Article pubs.acs.org/JPCA
Hydrogen Bonding in the Sulfuric Acid−Methanol−Water System: A Matrix Isolation and Computational Study Mark Rozenberg,† Aharon Loewenschuss,† and Claus J. Nielsen*,‡ †
Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem, Israel 919040 Department of Chemistry, University of Oslo, 1033 Blindern, N-0315 Oslo, Norway
‡
S Supporting Information *
ABSTRACT: Frozen core MP2 and DFT computations were carried out on possible configurations of 1:1 H2SO4·CH3OH and 1:1:1 H2SO4·CH3OH·H2O complexes. Minimum energy structures, stabilization energies, H-bond lengths and vibrational frequencies were calculated. The latter complex can exist in either sequential “linear” configurations involving four Hbonds or “cyclic” structures involving three H-bonds only. However, there is little difference in the energy of stabilization between these two possible forms, indicating a “cooperative effect” between the H-bonds in the latter. This effect is also evidenced by the calculated H-bond lengths. In the cyclic complex, the hydroxyl of either CH3OH or H2O may be the proton donor to the H-bond between them. Argon matrix isolation FTIR spectra of layers with various concentration ratios were recorded. In the hydroxyl stretch wavenumber regions several weak new bands were observed. Their position was found to fit best the cyclic structures. The observed red shifts exceed the corresponding calculated values. Together with the considerable observed bandwidths they are further manifestations of the cooperative effect between the H-bonds. The lower skeletal mode wavenumber regions show a number of sharper bands compatible with those previously reported for dimethyl sulfate and hydrogen methyl sulfate, indicating their formation in the vapor mixing region or at the solid matrix layer interface.
■
INTRODUCTION Sulfuric acid and methanol are both substances of significant atmospheric importance. Sulfuric acid is an important component of atmospheric aerosols which in turn impacts the environment. The major sources of atmospheric sulfuric acid are volcanic SO2 emanations and industrial activities. Atmospheric methanol is formed predominantly by the decay of organic matter. Its atmospheric impact is primarily due to its interaction with sulfuric acid and the consequent formation of sulfate grains, which in turn serve as nucleation centers for cloud formation.1−4 The atmospheric complexation processes between atmospheric contaminations such as sulfuric acid, water and others, including hydrogen bonding formation, have been recently reviewed.5 The matrix isolation studies of complexes formed between sulfuric acid and several molecules of atmospheric interest have been reported by us.6−11 Here we study the interactions in the H2SO4/CH3OH/H2O system by matrix isolation spectroscopic methods and theoretical computations. The observed complexes may be precursors in the atmospheric sulfate and bisulfate cluster formation. From a more theoretical point of view, the interest in the interaction of sulfuric acid and methanol is due to the similarities between the latter and water in their similar tendency to form H-bonds. In the liquid phase of both, © XXXX American Chemical Society
hydrogen bonds have an essential role in the intermolecular forces. The vaporization enthalpy of water, with its four Hbonds per molecule in the liquid phase, is 40.65 kJ mol−1 (373 K).12 In comparison, for methanol the vaporization enthalpy is 35.21 kJ mol−1 (337.7 K),13 with two H-bonds per molecule in the liquid. The H-bond energies of water dimer and methanol dimer are 15.014 and 24.8 kJ mol−1 (estimated from the dissociation energy of the methanol tetramer15), respectively. Both liquids are also similar in their acid/base properties, as shown by various acidity scales. For example, the covalent proton donor parameters are, respectively, 1.15 and 0.86 and the proton acceptor parameters are 0.78 and 0.74.16 Similarly, in the compilation of Kamlet17 of hydrogen bonding donor acidities, water is slightly more acidic than methanol, whereas according to Iogansen18 the opposite order is suggested. Likewise, the pKa values of both are very close 15.7 and 16.0,19 with methanol thus being slightly more acidic. We reported the matrix isolation spectra of the H-bonded sulfuric acid/water monohydrated complex8 and have also Special Issue: Markku Räsänen Festschrift Received: June 16, 2014 Revised: September 10, 2014
A
dx.doi.org/10.1021/jp505965z | J. Phys. Chem. A XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry A
Article
Table 1. Calculated Structures, H-Bond Lengths, and Stabilization Energies (and Enthalpies, kJ mol−1) for Complexes Related to the H2SO4/CH3OH/H2O System
B
dx.doi.org/10.1021/jp505965z | J. Phys. Chem. A XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry A
Article
Table 1. continued
C
dx.doi.org/10.1021/jp505965z | J. Phys. Chem. A XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry A
Article
Table 1. continued
a From B3LYP/aug-cc-pVTZ calculations. calculations. dReference 31.
