Guanidine and guanidinium cation in the excited state—theoretical investigation Ivana Antol, Zoran Glasovac, Rachel Crespo-Otero, and Mario Barbatti Citation: The Journal of Chemical Physics 141, 074307 (2014); doi: 10.1063/1.4892569 View online: http://dx.doi.org/10.1063/1.4892569 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/141/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Active and silent chromophore isoforms for phytochrome Pr photoisomerization: An alternative evolutionary strategy to optimize photoreaction quantum yields Struct. Dyn. 1, 014701 (2014); 10.1063/1.4865233 Vibrational and electronic excitations in fluorinated ethene cations from the ground up J. Chem. Phys. 138, 124301 (2013); 10.1063/1.4795428 Theoretical studies of UO 2 ( OH ) ( H 2 O ) n + , UO 2 ( OH ) 2 ( H 2 O ) n , NpO 2 ( OH ) ( H 2 O ) n , and PuO 2 ( OH ) ( H 2 O ) n + ( n ≤ 21 ) complexes in aqueous solution J. Chem. Phys. 131, 164504 (2009); 10.1063/1.3244041 Structure and excited-state dynamics of anthracene: Ultrahigh-resolution spectroscopy and theoretical calculation J. Chem. Phys. 130, 134315 (2009); 10.1063/1.3104811 Towards a photophysical model for 5-hydroxyflavone J. Chem. Phys. 124, 104506 (2006); 10.1063/1.2177256

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THE JOURNAL OF CHEMICAL PHYSICS 141, 074307 (2014)

Guanidine and guanidinium cation in the excited state—theoretical investigation Ivana Antol,1,a) Zoran Glasovac,1 Rachel Crespo-Otero,2,b) and Mario Barbatti2 1

Division of Organic Chemistry and Biochemistry, Ruder ¯ Boškovi´c Institute, P.O. Box 180, HR-10002 Zagreb, Croatia 2 Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim an der Ruhr, Germany

(Received 13 May 2014; accepted 28 July 2014; published online 18 August 2014) Diverse ab initio and density-functional-theory methods were used to investigate geometries, energies, and electronic absorption spectra of guanidine and its protonated form, as well as their photodeactivation processes. It was shown that the guanidine is a weakly absorbing species with the excitation spectrum consisting mostly of transitions to the Rydberg excited states and one valence n-π 4 state. The lowest energy band has a maximum at ca. 6.9 eV (∼180 nm). The protonation of guanidine affects its excitation spectrum substantially. A major shift of the Rydberg states to higher energies is clearly visible and strongly absorbing transitions from the ground state to the π 3 -π 4 and π 2 -π 4 states appears at 7.8 eV (∼160 nm). Three low-lying conical intersections (two for guanidine and one for protonated guanidine) between the ground state and the first excited singlet state were located. They are accessible from the Franck–Condon region through amino N–H stretching and out-ofplane deformations in guanidine and protonated guanidine, respectively. The relaxation of the π 3 -3s Rydberg state via amino N–H bond stretching was hindered by a barrier. The nondissociated conical intersection in protonated guanidine mediates the radiationless deactivation of the compound after excitation into the π 3 -π 4 state. This fact is detrimental for the photostability of guanidine, since its conjugate acid is stable in aqueous solution over a wide pH range and in protein environment, where guanidinium moiety in arginine is expected to be in a protonated form. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4892569] I. INTRODUCTION

The large interest on guanidinium species (Scheme 1) is motivated by the high guanidine’s basicity,1 the tendency toward both hydrogen-bonding and charge-pairing interactions,2 as well as by the importance for biological systems. In particular, guanidinium group in arginine is ubiquitous in enzymes that bind anionic substrates and is also involved in the stabilization of protein tertiary structures.3 The guanidinium moiety has been widely explored as strong and selective binder for anionic guests and was used in the design of anion receptors in supramolecular chemistry,4 anion transporters across lipid bilayers and membranes,5 colorimetric probes, and sensors.6 Guanidine organocatalysis7 and biological activities of guanidine compounds8 have been reviewed recently. Unsubstituted guanidine itself is one of compounds important for prebiotic and astro-chemistry as well. Namely, it may be involved in nitrogen fixation routes in early atmospheres and prebiotic synthesis of cytosine and uracil.9 Also, it has been identified as nitrogenous organic molecule in Titan’s atmosphere.10 Due to the large availability of short-waved UV radiation in prebiotic and many astrochemical conditions, interaction of guanidine with the irradiation could possibly lead to its chemical photoactivation. Nevertheless, photochemical properties of guanidine and its

derivatives have not received much attention. For example, the first report of a guanidinium group associated with an excitedstate proton transfer has been published only recently,11 although the excited-state proton transfer (ESPT) process was assumed as the most fundamental chemical photoreaction. It should be emphasized that valence excited states, Rydberg states, and ionic states of guanidine and its protonated form have not been investigated so far. To the best of our knowledge, there is only one report in the literature that deals with the ab initio investigation of the Rydberg states of an electron bound to the guanidinium cation.12 Excited-state electron and proton transfers in guanidine-chromophore systems, as well as photochemical and photophysical deactivation processes in these systems, have not been studied theoretically, constituting likewise another knowledge gap in the field. Therefore, we carried out quantum chemical investigations on vertical excitation processes and excited-state potential energy profiles with the aim of locating and characterizing energy minima, and of finding energetically accessible conical intersection seams. These investigations should serve as the foundation for further study of the larger systems, in which guanidine has auxochrome functionality or regulates photoenergy transfer.

a) Author to whom correspondence should be addressed. Electronic mail:

II. COMPUTATIONAL DETAILS

[email protected]. b) Present address: Department of Chemistry, University of Bath, Claverton Down, BA2 7AY Bath, United Kingdom.

