Experimental and theoretical study of the HXeI⋯HCl and HXeI⋯HCCH complexes Cheng Zhu, Masashi Tsuge, Markku Räsänen, and Leonid Khriachtchev Citation: The Journal of Chemical Physics 142, 144306 (2015); doi: 10.1063/1.4917167 View online: http://dx.doi.org/10.1063/1.4917167 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/142/14?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Environmental effects on noble-gas hydrides: HXeBr, HXeCCH, and HXeH in noble-gas and molecular matrices J. Chem. Phys. 139, 204303 (2013); 10.1063/1.4832384 Theoretical prediction of rare gas inserted hydronium ions: HRgOH2 + J. Chem. Phys. 138, 194308 (2013); 10.1063/1.4804623 Experimental and computational study of the HXeI⋯HY complexes (Y = Br and I) J. Chem. Phys. 138, 104314 (2013); 10.1063/1.4794309 Properties of the B+-H2 and B+-D2 complexes: A theoretical and spectroscopic study J. Chem. Phys. 137, 124312 (2012); 10.1063/1.4754131 Neutral rare-gas containing charge-transfer molecules in solid matrices. III. HXeCN, HXeNC, and HKrCN in Kr and Xe J. Chem. Phys. 109, 618 (1998); 10.1063/1.476599
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THE JOURNAL OF CHEMICAL PHYSICS 142, 144306 (2015)
Experimental and theoretical study of the HXeI · · · HCl and HXeI · · · HCCH complexes Cheng Zhu, Masashi Tsuge,a) Markku Räsänen, and Leonid Khriachtchevb) Department of Chemistry, University of Helsinki, P.O. Box 55, FI-00014 Helsinki, Finland
(Received 10 February 2015; accepted 27 March 2015; published online 13 April 2015) The HXeI · · · HCl and HXeI · · · HCCH complexes are studied computationally and experimentally in a Xe matrix. In the experiments, three bands of the HXeI · · · HCl complex and one band of the HXeI · · · HCCH complex in the H–Xe stretching region are observed. The monomer-tocomplex shifts are +94, +111, and +155 cm−1 for the HXeI · · · HCl complex and +49 cm−1 for the HXeI · · · HCCH complex. The bands of the complexed HCl molecules are also observed with large red shifts from the HCl monomer (−187, −252, and −337 cm−1). The ab initio calculations at the CCSD(T)/def2-TZVPPD level of theory predict two stable structures for the HXeI · · · HCl complex with interaction energies of −3.72 and −0.28 kcal mol−1 and one structure for the HXeI · · · HCCH complex with an interaction energy of −2.67 kcal mol−1 and the calculated monomer-to-complex shifts are in a good agreement with experiment (in the case of HXeI · · · HCl, for the stronger structure). The HXeI molecules are decomposed by broad-band infrared light; however, the decomposition is much more efficient for the HXeI monomer than for the complexes studied here as well as for the previously studied HXeI · · · HI and HXeI · · · HBr complexes. In fact, the decomposition efficiency decreases as the monomer-to-complex shift of the H–Xe stretching mode increases. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4917167]
I. INTRODUCTION
Non-covalent interactions are responsible for many properties of matter and play an important role in chemical reactions.1,2 These interactions can be studied by vibrational spectroscopy because they change the frequencies of the characteristic vibrational modes. In particular, infrared (IR) matrixisolation spectroscopy has been extensively used to study intermolecular interactions.3–16 For example, Barnes investigated various complexes between hydrogen chloride and other species (H2O, N2, benzene, etc.) trapped in argon matrices.6 The results show that for complexes with weak to medium strength bases, the red shift of the HCl frequency depends on the proton affinity of the bases. For hydrogen bonding system like HBr · · · HI, in which HI acts as a proton donor, the HI vibrational frequency decreases and its intensity increases upon complexation.17,18 In some cases, blue shifting hydrogen bonds are observed.19,20 Noble-gas hydrides HNgY (Ng = noble-gas atom and Y = electronegative fragment) have been mostly prepared in low-temperature matrices.21–25 These molecules are characterized by weak bonding and large dipole moments, which leads to a strong complexation effect on the vibrational properties of these molecules.26–30 In all HNgY complexes studied experimentally, the H–Ng stretching modes exhibit blue shifts, explained by the enhancement of charge separation in these molecules.28 The HXeI molecule is one of the least stable noblegas hydrides as judged by the very small dissociation energy a)Present address: Department of Applied Chemistry, National Chiao Tung
University, 1001 Ta-Hsueh Road, Hsinchu 30010, Taiwan.
b)E-mail address: [email protected]
0021-9606/2015/142(14)/144306/8/$30.00
(D0 = 8.30 kcal mol−1).31 It follows that the complexation effect on the properties of HXeI should be very strong;18,32 thus, the complexes of this molecule are particularly interesting. Recently, Tsuge et al. have studied the HXeY · · · H2O (Y = Cl, Br, and I) complexes.32 The frequency of the H–Xe stretching mode of the HXeI · · · H2O complex, indeed, has the largest blue shift (+138 cm−1) among these three species. The complexes of HXeI with HBr and HI have also been investigated.18 The interaction with HBr is found to induce larger blue shifts of the H–Xe stretching mode (up to +157 cm−1) than the interaction with HI (up to +96 cm−1). Another finding of that work is that the stability of HXeI under IR light increases upon complex formation. It is interesting to expand these data to complexes of HXeI with other molecules to follow the correlation between the interaction strength and the complexation effects. In the present work, we study the HXeI · · · HCl and HXeI · · · HCCH complexes in a Xe matrix by IR absorption spectroscopy. The experimental work is supported by quantum chemical calculations.
II. COMPUTATIONAL DETAILS AND RESULTS
The equilibrium structures, relative energies, and vibrational spectra are calculated at the CCSD(T) and MP2(full) levels of theory.33 The def2-TZVPPD basis sets, used for H, C, Cl, I, and Xe atoms,34 are taken from the EMSL basis set library using the Basis Set Exchange software.35 For I and Xe atoms, 28 electrons are replaced by an effective core potential. The interaction energy (Eint) is defined as the difference between the total energies of the complex and of the monomers (with the structures in the complex). The difference of zero-point
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vibrational energies (∆ZPVE) and the basis set superposition error (BSSE) (corrected using the counterpoise procedure (CP))36 is taken into account. The structural optimization and harmonic vibrational analysis at the CCSD(T) level of theory are performed using the MOLPRO program.37 The calculations at the MP2(full) level of theory, single point energy calculations and the BSSE correction at the CCSD(T) level of theory, and the natural population analysis at the CCSD/def2TZVPPD//CCSD(T)/def2-TZVPPD level of theory38 are performed using the GAUSSIAN 09 program.39 These methods have been previously used for similar studies.18,32,40 For the HXeI · · · HCl complex, the structural optimization at the CCSD(T)/def2-TZVPPD level of theory gives two structures with negative interaction energies and one structure with positive interaction energy (no imaginary frequencies) (see Figure 1 and Table I). The MP2(full) method leads to similar results for these three structures and features an additional structure with negative interaction energy. In the stable bent structures (A and B), the HCl molecule interacts with the iodine atom of HXeI. Structure A is stabilized by the H · · · I hydrogen bond whereas the less stable structure B has the Cl · · · I interaction. Structure D of this complex has positive interaction energy; therefore, we exclude it from the consideration. For the HXeI · · · HCCH complex, the MP2(full) method gives two stable structures (A and C); however, only structure A is obtained at the CCSD(T) level (see Figure 1 and Table I), and this complex is stabilized by the H · · · I hydrogen bond. The partial atomic charges are shown in Table II. In structure A, the HXe group becomes more positive than in the HXeI monomer (by +0.059e and by +0.039e for HXeI · · · HCl and HXeI · · · HCCH, respectively, at the CCSD level). In structure B of the HXeI · · · HCl complex, the complexation effect on the atomic charges is much weaker. Table III presents the calculated geometrical parameters of the complexes at the MP2(full) and CCSD(T) levels of theory and we describe below the CCSD(T) results. For the HXeI · · · HCl complex (structure A), the H–Xe bond is shortened by 0.038 Å and the Xe–I bond is lengthened by 0.031 Å compared to the HXeI monomer. In structure B, the bond lengths of HXeI change in the same direction but to a lesser extent. In structure D, the change is opposite. The HCl bond becomes longer in all structures, especially in structure A (by 0.021 Å). In the HXeI · · · HCCH complex (structure A), the H–Xe bond becomes shorter by 0.024 Å and the Xe–I bond is elongated by 0.017 Å. The bond lengths and angles of the HCCH moiety are not changed significantly; only a small elongation is seen for the H1–C bond (by 0.005 Å in structure A). The calculated frequencies of the H–Xe stretching mode are shown in Table IV. For the HXeI · · · HCl complex, the complexation-induced shift of this mode is +166.7 cm−1 in structure A, +19.5 cm−1 in structure B, and −98.4 cm−1 in structure D at the CCSD(T) level of theory, and the MP2(full) results are similar. The spectral shift in structure C is +71.1 cm−1 (MP2(full)). For the HXeI · · · HCCH complex, the shift is +103.5 cm−1 (CCSD(T)) and +70.1 cm−1 (MP2(full)) for structure A and it is +92.9 cm−1 (MP2(full)) for structure C. The H–Xe stretching intensity decreases upon complexation except for structure D. It should be mentioned that the obtained H–Xe stretching frequencies are presumably overestimated
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FIG. 1. Structures of the HXeI · · ·HY (Y = Cl and CCH) complexes. The selected structural parameters are given in Table III. Structure C of the HXeI · · ·HCCH complex is T-shaped (C2v symmetry).