b
In angstroms. cComplexation energies (kcal mol−1) from BSSE corrected MP2/aug-cc-pVTZ
The water content of the matrix depends on the degree of predrying (P2O5) and prepumping and is not fully reproducible. It could, however, be monitored by the intensity of the pure water bands. Argon was introduced by flowing it over the acid sample. To achieve the desired vapor pressure, the sample was heated by irradiating it with a small electrical bulb. The degree of heating was determined by monitoring the recorded spectral intensities. Methanol/Ar mixtures in ratios from 1:40 to 1:200 were prepared by standard manometric techniques. Samples were deposited on a CsI window, with deposition rates ranging from 10 to 1000 mmol of Ar/h and deposition window temperatures in the 17−24 K range. Cooling was provided by an Air Product Displex model 202A closed cycle helium refrigerator. To protect the window, a pure Ar layer was deposited for several minutes before the first warming of the acid drop. Deposition temperatures were monitored by an Au0.007%Fe/Chromel thermocouple and controlled with an APDE temperature controller. Deposited samples were temperature cycled to up to 40 K. Spectra were recorded on a Bruker Equinox 55 FTIR spectrometer with a DTGS detector at a resolution of 0.5 cm−1 and generally coadding 128 scans.
shown that water may be involved in other hydrogen bonded complexes of sulfuric acid.10 In the present study we extend the investigation to the methanol involvement in complex formation with sulfuric acid along with the involvement of water molecules in this process. Similarly to water, methanol tends to form dimers in its vapor phase. In a mixed vapor phase both homogeneous and heterogeneous dimers may be formed due to the similarity in the proton donation properties of both molecules. In the water−methanol 1:1 mixed dimers both molecules can, in principle, assume the role of proton acceptor or proton donor. Both methanol−methanol and methanol−water dimers have been investigated experimentally20−22 and theoretically.23,24 These studies are relevant to the present work because, as we shall show, the mixed 1:1 methanol−water dimer is involved in the complex formation with sulfuric acid. We present computational results of the various conformations of the 1:1 sulfuric acid−methanol and the 1:1:1 sulfuric acid− methanol−water complexes along with recalculations of the two possible 1:1 methanol−water dimers so as to have all results on the same computational level and ensure valid comparisons. Experimentally, we observed several new bands not recorded for either of the matrix isolated pure compounds or in the spectra of matrix isolated “wet” sulfuric acid.8 Of these, the bands indicating the formation of new hydrogen bonded species will be discussed and related to the computational results. In addition, spectral features associated with the results of a chemical reaction of the vapors will be indicated and discussed.
