Dimethyl Sulfoxide Complexes Detected at Ambient Conditions - The

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Dimethyl Sulfoxide Complexes Detected at Ambient Conditions Anne Schou Hansen, and Henrik Grum Kjaergaard J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b06102 • Publication Date (Web): 17 Jul 2017 Downloaded from http://pubs.acs.org on July 19, 2017

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The Journal of Physical Chemistry

Dimethyl Sulfoxide Complexes Detected at Ambient Conditions Anne S. Hansen and Henrik G. Kjaergaard∗ Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark E-mail: [email protected]

Phone: +45-35320334. Fax: +45-35320322

∗

To whom correspondence should be addressed

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Abstract Dimethyl sulfoxide (DMSO) is an important intermediate in the atmospheric oxidation of dimethyl sulfide (DMS). DMS, emitted from the ocean, is the main natural sulfur source and its oxidation products are essential in the formation of sulfate aerosols. The high atmospheric concentration of water makes hydrogen bonding with DMS and its oxidation products likely. Through hydrogen bonding, water can potentially catalyze and affect the steps of the oxidation. We investigate binary hydrogen bound complexes involving DMSO. Both water·DMSO and methanol·DMSO complexes are identified in an Ar matrix, and at room temperature, a Gibbs free energy of 0.7 kJ/mol for the formation of the methanol·DMSO complex is determined. Assuming a similar Gibbs free energy of the hydrate it would suggest a relatively high abundance of the DMSO hydrate relative to the monomer in the atmosphere. The effect of changing the atom divalently bound to the hydrogen bond accepting oxygen, from S to C (DMSO to acetone), is found to significantly decrease the equilibrium constant of complex formation.

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1

Introduction

The main sulfur sources in the atmosphere are sulfur dioxide (SO2 ) and dimethyl sulfide (DMS). 1 DMS is emitted from oceans and is the main natural sulfur source in times with limited volcanic activity. 2–4 In the atmosphere DMS reacts with the hydroxide (OH) or nitrate (NO3 ) radicals and various oxidation products like dimethyl sulfoxide (DMSO), methylsulfonylmethane, methanesulfonic acid, and sulfuric acid are formed. 1,3 The mechanisms of the atmospheric oxidation of DMS are complex, and the details and understanding of the them are far from complete. 5,6 However, the acid oxidation products from the sulfur cycle are known to be important in the formation of aerosols. 3,7–11 Due to the high water concentration in the atmosphere, hydrogen bonding between water and DMS and its oxidation products is likely to have an effect on the oxidation processes. The hydrogen bonding can potentially affect the reaction rates in sulfur cycle. For example, the reaction between SO3 and H2 O forming sulfuric acid is accelerated significantly by the presence of one additional water molecule. 12,13 Previously, hydrogen abstraction from DMS and DMSO by the OH radical was investigated in the absence and presence of water. 14 In both cases the presence of water was found to increase the reaction rate, especially for the reaction with DMSO. 14 This may be explained by stronger hydrogen bonding between water and DMSO than that between water and DMS. 14 As a plausible intermediate in the conversion of DMS to oxygenated sulfur compounds, DMSO and its monohydrate are relevant to the formation of sulfate aerosols and cloud condensation nuclei. Acetone is similar to DMSO, with the sulfur atom replaced by carbon. The oxygen atom in both DMSO and acetone can act as a hydrogen bond acceptor atom. Such hydrogen bonds have previously been investigated, 15–37 but investigations with DMSO are limited, and to our knowledge no studies in the gas phase at room temperature exist. This is probably caused by the low vapor pressure of DMSO (∼0.42 Torr at 20 ◦ C), which makes room temperature detection of its hydrogen bound complexes challenging. The previous condensed phase investigations focus primarily on vibrational frequencies in the 500 - 2000 cm−1 re3