because of the harmonic approximation. The previous calculations of HXeI show that taking into account the anharmonicity decreases the H–Xe stretching frequency by 155 cm−1, which improves the agreement between the experimental and calculated frequencies.41 The anharmonicity of the H–Xe stretching
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TABLE I. BSSE-corrected interaction energies E intCP (in kcal mol−1).a Structure A
Structure B
Structure C
Structure D
... −1.35(−0.20)
+0.31(+1.03) +0.65(+1.04)
... −2.31(−0.19)
... ...
HXeI · · ·HCl CCSD(T) MP2(full)
−4.94(−3.72) −6.40(−4.55)
−0.98(−0.28) −1.81(−0.36) HXeI · · ·HCCH
CCSD(T) MP2(full) a Interaction
−3.10(−2.67) −3.86(−1.79)
... ...
energies additionally corrected by ∆ZPVE (E intCP + ∆ZPVE) are given in parentheses.
vibration is probably somewhat smaller in the complexes than in the HXeI monomer; however, the difference should not be large. It follows that the anharmonicity does not have a strong effect on the monomer-to-complex shift. In the HXeI · · · HCl complex (structure A), the HCl frequency decreases by 265.6 cm−1 (CCSD(T)) as compared with the HCl monomer and the absorption intensity increases to ∼1000 km mol−1 from 52 km mol−1 for the monomer (MP2 (full)). In structure B, the change of the HCl frequency is rela-
tively small (−32.9 cm−1, CCSD(T)) and the absorption intensity even decreases (32 km mol−1, MP2(full)). The HCl frequency decreases in both structure C (−16.8 cm−1, MP2(full)) and structure D (−65.1 cm−1, MP2(full)) whereas the intensity increases to 76 (structure C) and 517 (structure D) km mol−1. For the HXeI · · · HCCH complex, the CH stretching mode is red shifted and the HCCH bending frequencies increase in both structures. The intensities of the HCCH modes are an order of magnitude smaller than that of the H–Xe stretching mode.
TABLE II. Partial atomic charges (in elementary charges) in the HXeI · · ·HCl and HXeI · · ·HCCH complexes. Monomers
Structure A
Structure B
Structure C
Structure D
... ... ... ... ...
+0.034 +0.665 −0.640 +0.283 −0.319
... ... ... ... ... ... ...
... ... ... ... ... ... ...
... ... ... ... ... ... ...
−0.021 +0.686 −0.659 +0.242 −0.248
+0.015 +0.672 −0.699 +0.262 −0.249
−0.083 +0.689 −0.602 +0.230 −0.233
+0.047 +0.659 −0.737 +0.239 −0.223 −0.223 +0.239
... ... ... ... ... ... ...
HXeI · · ·HCl
CCSD q(H)HXeI q(Xe)HXeI q(I)HXeI q(H)HCl q(Cl)HCl
−0.003 +0.643 −0.640 +0.322 −0.322
+0.034 +0.665 −0.664 +0.284 −0.319
−0.005 +0.649 −0.643 +0.242 −0.243
HXeI · · ·HCCHa q(H)HXeI q(Xe)HXeI q(I)HXeI q(H1)HCCH q(C2)HCCH q(C3)HCCH q(H4)HCCH
−0.003 +0.643 −0.640 +0.074 −0.074 −0.074 +0.074
+0.018 +0.661 −0.670 +0.254 −0.233 −0.261 +0.230 HXeI · · ·HCl
MP2(full) q(H)HXeI q(Xe)HXeI q(I)HXeI q(H)HCl q(Cl)HCl
−0.022 +0.678 −0.656 +0.252 −0.252
+0.017 +0.699 −0.661 +0.279 −0.334
HXeI · · ·HCCHa q(H)HXeI q(Xe)HXeI q(I)HXeI q(H1)HCCH q(C2)HCCH q(C3)HCCH q(H4)HCCH a The
−0.022 +0.678 −0.656 +0.230 −0.230 −0.230 +0.230
−0.000 +0.695 −0.679 +0.255 −0.250 −0.248 +0.227
... ... ... ... ... ... ...
H1 atom interacts with the I atom.
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TABLE III. Selected geometrical parameters of the HXeI · · ·HCl and HXeI · · ·HCCH complexes.a
CCSD(T)
HXeI, HCl
r (HHXeI–Xe)
r (Xe–I)
r (I · · ·HHY/Y)
r (HHXeI · · ·HHY/Y)
r (H–Cl)
angle 1b
angle 2b
1.768
3.024
...
...
1.275
...
...
1.730 1.763 1.797
3.055 3.024 3.009
2.651 4.111 ...
... ... 2.143
1.296 1.279 1.277
72.1 62.7 180.0
159.6 150.5 180.0
70.2
145.8
HXeI · · ·HCl Structure A Structure B Structure D HXeI · · ·HCCH
MP2(full)
Structure A
1.744
3.041
3.073
...
...
HXeI, HCl
1.709
2.975
...
...
1.269
...
...
1.677 1.700 1.692 1.728
3.012 2.974 2.993 2.944
2.495 3.829 ... ...
... ... 2.679 1.993
1.296 1.271 1.271 1.272
71.8 66.4 180.0 180.0
164.9 172.6 102.7 180.0
1.689 1.686
2.992 3.017
2.904 ...
... 2.346
... ...
69.2 90c
148.8 180c
HXeI · · ·HCl Structure A Structure B Structure C Structure D HXeI · · ·HCCH Structure A Structure C a The b See
bond lengths are in Å and angles in degrees. Figure 1. C of the HXeI · · ·HCCH complex is T-shaped (C2v symmetry).
c Structure
III. EXPERIMENTS A. Experimental details
HI was synthesized from 1, 2, 3, 4-tetrahydronaphthalene (tetralin) and iodine.42 HCl (≥99.8%, Linde), HCCH (≥99.8%,
AGA), and xenon (≥99.999%, AGA) were used without further purification. The gas mixtures with mixing ratios of HI/HCl(HCCH)/Xe ≈ 1/(0–2)/1000 were deposited onto a cold CsI substrate in a closed-cycle helium cryostat (RDK-408D2, SHI). The IR absorption spectra were measured at 3 K by a
TABLE IV. Calculated frequencies and monomer-to-complex shifts (in cm−1) of the HXeI · · ·HCl and HXeI · · ·HCCH complexes.a Monomer
Structure A
Structure B
Structure C
Structure D
... 1733.7, +71.1 (1483)
1228.8, −98.4 1542.2, −120.4 (4473)
... 3053.2, −16.8 (76)
2965.2, −42.1 3004.9, −65.1 (517)
HXeI· · · HCl CCSD(T) MP2(full)
1327.2 1662.6 (2791)
1493.9, +166.7 1788.8, +126.2 (2091)
1346.7, +19.5 1678.4, +15.8 (2630) HXeI· · ·HCl
CCSD(T) MP2(full)
3007.3 3070.0 (52)
2741.7, −265.6 2683.7, −386.3 (1033)
2974.4, −32.9 3049.9, −20.1 (32) HXeI· · · HCCH
CCSD(T) MP2(full)
1327.2 1662.6 (2791)
1430.7, +103.5 1732.7, +70.1, (2366)
... ...
... 1755.5, +92.9 (369)
... ...
... 3428.5, −16.8 (114)
... ...
... 1969.0, −8.4 (21)
... ...