■
ELECTRONIC STRUCTURE CALCULATIONS Frozen core MP225 and DFT calculations employing the Becke three-parameter26 and Lee−Yang−Parr27 (B3LYP) hybrid functional were carried out with the Gaussian 09 program.28 Dunning’s correlation-consistent aug-cc-pVXZ (X = D, T, Q) basis sets29,30 were employed in all calculations. The electronic structure calculations were carried out to obtain possible structures and configurations of the complexes and to assist in the spectral interpretation; the structures of all species considered are illustrated in Table 1; the Cartesian coordinates obtained in B3LYP/aug-cc-pVTZ calculations are presented in Table S1 in the Supporting Information. Complexation energies have been estimated as the energy of the complex minus the monomer energies, ΔEcomplex = E(A··· B···C) − E(A) − E(B) − E(C). The results are subsequently corrected for the basis set superposition error (BSSE) by the
■
EXPERIMENTAL SECTION Materials. Sulfuric acid (98%) was supplied by Frutarum, Israel. Analytical grade methanol was supplied by Aldrich. Argon gas (5.7 purity), supplied by AGA was used to produce the solid matrix layers. Sample Preparation. An acid drop was placed in a glass tube ending with a 2 mm pinhole and pumped for up to 100 h. Additional drying was attained by placing phosphorus pentoxide powder in the vapor path in front of the acid drop. D
dx.doi.org/10.1021/jp505965z | J. Phys. Chem. A XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry A
Article
counterpoise (CP) correction. The values obtained are also included in Table 1; energies obtained in B3LYP/aug-cc-pVTZ and MP2/aug-cc-pVTZ calculations are collected in Table S2 in the Supporting Information. It is noted that the counterpoise corrections are quite substantial in the MP2 calculations and that aug-cc-pVQZ basis set calculations are needed to bring the BSSE energies below 4 kJ mol−1. Vibrational wavenumbers and infrared intensities were obtained in the harmonic approximation and additional anharmonic vibrational wavenumbers were obtained directly from numerically derived cubic and quartic force constants as implemented in G09. The force fields were scaled according to 1/2 the procedure, Fscaled = Fcalc where αi and αj are scaling i,j i,j ·(αi·αj) parameters for the valence coordinates i and j, respectively. The scaling parameters are derived from fitting vibrational data of the monomeric compounds. The described frequency scaling has the advantage of being more mode specific than the application of a uniform scaling factor. The H2SO4 and D2SO4 matrix isolation spectra6 are reproduced within a few wavenumbers in both B3LYP and MP2 calculations employing six scaling factors. Similar agreements are obtained for the matrix isolation data for eight methanol isotopomers20 employing six scaling factors (Table S3 in the Supporting Information), and for the H2O matrix isolation data employing two scaling constants.11 These scaling factors were then applied to the various molecular complexes; unity scaling factors were applied to all interfragment valence coordinates. The harmonic and scaled wavenumbers of the complexes are collected in Tables S4−S9 in the Supporting Information. Dispersion forces are not described by the B3LYP functional, resulting in an underestimation of the interaction between the complexes studied. MP2 theory, on the other hand, overestimates the dispersion interaction. Incommensurable results may therefore be expected from the two methods. However, our previous results for various sulfuric acid complexes8−11 show that although the computed interaction energies differ, the scaling method outlined above aligns the B3LYP and MP2 calculated vibrational spectra to within a few wave numbers, which justifies our use of DFT methods for estimating the vibrational spectra of large molecular complexes for which MP2 calculations are unrealistic.
Figure 1. CH3OH·H2SO4 complex potential energy curve and barrier height of interconfigurational conversion.
■
Figure 2. 3520−3550 cm−1 range. New complex ν(OH) stretching mode bands are marked in red: A, a sulfuric acid/argon matrix layer; B, a methanol/argon (1:100) matrix layer; C, D, and E, matrix layers of H2SO4/CH3OH/H2O with successively increasing H2O contents. (Note growth of CH3OH-H2O dimer band, marked in cyan.)
