DOI: 10.17344/acsi.2015.1512 Acta Chim. Slov. 2015, 62, 565-573 565 Scientific paper H-Bonded CH3SO/H2SO4/H2O Complexes: A Quantum Chemical Study Simona Tusar and Antonija Lesar* Department of Physical and Organic Chemistry, Institute Jožef Stefan, Jamova c. 39, SI-1000 Ljubljana, Slovenia * Corresponding author: E-mail: antonija.lesar@ijs.si Received: 11-03-2015 Dedicated to prof. Jože Koller on the occasion of his 70th birthday. Abstract The structural, electronic, and spectroscopic properties of complexes of the methyl sulfinyl radical, sulphuric acid and water molecules have been studied by density functional theory and ab initio methods. The hydrogen bond interactions between the CH3SO radical, H2SO4 and H2O molecules have been characterised. The calculations predict relatively large binding energies for the complexes of 12.2 kcal mol-1 for the most stable CH3SO-H2SO4 complex, 19.1 kcal mol-1 for CH3SO-H2SO4-H2O complex and 28.8 kcal mol-1 for CH3SO-H2SO4-2H2O complex at 4he CBS-QB3 level of theory. The relatively high stabilisation of the complexes is likely to have significant effects on the overall processes that lead to the formation of new-particles in the atmosphere. Infrared spectroscopy is suggested to be a potentially useful tool for the detection of these complexes either in laboratory experiments or in atmospheric observations. The electronic spectra of the complexes have been examined, and their photochemical spectral features are discussed. The hydrated CH3SO-H2SO4 complexes can be expected to undergo photolysis in sunlight. Keywords: hydrogen-bond complexes, methyl sulfinyl radical, sulphuric acid, water, nucleation precursors, quantum-chemical methods 1. Introduction Sulphur-containing species are of significant interest in the atmospheric chemistry of the marine boundary layer. The methyl sulfinyl radical CH3SO is a key intermediate in the atmospheric oxidation of dimethyl sulphide CH3SCH3, which is the largest natural source of reactive sulphur emitted into the troposphere.1 The radical is relatively stable,2 but the concentration in the atmosphere is small, making its direct observation difficult. The recent work of Reisenauer et al.3 is dedicated to the matrix-isolation technique, investigating the spectrosco-pic properties of the CH3SO radical by UV/Vis spectroscopy. As the most abundant greenhouse gas, water can form complexes with this radical, affecting its stability and changing its photochemical features, which has been studied in detail.4 Sulphuric acid is one of the most important vapours leading to the formation of secondary aerosols.5 Sulphate aerosols have a large cooling effect on the global climate. In contrast to primary aerosols, which are released directly into the atmosphere from geogenic and anthropoge- nic sources, secondary aerosols are produced in the atmosphere by nucleation from gas-phase species.6 The new-particle formation is initiated by the interaction between various atmospheric constituents or/and contaminants. The atmospheric nucleation processes are not yet well understood and are difficult to probe by experimental means. A physical understanding of the nucleation process would enable researchers to predict the nucleation rate, an essential variable in improved atmospheric models. High-level density functional and ab initio calculations represent a powerful tool for gaining insight into the nucleation mechanism at the molecular level: for instance, they are able to describe the very first step of particle formation in the atmosphere. This work presents a comprehensive series of electronic structure calculations on H-bonded complexes of the CH3SO radical with one molecule of H2SO4 and their subsequent hydration with one or two molecules of water. The primary focus of the study is to characterise the structure, calculate the binding energy and predict the influence of H-bonding on the infrared spectra of the H-bonded OH stretching modes in these complexes. Finally, the vertical excited state energies of the complexes will be calculated to determine to what extent the complexation might influence the electronic spectra of radicals within the acid and water complexes. The CH3SO-H2SO4 complexes and their hydration have not, to the best of our knowledge, been reported before, either experimentally or theoretically. Due to their role in new-particle formation in the troposphere, proposed studies are highly desirable. 