HXeI· · ·HCCH CCSD(T) MP2(full)
3406.6 3445.3 (95)
3464.9, −41.7 3376.2, −69.1, (200)
... ... HXeI· · · HCCH
CCSD(T) MP2(full)
1996.6 1977.4 (0.0)
1987.8, −8.8 1960.9, −16.5 (5)
... ... HXeI· · ·HCCH (bend)
CCSD(T)
749.2b
MP2(full)
767.1b (89)
a The
762.3, +13.1 766.9, +17.7 792.5, +25.4 (55) 797.2, +30.1 (113)
...
...
...
...
771.3, +4.2 (81) 793.7, +26.6 (196)
...
infrared intensities (in km mol−1) calculated by the MP2(full) method are shown in parentheses. The presented frequencies correspond to the underlined groups of atoms. degenerated.
b Doubly
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FIG. 2. FTIR spectra of HI/HCl/Xe matrices (upper spectrum: ∼1/2/1000; lower spectrum: ∼1/0/1000) measured after deposition at 35 K. The spectra were measured at 3 K. Notice the multiplication factor in the right part.
FTIR spectrometer (Vertex 80V, Bruker) in the 4000–400 cm−1 range with 1 cm−1 resolution typically co-adding 200 scans. After deposition, the matrices were photolyzed at 3 K by an excimer laser (MSX-250, MPB) operating at 193 nm (∼10 mJ cm−2). The annealing-induced products were decomposed by a 193-nm excimer laser, a low-pressure mercury lamp (254 nm), and a 488-nm argon ion laser (Series 532, model 35 LAS 450 230, Melles Griot). B. HXeI · · · HCl complex
In a HI/Xe matrix, several HI monomer bands are observed after deposition (the strongest band at 2214 cm−1, Figure 2).43 Addition of HCl leads to two new bands at 2190 and 2187 cm−1 (Figure 2) shifted by −24 and −27 cm−1 from the strongest monomeric band. We calculate the shift from the strongest monomer band because the Q branch of HI in a Xe matrix is unknown, to our knowledge. This approximation should not affect the result much because the difference between the R (0) and Q branches of HI in a Kr matrix is ∼6 cm−1.44 These bands are assigned to the HI vibration in the HCl · · · HI complex, in which HI acts as a proton donor. In the HCl stretching region, a band at 2785 cm−1 is assigned to the HCl vibration of the HI · · · HCl complex, in which HCl acts as a proton donor; it is shifted by −53 cm−1 from the Q branch of HCl monomer at 2837.8 cm−1.44 These spectral shifts are in agreement with the calculated values of −24 cm−1 (HCl · · · HI) and −55 cm−1 (HI · · · HCl).17 The HI · · · HCl complex in an Ar matrix shows a shift of −89 cm−1,6 which also agrees with the present data. 300 pulses at 193 nm (∼10 mJ cm−2) decompose more than 80% of HI whereas HCl is decomposed with a much lower efficiency. The ionic species such as (ClHCl)−, (IHI)−, and (XeHXe)+ are observed after photolysis.45,46 Annealing at ∼40 K mobilizes H atoms in a Xe matrix,47,48 and the bands of HXeI (1193, 1215, and 1322.0 cm−1)18,49 and HXeH (1166 and 1181 cm−1)50 are observed. In addition to the known strong bands of the HXeI monomer, three new bands appear at 1287, 1304, and 1348 cm−1 in matrices containing both HI and HCl (Figure 3), and these bands are assigned to the HXeI · · · HCl complex. HXeI, its complexes, and HXeH are efficiently decomposed by a mercury lamp. Decomposed HXeI and its complexes can be partially recovered by annealing. Three bands at 2501, 2586 (possibly with a broad
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FIG. 3. FTIR difference spectra of a HI/HCl/Xe (1/2/1000) matrix showing the results of annealing at 60 K after 193-nm photolysis (top spectrum), irradiation of the annealed matrix by a mercury lamp (middle spectrum), and the second annealing at 60 K (bottom spectrum).
side band at ∼2599 cm−1), and 2651 cm−1 with a similar behavior are assigned to HCl in the HXeI · · · HCl complex. The intensity ratio of the H–Xe and HCl stretching bands of the HXeI · · · HCl complex is ∼2.5, which agrees with the calculated value for structure A (∼2.0). The HXeI · · · HI complex was identified previously with bands at 1230, 1268, and 1289 cm−1,18 and the former two bands appear in the spectra. The band at 1289 cm−1 of HXeI · · · HI is relatively weak and overlaps with the 1287 cm−1 band of HXeI · · · HCl. The H–Xe stretching band of the HXeCl monomer at 1648.7 cm−1 is also observed;49 however, no indication of the HXeCl · · · HI complex is found. C. HXeI · · · HCCH complex
After deposition of a HI/HCCH/Xe matrix, a new band in the HI region is observed at 2175 cm−1 (Figure 4), and it is assigned to the HCCH · · · HI complex, in which HI is a proton donor. The experimental monomer-to-complex shift of −39 cm−1 (calculated from the strongest HI monomer band) agrees well with both the experimental shift in an Ar matrix (−66 cm−1)3 and our calculated value of −24.8 cm−1 (T-shaped structure, CCSD(T)/def2-TZVPPD) (Table SI of the supplementary material).51 Although the calculated intensities of the
FIG. 4. FTIR spectra of HI/HCCH/Xe matrices (upper spectrum: ∼1/1/1000; lower spectrum ∼1/0/1000) measured after deposition at 35 K. The spectra were measured at 3 K.
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FIG. 5. FTIR difference spectrum of a HI/Xe (1/1000) matrix showing the result of annealing at 60 K after 193-nm photolysis (top spectrum); FTIR difference spectra of a HI/HCCH/Xe (1/1/1000) matrix showing the results of annealing at 60 K after 193-nm photolysis (middle spectrum) and irradiation of the annealed matrix by a 488-nm laser (bottom spectrum).
CH stretching and HCCH bending mode of the HCCH · · · HI complex are comparable to that of the HI vibration (MP2(full)), the frequency shifts from the monomeric values are small (−6.5 cm−1 for the CH stretching mode and +2.6/+11.4 cm−1 for the HCCH bending mode). This probably explains why we did not identify the HCCH · · · HI bands in the HCCH stretching and bending regions. The linear HI · · · HCCH complex has interaction energy of −0.01 kcal mol−1 at the CCSD(T)/def2TZVPPD level of theory,51 and it is not observed in the experiments. After 300 pulses of 193-nm light (∼10 mJ cm−2), more than 80% of the HI monomers and HCCH · · · HI complexes are decomposed, and the decomposition of HCCH is much less efficient. After annealing of a photolyzed HI/HCCH/Xe matrix at ≥40 K, a new acetylene-induced band appears at 1242 cm−1 with a side band at 1248 cm−1, which are blue-shifted from the HXeI monomer band (1193 cm−1). The 1242 and 1248 cm−1 bands are assigned to the H–Xe stretching mode of the HXeI· · · HCCH complex (Figure 5). These bands are efficiently decom-
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FIG. 7. Lifetime of HXeI in different complexes under broadband IR irradiation as a function of the shift of the H–Xe stretching mode. The data for the HXeI · · ·HI and HXeI · · ·HBr complexes are from Ref. 18.
posed by short irradiation at 488 nm (20 s with ∼10 mW cm−2) (Figure 5). No bands of HCCH are observed with a similar behavior. The formation of vinyl radical (HCCH2 at 1349 cm−1),52 HXeCCH (1486.4 cm−1), HXeCC (1478.0 cm−1), and HXeCCXeH (1301 cm−1)22 is also observed, but the complexes of these molecules with HI are not found. D. Photostability
The HXeI monomer is decomposed by IR radiation of the spectrometer (Figure 6), and the 2950-3800 cm−1 spectral region is responsible for this effect.31 The HXeI · · · HCl and HXeI · · · HCCH complexes can be decomposed by IR light of the spectrometer as well (Figure 6). No deviations from a single exponential function are observed for these decay curves. The HXeI monomer is decomposed most quickly, with a lifetime of (1.54 ± 0.05) h at the 1/e level. The lifetimes of the 1287, 1304, and 1348 cm−1 bands of the HXeI · · · HCl complex are (10.0 ± 0.2), (19.5 ± 0.4), and (129 ± 18) h, respectively. The corresponding HCl bands at 2651, 2586, and 2501 cm−1 have similar lifetimes (9.7 ± 0.2), (24.2 ± 2.2), and (115 ± 13) h, respectively. The HXeI · · · HCCH complex (1242 cm−1) has a lifetime of (3.0 ± 0.2) h. Figure 7 shows the lifetimes of the HXeI monomer and the HXeI complexes with HI, HBr, HCl, and HCCH as a function of the monomer-to-complex shift of the H–Xe stretching mode. The photostability of the species increases with the shift. The exception of this trend is the 1230.4 cm−1 band of the HXeI · · · HI complex with the smallest shift of +37.2 cm−1.
