso-Filho, G. G. B. de Souza, A. C. F. Santos, W. C. Stolte, and A. S. Schlachter, Chem. Phys. Lett. 543, 34 (2012). 7 W. C. Stolte, D. L. Hansen, M. N. Piancastelli, I. Dominguez Lopez, A. Rizvi, O. Hemmers, H. Wang, A. S. Schlachter, M. S. Lubell, and D. W. Lindle, Phys. Rev. Lett. 86, 4504 (2001). 8 D. L. Hansen, W. C. Stolte, O. Hemmers, R. Guillemin, and D. W. Lindle, J. Phys. B 35, L381 (2002). 9 D. L. Hansen, G. B. Armen, M. E. Arrasate, J. Cotter, G. R. Fisher, K. T. Leung, J. C. Levin, R. Martin, P. Neill, R. C. C. Perera, I. A. Sellin, M. Simon, Y. Uehara, B. Vanderford, S. B. Whitfield, and D. W. Lindle, Phys. Rev. A 57, R4090 (1998). 10 W. C. Stolte, R. Guillemin, I. N. Demchenko, G. Öhrwall, S.-W. Yu, J. A. Young, M. Taupin, O. Hemmers, M. N. Piancastelli, and D. W. Lindle, J. Phys. B 43, 155202 (2010). 11 R. Guillemin, S. Sheinerman, C. Bomme, T. Marin, L. Journel, T. Marchenko, R. K. Kushawaha, N. Trcera, M. N. Piancastelli, and M. Simon, Phys. Rev. Lett. 109, 013001 (2012) and references therein. 12 C. Bomme, R. Guillemin, S. Sheinerman, T. Marin, L. Journel, T. Marchenko, R. K. Kushawaha, N. Trcera, M. N. Picancastelli, and M. Simon, J. Phys. B 46, 215101 (2013). 13 R. Boden, J. C. Murrell, and H. Schafer, FEMS Microbiol. Lett. 322, 188 (2011). 14 J. Farrant, “General observations on cell preservation,” in Low-temperature Preservation in Medicine and Biology, edited by M. J. Ashwood-Smith (Pitman Medical Limited, Kent, England, 1980). 15 K. H. Sze, C. E. Brion, M. Tronc, S. Bodeur, and A. P. Hitchcock, Chem. Phys. 121, 279 (1988). 16 H. Bock and B. Solouki, Angew. Chem., Int. Ed. 11, 436 (1972). 17 R. N. S. Sodhi and R. G. Cavell, J. Electron Spectrosc. Relat. Phenom. 41, 1 (1986); 43, 215 (1987). 18 E. A. Drage, P. Cahillane, S. V. Hoffman, N. J. Mason, and P. Limão-Vieira, Chem. Phys. 331, 447 (2007). 19 W. C. Stolte, R. Guillemin, S.-W. Yu, and D. W. Lindle, J. Phys. B 41, 145102 (2008). 20 C. Reynaud, S. Bodeur, J. L. Maréchal, D. Bazin, P. Millié, I. Nenner, U. Rockland, and H. Baumgätel, Chem. Phys. 166, 411 (1992). 21 T. A. Ferrett, D. W. Lindle, P. A. Heimann, H. G. Kerkhoff, U. E. Becker, and D. A. Shirley, Phys. Rev. A 34, 1916 (1986). 22 S. Nemsak and C. S. Fadley, private communication (2012). 23 M. J. Frisch, G. W. Trucks, H. B. Schlegelet al., Gaussian 09, Revision C.1, Gaussian, Inc., Wallingford, CT, 2009. 24 V. Typke, J. Mol. Struct. 384, 35 (1996). 25 J. A. Pople, M. Head-Gordon, and K. Raghavachari, J. Chem. Phys. 87, 5968 (1987). 26 A. Schäfer, C. Huber, and R. Ahlrichs, J. Chem. Phys. 100, 5829 (1994). 27 C. Lee, W. Yang, and R. G. Parr, Phys. Rev. B 37, 785 (1988). 28 A. D. Becke, J. Chem. Phys. 98, 5648 (1993). 29 E. I. Solomon, B. Hedman, K. O. Hodgson, A. Dey, and R. K. Szilagyi, Coord. Chem. Rev. 249, 97 (2005). 