2. Computational Methods Electronic structure calculations for systems containing the methyl sulfinyl radical, sulphuric acid and water were performed with the GAUSSIAN 09 program.7 All complex geometries were optimised using the Becke three-parameter non-local exchange functional8 with the non-local correlation of Lee, Yang and Parr (B3LYP)910 and the People-type 6-311++G(2df,2pd)11 basis set. The proposed level of calculations has been proven to be an economical and accurate computational model for obtaining reliable results and has been employed widely.4 The spin contamination was monitored for all species, and the value showed insignificant deviation from the expectation value of 0.75 for open shell species. The harmonic and anharmonic frequencies of all species were computed at the same level of theory to confirm the nature of the stationary points and to determine the zero-point energies. The geometries of the certain complexes have been reoptimised by the coupled cluster with single and double excitation method (CCSD)12,13 in conjunction with the Dunning aug-cc-pVDZ basis set14,15 to further verify the reliability of the density functional method for the prediction ground-state geometrical parameters of hydrogen-bonded complexes. It is known that coupled cluster methods are well suited for determining hydrogen bonding interactions. The rotational constants were evaluated for the B3LYP/6-311++G(2df,2pd) geometrical parameters. The final energies of the complexes were improved using the CBS-QB3 level of theory.16 Additionally, for the purpose of comparison, the energies were also evaluated by the G4 method.17 Vertical excitation energies were calculated with time-dependent DFT (TDDFT).18 TDDFT calculations were performed with the B3LYP functional and the aug-cc-pVTZ basis set on the B3LYP/6-311++G(2df,2pd) geometry. 3. Results and Discussion The optimised geometry for the structures of the CH3SO radical, H2SO4 and H2O molecules at the B3LYP/6-311++G(2df,2pd) level of theory are shown in Figure 1. The structures of 1:1 CH3SO-sulphuric acid complexes are illustrated in Figure 2, and 1:1:1 and 1:1:2 CH3SO-sulphuric acid-water complexes are given in Figure 3 and Figure 4, respectively. The particular type of complexes are labelled as MS-SA, MS-SA-W and MS-SA-2W, respectively, where MS denotes methyl sul-finyl radical, SA denotes sulphuric acid and W denotes a water molecule. The descriptive bond lengths of the structures are displayed in the figures, and the CCSD/aug-cc-pVDZ parameters for a few complexes are also provided. The Cartesian coordinates for all of the studied structures (Table SI-1) are available in the Supplementary data. The harmonic and anharmonic vibrational frequencies, along with the IR intensities for the water, sulphuric acid and CH3SO radical (Table SI-2), and sulphuric acid-complexes, sulphuric acid-water complexes and sulphuric acid-2water complexes (Table SI-3, Table SI-4, Table SI-5, respectively), are also available in the Supplementary data. The reasonable agreement between the B3LYP and CCSD geometrical parameters indicate that more the economical B3LYP method would be relevant for the geometry prediction of these species. Figure 1. B3LYP/6-311++G(2df,2pd) optimised structures of the CH3SO radical, H2SO4 and H2O molecules. In parentheses are the CCSD/aug-cc-pVDZ parameters. Bond lengths are in À. Figure 2. B3LYP/6-311++G(2df,2pd) optimised structures of the CH3SO-H2SO4 complexes. In parentheses are the CCSD/aug-cc-pVDZ parameters. Bond lengths are in À. Figure 3. B3LYP/6-311++G(2df,2pd) optimised structures of the CH3SO-H2SO4-H2O complexes. In parentheses are the CCSD/aug-cc-pVDZ parameters. Bond lengths are in À. MS-SA-2Wa MS-SA-2Wb MS-SA-2Wc Figure 4. B3LYP/6-311++G(2df,2pd) optimised structures of the CH3SO-H2SO4-2H2O complexes. Bond lengths are in À. The hydrogen bond distances for the complexes are summarised in Table 1. Table 2 presents the binding energies for the CBS-QB3 and G4 composite methods, along with the B3LYP/6-311++G(2df,2pd) binding energies. The CBS-QB3 and G4 Gibbs free energies are also involved in Table 2. The comparison of the binding energies obtained with both compound methods shows good agreement between the two values, and the G4 values are on average 1.4 kcal mol-1 higher. The CBS-QB3 and G4 methods are estimated to be accurate to 0.8716 and 0.83 kcal mol-1,17 respectively. In the following discussion, the B3LYP/6-311++G(2df,2pd) geometrical parameters and the CBS-QB3 binding energies will be used unless stated otherwise. The equilibrium rotational constants calculated at the B3LYP/6-311++G(2df,2pd) level of theory are presented in Table 3 and would be valuable for eventual identification by microwave spectroscopy. Selected IR spectroscopic findings are collected in Table 4, and Table 5 presents the vertical excitation energies. 3. 1. Geometrical Parameters and Binding Energies CH3SO-H2SO4 Complexes. A large set of initial guess configurations for the CH3SO-sulphuric acid complexes have converged after full geometry optimisation to the three stable hydrogen-bonded structures, designated as MS-SAa, MS-SAb and MS-SAC, shown in Figure 2. In the MS-SAa cyclic complex, the shorter H-bond is 1.673 À long, resulting from the interaction between the lone pair on the oxygen atom of the MS and H atom of SA, thus with SA as hydrogen donor. The second H-bond is longer at 2.376 À with MS as the hydrogen donor. The complex is quite stable, and the binding energy of the complex is 12.2 kcal mol-1. The next structure, the MS-SAB complex, has slightly lower binding energy, 10.0 kcal mol-1, compared to the MS-SAa complex and possesses only a single H-bond with the SA molecule acting as a hydrogen donor. However, the MS-SAB complex is relatively stable, suggesting that H2SO4 forms a strong H-bond through the inte- raction of its H atom with the lone oxygen electron pair of CH3SO. In addition, the MS-SAB