IV. CONCLUDING DISCUSSION
FIG. 6. Time dependences of the integrated absorptions of the HXeI monomer, HXeI · · ·HCCH, and HXeI · · ·HCl bands under broadband IR radiation: (a) HXeI monomer (vH–Xe at 1193 cm−1); (b) HXeI · · ·HCCH complex (vH–Xe at 1242 cm−1); (c) HXeI · · ·HCl complex (vH–Xe at 1287 cm−1 and vH–Cl at 2651 cm−1); (d) HXeI · · ·HCl complex (vH–Xe at 1304 cm−1 and vH–Cl at 2586 cm−1); and (e) HXeI · · ·HCl complex (vH–Xe at 1348 cm−1 and vH–Cl at 2501 cm−1). The lines are single exponential fits of the H–Xe stretching modes. In (c)–(e), solid symbols stand for HXeI and open for HCl.
The bands at 2785 cm−1 and 2190/2187 cm−1 originate from the HI · · · HCl (HCl acts as a proton donor) and HCl · · · HI (HI acts as a proton donor) complexes in a Xe matrix, which are the precursors of the HXeI · · · HCl complex. Higher HCl and HI concentrations and/or higher deposition temperatures enhance the amount of these complexes. In different experiments, the intensities of six new bands observed after photolysis and annealing at 1287, 1304, 1348, 2501, 2586, and 2651 cm−1 correlate with the amount of the
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TABLE V. Experimental H–Xe stretching and HCl frequencies (in cm−1) of the HXeI · · ·HCl and HXeI · · ·HCCH complexes.a Species HXeI HCl HXeI· · · HCl
HXeI· · ·HCl
HXeI· · · HCCH
Frequencies 1193 2838 1287 (+94) 1304 (+111) 1348 (+155) 2501 (−337) 2586 (−252), 2599 (−239) 2651 (−187) 1242 (+49), 1248 (+55)
frequency shifts (in cm−1) are shown in parentheses. The presented frequencies correspond to the underlined groups of atoms.
a Monomer-to-complex
HI · · · HCl and HCl · · · HI complexes observed after deposition. The first three bands are shifted by +94, +111, and +155 cm−1 from the HXeI monomer band and the last three bands are shifted by −337, −252, and −187 cm−1 from the Q branch of the HCl monomer. These bands are assigned to the HXeI · · · HCl complex. The contribution of the HXeI· · ·(HCl)2 complex is ruled out because the relative intensity of these bands does not depend on the HCl concentration. The HXeCl · · · HI complex is not found presumably due to more efficient photodecomposition of HI as compared with HCl. The structural assignment of the HXeI · · · HCl complex is based on the comparison of the computational (Table IV) and experimental results (Table V). The H–Xe and HCl stretching shifts calculated for structure A are +166.7 and −265.6 cm−1 at the CCSD(T)/def2-TZVPPD level of theory, in agreement with the experimental results (the largest experimental shifts are +155 and −337 cm−1). The shifts calculated for structure B (+19.5 and −32.9 cm−1) disagree with the experimental values. Furthermore, the intensity ratio of the H–Xe and HCl stretching modes calculated for structure A is ∼2, which agrees with the experimental value of ∼2.5 whereas this ratio is ∼20 for structure B. It is seen that structures C and D do not fit the experimental data. Thus, we assign all six bands to the most stable structure A. For the same structure A, the interaction energies (without ZPVE correction, at the CCSD(T) level of theory) of the HXeI · · · HCl (−4.94 kcal mol−1) and HXeI · · · HBr (−4.69 kcal mol−1) complexes are similar but substantially larger than that of the HXeI · · · HI complex (−3.88 kcal mol−1).18 In accord, the H–Xe stretching shifts of the HXeI · · · HCl (up to +155 cm−1) and HXeI · · · HBr (up to +154 cm−1) complexes are also similar and larger than that of the HXeI· · · HI complex (up to +96 cm−1). The HXeI · · · HCl complexes are decomposed by broadband IR radiation of the spectrometer source (Figure 6). Because IR-induced decomposition is characteristic of HXeI,31 this observation supports the assignment of the observed bands to the HXeI · · · HCl complex. Moreover, the IR-decomposition experiments allow us to separate these six bands into three pairs of bands belonging to the same species (1287-2651, 1304-2586, and 1348-2501 cm−1). The large splitting of the H–Xe stretching and HCl stretching bands is most probably due to structural perturbations caused by different matrix morphologies (matrix-site effect). As shown above, only one
theoretical structure (structure A) can correspond to these experimental bands. As expected, the decrease of the HCl frequency correlates with the increase of the H–Xe stretching frequency. We can speculate that the largest shifts correspond to the most relaxed structure whereas in two other matrix sites the interaction of the complex is more perturbed by the matrix environment. A similarly extensive band splitting has been previously observed for the HKrCl · · · HCl,29 HXeI · · · HI, and HXeI · · · HBr complexes.18 The detailed explanation of this extensive matrix-site effect is a challenge for computational chemistry. The 1242 cm−1 band observed after photolysis and annealing of a HI/HCCH/Xe matrix is safely assigned to the HXeI · · · HCCH complex. The intensity of this band correlates with the amount of the photolyzed precursor, the HCCH· · · HI complex. The intensities of the CH stretching and CCH bending modes of the HXeI · · · HCCH complex are much lower than that of the H–Xe stretching mode (Table IV), which explains the lack of the experimental identification of the HCCH bands. The CCSD(T) results predict only one stable structure (structure A) for the HXeI · · · HCCH complex. The calculated blue shift of the H–Xe stretching mode (+103.5 cm−1 at the CCSD(T) level and +70.1 cm−1 at the MP2(full) level) reasonably agrees with the experimental result (+49 cm−1). It should be noted that the monomer-to-complex shift can be influenced by the matrix. The calculations are performed in vacuum whereas the experiments are made in a polarizable medium. The matrix can differently affect the H–Xe stretching frequency of the monomer and complex. The lifetime of different complexes under broadband IR radiation correlates with the H–Xe stretching frequency (Figure 7). The photostability of species strongly increases with the H–Xe stretching shift (by two orders of magnitude). The mechanism of this effect is worth discussing. According to the previous study by Pettersson et al., the decomposition of HXeI occurs via absorption of the second H–Xe stretching overtone at ∼3000 cm−1.31 The second overtones of the complexes are higher in frequency, for example, the frequency can be estimated as up to ∼3370 cm−1 for the HXeI · · · HCl complex if the anharmonicity of the HXeI monomer is used. First of all, it should be noted that the observed difference in photostabilities cannot be due to the change of the IR light intensity at these frequencies because the Globar intensity has the maximum above 4000 cm−1, i.e., the opposite trend should be expected from this contribution. We propose two mechanisms that can be responsible for the observed dependence (Figure 7). The first mechanism is due to an increase of the harmonicity of the H–Xe stretching mode upon complexation. The intensity of the second H–Xe stretching absorption of the complexes presumably decreases as the H–Xe stretching frequency increases, in qualitative agreement with the experimental dependence. To support this mechanism, anharmonic calculations were performed at the MP2(full)/def2-TZVPPD level of theory by using Gaussian 09 (Rev. D.01) program. According to the calculations, the intensity of the first overtone of the HXeI · · · HCCH and HXeI · · · HCl complexes is about 55% and 30% of that of the monomer (Table SIII of the supplementary material).51 The differences for the second overtone are probably larger;
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however, the second overtones cannot be calculated by the program used. The second proposed mechanism originates from the different excess energies available for the H atom after photodissociation, which changes the cage exit probability. Three-body dissociation energies of the HXeI monomer and HXeI · · · HY (Y = I, Br, Cl, and CCH) complexes were calculated at the CCSD(T)/def2-TZVPPD level of theory (Table SIV of the supplementary material).51 The absolute accuracy of these results is questionable; however, a reliable qualitative conclusion can be derived. In fact, the dissociation energy increases for the complexes compared to the HXeI monomer, and this increase is larger than the increase of the frequency of the second overtone. For example, for HXeI · · · HCl, the dissociation energy is larger than that of the monomer by ≥850 cm−1 whereas the second overtone frequency is larger only by 370 cm−1. It follows that the excess energy is about 500 cm−1 smaller for the complex. This change presumably decreases the cage exit probability of the H atom for the complex and increases the probability of the back-reaction. Other factors can also affect the photodecomposition rate. For example, the complexation increases the vibrational state density, which can enhance the intramolecular vibrational relaxation and compete with dissociation. ACKNOWLEDGMENTS
C.Z. thanks the CSC-China Scholarship Council for the Ph.D. grant. M.T. thanks the Academy of Finland for the postdoctoral Grant No. 1139105. This work is also a part of the Project KUMURA of the Academy of Finland (No. 1277993). Jere Renlund is thanked for technical assistance. The authors are grateful to the CSC-IT Center for Science for computer time. 1K.