RESULTS AND DISCUSSION The simplest sulfuric acid−methanol complex is of the 1:1 H2SO4·CH3OH configuration. Our calculations indicate two possible conformers, as shown in Table 1, with a minimal energy difference (less than 0.5 kJ mol−1) between them. Both involve two H-bonds in an analogous manner to the bonding in the H2SO4·H2O complex.8 The main H-bonding is generated by the “acidic” H atom of H2SO4 to the free electron pair of CH3OH, whereas a secondary hydrogen-bond is the result of the interaction between the hydroxyl H atom of CH3OH and the free electron pair of a sulfuric acid double-bonded oxygen. The barrier for conformational interconversion is relatively low, around 2.5 kJ mol−1; the potential is shown in Figure 1. It is reasonable to assume that similar potential barriers will result from scans of the potential surfaces of the other complexes discussed here. As the complexes studied involve two hydrophilic substances, on the one hand, and due to the natural presence of H2O in the sulfuric acid decomposition equilibrium, on the other, H2O molecules may be expected to get involved in the complex structures. Such has also been our previous experience.8 In the
present case, the simplest 1:1:1 sulfuric acid−methanol−water complex may assume two major configurationsa “linear” one, denoted as CH3OH·H2SO4·H2O or Me·Sa·Wa and two cyclic forms, denoted as H2SO4·CH3OH·H2O or Sa·Me·Wa and H2SO4·H2O·CH3OH or Sa·Wa·Me, respectively. The two cyclic forms have either the H2O or the CH3OH molecules as the proton acceptor from sulfuric acid. This bonding scheme also determines whether CH3OH or H2O is the proton donor to the H-bond between them, in the CH3OH·OH2 or HOH· O(H)CH3 moieties, respectively. This is very similar to the two bonding possibilities encountered for the 1:1 CH3OH·H2O complex.21 The possible structures described here are all shown in Table 1 along with their calculated H-bond lengths. While the four “linear” 1:1:1 complexes Me·Sa·Wa(n) have complexation energies spread of over 11 kJ mol−1, the spread for the “cyclic” configurations Sa·Wa·Me(n) is about 5 kJ mol−1. E
dx.doi.org/10.1021/jp505965z | J. Phys. Chem. A XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry A
Article
In spite of the different number of H-bonds involved in the cyclic and linear configurations, respectively, both forms are of similar stabilization energy values. This seeming discrepancy may be explained by a “cooperative effect” between the Hbonds in the cyclic configuration, resulting in shorter H-bond lengths and higher stabilization energy per bond. However, it is evident from a comparison between the experimentally observed “new” complex band positions and the calculated frequencies of both configurations, that the better agreement is obtained for the “cyclic” complexes. A possible explanation is that the formation of the “cyclic” complex occurs between a H2SO4 molecule and a CH3OH·H2O dimer (existing in the vapor) whereas the “linear” complex needs to be formed by two separate CH3OH and H2O molecules getting into the vicinity of the same H2SO4 molecule. From the experimental spectroscopic aspect, the H2SO4/ CH3OH/H2O ternary system poses a challenge where the identification of the complex bands is concerned. Even separately, all parent molecules produce a rich matrix isolation spectrum and therefore the spectral characterization of the complex species formed must rely on the appearance of new bands of low peak intensities, due to both the relatively low abundance of the complex and their greater bandwidths. These impediments also dictate the deposition of rather concentrated matrix layers and longer deposition times. Actually, an identifying feature of the bands assignable to complex species is their intensity dependence (relative to those of the parent molecules bands) on the deposition time and on the concentration ratios in the deposition mixtures (Figure 3). As may be expected, the frequency regions most indicative of new species formation are those where the hydroxyl groups of the parent molecules are active spectroscopically. Figure 2 compares five spectra of the 3520−3550 cm−1 wavenumber range. Curve A is the spectrum of a sulfuric acid/argon matrix layer, curve B is the spectrum of a methanol/argon (1:100) matrix layer, whereas curves C, D, and E show spectra of matrix layers of H2SO4/CH3OH/H2O with successively increasing H2O contents. Two new bands are identified in this region, which is dominated by bands of the bonded ν(OH) of the methanol dimer20,21 and of the CH3OH·H2O complex.22 The 3546 cm−1 band is rather prominent in its relative growth, whereas the lower frequency 3537 cm−1 band is almost overshadowed by the 3535 cm−1 band of the CH3OH·H2O dimer. Figure 3 illustrates the growth of bands attributable to OH stretching modes by deposition time. Trace A in Figure 3 is the spectrum of a sulfuric acid/argon matrix layer, trace B is the spectrum of a methanol/argon (1:100) matrix layer, whereas traces C and D show spectra of matrix layers of H2SO4/ CH3OH/H2O obtained after successively increasing deposition times. We consider the rather broad (∼50 cm−1) band, marked red in Figure 3 and centered at about 3020 cm−1, to represent a single spectral feature related to a strongly H-bonded hydroxyl, as further discussed in the text related to assignments. The effect of deposition time on the shape of this broad spectral feature supports the assertion that the complexes are formed at the vapor/solid interface. The thicker matrix layer may be of slightly higher temperature and allow more mobility of the molecules before their being frozen into the matrix. The relative growth of the sulfate bands in trace D (discussed below) is further evidence of this effect. The third spectral region of 2600−2770 cm−1 in which bands related to H-bonded hydroxyl groups were discerned, is shown
Figure 3. 2970−3055 cm−1 range: effect of increasing deposition times. New complex ν(OH) stretching mode bands are marked in red: A, a sulfuric acid/argon matrix layer; B, a methanol/argon (1:100) matrix layer; C, a matrix layer of H2SO4/CH3OH/H2O (18 min deposition time); D, a matrix layer of H2SO4/CH3OH/H2O (45 min deposition time).