complex is stabilised by the van der Waals interaction between the oxygen atom of sulphuric acid and the sulphur atom of the CH3SO radical. The last stable structure found, a cyclic MS-SAC complex, is formed by intermolecular C-S-H-O and C-H-O-S bonds with bond lengths of 2.468 À and 2.696 À, respectively. Due to the two weak bonds in the complex the binding energy is small, 4.2 kcal mol-1. CH3SO-H2SO4-H2O Complexes. The optimisation of the extensive set of initial geometrical structures for the CH3SO-H2SO4-H2O complex again yields three different structures, MS-SA-Wa, MS-SA-WB and MS-SA-WC, which are presented in Figure 3. The MS-SA-WA complex is a cyclic structure with triple H-bonds. H2O molecules in this complex act as an H acceptor from the H2SO4 molecule with a strong O-H-H bond with a length of 1.550 À and as an H donor to the oxygen atom of the CH3SO radical, also with a relatively strong O-H-O bond that is 1.783 À long. The third intermolecular H-bond is longer at 2.327 À and results from interaction of the methyl hydrogen atom and the sulphuric acid oxygen atom. The complex is fairly stable, with a binding energy of 19.1 kcal mol-1. Further, the MS-SA-WB complex, with a computed binding energy of 17.4 kcal mol-1, is held together by two hydrogen bonds and one van der Waals interaction. The nature of the two hydrogen bonds are similar to those in the MS-SA-W^ complex: thus, their bond lengths of 1.584 and 1.781 À are also comparable to those in the MS-SA-WA complex. The van der Waals interaction occurs between the oxygen atom of sulphuric acid and the sulphur atom of the CH3SO radical. The last three-body complex, MS-SA-WC, is again stabilised by three hydrogen bonds. The stronger bond with sulphuric acid as proton donor is 1.596 À long, whereas the H-bond with sulphuric acid as proton acceptor is 1.996 À long. The third H-bond is significantly longer, 2.666 À, for which water acts as the proton acceptor. The binding energy of the MS-SA-WC complex is predicted to be 16.2 kcal mol-1, which is 2.9 kcal mol-1 and only Table 1. Hydrogen bond distances (À) for the CH3SO-H2SO4, CH3SO-H2SO4-H2O and CH3SO-H2SO4-2H2O complexes. CSO...H SCH...O B3LYP / 6-311++G(2df,2pd) CS...H SOH...O SO...HO O...HO MS-SA A MS-SA B MS-SA C 1.673 1.678 2.376 2.696 2.468 MS-SA-W A MS-SA-W B MS-SA-W C 1.783 1.781 1.596 2.327 2.666 1.550 1.584 1.996 MS-SA-2W A MS-SA-2W B MS-SA-2W C 1.809 1.779 1.805 2.347 2.266 1.38 1.573 1.694 1.599 1.715 1.912 2.076 2.050 1.774 1.2 kcal mol 1 lower than the binding energies of the MS-SA-Wa and MS-SA-WB complexes, respectively. CH3SO-H2SO4-2H2O Complexes. We have studied the addition of a second water molecule to the CH3SO-H2SO4-H2O complexes, and all of our attempts with various initial guess configurations resulted in the three stable structures that are illustrated in Figure 4. In the MS-SA-2WA complex, the second water molecule acts as an H acceptor with a stronger H-bond (1.774 À) to the first water molecule and as an H-donor, forming a weak H bond (1.912 À) with the free electron pair of sulphuric acid double-bonded oxygen. The MS-SA-2Wb complex also involves a weak H-bond (2.076 À) between the water hydrogen atom and the free electron pair of sulphuric acid double-bonded oxygen, whereas the main H-bonding is generated by the acidic H atom of sulphuric acid and the oxygen atom of the second water molecule. The binding energy of both configurations is equal at 28.8 kcal mol-1. Similarly, the second water molecule in the third complex labelled as MS-SA-2Wc interacts with the free electron pair of sulphuric acid double-bonded oxygen and the acidic H atom of sulphuric acid, with bonds that are 2.050 and 1.715 À long, respectively. Its binding energy is only 1.8 kcal mol-1 lower than in the previous two structures. An inspection of the relative Gibbs free energies from Table 1 shows that the formation of the complexes, in particular those with the higher binding energies, are spontaneous processes. When we analyse the binding energies of the complexes studied, we can conclude that the lowest energy structure of CH3SO-sulphuric acid complex with a binding energy of 12.2 kcal mol-1 is relatively stable. The subsequent hydration of the CH3SO-sulphuric acid complex significantly increases the binding energy, to 19.1 kcal mol-1 and to 28.8 kcal mol-1, when one and two additional water molecules, respectively, participate in the hydration process. Table 3. Rotational constants (GHz) for the CH3SO radical, and for the CH3SO-H2SO4, CH3SO-H2SO4-H2O and CH3SO-H2SO4-2H2O complexes at the B3LYP/6-311++G(2df,2pd) level of theory. A B C CH3SO 27.367 8.336 6.660 MS-SAA 3.086 0.674 0.626 MS-SAa 4.108 0.564 0.554 MS-SAc 3.140 0.588 0.558 MS-SA-WA 1.816 0.497 0.436 MS-SA-Wa 1.967 0.454 0.402 MS-SA-Wc 1.664 0.596 0.528 MS-SA-2WA 1.056 0.475 0.371 MS-SA-2Wa 1.807 0.337 0.309 MS-SA-2Wc 1.181 0.394 0.382 3. 