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THE JOURNAL OF CHEMICAL PHYSICS 142, 144306 (2015)
Experimental and theoretical study of the HXeI · · · HCl and HXeI · · · HCCH complexes Cheng Zhu, Masashi Tsuge,a) Markku Räsänen, and Leonid Khriachtchevb) Department of Chemistry, University of Helsinki, P.O. Box 55, FI-00014 Helsinki, Finland
(Received 10 February 2015; accepted 27 March 2015; published online 13 April 2015) The HXeI · · · HCl and HXeI · · · HCCH complexes are studied computationally and experimentally in a Xe matrix. In the experiments, three bands of the HXeI · · · HCl complex and one band of the HXeI · · · HCCH complex in the H–Xe stretching region are observed. The monomer-tocomplex shifts are +94, +111, and +155 cm−1 for the HXeI · · · HCl complex and +49 cm−1 for the HXeI · · · HCCH complex. The bands of the complexed HCl molecules are also observed with large red shifts from the HCl monomer (−187, −252, and −337 cm−1). The ab initio calculations at the CCSD(T)/def2-TZVPPD level of theory predict two stable structures for the HXeI · · · HCl complex with interaction energies of −3.72 and −0.28 kcal mol−1 and one structure for the HXeI · · · HCCH complex with an interaction energy of −2.67 kcal mol−1 and the calculated monomer-to-complex shifts are in a good agreement with experiment (in the case of HXeI · · · HCl, for the stronger structure). The HXeI molecules are decomposed by broad-band infrared light; however, the decomposition is much more efficient for the HXeI monomer than for the complexes studied here as well as for the previously studied HXeI · · · HI and HXeI · · · HBr complexes. In fact, the decomposition efficiency decreases as the monomer-to-complex shift of the H–Xe stretching mode increases. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4917167]
I. INTRODUCTION
Non-covalent interactions are responsible for many properties of matter and play an important role in chemical reactions.1,2 These interactions can be studied by vibrational spectroscopy because they change the frequencies of the characteristic vibrational modes. In particular, infrared (IR) matrixisolation spectroscopy has been extensively used to study intermolecular interactions.3–16 For example, Barnes investigated various complexes between hydrogen chloride and other species (H2O, N2, benzene, etc.) trapped in argon matrices.6 The results show that for complexes with weak to medium strength bases, the red shift of the HCl frequency depends on the proton affinity of the bases. For hydrogen bonding system like HBr · · · HI, in which HI acts as a proton donor, the HI vibrational frequency decreases and its intensity increases upon complexation.17,18 In some cases, blue shifting hydrogen bonds are observed.19,20 Noble-gas hydrides HNgY (Ng = noble-gas atom and Y = electronegative fragment) have been mostly prepared in low-temperature matrices.21–25 These molecules are characterized by weak bonding and large dipole moments, which leads to a strong complexation effect on the vibrational properties of these molecules.26–30 In all HNgY complexes studied experimentally, the H–Ng stretching modes exhibit blue shifts, explained by the enhancement of charge separation in these molecules.28 The HXeI molecule is one of the least stable noblegas hydrides as judged by the very small dissociation energy a)Present address: Department of Applied Chemistry, National Chiao Tung
University, 1001 Ta-Hsueh Road, Hsinchu 30010, Taiwan.
b)E-mail address: [email protected]
0021-9606/2015/142(14)/144306/8/$30.00
(D0 = 8.30 kcal mol−1).31 It follows that the complexation effect on the properties of HXeI should be very strong;18,32 thus, the complexes of this molecule are particularly interesting. Recently, Tsuge et al. have studied the HXeY · · · H2O (Y = Cl, Br, and I) complexes.32 The frequency of the H–Xe stretching mode of the HXeI · · · H2O complex, indeed, has the largest blue shift (+138 cm−1) among these three species. The complexes of HXeI with HBr and HI have also been investigated.18 The interaction with HBr is found to induce larger blue shifts of the H–Xe stretching mode (up to +157 cm−1) than the interaction with HI (up to +96 cm−1). Another finding of that work is that the stability of HXeI under IR light increases upon complex formation. It is interesting to expand these data to complexes of HXeI with other molecules to follow the correlation between the interaction strength and the complexation effects. In the present work, we study the HXeI · · · HCl and HXeI · · · HCCH complexes in a Xe matrix by IR absorption spectroscopy. The experimental work is supported by quantum chemical calculations.
II. COMPUTATIONAL DETAILS AND RESULTS
The equilibrium structures, relative energies, and vibrational spectra are calculated at the CCSD(T) and MP2(full) levels of theory.33 The def2-TZVPPD basis sets, used for H, C, Cl, I, and Xe atoms,34 are taken from the EMSL basis set library using the Basis Set Exchange software.35 For I and Xe atoms, 28 electrons are replaced by an effective core potential. The interaction energy (Eint) is defined as the difference between the total energies of the complex and of the monomers (with the structures in the complex). The difference of zero-point
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vibrational energies (∆ZPVE) and the basis set superposition error (BSSE) (corrected using the counterpoise procedure (CP))36 is taken into account. The structural optimization and harmonic vibrational analysis at the CCSD(T) level of theory are performed using the MOLPRO program.37 The calculations at the MP2(full) level of theory, single point energy calculations and the BSSE correction at the CCSD(T) level of theory, and the natural population analysis at the CCSD/def2TZVPPD//CCSD(T)/def2-TZVPPD level of theory38 are performed using the GAUSSIAN 09 program.39 These methods have been previously used for similar studies.18,32,40 For the HXeI · · · HCl complex, the structural optimization at the CCSD(T)/def2-TZVPPD level of theory gives two structures with negative interaction energies and one structure with positive interaction energy (no imaginary frequencies) (see Figure 1 and Table I). The MP2(full) method leads to similar results for these three structures and features an additional structure with negative interaction energy. In the stable bent structures (A and B), the HCl molecule interacts with the iodine atom of HXeI. Structure A is stabilized by the H · · · I hydrogen bond whereas the less stable structure B has the Cl · · · I interaction. Structure D of this complex has positive interaction energy; therefore, we exclude it from the consideration. For the HXeI · · · HCCH complex, the MP2(full) method gives two stable structures (A and C); however, only structure A is obtained at the CCSD(T) level (see Figure 1 and Table I), and this complex is stabilized by the H · · · I hydrogen bond. The partial atomic charges are shown in Table II. In structure A, the HXe group becomes more positive than in the HXeI monomer (by +0.059e and by +0.039e for HXeI · · · HCl and HXeI · · · HCCH, respectively, at the CCSD level). In structure B of the HXeI · · · HCl complex, the complexation effect on the atomic charges is much weaker. Table III presents the calculated geometrical parameters of the complexes at the MP2(full) and CCSD(T) levels of theory and we describe below the CCSD(T) results. For the HXeI · · · HCl complex (structure A), the H–Xe bond is shortened by 0.038 Å and the Xe–I bond is lengthened by 0.031 Å compared to the HXeI monomer. In structure B, the bond lengths of HXeI change in the same direction but to a lesser extent. In structure D, the change is opposite. The HCl bond becomes longer in all structures, especially in structure A (by 0.021 Å). In the HXeI · · · HCCH complex (structure A), the H–Xe bond becomes shorter by 0.024 Å and the Xe–I bond is elongated by 0.017 Å. The bond lengths and angles of the HCCH moiety are not changed significantly; only a small elongation is seen for the H1–C bond (by 0.005 Å in structure A). The calculated frequencies of the H–Xe stretching mode are shown in Table IV. For the HXeI · · · HCl complex, the complexation-induced shift of this mode is +166.7 cm−1 in structure A, +19.5 cm−1 in structure B, and −98.4 cm−1 in structure D at the CCSD(T) level of theory, and the MP2(full) results are similar. The spectral shift in structure C is +71.1 cm−1 (MP2(full)). For the HXeI · · · HCCH complex, the shift is +103.5 cm−1 (CCSD(T)) and +70.1 cm−1 (MP2(full)) for structure A and it is +92.9 cm−1 (MP2(full)) for structure C. The H–Xe stretching intensity decreases upon complexation except for structure D. It should be mentioned that the obtained H–Xe stretching frequencies are presumably overestimated
J. Chem. Phys. 142, 144306 (2015)
FIG. 1. Structures of the HXeI · · ·HY (Y = Cl and CCH) complexes. The selected structural parameters are given in Table III. Structure C of the HXeI · · ·HCCH complex is T-shaped (C2v symmetry).
because of the harmonic approximation. The previous calculations of HXeI show that taking into account the anharmonicity decreases the H–Xe stretching frequency by 155 cm−1, which improves the agreement between the experimental and calculated frequencies.41 The anharmonicity of the H–Xe stretching
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TABLE I. BSSE-corrected interaction energies E intCP (in kcal mol−1).a Structure A
Structure B
Structure C
Structure D
... −1.35(−0.20)
+0.31(+1.03) +0.65(+1.04)
... −2.31(−0.19)
... ...