Geometries of guanidine and its protonated form (guanidinium cation) were optimized in the ground state with

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NH

NH2 +

C H2N

C NH2

guanidine

H2N

NH2

protonated guanidine SCHEME 1.

density functional theory (DFT) at B3LYP/aug-cc-pVTZ level.13 The nature of the minima was checked by vibrational analysis at the same theoretical level. The Cartesian coordinates of all optimized structures are given in the supplementary material.14 Vertical excitation energies for singlet excited states and oscillator strengths were calculated with different methods: time-dependent (TD) DFT15 with the CAM-B3LYP functional,16 resolution-of-the-identity coupled cluster to approximated second order (RI-CC2),17 configuration interactions based on DFT (DFT/MRCI),18 equation-of-motion coupled cluster with singles and doubles (EOM-CCSD),19 and complete active space perturbation theory to second order (CASPT2)20 in conjunction with the aug-cc-pVTZ basis set.21 The CASPT2 calculations for guanidine were based on the restricted active space self-consistent field (RASSCF) wavefunction with active space that encompasses 8 electrons and 5 valence orbitals (π 1 , π 2 , n, π 3 , and π 4 ) in the complete active space (CAS/RAS2), and additionally 1 (3s), 4 (3s and 3p), or 9 (3s, 3p, and 3d) virtual Rydberg orbitals in the auxiliary (AUX/RAS3) space (see Fig. 1(a)). Only single excitations from CAS to AUX space were allowed. These calculations will be denoted as RASPT2(5+1), RASPT2(5+4), and RASPT2(5+9), respectively. In the case of protonated guanidine, the CAS space was reduced to 6 electrons and 4 valence orbitals (π 1 , π 2 , π 3 , and π 4 ), while the AUX space remained unchanged (see Fig. 1(b)). Following the same notation, the calculations for protonated guanidine will be denoted RASPT2(4+1), RASPT2(4+4), and RASPT2(4+9). To test the stability of the electronic states against higher order of excitations, RASPT2(5+9) and RASPT2(4+9) calculations allowing single and double excitations to the AUX space

were conducted as well. These calculations will be denoted as RASPT2(5+9)sd and RASPT2(4+9)sd . State averaging (SA) procedure at RASSCF and multi-state (MS) RASPT2 was used for calculating vertical excitation energies with up to 30 roots for guanidine and 21 roots for its protonated form. The RASPT2 vertical energies were calculated by using level shift of 0.3 a.u.22 and default ionization potential–electron affinity (IPEA) shift (0.25 a.u.).23 Also, in all RASPT2 calculations, the core electrons (8 electrons, 4 orbitals) were kept frozen and the oscillator strengths were calculated with the RAS state interaction method (RASSI).24 The RASSI calculations were done following MS-RASPT2 calculations. To calculate vertical ionization potentials, the energies of the ionic states (doublets) were computed with the CASPT2/aug-ccpVTZ method with 7 and 5 electrons distributed in 5 and 4 valence CAS orbitals for guanidine and protonated guanidine, respectively (denoted CASPT2(5) and CASPT2(4)). A search for stationary points on the potential energy surface of the first excited singlet state and the first ionic state (doublet) has been conducted with the RASPT2 (or CASPT2 for protonated form and ionic states) method using numerically calculated gradients.25 The number of AUX active orbitals was reduced to 1 and 0 in the case of the excited state of guanidine and its protonated form, respectively. Also, only the complete active space was kept in the calculations of ionic states. A shift in the external part of the zero-order Hamiltonian was no more necessary (and, therefore, not used) due to large reduction in the number of roots in state average procedure, accounting to 1, 2, 2, and 3 for the ionic guanidine, the first excited guanidine, the ionic guanidinium cation and the excited guanidinium cation, respectively. The aug-ccpVDZ basis set26 was used for optimizations and harmonic vibrational frequency analyses, while the aug-cc-pVTZ26 was assigned for energy refinement in subsequent single-point calculations. In the case of guanidine, rigid scans along different bonddissociation reaction coordinates were performed. To describe the potential energy surface (PES) region where the bond is strongly stretched properly, the complete active space included two additional electrons and two additional orbitals

FIG. 1. The active orbitals used for the calculation of RASPT2 vertical excitation energies in guanidine (a) and protonated guanidine (b). The orbitals in protonated guanidine are classified according to the irreducible representations in the Abelian subgroup C2 of D3 symmetry.

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TABLE I. Vertical excitation energies (eV) for selected excited states of guanidine calculated at different levels of theory with aug-cc-pVTZ basis set. Oscillator strengths are given in parentheses.