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gion.The OH-stretching vibration of 2-mercaptoethanol in CCl4 was redshifted ∼290 cm−1 as DMSO was added to the solution. 21 As different o-methoxyphenols (2-methoxyphenol, 4-methoxyphenol, 2,4-dimethoxyphenol and 2,6-dimethoxyphenol) were dissolved in CCl4 and DMSO, the OH-stretching frequency was redshifted up to ∼1100 cm−1 relative to the OH-stretching vibration of the phenol monomers dissolved in CCl4 . 20 For comparison the OH-stretching vibration in ethanol (EtOH) and phenol is redshifted by 111 cm−1 and 210 cm−1 , respectively, upon complexation with dimethyl ether (DME) in the gas phase. 38,39 The observed frequency shifts suggest that the sp2 hybridized oxygen atom in DMSO acts as a stronger hydrogen bond acceptor than the sp3 hybridized oxygen atom in DME. 40,41 Here, hydrogen bound complexes with DMSO are investigated. Matrix isolation infrared (IR) spectra of complexes with water (H2 O·DMSO) and methanol (MeOH·DMSO) were initially recorded in the OH-stretching region. Subsequently, the MeOH complex was detected in the gas phase at room temperature. We did not detect the hydrated complexes at room temperature due to detector saturation and pressure dependence of the sharp rotational lines from the water monomer, which prevents accurate subtraction. We therefore use the MeOH·DMSO complex as a model system for the water complex. Complexes with acetone (H2 O·acetone and MeOH·acetone) were also investigated to show the effect of changing the atom divalent bound to the oxygen acceptor atom. Spectral assignment was facilitated by Local Mode Perturbation Theory (LMPT) frequency calculations of the optimized structures. 42–44 The abundance (equilibrium constant) of the complexes is determined from the measured gas phase OH-stretching intensity in combination with a LMPT calculated OHstretching intensity. 43,45,46

2

Experimental Section

Water (demineralized, in-house), MeOH (Aldrich 154903, ≥99.9%), EtOH (Kemetyl anhydrous, 99.9%), acetone (Aldrich 650501, ≥99.9%) and DMSO (Aldrich D8418, ≥99.9%, vapor

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pressure ∼0.42 Torr at 20 ◦ C) were purified by freeze, pump, and thaw cycles. The matrix isolation spectra were recorded with our Ar matrix isolation Fourier transform infrared spectroscopy (FTIR) setup. 47,48 Low temperatures (12 K) were obtained using a closed-cycle helium-compressor-cooled cryostat (CS202SI, Advanced Research Systems, Inc.), which was housed in a vacuum chamber with a base pressure of less than 5×10−6 Torr, measured with a Balzers compact full-range Pirani cold cathode (4×10−9 - 750 Torr, PKR250) gauge. Highpurity Ar (Air Liquide, ≥99.999%) was used as the matrix gas. Mixtures (e.g. MeOH + DMSO + Ar) were prepared at room temperature in a 1 L glass bulb connected to a vacuum line (J. Young, base pressure of 5×10−5 - 1×10−4 Torr). The samples were connected to the glass vacuum line using 3/8 inch vacuum fittings (Swagelok). Sample and base pressures were measured with a Varian and Agilent Technologies Pirani capacitance diaphragm (4×10−5 1125 Torr, PCG750) and Agilent Technologies capacitance diaphragm (1×10−3 - 10 Torr, CDG500) pressure gauges connected to the vacuum line. We estimate the uncertainty of the measured sample pressures to be less than 10 %. The uncertainty of the pressure was evaluated as the fluctuation in pressure read-out on the gauges. The gas mixture was deposited onto a CaF2 window maintained at 12 K. This temperature was measured with a silicon diode sensor (LakeShore) and regulated by a temperature controller (model 32, Cryocon). The flow of the gas deposition was controlled by a leak valve (model 203021, Brooks). The deposition was performed at a rate of approximately 20 - 30 mmol/h and lasted for about 60 minutes. A FTIR spectrometer (VERTEX 80 Bruker) fitted with CaF2 beam splitter, MIR light source and an MCT detector was used to record the spectra with a 0.5 cm−1 resolution and 256 scans. The spectrometer was purged with dry nitrogen gas to minimize the interference from atmospheric vapors. After recording a spectrum at 12 K, the matrix was annealed to 25 K, kept at that temperature for 20 minutes. Then is was cooled back to 12 K and a second spectrum was recorded. Finally, the matrix was annealed to 35 K for 20 minutes and cooled back to 12 K, and a third spectrum was recorded. The room temperature gas phase IR spectra were recorded at with a FTIR spectrometer