2. Infrared Spectra For the sake of completeness the harmonic and an-harmonic frequencies, along with the IR intensities calculated at the B3LYP/6-311++G(2df,2pd) level of theory for the CH3SO radical, sulphuric acid and water molecule are presented in Table SI-2 of the Supplementary data, where the available experimental values for the CH3SO radical,3 sulphuric acid19 and water molecule20,21 are also quoted. The data for the CH3SO-H2SO4, CH3SO-H2SO4-H2O and CH3SO-H2SO4-2H2O complexes are given in Tables SI-3, SI-4 and SI-5, respectively. The calculated an-harmonic frequencies of the CH3SO radical, sulphuric acid and water molecule are in very good agreement with the observed gas-phase fundamental frequencies, which has been extensively discussed in our previous work.4 The harmonic and anharmonic OH stretching vibrations and IR intensities of H2SO4, water and the complexes studied are summarised in Table 4. An examination of the calculated frequencies and intensities shows that the frequencies and intensities of the hydrogen-bonded OH stretching regions are most affected by complex forma- Table 2. Binding energies (Do, in kcal mol ') and Gibbs free energies (Go, in kcal mol ') for the CH3SO-H2SO4, CH3SO-H2SO4-H2O and CH3SO-H2SO4-2H2O complexes. B3LYP 6-311++G(2df,2pd) D0 (kcal mol1) CBS-QB3 G4 G0 (kcal mol-1) CBS-QB3 G4 MS-SA A 9.7 12.2 13.6 -2.4 -4.5 MS-SA a 7.7 10.0 11.2 -0.9 -2.0 ms-sa c 1.4 4.2 5.4 5.9 3.1 ms-sa-w A 16.3 19.1 20.7 -1.5 -3.7 MS-SA-W a 15.0 17.4 19.4 1.1 -1.8 ms-sa-w c 12.8 16.2 17.4 2.1 1.2 MS-SA-2W A 25.3 28.8 30.4 -0.4 -3.2 MS-SA-2W a 24.8 28.8 29.7 -2.6 -2.8 MS-SA-2W c 22.4 27.0 28.1 -1.6 -2.0 tion: thus, the large frequency shift is mainly related to the OH bonds involved in the hydrogen-bonding interaction. CH3SO-H2SO4 Complexes. This particular complex has six modes that correspond to the unique intermolecu-lar modes. A comparison of the OH stretching modes in the MS-SAa, MS-SAb and S-SAC complexes with the sul- phuric acid monomer shows that the OH stretching frequencies for the 'free' OH bonds in the complexes are only slightly blue-shifted. By contrast, for the MS-SAA and MS-SAB complexes, the H-bonded OH-stretching frequencies are red-shifted by approximately 650 cm-1, and for MS-SAC complex, the shift is lower at 208 cm-1. At the Table 4. Harmonic (H) and nharmonic (A) OH-stretching vibrations (cm 1) with IR intensities (I, km mol 1) for the CH3SO-H2SO4, CH3SO-H2SO4-H2O and CH3SO-H2SO4-2H2O complexes at the B3LYP/6-311++G(2df,2pd) level of theory. Moiety in complex H2O OH-free OH-bonded OH-free OH-bonded OH-free OH-bonded H2SO4 H2O H2Osecond H2SO4 H 3774 3770 A 3578 3574 I 50 210 H2O H A I 3921 3736 64 3818 3647 MS-SAA H 3778 3230 A 3583 2929 I 119 1733 shift 5 -645 MS-SAB H 3791 3226 A 3593 2941 I 100 2027 shift 15 -633 MS-SAC H 3781 3588 A 3613 3366 I 115 719 shift 35 -208 MS-SA-WA H 3780 2775 3857 3527 A 3591 2342 3683 3274 I 120 1981 118 839 shift 13 -1232 -53 -373 MS-SA-WB H 2790 2936 3877 3500 A 3603 2489 3682 3324 I 99 1903 116 1048 shift 25 -1085 -54 -323 MS-SA-WC H 3782 2990 3892 3737 A 3573 2624 3683 3549 I 132 2253 116 254 shift -5 -950 -53 -98 MS-SA-2Wa H 1871 3780a 3563b 3436 3885 3651 A n/a n/a n/a n/a n/a n/a I 2666 120 913 607 112 474 MS-SA-2WB H 2882 3194 3870 3520 3879 3702 A 2697 2964 3694 3311 3685 3507 I 2158 1121 116 867 120 176 shift -877 -614 -42 -336 -51 -140 MS-SA-2WC H 2988 3250 3878 3538 3877 3700 A 2579 3051 3618 3326 3683 3505 I 1634 1087 101 923 139 201 shift -995 -527 -118 -321 -53 -142 a OH-free, b OH-bonded same time, the intensities are significantly larger, implying the strong participation of the sulphuric acid OH in the H-bonding. CH3SO-H2SO4-H2O Complexes. An examination of the anharmonic frequencies of the H-bonded OH-stretching modes in the MS-SA-WA, MS-SA-WB and MS-SA-WC complexes also reveals strong participation of the sulphuric acid OH in the H-bonding: the red shifts are 1232, 1085 and 950 cm-1, respectively. For the water subunit in the complexes, the OH stretching frequencies in the H-bonding are changed by 373, 323 and 98 cm-1, respectively, which is in line with the weaker H-bonds. CH3SO-H2SO4-2H2O Complexes. When the anhar-monic frequencies of the OH-stretching modes in the MS-SA-2Wa, MS-SA-2Wb, MS-SA-2Wc complexes are considered, the red shift on average of 940 cm-1 for the OH modes of the sulphuric acid moiety involved in H-bonding is observed. The OH-stretching modes of both water subunits in the complex are also affected by com-plexation. The OH modes participating in H-bonding are shifted by approximately 140 cm-1 to the red region of the spectrum compared to the symmetric stretching frequencies of the free water molecule. The present data provide strong evidence that the complexation of the methyl sulphinyl radical with sulphuric acid, as well as the subsequent hydration of these complexes with one or two water molecules, induces large frequency shifts and an intensity enhancement of the H-bonded OH-stretching vibrations in relation to that of the corresponding parent monomers. The modes that are similar to the isolated monomers are changed with respect to the monomers, mainly due to the geometry modification induced by the new interaction with the other atoms in the complexes. 3. 