Figure 4. 2570−2770 cm−1 range: effect of increasing amount of water in the matrix. New complex ν(OH) stretching mode bands are marked in red: A, a methanol/argon (1:100) matrix layer; B, a sulfuric acid/ H2O/argon matrix layer; C, a matrix layer of H2SO4/CH3OH/H2O; D, a matrix layer of H2SO4/CH3OH/H2O with higher H2O content.
The ternary complex “linear” bonding scheme H2O·H2SO4· CH3OH, has four possible configurations, which involve four H-bonds each. In comparison, the cyclic configurations have only three H-bonds each. Table 1 also shows the calculated stabilization energies for all considered structures. Stabilization enthalpies may also be estimated from their correlation with Hbond lengths.31 The results of this correlation, as applied to the calculated bond lengths, are also listed in Table 1 (the latter values are evaluated for room temperature). For the enthalpies obtained by correlation, the spread of values is in 0.5−5 kJ mol−1. F
dx.doi.org/10.1021/jp505965z | J. Phys. Chem. A XXXX, XXX, XXX−XXX
a
G
(111) (108) (112) (110)
(105) (103) (114) (104)
3575 3577 3577 3578
3575 3576 3582 3576
See Table 1 for structure nomenclature.
Sa·Me·Wa1 Sa·Me·Wa2 Sa·Me·Wa3 Sa·Me·Wa4 observed Sa·Wa·Me1 Sa·Wa ·Me2 Sa·Wa·Me3 Sa·Wa·Me4 observed
Me·Sa·Me3
Me·Sa·Me2
3577 (117) 3577 (121)
Sa·Me1 Sa·Me2 Me·Sa·Wa1 Me·Sa·Wa2 Me·Sa·Wa3 Me·Sa·Wa4 Me·Sa·Me1
3606 (138) 3613 (149)
3684 (44)
Me·Wa1 Me·Wa2
ν(OH)free (CH3OH)
3683 (43)
ν(OH)free (H2SO4)
Me·Me
structurea 3530 (517) 3535.2, 3540.9
ν(MeOH···O(Me)
3634 (7)s 3728 (83)as
ν(OH) (H2O)
3689 3695 3692 3692
3701 3706 3704 3700
3699 3699 3698 3698
(87) (78) (80) (83)
(108) (111) (113) (115)
(111) (112) (117) (118)
3707 (83)
ν(OH)free (H2O)
(1024) (1039) (834) (1062)
2572 (2094) 2560 (2175) 2520 (2294) 2540 (2192) 2744
3008 3006 3014 3012
ν(SOH···OH2)
2820 (1822) 2814 (1814) 2848 (1990) 2842 (2062) 2909 (1698) 2839 (2088) 2884 (240)s 2855 (3260)as 2884 (154)s 2854 (3446)as 2884 (35)s 2853 (3640)as 2344 (2743) 2393 (2647) 2395 (2708) 2358 (2688) 2675, 2615
ν(SOH···O(Me))
3545 (128) 3548 (127) 3543 (132) 3545 (134) 3589 (192)s 3587 (150)as 3588 (152)s 3585 (200)as 3588 (79)s 3568 (286)as 3468 (522) 3477 (515) 3472 (530) 3474 (522) 3537 3527 (586) 3528 (595) 3521 (606) 3527 (583) 3546
ν(OH···OS)
Table 2. Scaled Calculated (MP2/aug-cc-pVTZ) and Observed Frequencies for Complexes Related to the H2SO4/CH3OH/H2O System
3131 (990) 3169 (913) 3162 (917) 3132 (992) 2996.0
3492 (426)
ν(OH)H2O···O(Me)
3254 (840) 3281 (788) 3276 (806) 3255 (831) 3016.6
3560 (421) 3535.2, 3537.1
ν(OH)CH3OH···OH2
The Journal of Physical Chemistry A Article
dx.doi.org/10.1021/jp505965z | J. Phys. Chem. A XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry A
Article
position of the four Sa·Me·Wa complexes is 2371 cm−1. The difference of 177 cm−1 is not far from the wavenumber difference between the observed bands at 2615 and 2675 cm−1 and the band at 2744 cm−1 (Table 2). Although it is rather clear that all three bands are due to the H-bonded hydroxyl of sulfuric acid, their similar wavenumber values do not allow a confident assignment to a certain configuration of the cyclic 1:1:1 sulfuric acid/methanol/water complex. Table 2 also lists the calculated frequencies for the Me·Me dimer and Me·Wa heterodimer. Their