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(VERTEX 70 Bruker) , fitted with either a CaF2 or a KBr beamsplitter, an MCT detector and an MIR light source. Spectra were recorded with a 1 cm−1 resolution, 500 scans and a 20 cm cell equipped with KBr windows or a 16 m multireflection White cell (Infrared Analysis, Inc.) equipped with KCl windows and Au coated mirrors. A glass vacuum line, equipped with PCG750 and CDG500 pressure gauges, was also used for sample preparation of the room temperature gas phase measurements. Spectra of the gas phase complexes were obtained by subtracting the individual monomer spectra from the spectrum of the mixture (e.g. MeOH + DMSO), as illustrated in previous work and shown in Section S1.4. 45,47,49–53 The monomer spectra were recorded at a slightly different pressure relative to the pressure of each monomer in the mixture. The monomer spectra were scaled and then subtracted from the mixture, and an accurate subtraction was found when a straight baseline was obtained in the regions where only one of the monomers absorb. The individual monomer pressures in the mixture were obtained from the spectral subtraction, by multiplying the scaling factor and the monomer pressure from the individual monomer spectra. The integrated absorbance of the OH-stretching vibration in the MeOH·DMSO and MeOH·acetone complexes were obtained by drawing a straight baseline between two points on either side of the observed band and integrating the band, as shown in Figure S1. We estimate an uncertainty of less than 10 % for the integrated absorbance. The uncertainty was determined by evaluating the effect of a shift in baseline. The integration range for the MeOH·DMSO and MeOH·acetone complexes was 3200 - 3615 cm−1 and 3500 - 3670 cm−1 , respectively. Spectral subtraction and analyses were performed with OPUS 6.5 (Bruker) and OriginPro 9.1. Additional experimental details are summarized in Section S1.

3

Computational Details

Optimizations of the complexes and the corresponding monomers were performed with the CCSD(T)-F12a/VDZ-F12 method in Molpro2012 with a global energy threshold of 10−9 a.u.,

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default optimization threshold criteria and the correlation factor: (1/β)exp(-βr12 ), where β = 0.9. 54,55 In addition, optimizations were performed in Gaussian09, with the density functional theory (DFT) functionals: B3LYP, B3LYP-D3, ωB97X-D and M06-2X and the Møller Plesset (MP2) method using the aug-cc-pVTZ basis set. 56–61 The option "opt=verytight" was used in the DFT and MP2 optimizations, and for the DFT optimizations "integral=ultrafine" was also used. Subsequently, harmonic frequency calculations were performed with each method, using the same options as for the optimization. At the optimized structures, Non-Covalent Interactions (NCI) were calculated with NCIPLOT. 62–64 The wavefunction used in the NCI calculations was calculated in Molpro2012. For more detail on the NCI theory see Section S2.5. 62–64 OH-stretching frequencies (˜ ν ) and oscillator strengths (f ) were calculated with the Local Mode Perturbation Theory (LMPT) model. 42,43 Originally, the model was applied to a series of hydrated complexes, in which the water unit was described by a three dimensional local mode (LM) model, 65 where the effect of the six intermolecular modes on the donor vibrational modes were included. For hydrated complexes, the LMPT model calculates hydrogen bound OH-stretching frequencies to within 10 cm−1 of experimental values. 42,66 Experimental intensities are scarce and have large uncertainties, which makes it difficult to quantify the accuracy of the LMPT calculated oscillator strengths. For the water dimer, the LMPT oscillator strength of the fundamental bound OH-stretching transition calculated at the CCSD(T)-F12a/VTZ-F12 level of theory is 4.1×10−5 , which compares well with experimental values of 2.7 - 4.3×10−5 and a full dimensional (VPT2) calculated value of 2.8×10−5 . 43,66–68 This suggests that the uncertainty of our LMPT calculated oscillator strength is less than 35 %. For the MeOH·DMSO and MeOH·acetone complexes, a modified version of the LMPT model was used, in which a two dimensional LM model of the OHstretching and the COH-bending oscillators is employed, and only the effect of the two most important intermolecular modes is included. 44 We refer to this method as the 2D+2D LMPT model. Details on the displacements performed to generate the potential energy and dipole

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moment surfaces used in the LMPT calculations are given in Section S2.1. For the results presented in the paper, all potential energies and dipole moments required in the LMPT model were calculated at the CCSD(T)-F12a/VDZ-F12 level of theory in Molpro2012. 54,55