3. Vertical Excitation Energies It is also important to consider the photochemistry of the system in the atmosphere: thus, an investigation of the excited states of the complexes can provide other spectroscopic features that should aid in an experimental characterisation. In this study, we will especially concentrate on determining to what extent the complexation might affect the electronic spectra of the CH3SO radical within the sulphuric acid and water complexes. Electronic excitations in water and sulphuric acid require very high energy, and the transitions occur in the VUV region, well above those available from the sun in the troposphere. Water has its first electronic transition at approximately 180 nm,22 whereas for sulphuric acid, the electronic excitations are below 150 nm.23,24 The vertical excitation energies for three low-lying singlet electronically excited states and the oscillator strengths calculated from the TDDFT B3LYP/aug-cc-pVTZ calculations on the B3LYP/6-311++G(2df,2pd) geome- tries for the CH3SO radical and for its complexes with sulphuric acid and water are summarised in Table 5. The UV/Vis absorption spectrum of CH3SO shows two absorption bands.3 One is a very weak, broad band starting at 635 nm and terminating at approximately 450 nm, with maximum at approximately 530 nm. The second is much more intense, starting near 320 nm with a maximum at approximately 260 nm. Both bands are in reasonable agreement with our computed electronic transitions, found at 556 nm and 234 nm, respectively. From comparison of the experimental and calculated absorption bands, we have estimated that the excitation energies would be provided to within 0.3 eV or 25 nm. Thus, we can expect that the excitation energies for the complexes predicted by our calculations would be sufficiently reliable for qualitative prediction of the general trends or shift of the absorption energies. Among the CH3SO-H2SO4 complexes, the most intense band is calculated at 258 nm for the relatively least stable structure, the MS-SAC complex. Although the photolysis in the sunlight occurs at the threshold of approximately 300 nm, this complex is expected not to photolyse under sunlight. Eventually, the MS-SAB complex with the third singlet electronic transition calculated at 301 nm can undergo photolysis. For the CH3SO-H2SO4-H2O complexes, the highly intense absorption band appears at 321 nm for MS-SA-WC structure. Further, the MS-SA-2Wa and MS-SA-2WB structures for the CH3SO-H2SO4-AH2O complexes have electronic transitions slightly above 300 nm, at 302 and 312 nm, respectively, with moderate intensities. When the calculated electronic transitions related to the first singlet exited states of the CH3SO-comple-xes and CH3SO radical are compared, it is found that the transitions in the complexes are approximately 35 nm blue-shifted relative to those for the free radical, except in the case of the MS-SAC complex, for which the transition is red-shifted by 32 nm. TTDFT calculations demonstrate that the character of the first electronic transitions is the same in the radical and in all complexes and that they correspond to the HOMO-1 ^ LUMO excitations or an n(O),o(S-O) ^ n*(S-O) type transitions. The vertical transition energies for the most intense peaks in the radical at 234 nm and in the MS-SAC complex at 258 nm are considered to be n(S-O) ^ n*(S-O) type transitions. The second electronic transition for the MS-SA-W complexes and also for the MS-SA-2W complexes are associated with the n(O) ^ n*(S-O) type transitions with the difference that, for the former complexes, the n(O) type lone-pair orbitals are at water oxygen centres, whereas in the latter, the complexes the n(O) lone-pair orbitals are at sulphuric acid oxygen centres. For example, this type of transition is computed to be located at 321 and 312 nm for the MS-SA-WC and MS-SA-2WB complexes, respectively. Table 5. TDDFT vertical excitation energies (in eV and in nm) and oscillator strengths f for the CH3SO radical (experimental values from ref. 3.) and for the CH3SO-H2SO4, CH3SO-H2SO4-H2O and CH3SO-H2SO4-2H2O complexes at the B3LYP/aug-cc-pVTZ level of theory. Transition AE [eV] X max [nm] f Exp. CH3SO 16ß — 17ß 2.23 556 0.0005 530 nm 17a — 19a 4.92 252 0.0001 15ß—> 17ß 5.29 234 0.0019 260 nm MS-SAa 41 ß —42ß 2.38 521 0.0006 40ß—42ß 4.16 298 0.0005 38ß —42ß 4.53 274 0.0006 MS-SAB 41 ß —42ß 2.39 519 0.0004 40ß—42ß 3.88 319 0.0000 39ß —42ß 4.12 301 0.0015 MS-SAC 41 ß —42ß 2.Л 588 0.0008 37ß—42ß 4.81 258 0.0432 40ß—42ß 4.93 251 0.0015 MS-SA-WA 46ß—47ß 2.36 525 0.0005 A 45ß—47ß 4.05 306 0.0009 44ß—47ß 4.66 266 0.0003 MS-SA-WB 46ß—47ß 2.36 525 0.0003 45ß—47ß 4.04 307 0.0003 44ß—47ß 4.28 290 0.0005 MS-SA-WC 46ß—47ß 2.53 490 0.0007 C 45ß—47ß 3.86 321 0.0094 44ß—47ß 4.19 296 0.0004 MS-SA-2WA 51ß—52ß 2.36 526 0.0005 50ß—52ß 4.11 302 0.0012 49ß—52ß 4.64 267 0.0025 MS-SA-2WB 51ß—52ß 2.37 523 0.0005 50ß—52ß 3.97 312 0.0016 49ß—52ß 4.55 273 0.0002 MS-SA-2WC 51ß—52ß 2.35 527 0.0003 50ß—52ß 3.84 3 23 0.0001 49ß—52ß 4.12 301 0.0001 4. Conclusions The primary aim of this study was to characterise structural and spectroscopic properties of complexes involving the CH3SO radical, sulphuric acid and water molecules. Quantum chemical calculations at the density functional theory (B3LYP) level in conjunction with the 6-311++G(2df,2pd) basis set determined multiple H-bon-ded cyclic complexes for all studied stoichiometries, with H-bond lengths