calculated ν(OH) wavenumber values are very similar. The observed values are similarly close, both in this and in previous studies21 and thus do not allow a confident distinction in their assignment. The second strongest H-bonded hydroxyl group is between the two possible configurations of the CH3OH/H2O hydrogen bonded moieties, namely CH3OH·OH2 and H2O·O(H)CH3 in the respective 1:1:1 H2SO4·CH3OH·H2O and H2SO4·H2O· CH3OH complexes. As seen in Table 2 and Figure 3, this spectral feature centered at 3016 cm−1, is shifted to lower frequencies than those predicted by the calculations, as discussed in the computational section. We have previously encountered such underestimations of the H-bonding induced red shifts by theoretical calculations.8−11 The observed red shift of this broad band places it also at a considerably lower frequency than the analogous bands in the 1:1 methanol/water complexes at 3537 cm−1 (Figure 2) as assigned.21,22 This additional red shift is attributed to the “cooperative effect”32,33 between the three H-bonds of these closed cycle configurations. The actual width of this band is difficult to estimate, as its position coincides with that of the very strong ν(CH3) modes of the parent methanol molecules. However, it is of considerable width, which may also be due to the cooperative effect. The weakest and least shifted ν(O−H) mode bands are those between “free” oxygen electrons of a sulfuric acid SO bond and a hydroxyl hydrogen of either methanol or water, in the H2SO4·H2O·CH3OH and H2SO4·CH3OH·H2O complexes, respectively The two bands, at 3536 and 3547 cm−1, marked in red in Figure 2, are assigned to this mode. The 3536 cm−1 band is shifted by 130 cm−1 from the ν(OH) band of free matrix isolated methanol at 3666 cm−1 (this work). The 3547 cm−1 band is red-shifted by 140 cm−1 from the mean position of the ν(OH) modes of matrix isolated H2O molecules at 3687 cm−1.34 This latter shift is also considerably higher than that of the analogous band in 1:1 H2SO4·H2O complex at 3582 cm−1.8 This larger shift is, again, attributed to the “cooperative effect” with the other H-bonds of this configuration. Table 2 shows the calculated frequency values for these modes and, again, the computed values underestimate the experimental red shift. However, the calculated frequencies retain the order of the above band assignments. In addition to the bands attributed to hydrogen bonded complex formation in the ν(OH) region, the skeletal mode range shows additional new bands. This is illustrated in Figure 5, in which trace A is the spectrum of methanol/argon (1:100) matrix layer, and trace B is the spectrum of sulfuric acid/argon matrix layer. Trace C shows the spectrum of a matrix layer of H2SO4/CH3OH/H2O, in which reaction product bands marked with wavenumber values. These were noted to increase at long deposition time as compared to the ν(OH) complexation related bands and even relative to the pure methanol bands. These bands fit well the positions of bands of matrix
Figure 5. 780−1480 cm−1 skeletal mode range: A, a methanol/argon (1:100) matrix layer (red curve); B, a sulfuric acid/argon matrix layer (blue curve); C, a matrix layer of H2SO4/CH3OH/H2O (balck curve). Reaction product bands are marked with wavenumber values.