4

Result and Discussion

In Figure 1, we show the two most abundant CCSD(T)-F12a/VDZ-F12 optimized conformers of the MeOH·DMSO (MD1 and MD2) complex. The MD1 conformer has a mirror plane and Cs symmetry. Keeping MeOH fixed, the main structural difference between MD1 and MD2 is a torsion along the O=S bond of DMSO, which leads to a loss of symmetry in the MD2 conformer. Based on the Gibbs free energies, the abundance of the MD1 and MD2 conformers is roughly 50%/50%. The less abundant conformers (MD3 and MD4) are shown in Section S2.2. The MD2 conformer becomes equally abundant with MD1, due to a larger entropy contribution, to the Gibbs free energy, caused by a low frequency mode. In Figure 1, we also show the two corresponding conformers for the hydrated complex (HD1 and HD2), which have the same structural difference as MD1 and MD2. One additional conformer (HD3) was found for the H2 O·DMSO complex, see Section S2.3. In Table 1, we compare the calculated binding energies. The binding strength of the hydrated complexes is similar to that of the MeOH complexes, and we expect the two complexes to have similar equilibrium constants. Complexes with MeOH and water as hydrogen bond donors have previously been found to form complexes with similar stabilities. 69 In a theoretical investigation of alcoholwater complexes, comparison of the MeOH dimer and H2 O·MeOH complex showed that their binding energies where within 0.4 kJ/mol. 69

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4.1

Spectra of Complexes with DMSO as Hydrogen Bond Acceptor

The H2 O·DMSO and MeOH·DMSO complexes were initially investigated with matrix isolation FTIR spectroscopy. In the spectra of the mixtures (e.g. MeOH + DMSO), new bands appear that are not present in the monomer spectra, which indicate complex formation and confirm an interaction between the monomers, see Figure S3. Gas phase room temperature detection followed the matrix isolation experiments. Compared to the matrix isolation measurements, experimental detection of gas phase complexes at room temperature can be difficult. Here the formation of complex depends on the product of the pressures of the two monomers and the equilibrium constant, which for weakly bound complexes is small. 47,53 The vapor pressure of DMSO is small and thus a high MeOH pressure is used to form sufficient complex to facilitate detection. However, we also want to keep the MeOH pressure low to minimize MeOH dimer formation and detection saturation in the OH-stretching region. In Figure 2, we show spectra of the MeOH·DMSO complex in the OH-stretching region recorded at different monomer pressures. We find a linear correlation between the integrated absorbance, of the band observed in the OH-stretching region, and the multiplied monomer pressures, which indicates the presence of a binary hydrogen bound complex. At room temperature conditions the MeOH dimer has a band centered around 3599 cm−1 with a width of about 45 cm−1 , as shown previously 47 (Figure 4 and Figure S3, notice there is an error in this caption to S3, all 1 cm path lengths should be 10 cm). This is at the edge of the observed MeOH·DMSO band and would thus not affect the observed band significantly. In addition, the reference spectrum that is used to subtract the MeOH spectrum from that of the mixture is recorded at a pressure that is close to the MeOH pressure in the mixture. Thus the amount of the MeOH dimer in the spectrum of the mixture and that of MeOH monomer will be very similar and spectral subtraction will eliminate most of the signal from MeOH dimer. For the experiment with the highest MeOH pressure (Expt. H, 20 Torr) the scaling factor was 1.0 indicating that the MeOH reference spectrum had the same MeOH pressure as in the mixture and the effect of MeOH dimer on the MeOH·DMSO spectrum 10


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will be minimal. In the spectrum of the MeOH·DMSO complex two clear peaks are observed in the OHstretching region. These peaks are redshifted relative to that in the MeOH monomer and are assigned to the fundamental OH-stretching vibration in the two dominant conformers of the complex. A weak side band is observed at ∼3323 cm−1 and assigned to a hot band, similar to previous observation for the MeOH·DMA and MeOH·TMA complexes. 51,52,70,71 In Table 2, we summarize the observed and 2D+2D LMPT calculated OH-stretching frequencies, redshifts and calculated intensities of the MD1 and MD2 conformers of MeOH·DMSO. Calculated 2D+2D LMPT OH-stretching frequencies have previously been found to underestimate observed transition frequencies by about 10 - 40 cm−1 for a series of MeOH complexes. 44 Based on this and the calculated frequencies we assign the lower and higher frequency peaks observed to the OH-stretching vibration of MD1 and MD2, respectively. From the experimental and calculated intensities we estimate a 40/60 ratio of the two conformers, MD1/MD2, which is similar to the estimated 50 %/50 % abundance based on the calculated Gibbs free energies, see Section S2.2.