Method CAM-B3LYP RI-CC2 DFT/MRCI RASPT2(5+9) RASPT2(5+9)sd RASPT2(5+4) RASPT2(5+1) EOM-CCSD

π 3 -3s Rydberg

n-3s Rydberg

π 2 -3s Rydberg

n-π 4 Valence

5.60 (0.0026) 5.60 (0.0046) 5.59 (0.0022) 5.69 (0.0005) 5.78 (0.0008) 5.64 (0.0010) 5.69 (0.0018) 5.81

6.53 (0.0615) 6.38 (0.0577) 6.57 (0.0990) 6.68 (0.0367) 6.76 (0.0445) 6.64 (0.0438) 6.62 (0.0156) 6.75

6.67 (0.0485) 6.64 (0.0584) 6.80 (0.1840) 6.85 (0.0102) 6.92 (0.0101) 6.85 (0.0165) 6.84 (0.0102) 6.89

6.90 (0.0232) 6.93 (0.0423) 6.46 (0.0386) 7.04 (0.0523) 7.03 (0.0556) 6.94 (0.0227) 7.01 (0.0163) 7.09

(CAS(7)). One of them was a σ bonding orbital located along the bond under consideration. Also Rydberg 3s orbital was moved from AUX to CAS because it slowly changed its shape during dissociation and became a contracted σ ∗ at long distances. Accordingly, no AUX orbitals were used. Minimizations on conical intersection seams between the first excited and the ground states were performed by MR-CIS method based on the complete active space selfconsistent field (CASSCF) wavefunction generated by the distribution of 10 electrons in 7 orbitals for guanidine and 6 electrons in 4 orbitals for protonated guanidine. The CI expansion space was constructed by allowing all the single excitations from the reference configurations into all the internal and virtual orbitals. As in the CASPT2 calculations, the four core orbitals were kept frozen. On fully optimized minima on the crossing seam (MXS) structures, the energies were recalculated at the CASPT2/aug-cc-pVTZ level, which resulted in an energy split between ground and excited state. Therefore, the MXS energies reported in the text correspond to the average between two ground and excited energy values. The DFT and TDDFT electronic data were obtained using the Gaussian 09 program.27 The RASPT2/RASSCF and CASPT2/CASSCF calculations were performed with the Molcas 7.8 software.28–30 Columbus31 and Turbomole32 were used for the MR-CIS and RI-CC2 calculations, respectively, while Vega-ZZ33 and Molden34 programs were used for visualization and geometry manipulations.

ergy excited states is given in Table I. The RASPT2(5+9) data were used for absorption spectrum simulation in Fig. 2. All methods agree well that Rydberg states dominate the spectrum and play a key role in the interaction of guanidine with UV radiation. In the 30 calculated roots, excitations from the three highest occupied orbitals (π 3 , n, and π 2 ) to Rydberg orbitals were observed. The lowest excited state is the Rydberg π 3 -3s state with excitation energy around 5.7 eV. All calculated values for this state with different methods are in range between 5.59 and 5.81 eV. Its oscillator strength is low (spans from 0.0005 at RASPT2(5+9) to 0.0046 at RICC2 level). At the energies above 6.4 eV, more intensive Rydberg states are located. Distinguishing between states belonging to the different Rydberg series converging to three ionization potentials was not an easy task because of the state mixing resulting in low-weight leading configurations (see detailed analysis of RASPT2(5+9) wavefunction for all calculated roots in Table SI1 of the supplementary material).14 Therefore, the states were tentatively grouped in three series converging to different ionization potentials. Only the lowest

III. RESULTS AND DISCUSSION A. Vertical excitation energies and photo-deactivation in guanidine

To the best of our knowledge, experimental spectroscopic data on guanidine are very limited. The only available UV spectra of guanidine hydrochloride in water was published without any comment and assignment as early as 1947.35 Also a search for the gas-phase UV absorption spectrum of unsubstituted guanidine has not yielded any result. Due to this lack of experimental results, we thought it was worthwhile calculating guanidine vertical excitation energies and oscillator strengths using a wide range of methods from different theoretical backgrounds (TDDFT, RI-CC2, DFT/MRCI, EOMCCSD, and RASPT2). A short summary with the lowest en-

FIG. 2. Vertical excitation energies of singlet excited states calculated with SA-30-RASPT2(5+9)/aug-cc-pVTZ method. Oscillator strengths for states contributing the most for absorption are given in parentheses. A simple convoluted spectrum is represented by dotted line in arbitrary units.