HXeI · · ·HCl CCSD(T) MP2(full)
−4.94(−3.72) −6.40(−4.55)
−0.98(−0.28) −1.81(−0.36) HXeI · · ·HCCH
CCSD(T) MP2(full) a Interaction
−3.10(−2.67) −3.86(−1.79)
... ...
energies additionally corrected by ∆ZPVE (E intCP + ∆ZPVE) are given in parentheses.
vibration is probably somewhat smaller in the complexes than in the HXeI monomer; however, the difference should not be large. It follows that the anharmonicity does not have a strong effect on the monomer-to-complex shift. In the HXeI · · · HCl complex (structure A), the HCl frequency decreases by 265.6 cm−1 (CCSD(T)) as compared with the HCl monomer and the absorption intensity increases to ∼1000 km mol−1 from 52 km mol−1 for the monomer (MP2 (full)). In structure B, the change of the HCl frequency is rela-
tively small (−32.9 cm−1, CCSD(T)) and the absorption intensity even decreases (32 km mol−1, MP2(full)). The HCl frequency decreases in both structure C (−16.8 cm−1, MP2(full)) and structure D (−65.1 cm−1, MP2(full)) whereas the intensity increases to 76 (structure C) and 517 (structure D) km mol−1. For the HXeI · · · HCCH complex, the CH stretching mode is red shifted and the HCCH bending frequencies increase in both structures. The intensities of the HCCH modes are an order of magnitude smaller than that of the H–Xe stretching mode.
TABLE II. Partial atomic charges (in elementary charges) in the HXeI · · ·HCl and HXeI · · ·HCCH complexes. Monomers
Structure A
Structure B
Structure C
Structure D
... ... ... ... ...
+0.034 +0.665 −0.640 +0.283 −0.319
... ... ... ... ... ... ...
... ... ... ... ... ... ...
... ... ... ... ... ... ...
−0.021 +0.686 −0.659 +0.242 −0.248
+0.015 +0.672 −0.699 +0.262 −0.249
−0.083 +0.689 −0.602 +0.230 −0.233
+0.047 +0.659 −0.737 +0.239 −0.223 −0.223 +0.239
... ... ... ... ... ... ...
HXeI · · ·HCl
CCSD q(H)HXeI q(Xe)HXeI q(I)HXeI q(H)HCl q(Cl)HCl
−0.003 +0.643 −0.640 +0.322 −0.322
+0.034 +0.665 −0.664 +0.284 −0.319
−0.005 +0.649 −0.643 +0.242 −0.243
HXeI · · ·HCCHa q(H)HXeI q(Xe)HXeI q(I)HXeI q(H1)HCCH q(C2)HCCH q(C3)HCCH q(H4)HCCH
−0.003 +0.643 −0.640 +0.074 −0.074 −0.074 +0.074
+0.018 +0.661 −0.670 +0.254 −0.233 −0.261 +0.230 HXeI · · ·HCl
MP2(full) q(H)HXeI q(Xe)HXeI q(I)HXeI q(H)HCl q(Cl)HCl
−0.022 +0.678 −0.656 +0.252 −0.252
+0.017 +0.699 −0.661 +0.279 −0.334
HXeI · · ·HCCHa q(H)HXeI q(Xe)HXeI q(I)HXeI q(H1)HCCH q(C2)HCCH q(C3)HCCH q(H4)HCCH a The
−0.022 +0.678 −0.656 +0.230 −0.230 −0.230 +0.230
−0.000 +0.695 −0.679 +0.255 −0.250 −0.248 +0.227
... ... ... ... ... ... ...
H1 atom interacts with the I atom.
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TABLE III. Selected geometrical parameters of the HXeI · · ·HCl and HXeI · · ·HCCH complexes.a
CCSD(T)
HXeI, HCl
r (HHXeI–Xe)
r (Xe–I)
r (I · · ·HHY/Y)
r (HHXeI · · ·HHY/Y)
r (H–Cl)
angle 1b
angle 2b
1.768
3.024
...
...
1.275
...
...
1.730 1.763 1.797
3.055 3.024 3.009
2.651 4.111 ...
... ... 2.143
1.296 1.279 1.277
72.1 62.7 180.0
159.6 150.5 180.0
70.2
145.8
HXeI · · ·HCl Structure A Structure B Structure D HXeI · · ·HCCH
MP2(full)
Structure A
1.744
3.041
3.073
...
...
HXeI, HCl
1.709
2.975
...
...
1.269
...
...
1.677 1.700 1.692 1.728
3.012 2.974 2.993 2.944
2.495 3.829 ... ...
... ... 2.679 1.993
1.296 1.271 1.271 1.272
71.8 66.4 180.0 180.0
164.9 172.6 102.7 180.0
1.689 1.686
2.992 3.017
2.904 ...
... 2.346
... ...
69.2 90c
148.8 180c
HXeI · · ·HCl Structure A Structure B Structure C Structure D HXeI · · ·HCCH Structure A Structure C a The b See
bond lengths are in Å and angles in degrees. Figure 1. C of the HXeI · · ·HCCH complex is T-shaped (C2v symmetry).
c Structure
III. EXPERIMENTS A. Experimental details
HI was synthesized from 1, 2, 3, 4-tetrahydronaphthalene (tetralin) and iodine.42 HCl (≥99.8%, Linde), HCCH (≥99.8%,
AGA), and xenon (≥99.999%, AGA) were used without further purification. The gas mixtures with mixing ratios of HI/HCl(HCCH)/Xe ≈ 1/(0–2)/1000 were deposited onto a cold CsI substrate in a closed-cycle helium cryostat (RDK-408D2, SHI). The IR absorption spectra were measured at 3 K by a
TABLE IV. Calculated frequencies and monomer-to-complex shifts (in cm−1) of the HXeI · · ·HCl and HXeI · · ·HCCH complexes.a Monomer
Structure A
Structure B
Structure C
Structure D
... 1733.7, +71.1 (1483)
1228.8, −98.4 1542.2, −120.4 (4473)
... 3053.2, −16.8 (76)
2965.2, −42.1 3004.9, −65.1 (517)
HXeI· · · HCl CCSD(T) MP2(full)
1327.2 1662.6 (2791)
1493.9, +166.7 1788.8, +126.2 (2091)
1346.7, +19.5 1678.4, +15.8 (2630) HXeI· · ·HCl
CCSD(T) MP2(full)
3007.3 3070.0 (52)
2741.7, −265.6 2683.7, −386.3 (1033)
2974.4, −32.9 3049.9, −20.1 (32) HXeI· · · HCCH
CCSD(T) MP2(full)
1327.2 1662.6 (2791)
1430.7, +103.5 1732.7, +70.1, (2366)
... ...
... 1755.5, +92.9 (369)
... ...
... 3428.5, −16.8 (114)
... ...
... 1969.0, −8.4 (21)
... ...
HXeI· · ·HCCH CCSD(T) MP2(full)
3406.6 3445.3 (95)
3464.9, −41.7 3376.2, −69.1, (200)
... ... HXeI· · · HCCH
CCSD(T) MP2(full)
1996.6 1977.4 (0.0)
1987.8, −8.8 1960.9, −16.5 (5)
... ... HXeI· · ·HCCH (bend)
CCSD(T)
749.2b
MP2(full)
767.1b (89)
a The
762.3, +13.1 766.9, +17.7 792.5, +25.4 (55) 797.2, +30.1 (113)
...
...
...
...