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0.12

0.10

Absorbance

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0.08

0.06

0.04

0.02

0.00 3200

3300

3400

3500

3600

Wavenumber / cm-1 Figure 2: Spectra of the OH-stretching region in the MeOH·DMSO complex recorded with a 16 m path length cell. The pressures used in each measurement are given in the supporting information Table S2. For the highest absorbance curve (blue) PDMSO = 0.29 Torr and PMeOH = 20.2 Torr. Table 2: Observed and calculated fundamental OH-stretching frequencies (˜ ν in cm−1 ), redshifts (∆˜ ν in cm−1 ) and intensities (f ) for the MeOH monomer and the MeOH·DMSO complex. Calculateda

Observed Compound

ν˜max

∆˜ νb

MeOH 3681c MeOH·DMSO 3441 240 3517 164

Conformerd MeOH MD1 MD2

ν˜

∆˜ νb

f

3683 3.59×10−6 3403 280 1.09×10−4 3491 192 1.11×10−4

a: Calculated with the 2D+2D LMPT model and the CCSD(T)F12a/VDZ-F12 method. 44 The MeOH monomer frequency is calculated with a 2D local mode model. b: ∆˜ ν = ν˜Alcohol - ν˜complex . c: Location of the Q-branch. d: The conformer abbreviations are defined in Figure 1. 12


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The appearance of the OH-stretching vibration in the MeOH·DMSO complex was confirmed by recording spectra of the EtOH·DMSO complex. This approach is a useful verification of the OH-stretching mode, as spectra of MeOH and EtOH hydrogen bound complexes are similar. 52,71 In the spectrum of the EtOH·DMSO complex, we also observe two distinct peaks, see SectionS1.4. The OH-stretching vibration in the MeOH·acetone complex was also investigated. Two close lying peaks, at 3578 cm−1 and 3612 cm−1 , are observed in the OH-stretching region, see Figure S24. The observed peaks agree well with the previous lower resolution gas phase detection, which found one broad band at 3598 cm−1 . 72 In matrix isolation two bands at 3503 cm−1 and 3518 cm−1 are observed. 33 In the gas phase under jet cooled conditions a single band is found at 3530 cm−1 , which compares well with the 2D + 2D LMPT calculated value (Table S7) and suggests that only one of the conformers are found in the jet. 35 In Section S1.5, we further compare and discuss our and the previous observations. Based on the OH-stretching redshifts observed for the complexes with acetone (∼100 cm−1 ) and DMSO (∼200 cm−1 ) as hydrogen bond acceptors we find that the OH· · · O interaction is stronger between MeOH and DMSO than between MeOH and acetone, in agreement with previous CCl4 solution phase experiments. 21

4.2

The Hydrogen Bond Strength

The magnitude of the OH-stretching redshift is an indication of the hydrogen bond strength, where the greater the shift the stronger the hydrogen bond. 41 The observed and calculated OH-stretching redshifts indicate that the OH· · · O hydrogen bond in the MeOH·DMSO conformer MD1 is stronger than that in MD2. We have used the NCI theory to visualize the differences in the interactions present in the MD1 and MD2 conformers. 62–64 In Figure 3, 2D NCI plots and 3D NCI isosurfaces are shown for the MD1 and MD2 conformers. A clear attractive interaction is found in both conformers (blue trough and isosurface), which corresponds to the OH· · · O hydrogen bond interaction. Secondary interactions in MD1 and 13




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unable to measure PAB directly in our experiments, as it is small with a maximum value of 0.04 Torr on a larger background, ∼20 - 80 Torr. We determine PAB indirectly by: 74

PAB = 2.6935 × 10 (K −9

where

R

−1

Torr m cm)

T

R

A(˜ v ) d˜ v , fcalc l

(2)

A(˜ v ) d˜ v is the measured integrated absorbance of the OH-stretching band of the com-

plex and fcalc is the corresponding calculated oscillator strength. We use the OH-stretching intensity of the lowest energy conformer, as the calculated intensity differs less than 2 % between the two conformers, see Table 2. Based on the uncertainties related to the determined integrated absorbance (10 %) and calculated oscillator strength (35 %), we estimate the uncertainty of our determined KP values to be 45 %. This results in an uncertainty in our determined ∆G values of approximately −1 kJ/mol and +1.5 kJ/mol. To facilitate comparison between the KP values determined in the present and previous investigations, we have determined the KP values for the MeOH·DMS and MeOH·DME complexes using the already published experimental data and CCSD(T)-F12a/VDZ-F12 calculated oscillator strengths with the 2D+2D LMPT model. 38,44 In Table 3, the KP value of the MeOH·DMSO complex is compared to values for other MeOH complexes. A KP value of 0.75 is determined for the MeOH·DMSO complex, which is a factor of ∼25 larger than that for the MeOH·acetone complex. The KP value for the MeOH·DMSO complex is a factor of 45 larger than that determined for the MeOH·DMS complex (0.016). 38 This difference agrees with the large difference in the calculated CCSD(T)-F12a/VDZ-F12 binding energies of -42.9 kJ/mol and -24.0 kJ/mol for the complexes with DMSO and DMS, respectively, 44 and indicates that hydrogen bonds to DMSO are significantly stronger than to DMS. Previously, a theoretical investigation discussed the hydrogen abstraction reactions by the OH radical in sulfur containing compounds (DMS, DMSO, and DMSO2 ) and their monohydrates. 14 Concentration ratios (C(H2 O·DMSO)/C(HO·H2 O)) were estimated, based on ab initio calculated KP values and atmospheric concentrations of the individual compounds, to determine the impact of these hydrated complexes on the atmospheric sulfur cycle. 14,75,76 15