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energy Rydberg 3s states were assigned for n-Ryd and π 2 Ryd series in all calculations (see Table I). The mixing due to the high density of Rydberg states led to a redistribution of their properties and an expected decrease in the magnitude of oscillator strength along each of the Rydberg series was not observed (see Table SI1 in the supplementary material). It should be noted that addition of double excitations from the CAS to the AUX space in the RASSCF(5+9)sd wavefunction gave very similar results to those obtained by RASPT2(5+9) method (Table I). All Rydberg 3s states are systematically shifted toward higher energies by 0.1 eV, while the excitation energy of valence n-π 4 state differs by only 0.01 eV. The average values for the excitation energy of the n-3s and π 2 -3s Rydberg states are 6.6 and 6.8 eV, respectively. The agreement between methods for the calculated excitation energy of the n-3s state was a little deteriorated with respect to the first excited state. In particular, the RI-CC2 method gave the lowest value, 6.23 eV. The same holds for the π 2 -3s state, for which the RI-CC2 value of 6.64 eV was calculated. Nevertheless, all methods predicted that the spacing between the n-3s and π 2 -3s states was ca. 0.2 eV. Ionisation potentials can be calculated as the energy difference between the singlet ground state S0 (. . . (π 2 )2 , n2 , (π 3 )2 ) and of the ionized doublet states D1 , D2 , D3 , . . . (for example, doublet cation, (. . . (π 2 )2 , n2 , (π 3 )1 ). Using the geometry in the ground state minimum, the energy difference (Ev ) corresponds to the vertical ionization potential. In the case of guanidine, three vertical ionisation potentials (IP1 , IP2 , and IP3 , see Table II and Fig. 2) for ejection of an electron from π 3 , n, and π 2 orbitals were calculated at the CASPT2 level. The energy gaps between Rydberg 3s states (0.9 and 0.2 eV) are in agreement with relative positions of ionisation potentials (gaps between IP1 –IP2 and IP2 –IP3 are 0.9 and 0.4 eV, respectively). The experimentally determined (by electron impact technique) ionization energy for guanidine is 9.10 ± 0.05 eV.36 This value is in excellent agreement with the first vertical ionization potential IP1 calculated by the CASPT2 method. Guanidine has a system of three occupied and one unoccupied π orbital. Similar to other small organic molecules with similar number of π orbitals,37–39 a valence π -π ∗ state could be expected to dominate the absorption band in UV spectrum. However, only one valence n-π 4 state with oscilTABLE II. Vertical and adiabatic ionization potentials calculated at CASPT2/aug-cc-pVTZ//CASPT2/aug-cc-pVDZ level. Excitation energies for first excited state (R1 ) in guanidine are given as well. The active space is defined for each species in Sec. II. Ev (eV)

Ea (eV)

Guanidine D1 (. . . (π 2 )2 , n2 , (π 3 )1 ) D2 (. . . (π 2 )2 , n1 , (π 3 )2 ) D3 (. . . (π 2 )1 , n2 , (π 3 )2 ) R1 (. . . (π 2 )2 , n2 , (π 3 )1 ,(π 4 )1 )

9.09 9.95 10.31 5.76

8.48

Protonated guanidine D1 (. . . (π 2 )2 , (π 3 )1 ) D2 (. . . (π 2 )1 , (π 3 )2 )

16.03 16.05

15.91

4.97

lator strength 0.0523 was located at 7.04 eV among Rydberg states in the region of first absorption band between 170 and 200 nm as calculated by RASPT2(5+9) method (see Fig. 2). Surprisingly, the valence π 3 -π 4 excited state was not found among 30 roots calculated up to 10 eV. One may be tempted to conclude that the absence of the π 3 -π 4 in guanidine at the RASPT2 level is result of its poor description at the reference RASSCF wavefunction, due to well known problem in describing π -π ∗ states with strong ionic character.39, 40 Nevertheless, the results of the TD-CAM-B3LYP and RI-CC2 calculations confirm the absence of that state. The calculated excitation energy for the n-π 4 state was 6.90 (TD-CAMB3LYP) and 6.93 eV (RI-CC2), and the π 3 -π 4 state was not found up to 9.8 and 9.3 eV, respectively. Comparison of the DFT/MRCI results with those obtained at the RASPT2(5+9), CAM-B3LYP, and RI-CC2 levels showed that the former method gave a transition energy of the n-π 4 state ca. 0.5 eV too low. It changed the relative ordering of the excited states— for example, the n-π 4 state became lower in energy than the n3s state. Additionally, calculated oscillator strengths for Rydberg n-3s and π 2 -3s states were strongly increased (to 0.0990 and 0.1840) with respect to results of other methods. Strong Rydberg-valence mixing at the orbital level was observed as well. To explore the possibility of using smaller active spaces for excited-state geometry optimizations, the reduction of active size in the RASPT2 method was tested. The reduction of the number of Rydberg orbitals from 9 to 4 had a systematic impact on the vertical energies, which were lowered for most of the states. Further reduction from 4 to only one 3s orbital increased the vertical excitation energy of Rydberg states and decreased it for valence states. All changes in the lowest states were, however, very small (maximum discrepancy was 0.1 eV, see Table I). Keeping this in mind, we proceeded to optimization of guanidine geometry on various cuts on the PES. The structures of guanidine in the ground state, first ionic state (cation doublet), and first Rydberg state were optimized with the CASPT2/aug-cc-pVDZ (or RASPT2/aug-cc-pVDZ for Rydberg state) method (Fig. 3). The active space encompassed five valence orbitals (π 1 , π 2 , n, π 3 , and π 4 ) and, in the case of Rydberg excited state, one additional Rydberg 3s orbital in the AUX. The ground-state equilibrium structure is nonplanar with strongly pyramidalized NH2 groups. It is in accordance with the results of previous quantum chemical studies41 and single-crystal X-ray diffraction data.42 The geometries of ionized and excited guanidine in the 3s Rydberg state are almost identical. The major change with respect to the ground-state geometry is planarization of amino nitrogen atoms and strong increase of C=N bond length from 1.29 Å in the ground state to 1.39 Å in ionic and Rydberg states. Also the lengths of the C1–N3 and C1–N4 bonds were decreased by 0.07 Å. The energies have been re-calculated with aug-ccpVTZ basis set and adiabatic values for the first ionization potential and the first excited state were estimated to 8.48 and 4.97 eV, respectively (Table I). Using the excited-state minimum structure, the emission energy of 4.39 eV and oscillator strength of 0.0002 were calculated. This emission strength, corresponding to a fluorescence lifetime of 6 μs,43 implies that we may expect occurrence of intersystem crossing or, if