771.3, +4.2 (81) 793.7, +26.6 (196)
...
infrared intensities (in km mol−1) calculated by the MP2(full) method are shown in parentheses. The presented frequencies correspond to the underlined groups of atoms. degenerated.
b Doubly
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FIG. 2. FTIR spectra of HI/HCl/Xe matrices (upper spectrum: ∼1/2/1000; lower spectrum: ∼1/0/1000) measured after deposition at 35 K. The spectra were measured at 3 K. Notice the multiplication factor in the right part.
FTIR spectrometer (Vertex 80V, Bruker) in the 4000–400 cm−1 range with 1 cm−1 resolution typically co-adding 200 scans. After deposition, the matrices were photolyzed at 3 K by an excimer laser (MSX-250, MPB) operating at 193 nm (∼10 mJ cm−2). The annealing-induced products were decomposed by a 193-nm excimer laser, a low-pressure mercury lamp (254 nm), and a 488-nm argon ion laser (Series 532, model 35 LAS 450 230, Melles Griot). B. HXeI · · · HCl complex
In a HI/Xe matrix, several HI monomer bands are observed after deposition (the strongest band at 2214 cm−1, Figure 2).43 Addition of HCl leads to two new bands at 2190 and 2187 cm−1 (Figure 2) shifted by −24 and −27 cm−1 from the strongest monomeric band. We calculate the shift from the strongest monomer band because the Q branch of HI in a Xe matrix is unknown, to our knowledge. This approximation should not affect the result much because the difference between the R (0) and Q branches of HI in a Kr matrix is ∼6 cm−1.44 These bands are assigned to the HI vibration in the HCl · · · HI complex, in which HI acts as a proton donor. In the HCl stretching region, a band at 2785 cm−1 is assigned to the HCl vibration of the HI · · · HCl complex, in which HCl acts as a proton donor; it is shifted by −53 cm−1 from the Q branch of HCl monomer at 2837.8 cm−1.44 These spectral shifts are in agreement with the calculated values of −24 cm−1 (HCl · · · HI) and −55 cm−1 (HI · · · HCl).17 The HI · · · HCl complex in an Ar matrix shows a shift of −89 cm−1,6 which also agrees with the present data. 300 pulses at 193 nm (∼10 mJ cm−2) decompose more than 80% of HI whereas HCl is decomposed with a much lower efficiency. The ionic species such as (ClHCl)−, (IHI)−, and (XeHXe)+ are observed after photolysis.45,46 Annealing at ∼40 K mobilizes H atoms in a Xe matrix,47,48 and the bands of HXeI (1193, 1215, and 1322.0 cm−1)18,49 and HXeH (1166 and 1181 cm−1)50 are observed. In addition to the known strong bands of the HXeI monomer, three new bands appear at 1287, 1304, and 1348 cm−1 in matrices containing both HI and HCl (Figure 3), and these bands are assigned to the HXeI · · · HCl complex. HXeI, its complexes, and HXeH are efficiently decomposed by a mercury lamp. Decomposed HXeI and its complexes can be partially recovered by annealing. Three bands at 2501, 2586 (possibly with a broad
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FIG. 3. FTIR difference spectra of a HI/HCl/Xe (1/2/1000) matrix showing the results of annealing at 60 K after 193-nm photolysis (top spectrum), irradiation of the annealed matrix by a mercury lamp (middle spectrum), and the second annealing at 60 K (bottom spectrum).
side band at ∼2599 cm−1), and 2651 cm−1 with a similar behavior are assigned to HCl in the HXeI · · · HCl complex. The intensity ratio of the H–Xe and HCl stretching bands of the HXeI · · · HCl complex is ∼2.5, which agrees with the calculated value for structure A (∼2.0). The HXeI · · · HI complex was identified previously with bands at 1230, 1268, and 1289 cm−1,18 and the former two bands appear in the spectra. The band at 1289 cm−1 of HXeI · · · HI is relatively weak and overlaps with the 1287 cm−1 band of HXeI · · · HCl. The H–Xe stretching band of the HXeCl monomer at 1648.7 cm−1 is also observed;49 however, no indication of the HXeCl · · · HI complex is found. C. HXeI · · · HCCH complex
After deposition of a HI/HCCH/Xe matrix, a new band in the HI region is observed at 2175 cm−1 (Figure 4), and it is assigned to the HCCH · · · HI complex, in which HI is a proton donor. The experimental monomer-to-complex shift of −39 cm−1 (calculated from the strongest HI monomer band) agrees well with both the experimental shift in an Ar matrix (−66 cm−1)3 and our calculated value of −24.8 cm−1 (T-shaped structure, CCSD(T)/def2-TZVPPD) (Table SI of the supplementary material).51 Although the calculated intensities of the
FIG. 4. FTIR spectra of HI/HCCH/Xe matrices (upper spectrum: ∼1/1/1000; lower spectrum ∼1/0/1000) measured after deposition at 35 K. The spectra were measured at 3 K.
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FIG. 5. FTIR difference spectrum of a HI/Xe (1/1000) matrix showing the result of annealing at 60 K after 193-nm photolysis (top spectrum); FTIR difference spectra of a HI/HCCH/Xe (1/1/1000) matrix showing the results of annealing at 60 K after 193-nm photolysis (middle spectrum) and irradiation of the annealed matrix by a 488-nm laser (bottom spectrum).
CH stretching and HCCH bending mode of the HCCH · · · HI complex are comparable to that of the HI vibration (MP2(full)), the frequency shifts from the monomeric values are small (−6.5 cm−1 for the CH stretching mode and +2.6/+11.4 cm−1 for the HCCH bending mode). This probably explains why we did not identify the HCCH · · · HI bands in the HCCH stretching and bending regions. The linear HI · · · HCCH complex has interaction energy of −0.01 kcal mol−1 at the CCSD(T)/def2TZVPPD level of theory,51 and it is not observed in the experiments. After 300 pulses of 193-nm light (∼10 mJ cm−2), more than 80% of the HI monomers and HCCH · · · HI complexes are decomposed, and the decomposition of HCCH is much less efficient. After annealing of a photolyzed HI/HCCH/Xe matrix at ≥40 K, a new acetylene-induced band appears at 1242 cm−1 with a side band at 1248 cm−1, which are blue-shifted from the HXeI monomer band (1193 cm−1). The 1242 and 1248 cm−1 bands are assigned to the H–Xe stretching mode of the HXeI· · · HCCH complex (Figure 5). These bands are efficiently decom-
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FIG. 7. Lifetime of HXeI in different complexes under broadband IR irradiation as a function of the shift of the H–Xe stretching mode. The data for the HXeI · · ·HI and HXeI · · ·HBr complexes are from Ref. 18.
posed by short irradiation at 488 nm (20 s with ∼10 mW cm−2) (Figure 5). No bands of HCCH are observed with a similar behavior. The formation of vinyl radical (HCCH2 at 1349 cm−1),52 HXeCCH (1486.4 cm−1), HXeCC (1478.0 cm−1), and HXeCCXeH (1301 cm−1)22 is also observed, but the complexes of these molecules with HI are not found. D. Photostability
The HXeI monomer is decomposed by IR radiation of the spectrometer (Figure 6), and the 2950-3800 cm−1 spectral region is responsible for this effect.31 The HXeI · · · HCl and HXeI · · · HCCH complexes can be decomposed by IR light of the spectrometer as well (Figure 6). No deviations from a single exponential function are observed for these decay curves. The HXeI monomer is decomposed most quickly, with a lifetime of (1.54 ± 0.05) h at the 1/e level. The lifetimes of the 1287, 1304, and 1348 cm−1 bands of the HXeI · · · HCl complex are (10.0 ± 0.2), (19.5 ± 0.4), and (129 ± 18) h, respectively. The corresponding HCl bands at 2651, 2586, and 2501 cm−1 have similar lifetimes (9.7 ± 0.2), (24.2 ± 2.2), and (115 ± 13) h, respectively. The HXeI · · · HCCH complex (1242 cm−1) has a lifetime of (3.0 ± 0.2) h. Figure 7 shows the lifetimes of the HXeI monomer and the HXeI complexes with HI, HBr, HCl, and HCCH as a function of the monomer-to-complex shift of the H–Xe stretching mode. The photostability of the species increases with the shift. The exception of this trend is the 1230.4 cm−1 band of the HXeI · · · HI complex with the smallest shift of +37.2 cm−1.