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However, quantum chemical KP values are difficult to calculate, mainly due to the vibrational entropy term and the presence of very low frequency vibrations and are known to depend on the electronic structure method. 77–79 Our determined KP values does not suffer from the same problems since our calculated 2D+2D LMPT intensities, used to determine KP , and are to a large extent method insensitive. Based on the binding energies of the water and MeOH complexes in Table 1 it seems reasonable to assume that water and MeOH form complexes with similar KP values. 69 Hence, our determined KP values indicate that the concentration of the hydrated DMSO complex is significantly greater than that of H2 O·DMS. With literature atmospheric monomer concentrations and assuming our KP values for the MeOH complexes, concentrations of 0.1 ppt and 1 ppt are estimated for the hydrated DMS and DMSO complexes, respectively. 75 The concentration of DMS (200 ppt) is 2000 times larger than that of its hydrated complex. 75 Hence, the rate constant for the OH oxidation reaction of the H2 O·DMS complex must be 2000 times faster than that of isolated DMS to be important in the sulfur cycle. However, a smaller concentration difference is estimated for H2 O·DMSO and DMSO (50 ppt), and the addition of a water molecule only needs to accelerate the reaction by a factor of 50 for the complex to be important in the atmosphere. A factor of 50 corresponds to a reaction barrier change of only 10 kJ/mol, which is easily obtained through formation of an additional hydrogen bond. Thus, H2 O·DMSO is likely to affect the atmospheric sulfur cycle. Table 3: Determined KP values for a series of MeOH complexes. DMSO >S=O KP

0.75

Acetone DME >C=O −O− 0.029

DMS −S−

0.021a 0.016a

a: References 38 and 44.

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5

Conclusions

Evidence of the OH· · · O hydrogen bond interaction in water and MeOH complexes with DMSO was found in Ar matrix isolations measurements. The MeOH·DMSO complex was also detected in the gas phase at room temperature, and confirmed by detection of the EtOH·DMSO complex. Two peaks, corresponding to two conformers, were observed in the OH-stretching region and found to be redshifted relative to that in the MeOH monomer by 240 and 164 cm−1 for the MeOH·DMSO complex. Combining the measured OH-stretching intensity with a calculated intensity, an equilibrium constant of 0.75 (or Gibbs free energy of 0.7 kJ/mol) were determined for the MeOH·DMSO complex. Compared to complexes with a divalent bound oxygen as acceptor atom (DME) the carbonylic bound oxygen atom in acetone is found to be a slightly better acceptor and that in DMSO is significantly better. The determined equilibrium constants also show that hydrogen bonding with DMSO is much more efficient than with DMS, which supports previous theoretical claims that DMS and not its hydrated complex is important for the atmospheric sulfur cycle, whereas the hydrated DMSO complex is more important in the sulfur cycle than isolated DMSO.

Supporting Information Available Band integration; Ar matrix spectra; Room temperature spectra of monomers and complexes; Additional optimized conformers; NCI details; Calculated frequencies and intensities. This material is available free of charge via the Internet at http://pubs.acs.org/.

Acknowledgement We thank Kasper Mackeprang, Benjamin N. Frandsen, and Kristian H. Møller for assistance with the calculated LMPT frequencies and intensities. We also thank Zeina Maroun, Malte F. Jespersen, Lin Du, and Joseph R. Lane for helpful discussions. We acknowledge the financial support from The Danish Council for Independent Research - Natural Sciences,

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the Danish Center for Scientific Computing and the Center for Exploitation of Solar Energy funded by the University of Copenhagen.

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