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FIG. 3. Comparison of ground-state (a), first ionic (b), and excited-state (c) geometries. Optimizations were performed at CASPT2(5) or RASPT2(5+1) level of theory (no shift) with aug-cc-pVDZ basis set. Bond distances are given in Å and angles in degrees. The atoms are colour coded as follows: green = C, blue = N, and gray = H.

the excitation energy is enough, radiative decay through the conical intersection discussed below. To predict possible photo-deactivation processes upon excitation, potential energy surface for the first excited state was scanned along three dissociation channels at the CASPT2/aug-cc-pVDZ level of theory (Fig. 4). We performed the rigid stretching of C=N double bond (highly weakened in the excited state) and the stretching of N– H bonds on amino (N3–H5) and imino (N2–H9) sites of the molecule. The conjecture that this kind of deformation may lead to low-lying intersections between the ground- and excited-state surfaces was based on several previously published investigations.44 In the case of C=N bond cleavage (Fig. 4(a)), the potential energy curves for the ground and the excited state merged at distances larger than 3 Å. There, both states are strongly destabilized with respect to the excitedstate minimum and, therefore, cleavage of C=N bond is not a probable deactivation channel. The same conclusion was valid for stretching of the imino N–H bond (Fig. 4(b)), since the potential energy curve for the excited state is strongly destabilized and the gap between excited and the ground state was not substantially decreased in any point along this reaction coordinate. When the amino NH bond was dissociated (Fig. 4(c)), a crossing of potential energy curves between the ground and the excited state was observed at 2.1 Å and 0.03 eV below excited state minimum. However, the crossing is separated from the minimum by an energy barrier. Based on this rigid scan (Fig. 4(c)), we estimate that this barrier is smaller than 0.62 eV. To describe the dissociation channel more precisely, a transition structure for the N3–H5 dissociation in the first excited state was fully optimized and search for minimum on the conical intersection seam between the ground and excited state related to the N3–H5 stretching was started at the

FIG. 4. Potential energy scans for the first excited state have been calculated for the dissociation of (a) C1–N2, (b) N2–H9, and (c) N3–H5 bonds at CASPT2/aug-cc-pVDZ level of theory. The numbering scheme is given in Fig. 3(a).

MR-CIS/aug-cc-pVDZ level of theory. The TS1 and MXS1 structures were found and they are shown in Fig. 5. The TS1 structure has slightly pyramidalized C atom (∼8◦ ) and pyramidalization on NH2 groups is quite more pronounced (∼34◦ ). The H9–N2–C1–N3 dihedral angle amounts 43.6◦ . The C1–N3 bond becomes shorter than in the excited state minimum and, as expected for the N3–H5 dissociation channel, the distance between N3 and H5 atoms increases to 1.235 Å. The stationary point TS1 has been positively identified as the transition state with one imaginary frequency corresponding to N3–H5 stretching. (We note in passing that planarization of TS1 yielded second order saddle point with two imaginary frequencies one as expected N3–H5 stretching and another out of plane deformation.) The MXS1 structure had a geometry strongly deformed from planarity with even more shorter C1–N3 bond than in TS1. The C1–N4 and

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FIG. 5. MR-CIS optimized structures of the saddle point on the N3–H5 dissociation channel (a) and two minima on the conical intersection seam between the ground and first excited state related to the N3–H5 bond stretching. Bond distances are given in Å and dihedral angles in degrees.

C1–N2 bonds became almost equal (1.406 and 1.405 Å, respectively). The energies of TS1 and MXS1 were recalculated at the CASPT2/aug-cc-pVTZ level of theory. The energy gap in the MXS1 point at the CASPT2 level was 0.10 eV. It is interesting to note that the MXS1 has higher energy than the relative energy of the saddle point TS1 by 0.45 eV (0.18 eV at the MR-CIS level). This means that passing to the ground state through conical intersections in this region of the seam would need more energy than dissociation process itself. Additionally, another search for a minimum on the conical intersection seam was started from planar geometry with the N3–H5 distance equal to 2.1 Å, where in CASPT2 scan (Fig. 4(c)) the ground state and the excited state curves cross each other. The almost planar structure MXS2 was obtained (Fig. 5(c)). Different from the geometry in the excited-state minimum, where both C1–N3 and C1–N4 bonds were equal, the C1–N3 bond became shorter than C1–N4 bond in the MXS2 structure. As expected, the N3–H5 bond is heavily stretched to 1.993 Å. Interestingly, the MXS2 structure has 0.48 and 0.03 eV below MXS1 and TS1 energies, respectively (0.76 and 0.58 eV at the MR-CIS level). The energy gap in the MXS2 point at the CASPT2 level is 0.02 eV. The transfer of excitation energy into amino NH vibrational motion would lead to the radiationless photo-deactivation of guanidine molecule if the dissociation on the excited state potential energy surface happened as a first process on this deactivation channel. In this case, the excited -state dissociation would be the rate determining step. B. Vertical excitation energies and photo-deactivation in protonated guanidine

For protonated guanidine the excitation spectrum, presented exemplarily for RASPT2(4+9)/aug-cc-pVTZ level