IV. CONCLUDING DISCUSSION
FIG. 6. Time dependences of the integrated absorptions of the HXeI monomer, HXeI · · ·HCCH, and HXeI · · ·HCl bands under broadband IR radiation: (a) HXeI monomer (vH–Xe at 1193 cm−1); (b) HXeI · · ·HCCH complex (vH–Xe at 1242 cm−1); (c) HXeI · · ·HCl complex (vH–Xe at 1287 cm−1 and vH–Cl at 2651 cm−1); (d) HXeI · · ·HCl complex (vH–Xe at 1304 cm−1 and vH–Cl at 2586 cm−1); and (e) HXeI · · ·HCl complex (vH–Xe at 1348 cm−1 and vH–Cl at 2501 cm−1). The lines are single exponential fits of the H–Xe stretching modes. In (c)–(e), solid symbols stand for HXeI and open for HCl.
The bands at 2785 cm−1 and 2190/2187 cm−1 originate from the HI · · · HCl (HCl acts as a proton donor) and HCl · · · HI (HI acts as a proton donor) complexes in a Xe matrix, which are the precursors of the HXeI · · · HCl complex. Higher HCl and HI concentrations and/or higher deposition temperatures enhance the amount of these complexes. In different experiments, the intensities of six new bands observed after photolysis and annealing at 1287, 1304, 1348, 2501, 2586, and 2651 cm−1 correlate with the amount of the
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TABLE V. Experimental H–Xe stretching and HCl frequencies (in cm−1) of the HXeI · · ·HCl and HXeI · · ·HCCH complexes.a Species HXeI HCl HXeI· · · HCl
HXeI· · ·HCl
HXeI· · · HCCH
Frequencies 1193 2838 1287 (+94) 1304 (+111) 1348 (+155) 2501 (−337) 2586 (−252), 2599 (−239) 2651 (−187) 1242 (+49), 1248 (+55)
frequency shifts (in cm−1) are shown in parentheses. The presented frequencies correspond to the underlined groups of atoms.
a Monomer-to-complex
HI · · · HCl and HCl · · · HI complexes observed after deposition. The first three bands are shifted by +94, +111, and +155 cm−1 from the HXeI monomer band and the last three bands are shifted by −337, −252, and −187 cm−1 from the Q branch of the HCl monomer. These bands are assigned to the HXeI · · · HCl complex. The contribution of the HXeI· · ·(HCl)2 complex is ruled out because the relative intensity of these bands does not depend on the HCl concentration. The HXeCl · · · HI complex is not found presumably due to more efficient photodecomposition of HI as compared with HCl. The structural assignment of the HXeI · · · HCl complex is based on the comparison of the computational (Table IV) and experimental results (Table V). The H–Xe and HCl stretching shifts calculated for structure A are +166.7 and −265.6 cm−1 at the CCSD(T)/def2-TZVPPD level of theory, in agreement with the experimental results (the largest experimental shifts are +155 and −337 cm−1). The shifts calculated for structure B (+19.5 and −32.9 cm−1) disagree with the experimental values. Furthermore, the intensity ratio of the H–Xe and HCl stretching modes calculated for structure A is ∼2, which agrees with the experimental value of ∼2.5 whereas this ratio is ∼20 for structure B. It is seen that structures C and D do not fit the experimental data. Thus, we assign all six bands to the most stable structure A. For the same structure A, the interaction energies (without ZPVE correction, at the CCSD(T) level of theory) of the HXeI · · · HCl (−4.94 kcal mol−1) and HXeI · · · HBr (−4.69 kcal mol−1) complexes are similar but substantially larger than that of the HXeI · · · HI complex (−3.88 kcal mol−1).18 In accord, the H–Xe stretching shifts of the HXeI · · · HCl (up to +155 cm−1) and HXeI · · · HBr (up to +154 cm−1) complexes are also similar and larger than that of the HXeI· · · HI complex (up to +96 cm−1). The HXeI · · · HCl complexes are decomposed by broadband IR radiation of the spectrometer source (Figure 6). Because IR-induced decomposition is characteristic of HXeI,31 this observation supports the assignment of the observed bands to the HXeI · · · HCl complex. Moreover, the IR-decomposition experiments allow us to separate these six bands into three pairs of bands belonging to the same species (1287-2651, 1304-2586, and 1348-2501 cm−1). The large splitting of the H–Xe stretching and HCl stretching bands is most probably due to structural perturbations caused by different matrix morphologies (matrix-site effect). As shown above, only one
theoretical structure (structure A) can correspond to these experimental bands. As expected, the decrease of the HCl frequency correlates with the increase of the H–Xe stretching frequency. We can speculate that the largest shifts correspond to the most relaxed structure whereas in two other matrix sites the interaction of the complex is more perturbed by the matrix environment. A similarly extensive band splitting has been previously observed for the HKrCl · · · HCl,29 HXeI · · · HI, and HXeI · · · HBr complexes.18 The detailed explanation of this extensive matrix-site effect is a challenge for computational chemistry. The 1242 cm−1 band observed after photolysis and annealing of a HI/HCCH/Xe matrix is safely assigned to the HXeI · · · HCCH complex. The intensity of this band correlates with the amount of the photolyzed precursor, the HCCH· · · HI complex. The intensities of the CH stretching and CCH bending modes of the HXeI · · · HCCH complex are much lower than that of the H–Xe stretching mode (Table IV), which explains the lack of the experimental identification of the HCCH bands. The CCSD(T) results predict only one stable structure (structure A) for the HXeI · · · HCCH complex. The calculated blue shift of the H–Xe stretching mode (+103.5 cm−1 at the CCSD(T) level and +70.1 cm−1 at the MP2(full) level) reasonably agrees with the experimental result (+49 cm−1). It should be noted that the monomer-to-complex shift can be influenced by the matrix. The calculations are performed in vacuum whereas the experiments are made in a polarizable medium. The matrix can differently affect the H–Xe stretching frequency of the monomer and complex. The lifetime of different complexes under broadband IR radiation correlates with the H–Xe stretching frequency (Figure 7). The photostability of species strongly increases with the H–Xe stretching shift (by two orders of magnitude). The mechanism of this effect is worth discussing. According to the previous study by Pettersson et al., the decomposition of HXeI occurs via absorption of the second H–Xe stretching overtone at ∼3000 cm−1.31 The second overtones of the complexes are higher in frequency, for example, the frequency can be estimated as up to ∼3370 cm−1 for the HXeI · · · HCl complex if the anharmonicity of the HXeI monomer is used. First of all, it should be noted that the observed difference in photostabilities cannot be due to the change of the IR light intensity at these frequencies because the Globar intensity has the maximum above 4000 cm−1, i.e., the opposite trend should be expected from this contribution. We propose two mechanisms that can be responsible for the observed dependence (Figure 7). The first mechanism is due to an increase of the harmonicity of the H–Xe stretching mode upon complexation. The intensity of the second H–Xe stretching absorption of the complexes presumably decreases as the H–Xe stretching frequency increases, in qualitative agreement with the experimental dependence. To support this mechanism, anharmonic calculations were performed at the MP2(full)/def2-TZVPPD level of theory by using Gaussian 09 (Rev. D.01) program. According to the calculations, the intensity of the first overtone of the HXeI · · · HCCH and HXeI · · · HCl complexes is about 55% and 30% of that of the monomer (Table SIII of the supplementary material).51 The differences for the second overtone are probably larger;
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however, the second overtones cannot be calculated by the program used. The second proposed mechanism originates from the different excess energies available for the H atom after photodissociation, which changes the cage exit probability. Three-body dissociation energies of the HXeI monomer and HXeI · · · HY (Y = I, Br, Cl, and CCH) complexes were calculated at the CCSD(T)/def2-TZVPPD level of theory (Table SIV of the supplementary material).51 The absolute accuracy of these results is questionable; however, a reliable qualitative conclusion can be derived. In fact, the dissociation energy increases for the complexes compared to the HXeI monomer, and this increase is larger than the increase of the frequency of the second overtone. For example, for HXeI · · · HCl, the dissociation energy is larger than that of the monomer by ≥850 cm−1 whereas the second overtone frequency is larger only by 370 cm−1. It follows that the excess energy is about 500 cm−1 smaller for the complex. This change presumably decreases the cage exit probability of the H atom for the complex and increases the probability of the back-reaction. Other factors can also affect the photodecomposition rate. For example, the complexation increases the vibrational state density, which can enhance the intramolecular vibrational relaxation and compete with dissociation. ACKNOWLEDGMENTS
C.Z. thanks the CSC-China Scholarship Council for the Ph.D. grant. M.T. thanks the Academy of Finland for the postdoctoral Grant No. 1139105. This work is also a part of the Project KUMURA of the Academy of Finland (No. 1277993). Jere Renlund is thanked for technical assistance. The authors are grateful to the CSC-IT Center for Science for computer time. 1K.
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