30 S. V. Didziulis, S. L. Cohen, A. A. Gewirth, and E. I. Solomon, J. Am. Chem. Soc. 110, 250 (1988). 31 A. A. Gewirth, S. L. Cohen, H. J. Schugar, and E. I. Solomon, Inorg. Chem. 26, 1133 (1987). 32 D. Rokhsana, A. E. Howells, D. M. Dooley, and R. K. Szilagyi, Inorg. Chem. 51, 3513 (2012). 33 D. Rokhsana, D. M. Dooley, and R. K. Szilagyi, J. Biol. Inorg. Chem. 13, 371 (2008). 34 E. I. Solomon, S. I. Gorelsky, and A. J. Dey, J. Comput. Chem. 27, 1415 (2006). 35 E. I. Solomon, Inorg. Chem. 45, 8012 (2006). 36 R. K. Szilagyi and M. A. Winslow, J. Comput. Chem. 27, 1385 (2006). 37 R. Sarangi and S. Gorelsky, J. Am. Chem. Soc. 130, 3866 (2008). 38 R. F. W. Bader, Atoms in Molecules: A Quantum Theory (Oxford University Press, Oxford, 1990). 39 R. F. W. Bader, Chem. Rev. 91, 893 (1991). 40 T. A. Keith, AIMALL, version 13.05.06, TK Gristmill Software, Overland Park, KS, USA, 2013. 3 M.
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 137.149.200.5 On: Sun, 30 Nov 2014 08:28:49
024314-8 41 J.
Coutinho et al.
P. Foster and F. Weinhold, J. Am. Chem. Soc. 102, 7211 (1980). Neese, Interdiscip. Rev. Comput. Mol. Sci. 2, 73 (2012). 43 G. S. DeBeer, T. Petrenko, and F. Neese, J. Phys. Chem. A 112, 12936 (2008). 44 G. S. DeBeer, T. Petrenko, and F. Neese, Inorg. Chim. Acta 361, 965 (2008). 45 G. S. DeBeer and F. Neese, Inorg. Chem. 49, 1849 (2010). 46 J. W. Cubbage and W. S. Jenks, J. Phys. Chem. A 105, 10588 (2001). 47 R. Thomas, C. B. Shoemaker, and K. Eriks, Acta Crystallogr. 21, 12 (1966). 48 J. A. Dobado, H. Martnez-Garca, and M. R. Sundberg, J. Am. Chem. Soc. 121, 3156 (1999). 49 W. C. Stolte and G. Öhrwall, J. Chem. Phys. 133, 014306 (2010). 50 D. L. Hansen, M. E. Arrasate, J. Cotter, G. R. Fisher, K. T. Leung, J. C. Levin, R. Martin, P. Neill, R. C. C. Perera, I. A. Sellin, M. Simon, Y. 42 F.
J. Chem. Phys. 140, 024314 (2014) Uehara, B. Vanderford, S. B. Whitfield, and D. W. Lindle, Phys. Rev. A 57, 2608 (1998). 51 W. C. Stolte, R. Guillemin, A. Wolska, R. Feng, I. C. Tran, and D. W. Lindle, J. Electron Spectrosc. Relat. Phenom. 144–147, 157 (2005). 52 H.-Q. Zhao, Y.-S. Cheung, D. P. Heck, C. Y. Ng, T. Tetzlaff, and W. S. Jenks, J. Chem. Phys. 106, 86 (1997). 53 Q. Zha, T. Nihimura, and G. G. Meisels, Int. J. Mass Spectrom. Ion Process. 83, 1 (1988). 54 W. C. Stolte, I. Dimitriu, S.-W. Yu, G. Öhrwall, M. N. Piancastelli, and D. W. Lindle, J. Chem. Phys. 131, 174306 (2009). 55 K. P. Bowen, W. C. Stolte, A. F. Lago, J. Z. Dávalos, M. N. Picancastelli, and D. W. Lindle, J. Chem. Phys. 137, 204313 (2012). 56 A. F. Lago, A. C. F. Santos, W. C. Stolte, A. S. Schlachter, and G. G. B. de Souza, J. Electron Spectrosc. Relat. Phenom. 144–147, 161 (2005).
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 137.149.200.5 On: Sun, 30 Nov 2014 08:28:49