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FIG. 6. Vertical excitation energies of singlet excited states calculated with SA-21-RASPT2(4+9)/aug-cc-pVTZ method. Oscillator strengths for states contributing the most for absorption are given in parentheses. A simple convoluted spectrum is represented by dotted line in arbitrary units.

of theory in Fig. 6 and Table SI2 in the supplementary material,14 is completely different from those of the guanidine case. It is characterized by a very strong band between 170 and 150 nm (energy range 7.3 and 8.3 eV). The π 3 –π 4 and π 2 –π 4 valence states (V1 and V2 ), not observed in guanidine, are the dominant contributions to the lower portions of the spectrum. Closely lying degenerated Rydberg 3s states (R1 and R2 ) do not contribute to the band intensity since their oscillator strengths were much smaller than those of the valence states. Also the n-Rydberg series and the n-π 4 valence state found in guanidine did not appear here. In particular, the lone electron pair in the N2 atom of guanidine was involved in N–H bond in protonated guanidine. For this reason, it is strongly stabilized as the other σ orbitals in the six equivalent NH bonds. Comparisons of the vertical excitation energies and oscillator strengths for the lowest energy excited states of protonated guanidine calculated at different levels of theory with aug-cc-pVTZ basis set are presented in Table III. Both the π 3 –π 4 and π 2 –π 4 states have similar vertical excitation energy around 7.8 eV. The exception is RI-CC2, whose valence states energy is about 7.6 eV. The results of calculations cannot give a definite answer about relative ordering of valence and Rydberg 3s states. The EOM–CCSD and RI-CC2 values for excitation energy of the R1 and R2 state are 0.2 eV higher than energy of the π 3 –π 4 and π 2 –π 4 valence states. On the other hand, DFTbased methods CAM-B3LYP and DFT/MRCI predict that the Rydberg states are the lowest energy states with more pronounced R1 /R2 − V1 /V2 energy difference at DFT/MRCI level (0.24 eV vs. 0.08 eV). We did additional tests using BMK,45 PBE0,46 and M06-2X47 functional. The choice of density functionals used in the present study was based on the elaborate benchmarking studies in the literature. For a

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TABLE III. Vertical excitation energies for selected excited states of protonated guanidine calculated at different levels of theory with aug-cc-pVTZ basis set. Oscillator strengths are given in parentheses. Method CAM-B3LYP BMK PBE0 M06-2X RI-CC2 DFT/MRCI RASPT2(4+9) RASPT2(4+9)sd RASPT2(4+4) RASPT2(4+1) EOM-CCSD

Val. π 3 –π 4

Val. π 2 –π 4

R1 π 3 –3s

R2 π 2 –3s

R3

R4

7.84 (0.2682) 7.96 (0.2789) 7.84 (0.2686) 7.86 (0.2085) 7.61 (0.3067) 7.81 (0.4160) 7.79 (0.4278) 7.94 (0.3926) 7.82 (0.4128) 7.94 (0.4483) 7.79 (0.2987)

7.84 (0.2682) 7.96 (0.2789) 7.84 (0.2686) 7.86 (0.2075) 7.61 (0.3067) 7.92 (0.4398) 7.89 (0.3793) 7.97 (0.3954) 7.83 (0.3902) 7.91 (0.4332) 7.79 (0.2987)

7.76 (0.0013) 8.22 (0.0052) 7.69 (0.0002) 7.83 (0.0599) 7.85 (0.0075) 7.69 (0.0001) 7.94 (0.0036) 8.03 (0.0030) 7.80 (0.0037) 7.51 (0.0023) 7.98 (0.0098)

7.76 (0.0013) 8.22 (0.0052) 7.69 (0.0002) 7.83 (0.0589) 7.85 (0.0075) 7.74 (0.0128) 8.02 (0.0030) 8.18 (0.0044) 7.84 (0.0029) 7.43 (0.0106) 7.98 (0.0098)

8.83 (0.0000) 9.30 (0.0000) 8.82 (0.0000) 8.83 (0.0000) 9.03 (0.0000) 8.75 (0.0000) 9.00 (0.0000) 9.09 (0.0000) 8.90 (0.0001) ... 9.14 (0.0000)

8.99 (0.0648) 9.40 (0.0712) 8.90 (0.0580) 9.02 (0.0621) 9.10 (0.0773) 8.87 (0.0585) 8.99 (0.0877) 9.22 (0.0794) 9.04 (0.0647) ... 9.25 (0.0751)