Table 3. Matrix Isolation Bands Resulting from H2SO4/ CH3OH Reactions band due to H2SO4/CH3OH/Ar depositiona
dimethyl sulfate bandsb
methyl hydrogen sulfate 3614.4c
2973.2 1427.7 1212.3 1017.7 855.5 785.6 581.1
2968.0 1415.5 1216.0 1025.1 821.9 755.8 597.9
1020d 790d
assignments ν(OH)c ν(CH3)b ν(SO)asb ν(SO)sb ν(CO)s ν(CO)asb ν(SO)sb γ(OSO)b
a
This work. New bands, not observed for parent molecules. bAr matrix. Reference 35. cAr matrix. Reference 9. dSulfuric acid solution. Reference 3.
in Figure 4, illustrating the effect of increasing the amount of water in the matrix. Trace A is the spectrum of a methanol/ argon (1:100) matrix layer, trace B is the spectrum of sulfuric acid/H2O/argon matrix layer. Trace C is the spectrum of a matrix layer of H2SO4/CH3OH/H2O, and the spectrum shown in trace D is obtained with higher water content. The most prominent bands at 2722 and 2593 cm−1, clearly visible in trace B of Figure 4, coincide with the position of the highly redshifted ν(O−H) bands of sulfuric acid, H-bonded to the oxygen of H2O, in the H2SO4·H2O monohydrate complex.8 In addition, three new peaks of intermediate width (10−15 cm−1) are identified at 2744, 2675, and 2615 cm−1. An additional band at 2632 cm−1 is too sharp to be assigned to a fundamental of an H-bonded moiety and is probably due to an overtone or combination band of methanol or dimethyl sulfate, also formed during deposition and further discussed below. Three observed peaks, marked in red in traces C and D of Figure 4, are therefore also assigned to the ν(OH) mode of H2SO4, H-bonded to the hydroxyl oxygen of either CH3OH or H2O in the cyclic ternary sulfuric acid/methanol/water complex. Possibly the highest wavenumber band in this region (2744 cm−1) is due to the SO−H···OH2 stretch of the 1:1:1 H2SO4·H2O·CH3OH and the lowest one at 2615 cm−1 is due to the H-bonding of the sulfuric acid hydroxyl to the somewhat more acidic methanol in the H2SO4·CH3OH·H2O complexes. The average of the calculated frequencies of the four Sa·Wa·Me complexes is 2548 cm−1. Similarly, the average calculated H
dx.doi.org/10.1021/jp505965z | J. Phys. Chem. A XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry A
■
isolated dimethyl sulfate, as reported by Borba et al.,35 Table 3 (calculated wavenumbers given in Table S11 in the Supporting Information). Several of them are also close to the position of some bisulfate bands,2,3 but the ν(OH) band of the latter was not observed in our experiments. Often the H-bonded species are formed by migration in the matrix of the lighter parent molecules. The H2SO4/CH3OH reaction involves high molecular mass molecules, unlikely to diffuse in the cold matrix layer. It must be concluded that here this reaction occurs in the vapor phase or at the interface between the deposition vapor and the solid layer, where the gas jets mix just before being frozen into the matrix. The formation of methyl hydrogen sulfate as a result of the reaction of sulfuric acid methanol was indeed observed in condensed phases and at the interface of vapor and liquid phases.2−4 Dimethyl sulfate is considerably less volatile than either methanol or hydrogen methyl sulfate and its formation here may indicate that it is also formed under atmospheric conditions and thus playing the role of nucleation centers in cloud formation. In summary, we have shown that matrix isolation layers produced from water/sulfuric acid/Ar and CH3OH/H2O mixtures show new bands in the ν(OH) region assignable to a 1:1:1 H2SO4/CH3OH/H2O complex of cyclic configuration. The red shifts and calculated stabilization energies and bond lengths indicate a cooperative effect between the H-bonds of this complex. In addition, we observed skeletal mode bands assignable to dimethyl sulfate (and possibly hydrogen methyl sulfate) in the vapor phase or at its interface with the matrix layer.