in the range of 1.38 to 2.70 À. The CBS-QB3 level of theory predicts that the complexes are bonded strongly, with binding energies of 12.2, 19.1 and 28.8 kcal mol-1 for the minimum-energy structure CH3SO-H2SO4 (MS-SAA), H3SO-H2SO4-H2O (MS-SA-Wa) and CH3SO-H2SO4-2H2O (MS-SA-2Wa) complexes, respectively. From the calculated vibrational frequencies and the IR intensities, it follow that complex formation through H-bonding induces a large spectral red-shift and enhancement of the IR intensities for the H-bonded OH stretching vibrational mode, relative to the modes in the monomers forming the complex. TDDFT calculations of the vertical excitation energies for the CH3SO-sulphuric acid and CH3SO-sulphuric acid-water complexes indicate significant spectral shifts in comparison to the free CH3SO radical, which suggests that the radical and complexes are experimentally distinguishable using standard UV/Vis absorption spectroscopy. In the troposphere, complexes of the MS-SA-W and MS-SA-2W types can be expected to undergo photolysis in the sunlight. 5. Acknowledgements This research was funded by the Slovene Research Agency, program grant numbers P2-0148 and P2-0393, and the Young Researcher program grant number PR-05022. 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Phys. 2003, 118, 7226-7232. http://dx.doi.org/10.1063/L1561852 Povzetek V tem delu smo preučevali strukturne, elektronske in spektroskopske lastnosti kompleksov metil-sulfilnega radikala, žveplove kisline in vodne molekule s pomočjo teorije gostotnega funkcionala in ab initio metod. Določili smo vodikovo med vez med CH3SO radikalom, H2SO4 in H2O. Dobili smo relativno velike vezne energije kompleksov in sicer 12,2 kcal mol-1 za najstabilnejši CH3SO-H2SO4 skupek, 19,1 kcal mol-1 za CH3SO-H2SO4-H2O in 28,8 kcal mol-1 za CH3SO-H2SO4-2H2O pri CBS-QB3 približku. Relativno visoka stabilizacija skupkov je verjetno razlog za tvorjenje novih struktur v atmosferi. S pomočjo infrardeče spektroskopije lahko opazimo te komplekse v laboratoriju kot tudi v atmosferi. Določili in razložili smo tudi elektronski spekter preučevanih skupkov ter fotokemični spekter. Hidratiran CH3SO-H2SO4 kompleks verjetno razpade s fotolizo na sončni svetlobi. H-Bonded CH3SO/H2SO4/H2O Complexes: Quantum Chemical Study Simona Tušar and Antonija Lesar * Department of Physical and Organic Chemistry, Institute Joef Stefan, Jamova c. 39, SI-1000 Ljubljana, Slovenia * Corresponding Author: E-mail: antonija.lesar@ijs.si Dedicated to Prof. Dr. Joe Koller on the occasion of his 70th birthday Supplementary Data Table SI-1: Cartesian coordinate in A for all structures studied in this work. Table SI-2: B3LYP/6-311++G(2df,2pd) harmonic and anharmonic (cm-1) frequencies together with IR intensities (I, in km mol-1) of CH3SO radical, H2SO4 and H2O molecule. Table SI-3: B3LYP/6-311++G(2df,2pd) harmonic and anharmonic (cm-1) frequencies together with IR intensities (I, in km mol-1) for CH3SO-H2SO4 complexes. Table SI-4: B3LYP/6-311++G(2df,2pd) harmonic and anharmonic (cm-1) frequencies together with IR intensities (I, in km mol-1) for CH3SO-H2SO4-H2O complexes. Table SI-5: B3LYP/6-311++G(2df,2pd) harmonic and anharmonic (cm-1) frequencies together with IR intensities (I, in km mol-1) for CH3SO-H2SO4-2H2O complexes. Table SI-1. Cartesian coordinate in A for all structures studied in this work. ch3so B3LYP/6-311 ++G(2df,2pd) CCSD/aug-cc-pVDZ O 1.245682 0.607973 0.000015 1.259622 0.628078 -0.000006 S 0.226446 -0.501169 0.000008 0.218014 -0.523920 -0.000010 C -1.404128 0.300882 -0.000030 -1.402817 0.320911 -0.000002 H -2.160317 -0.483992 -0.001712 -2.186249 -0.455515 0.000091 H -1.500815 0.918097 -0.891509 -1.481282 0.944148 -0.904076 H -1.502681 0.915529 0.893153 -1.480759 0.943995 0.904210 h2so4 B3LYP/6-311 ++G(2df,2pd) CCSD/aug-cc-pVDZ O -0.515762 0.496061 0.657449 -0.002745 0.003610 -0.039658 S 0.194842 -0.332201 1.576467 0.008355 0.000037 1.422287 O 1.600978 -0.567132 1.520097 1.212291 -0.003655 2.251710 O -0.096703 0.184158 3.063908 -0.880159 1.265122 1.953102 H -0.960199 0.621629 3.081825 -1.535727 1.463274 1.263661 S -0.562314 -1.742955 1.559748 -0.929696 -1.264900 1.860041 H 0.021913 -2.418467 1.933516 -0.726983 -1.462902 2.789592 h2o B3LYP/6-311 ++G(2df,2pd) O 0.000000 0.000000 0.116800 H 0.000000 0.763077 -0.467198 H 0.000000 -0.763077 -0.467198 CCSD/aug-cc-pVDZ 0.000000 0.000000 0.118740 0.000000 0.760299 -0.474961 0.000000 -0.760299 -0.474961 ms-saa B3LYP/6-311 ++G(2df,2pd) O 0.332699 0.804917 -0.125607 S 0.029616 0.757528 1.356394 C 1.459144 0.008029 2.167803 H 1.209643 -0.083978 3.224687 H 2.325662 0.656003 2.041851 H 1.653942 -0.967579 1.725753 H 1.196162 -0.058090 -1.268840 O 1.644165 -0.534379 -2.019223 S 2.660546 -1.609410 -1.494595 O 2.197937 -2.160583 -0.255399 O 3.017024 -2.422941 -2.613467 O 3.938397 -0.717330 -1.086710 H 4.443219 -0.515773 -1.887212 ms-sab B3LYP/6-311 ++G(2df,2pd) CCSD/aug-cc-pVDZ O 0.060500 -0.218138 0.858442 -0.077942 -0.147128 0.038226 S 0.994610 0.689066 1.637853 0.009682 0.339633 1.518190 C 2.445695 -0.337641 1.967804 1.741673 -0.016586 1.954052 H 3.153597 0.268155 2.532568 1.891748 0.294286 3.001146 H 2.885647 -0.650355 1.022369 2.401869 0.555450 1.284175 H 