recent review on the topic see Ref. 48. Generally, hybrid functionals with high percentage of exact exchange gave the best agreement between calculated and measured vertical excitation energies. The ordering of the excited states in protonated guanidine was V1 /V2 followed by R1 /R2 at BMK, R1 /R2 followed by V1 /V2 at PBE0. The four states are almost degenerated with M06-2X (valence − Rydberg gap was 0.03 eV). The RASPT2 energy of R1 state fluctuates between 7.51 and 8.03 eV, depending on the number of Rydberg orbitals in active space, state-averaged states, and CAS to AUX excitation level. The same fluctuation holds for the R2 state as well. It should be noted that most of the methods predicted the energy separation that is similar to the generally acceptable errors (within 0.1–0.2 eV) for excitation energies regardless of the method. All methods utilized here showed that the third Rydberg excited state R3 (3p) was separated from the R2 state by about 1 eV and has zero oscillator strength. It is followed by the R4 state (0.1–0.2 eV higher in energy) with calculated oscillator strength between 0.06 and 0.09. The remaining Rydberg states (R5 –R18 ) do not contribute to the absorption spectrum significantly. In conclusion, the Rydberg states of protonated guanidine are shifted by several eV to higher excitation energies as compared to guanidine. Similar effect of protonation on the Rydberg states was observed earlier in the case of formaldehyde, where the main reason for the large upward shift of the Rydberg states was the large ionization energy.49 Indeed, at the CASPT2/aug-ccpVTZ level of theory, we have computed energies of the D1 (12 A, . . . (π 3 )1 ) and D2 (12 B, . . . (π 2 )1 ) ionized states and predicted the vertical ionization potentials (IP1 and IP2 ) of 16.02 and 16.05 eV. The value of vertical IP1 is increased by ca. 7 eV with respect to guanidine. It is interesting to note that the excitation energy of the valence π –π ∗ state in formaldehyde changed only slightly.49 It was located well below the first Rydberg 3s state.49 Accordingly, it is not surprising that π 3 –π 4 and π 2 –π 4 states appeared in the spectrum of protonated guanidine among the Rydberg states. In the next step, the structures of protonated guanidine in the ground state and the first ionic state (cation doublet) were optimized with CASPT2/aug-cc-pVDZ method with active space reduced to valence CAS(4) (6 electrons

in 4 valence orbitals). Symmetry constraints were removed from the calculations. In accordance with previously published investigations12, 50, 51 as well as with the experimental X-ray analysis,52 the calculated minimum structure of protonated guanidine in the ground state is nonplanar and has D3 symmetry. The carbon and the three nitrogen atoms lie in a common plane, but the amino groups are rotated out of this plane by 11◦ (Fig. 7(a)). On the PES of the first ionized state D1 , two stationary points were located 15.60 and 15.61 eV above the ground-state minimum. With aug-cc-pVTZ basis set, these energies increase to 15.91 and 15.92 eV (Table II). The geometries are shown in Figs. 7(b) and 7(c). Both structures are planar and slightly asymmetric. In the first structure, one C–N bond was shortened by 0.03 Å, while two C–N bonds were stretched by 0.04 Å with respect to the ground state. In the second structure, the opposite change happened: one bond is stretched to 1.405 Å, while two bonds were shortened by 0.01. It is interesting to note that the structure with one long and two short C–N bonds had one imaginary frequency (157 cm−1 , see Fig. 7(d)) following the reaction path for in-plane interconversion schematically depicted in Fig. 7(e). Since UV irradiation of the protonated guanidine should mostly populate the lowest energy valence excited state (π 3 –π 4 ), this state should be considered as the starting point for the mechanistic study of protonated guanidine photophysics. A search for a minimum on its PES was started from the Franck-Condon point with CASPT2 method with state averaging over three roots. During the optimization of protonated guanidine in the π 3 –π 4 excited state, its structure changed significantly. Strong pyramidalization on the central C atom and on all three N atoms was observed. Besides that, rotations of NH2 groups around the C1–N4 and C1–N3 bonds led to an almost Cs -symmetrical structure with mirror plane including the C1–N2 bond. Also, the energy gap between the S1 and the ground state decreased to ca. 0.6 eV causing root flipping problems, which was an indication of a conical intersection nearby. It is interesting to note that full optimization of the excited-state geometry was possible on the CASSCF level of theory and vibrational analysis confirmed its minimum nature. Also, very similar structure was found upon minimization at the MR-CIS level of theory. The latter structure

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FIG. 7. Stationary points on the ground state and first ionic state of protonated guanidine. Optimizations were performed at CASPT2(4) level of theory (no shift) by using aug-cc-pVDZ basis set. Bond distances are given in Å and angles in degrees. Displacement vectors for imaginary frequency in D1 TS structure and scheme for in plane interconversion between minima on D1 PES are given as well.

is shown in (Fig. 8(a)). The ground-state–excited state energy gap was 0.76 and 0.40 eV at the CASSCF and MR-CIS levels, respectively. Further, a search for conical intersections between the ground and excited state was started at the MR-CIS level of theory. The minimum on the seam (MXS3, Fig. 8(b)) was eas-

FIG. 8. MR-CIS optimized structures of (a) the first excited state minimum and (b) the minima on the conical intersection seam between the S1 and the ground state. CASSCF values of selected geometrical parameters are given in parentheses. Bond distances are given in Å and dihedral angles in degrees.

ily found 4.46 eV above the ground state and only 0.01 eV above the excited state minimum, calculated at the same theoretical level. The C1–N2, C1–N3, and C1–N4 bonds were stretched by only 0.006, 0.004, and 0.004 Å, and pyramidalization on C1 and N2 atoms was decreased by 2.4◦ and 3.2◦ relative to the excited state minimum structure (S1 MIN, Fig. 8(a)). As the MXS3 lies very close to the S1 minimum, very fast radiationless deactivation to the ground state can be expected. This finding implies that protonated guanidine and neutral guanidine follow completely different deactivation channels. While protonated guanidine should preferentially relax through a nondissociative radiationless mechanism, neutral guanidine will relax either by photoemission at low excitation energies or by a dissociative radiationless mechanism at higher energies (see Sec. III A). Moreover, the nondissociative radiationless mechanism in protonated guanidine should enhance the photo-stability of guanidine in aqueous solution over a wide pH range due to the very high guanidine basicity in the ground state and, also, in protein environment, where guanidinium moiety in arginine is expected to be in a protonated form. The present results for protonated guanidine cation may be put in perspective by comparison with Ehrenfest dynamics investigations of processes occurring on PESs of different electronic states of the (Ala-Arg + 2H)+• cation radical upon electron attachment to (Ala-Arg + 2H)2+ dication,53 used for the interpretation of experimentally observed fragmentations in electron-transfer dissociation and electron-capture