2.145576 -1.208349 2.548298 1.922574 -1.096795 1.843231 H -1.336863 0.358737 0.130478 -1.453108 0.294367 -0.855073 O -2.143641 0.683760 -0.352847 -2.244873 0.581695 -1.382314 S -2.749385 1.944700 0.366691 -3.515369 0.580224 -0.397403 O -1.797077 2.424353 1.337118 -3.031041 0.664083 0.996700 O -3.994503 1.324733 1.170627 -4.120360 -0.930388 -0.602612 H -3.689522 1.026664 2.038094 -3.723255 -1.484818 0.087744 O -3.322807 2.785732 -0.627063 -4.507043 1.486569 -0.960210 ms-sac B3LYP/6-311 ++G(2df,2pd) O 1.078258 2.019338 0.735267 S 0.587920 0.928130 1.648910 C 1.833292 -0.387384 1.640158 H 1.490050 -1.159140 2.326950 H 2.785322 0.022533 1.972639 H 1.925216 -0.780177 0.629314 O -0.212680 -0.847080 4.394369 S -0.506117 0.085014 5.452599 O -0.123019 1.543755 4.943306 H 0.035761 1.520920 3.980504 O 0.003832 -0.047048 6.770887 O -2.094355 0.182343 5.602769 H -2.513425 -0.080402 4.771370 Table SI-1. Continued. ms-sa-wa B3LYP/6-311 ++G(2df,2pd) CCSD/aug-cc-pVDZ O 1.555753 -1.344575 0.394866 -0.178659 -0.032691 0.007460 S 1.614859 -0.815221 1.810859 -0.042652 0.050075 1.555042 C 0.694456 0.739386 1.833657 1.748858 0.034504 1.881440 H 0.766708 1.133205 2.847503 1.877289 0.059593 2.976182 H -0.347751 0.555902 1.573705 2.215085 0.918947 1.422955 H 1.148552 1.434945 1.129122 2.172146 -0.894918 1.468693 H -1.103516 -0.147747 -1.698770 3.029102 1.121014 -1.510682 O -1.858475 0.515273 -1.517526 3.922584 1.452200 -1.174488 S -2.897915 -0.112963 -0.528105 3.661362 2.910761 -0.553407 O -3.233735 -1.442860 -0.961941 2.862184 3.703144 -1.504675 O -4.133990 0.861801 -0.790436 5.190876 3.460546 -0.524521 H -4.561755 0.615462 -1.622533 5.421282 3.720672 -1.431746 O -2.510884 0.125734 0.831158 3.259828 2.809987 0.854458 H 0.626524 -1.216734 -1.121415 0.974268 0.337260 -1.340200 O 0.060868 -1.147403 -1.917239 1.617754 0.601847 -2.027205 H -0.268617 -2.034697 -2.091341 1.174891 1.296146 -2.531437 ms-sa-wb B3LYP/6-311 ++G(2df,2pd) O 0.193683 -0.305100 -0.131485 S -0.308718 0.007102 1.263951 C 1.193514 0.210378 2.253955 H 0.885454 0.433584 3.275049 H 1.782051 1.032525 1.850088 H 1.770604 -0.712470 2.227127 H -2.728931 0.056651 -2.316272 O -3.616016 0.539529 -2.266886 S -4.326991 0.278464 -0.899159 O -3.338425 -0.083129 0.088043 O -5.207730 -1.036774 -1.191593 H -4.672567 -1.816678 -0.992191 O -5.265494 1.327302 -0.684583 H -0.812934 -0.561681 -1.578763 O -1.339532 -0.699856 -2.393762 H -0.761614 -0.495936 -3.133740 ms-sa-wc B3LYP/6-311 ++G(2df,2pd) O 0.434094 -0.071181 0.044610 S 0.109631 -0.689251 1.391578 O 1.662267 -0.805452 2.303244 H 1.415653 -1.177083 3.296931 H 2.109604 0.182554 2.377883 H 2.328942 -1.498723 1.792031 H 1.647926 0.820135 -0.483299 O 2.409579 1.368340 -0.851270 S 2.520111 2.759087 -0.142660 O 1.628997 3.713204 -0.735317 O 4.014041 3.128791 -0.556216 H 3.991441 3.631851 -1.382340 O 2.535996 2.576862 1.285134 H 1.078342 2.478267 2.644517 O 0.436223 2.070015 3.241874 H -0.224795 2.745896 3.410334 MS-SA-2Wa B3LYP/6-311++G(2df,2pd) O 0.000000 0.000000 0.000000 S 0.000000 0.000000 1.512599 C 1.727805 0.000000 2.042981 H 1.722605 -0.042617 3.132266 H 2.219258 0.911227 1.703035 H 2.227882 -0.878301 1.637022 H 2.960402 0.934273 -1.548180 O 3.899895 1.265529 -1.126314 S 3.845341 2.710122 -0.590571 O 3.379670 3.612599 -1.625915 O 5.396519 2.995509 -0.347697 H 5.832504 3.137963 -1.199589 O 3.236160 2.776112 0.705809 H 1.117837 0.294114 -1.391764 O 1.762140 0.509897 -2.093735 H 1.397434 1.262137 -2.611901 H 1.927359 3.338375 -2.838720 O 1.248385 2.867407 -3.351660 H 0.453494 3.405167 -3.311901 ms-sa-2wb B3LYP/6-311++G(2df,2pd) O 0.258473 -0.909281 0.022669 S 0.347507 -1.054643 1.526146 C 2.020196 -0.567594 2.003404 H 2.087729 -0.685666 3.084909 H 2.187438 0.471959 1.722781 H 2.738365 -1.221298 1.510071 H 2.569010 1.533828 -1.549507 O 3.278554 2.199821 -1.262837 S 2.696004 3.249516 -0.258426 O 1.853168 4.191563 -0.966833 O 4.021164 3.972504 0.169360 H 4.133597 4.802682 -0.371482 O 2.156506 2.580126 0.892528 H 1.093741 -0.041262 -1.287030 O 1.519190 0.457744 -2.013899 H 0.806506 0.794253 -2.564377 H 2.952918 5.849934 -1.558907 O 3.869938 6.124035 -1.398909 H 3.830293 7.010426 -1.028811 Table SI-1. Continued. ms-sa-2wc B3LYP/6-311++G(2df,2pd) O 0.228145 0.026523 -0.030226 S 0.228233 0.209150 1.473832 C 1.965731 0.478786 1.906043 H 2.017218 0.616212 2.985745 H 2.328364 1.369306 1.395424 H 2.550574 -0.389586 1.607424 H -3.162762 0.844126 -0.810372 O -3.803052 1.418233 -0.284354 S -4.227418 0.737666 1.058387 O -3.154930 -0.141502 1.499153 O -5.451775 -0.172001 0.640074 H -5.174034 -1.126115 0.631380 O -4.714984 1.765130 1.917995 H -1.249013 -0.053393 -1.064443 O -2.070242 -0.020827 -1.594535 H -1.819155 0.260625 -2.478180 H -3.597365 -2.124092 1.225552 O -4.345712 -2.611839 0.846391 H -4.667280 -3.196686 1.538433 Table SI-2. Harmonic and anharmonic frequencies (cm *) with IR int. (km mol *) for CH3SO, H2SO4 and H2O molecules. CH3SO H2SO4 H2O H A IR int. Exp" H A IR int. Expb H A IR int. Expc d 3131 2981 2 2995 vw 3130 2979 4 3037 2917 3 2919 vw 1460 1419 8 1417 w 1446 1404 10 1405 w 1323 1299 1 1289 w 1061 1408 47 1068 vs 947 921 11 927 m 887 869 1 868 vw 662 646 14 670 w 332 333 7 341 w 137 93 0 n.o. 3774 3578 50 3609 3770 3574 210 1449 1418 304 1465 1201 1180 165 1220 1166 1124 84 1157 1152 1116 79 1138 851 831 342 891 799 781 113 834 540 532 24 568 531 525 39 550 485 476 42 429 397 17 365 346 2 281 327 265 59 251 219 98 216 3818 3647 8 3661 1630 1575 72 1596 3921 3736 64 3751 a Reisenauer, H.P.; Romanski, J.; Mloston, G.; Schreiner, P.R. ChemComm 2013, 49, 9467-9469. b Miller, Y.; Chaban, G. M.; Gerber, R.B. J. Phys. Chem. A 2005,109, 6565-6574. c Forney, D.; Jacox, M.E.; Thompson, W.E. Mol. Spectrosc. 