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dissociation mass spectrometry.54 That study indicated that branching into different dissociation channels was primarily determined by the electronic states being accessed. Of particular interest to the present discussion is electron attachment to the excited states localized on guanidinium functionality in the (Ala-Arg + 2H)+• cation radical. Tureˇcek and coworkers53 investigated two of such guanidinium-localized states (denoted LUMO and LUMO+2). In the first, electron was bound to guanidinium π ∗ orbital, which corresponds to π 4 orbital in our calculations on protonated guanidine. In the second, electron was bound to an orbital localized on the guanidinium group, which had a σ ∗ nodality. The ordering of the states is in accordance with herein calculated vertical excitation energies obtained by EOM-CCSD, RI-CC2, and RASPT2(4+9) methods. The Ehrenfest dynamics calculations with LUMO initial state initiated an umbrella vibration of the guanidinium central carbon, out of plane relative to the surrounding nitrogen atoms,53 completely analogous to the strong out-of-plane deformation found in the π 3 – π 4 excited-state minimum and MXS3 of guanidinium cation herein. No fragmentation was observed within the simulation time (up to 300 fs). As a final comment, the excited-state acid/base properties of guanidine should be mentioned. The excited-state gas phase proton affinity can approximately be deduced using simple Föster thermodynamic cycle55 in which the change of gas phase proton affinity upon excitation from the ground to the excited state is related to the shift of maxima in the vertical excitation spectrum between protonated and deprotonated species. From the difference in the position of the lowest energy band between guanidine and protonated guanidine in simulated spectra in this work (Eexc ∼ 0.9 eV, see Figs. 2 and 6), the difference between gas phase proton affinity of guanidine in the excited and the ground state was evaluated to −20.8 kcal mol−1 . If we refer to the experimentally obtained ground state proton affinity of guanidine of 235.7 kcal mol−1 ,56 the excited state proton affinity of guanidine can be estimated to 214.9 kcal mol−1 . The result simply means that guanidine loses its high basicity upon excitation. The more reliable insight to acid/base properties would require usage of Born-Haber thermodynamic cycle which is valid if the equilibrium between guanidine and protonated guanidine in the excited state could be established. In our case ultrafast internal conversion of protonated guanidine disable establishment of such equilibrium and therefore this thermodynamic cycle is not appropriate for determination of the excited-state basicity of guanidine. IV. CONCLUSIONS

Different theoretical methods (ab initio and DFT approaches) were used to predict singlet excited states, ionization potentials, and photo-deactivation processes of guanidine and its protonated form. The calculations revealed that guanidine is a weakly absorbing species with the excitation spectrum consisting mostly of transitions to Rydberg excited states and one valence n-π 4 state in the far UV range. The lowest energy band had maximum at ca. 6.9 eV (∼180 nm). A minimum on the PES of the lowest excited state (the Rydberg π 3 -3s state) was found. The channel for the excited state hy-

J. Chem. Phys. 141, 074307 (2014)

drogen abstraction from NH2 amino site of molecule as well as radiationless decay to the electronic ground state via conical intersection seam related to the amino N–H stretching was protected by an energy barrier raised from avoided crossing between the Rydberg π 3 -3s and the valence π 3 -σ ∗ state. The guanidine is thus potentially photo-emitting species with the calculated fluorescence at 4.39 eV (∼280 nm). Protonation in most cases leads to the significant charge reorganization and change in orbital shapes and orbital energies.57 Subsequently, the protonation of guanidine affected its excitation spectrum substantially. The major shift of the Rydberg states to higher energies is clearly visible and strongly absorbing transitions from the ground state to the π 3 –π 4 and π 2 –π 4 states appeared at 7.8 eV (∼160 nm). The present calculations cannot provide a clear-cut answer on the main character of the wavefunction of the lowest excited state. Nevertheless, we expect that the photo-physics of protonated guanidine is determined by properties of valence excited states due to their higher oscillator strength in comparison to the strength of the Rydberg states. Following the vertical excitation of the π 3 –π 4 excited state, protonated guanidine can relax without a barrier to the ground state through a conical intersection. The initial motion for an approach to this conical intersection is strong out-of-plane deformation (pyramidalization on the central C atom and all three N atoms, as well as the rotations of NH2 groups around the C1–N4 and C1–N3 bonds).

ACKNOWLEDGMENTS

This work was supported by Ministry of Science, Education and Sports (MSES) of Croatia through Project 098-0982933-2920. The calculations were performed on the Isabella cluster (isabella.srce.hr) at the University of Zagreb Computing Center (SRCE). Funds for GermanyCroatia bilateral collaboration from the German Academic Exchange Service (DAAD, Project 54368738) and MSES (Project: Optimizing bidirectional “highway” for photoactive response through guanidine-chromophore junction) are also acknowledged. 1 N.

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