1993,157, 479-493. d Tsuge, M.; Tsuji, K.; Kawai, A.; Shibuya, K. J. Phys. Chem. A 2007,111, 3540-3547. Table SI-3. Harmonic and anharmonic frequencies (cm *) with IR int. (km mol *) for MS-SA complexes. MS-SAA MS-SAB MS-SAc H A IR int. H A IR int. H A IR int 3778 3583 119 3791 3593 100 3781 3613 115 3230 2929 1733 3226 2941 2028 3588 3366 719 3139 2990 2 3136 2983 2 3147 2999 0 3128 2983 3 3134 2983 0 3136 2988 1 3036 2920 4 3040 2918 1 3043 2926 2 1462 1423 2 1457 1413 6 1457 1417 17 1443 1408 51 1445 1405 13 1451 1409 4 1428 1398 286 1437 1404 304 1439 1408 338 1336 1289 4 1331 1284 1 1326 1295 2 1288 1249 62 1301 1263 97 1205 1170 160 1194 1172 185 1183 1162 196 1173 1152 60 1157 1122 96 1150 1105 57 1146 1115 66 1066 1049 106 1046 1033 95 1058 1045 41 961 938 18 959 933 12 958 937 11 897 878 161 892 873 193 897 887 2 893 877 81 891 873 140 864 843 370 802 783 191 807 785 182 806 788 94 764 705 23 771 701 23 667 673 8 673 659 9 676 653 16 544 535 27 544 538 33 546 537 38 532 525 26 538 530 40 535 528 31 503 503 7 510 503 10 509 501 23 450 443 66 421 381 31 419 394 14 403 399 39 379 358 2 376 363 39 356 308 4 346 351 8 362 344 2 333 335 5 270 170 61 219 130 78 192 129 63 180 175 40 156 136 12 141 117 0 157 204 3 141 78 1 93 93 2 107 108 0 120 106 5 69 95 9 93 94 5 52 24 3 62 51 2 59 75 1 45 29 3 47 51 9 36 48 2 35 -10 1 29 66 3 20 106 3 24 -31 2 19 -7 3 Table SI-4. Harmonic and anharmonic frequencies (cm *) with IR int. (km mol *) for MS-SA-W complexes. MS-SA-W A___MS-SA-W B___MS-SA-WC H A IR int. H A IR int. H A IR int. 3857 3683 118 3877 3682 116 3892 3683 116 3780 3591 120 3790 3603 99 3782 3573 132 3527 3274 839 3500 3324 1048 3737 3549 254 3129 2978 29 3133 2985 2 3152 3000 5 3128 2982 2 3132 2987 1 3133 2988 1 3031 2931 22 3038 2923 2 3043 2920 8 2775 2342 1981 2936 2489 1903 2990 2624 2253 1663 1618 38 1661 1633 43 1645 1582 58 1467 1427 7 1467 1418 189 1452 1387 7 1446 1400 10 1457 1413 9 1442 1385 10 1406 1149 180 1446 1405 11 1383 1372 301 1371 1371 241 1369 1354 236 1334 1285 15 1339 1297 2 1328 1282 0 1322 1254 91 1186 1163 147 1183 1162 249 1186 1171 169 1158 1126 123 1150 1107 55 1159 1132 105 1074 1004 22 1050 1039 92 1062 1044 91 1062 1032 138 955 932 13 960 940 19 960 938 11 938 862 203 907 887 322 910 892 283 910 888 188 896 876 1 897 879 3 888 873 1 832 782 123 822 803 143 801 783 148 811 707 101 704 694 129 711 673 52 677 653 8 673 661 8 672 650 17 547 535 19 549 539 38 554 543 23 543 540 58 543 534 22 540 526 27 518 514 8 517 513 32 536 527 34 477 470 102 463 485 34 476 468 51 407 396 30 413 363 30 419 391 4 391 365 40 405 403 17 367 348 1 351 357 15 343 345 15 363 359 29 321 369 92 330 241 102 297 202 134 225 226 62 279 244 22 270 244 35 202 200 58 248 114 74 209 139 48 151 37 18 183 104 3 162 148 15 140 68 23 175 165 35 142 124 0 118 55 12 99 93 14 101 85 6 113 -183 62 77 101 8 65 42 2 89 142 5 67 74 14 52 24 6 60 79 2 47 78 3 43 20 2 46 65 3 30 42 3 34 -14 1 31 98 4 26 31 1 29 26 2 26 99 2 23 22 3 21 -45 4 20 51 3 Table SI-5. Harmonic and anharmonic frequencies (cm *) with IR int. (km mol *) for MS-SA-2W complexes. MS-SA-2Wa _MS-SA-2WB__MS-SA-2WC H A IR int. H A IR int. H A IR int. 3885 n/a 112 3879 3685 120 3878 3618 101 3780 120 3870 3694 116 3877 3683 139 3651 474 3702 3507 176 3700 3505 201 3563 913 3520 3311 867 3538 3326 923 3436 607 3194 2964 1121 3250 3051 1087 3129 19 3130 2987 56 3134 2982 2 3128 5 3128 2974 4 3133 2977 1 3031 17 3030 2914 41 3039 2913 1 1871 2666 2882 2697 2158 2988 2579 1634 1676 79 1665 1615 38 1657 1601 40 1643 56 1633 1578 62 1633 1580 53 1468 52 1469 1410 6 1457 1411 11 1464 69 1445 1387 16 1452 1393 224 1446 22 1428 1381 202 1446 1398 9 1355 377 1361 1296 122 1375 1314 79 1339 8 1337 1292 1 1329 1280 2 1300 68 1305 1265 256 1321 1277 212 1158 206 1166 1147 178 1149 1133 212 1156 112 1063 1047 78 1051 1037 76 1063 73 1033 901 80 957 903 49 962 204 961 936 12 953 898 165 960 77 931 905 355 924 896 278 897 3 901 882 2 889 857 1 874 114 859 841 76 843 825 86 829 195 804 748 86 778 626 69 730 45 697 623 102 675 628 68 673 9 674 661 9 670 630 39 626 176 584 513 183 587 516 135 590 16 554 540 18 558 540 41 554 25 551 545 40 548 507 68 535 87 526 502 43 542 533 33 522 34 454 509 24 467 384 14 420 60 414 409 11 411 376 28 413 109 387 371 46 367 357 11 388 29 352 258 109 361 340 27 344 2 344 338 13 328 192 93 321 97 285 367 97 285 20 141 244 160 270 260 11 261 231 57 238 43 256 135 113 247 209 28 227 26 217 194 17 208 183 22 180 1 182 87 0 161 126 18 161 31 172 153 35 153 85 17 148 3 152 117 46 139 137 0 115 18 113 85 8 102 14 1 91 2 90 94 2 65 -23 2 76 4 72 27 1 49 -45 7 54 2 48 55 8 36 -21 2 47 1 42 39 2 32 -101 3 38 6 28 -7 3 27 -180 1 29 3 15 5 2 20 -131 2 26 1 14 -52 1 8 -340 1