■
REFERENCES
(1) Schobesberger, S.; Junninen, H.; Bianchi, F.; Lönn, G.; Ehn, M.; Lehtipalo, K.; Dommen, J.; Ehrhart, I.; Ortega, K.; Franchin, A.; et al. Molecular Understanding of Atmospheric Particle Formation from Sulfuric Acid and Large Oxidized Organic Molecules. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 17223−17228. (2) Van Loon, L. L.; Allen, H. C. Methanol Reaction with Sulfuric Acid: A Vibrational Spectroscopic Study. J. Phys. Chem. A 2004, 108, 17666−17674. (3) Van Loon, L. L.; Allen, H. C. Uptake and Surface Reaction of Methanol by Sulfuric Acid Solutions Investigated by Vibrational Sum Frequency Generation and Raman Spectroscopies. J. Phys. Chem. A 2008, 112, 7873−7880. (4) Van Loon, L. L.; Allen, H. C. Effective Diffusion Coefficients for Methanol in Sulfuric Acid Solutions Measured by Raman Spectroscopy. J. Phys. Chem. A 2008, 112, 10758−10763. (5) Klemperer, W.; Vaida, V. Molecular Complexes in Close and Far Away. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 10584−10588. (6) Givan, A.; Larsen, L. A.; Loewenschuss, A.; Nielsen, C. J. Matrix Isolation Mid- and Far-Infrared Spectra of Sulfuric Acid and Deuterated Sulfuric Acid Vapors. J. Mol. Struct. 1999, 509, 35−47. (7) Givan, A.; Larsen, L. A.; Loewenschuss, A.; Nielsen, C. J. Matrix Isolation Mid- and Far-Infrared Spectra of Sulfuric Acid and Deuterated Sulfuric Acid Vapors. J. Chem. Soc. Faraday Trans. 1998, 94, 827−835. (8) Rozenberg, M.; Loewenschuss, A. Matrix Isolation Infrared Spectrum of the Sulfuric Acid-Monohydrate Complex: New Assignments and Resolution of the “Missing H-Bonded n(OH) Band” Issue. J. Phys. Chem. A 2009, 113, 4963−4971. (9) Rozenberg, M.; Loewenschuss, A.; Nielsen, C. J. Complexes of Molecular and Ionic Character in the Same Matrix Layer: Infrared Studies of the Sulfuric Acid/Ammonia System. J. Phys. Chem. A 2011, 115, 5759−5766. (10) Rozenberg, M.; Loewenschuss, A.; Nielsen, C. J. H-Bonded Clusters in the Trimethylamine/Water System: A Matrix Isolation and Computational Study. J. Phys. Chem. A 2012, 116, 4089−4096. (11) Rozenberg, M.; Loewenschuss, A.; Nielsen, C. J. Trimethylamine/Sulfuric Acid/Water Clusters: A Matrix Isolation Infrared Study. J. Phys. Chem. A 2014, 118, 1004−1011. (12) Marsh, K. N., Ed. Recommended Reference Materials for the Realization of Physicochemical Properties; Blackwell: Oxford, U.K., 1987. (13) Majer, V.; Svoboda, V. Enthalpies of Vaporization of Organic Compounds: A Critical Review and Data Compilation; Blackwell Scientific Publications: Oxford, U.K., 1985. (14) Curtiss, L. A.; Frurip, D. J.; Blander, M. Studies of Molecular Association in H2O and D2O Vapors by Measurement of Thermal Conductivity. J. Chem. Phys. 1979, 71, 2703−2711. (15) Renner, T. A.; Kucera, G. H.; Blander, M. A Study of Hydrogen Bonding in Methanol Vapor by Measurement of Thermal Conductivity. J. Chem. Phys. 1977, 66, 177−184. (16) Joerg, S.; Drago, R. S.; Adams, J. Donor-acceptor and Polarity Parameters for Hydrogen Bonding Solvents. J. Chem. Soc., Perkin Trans. 2 1997, 2431−2438. (17) Kamlet, M. J.; Abboud, J-L. M.; Abraham, M. H.; Taft, R. W. Linear Solvation Energy Relationships. 23. A Comprehensive Collection of the Solvatochromic Parameters, π*, a and b, and Some Methods for Simplifying the Generalized Solvatochromic Equation. J. Org. Chem. 1983, 48, 2884. (18) Iogansen, A. V. Rule of Products of Acid-Base Functions of Molecules at their Association by H-bonds in CCl4 Solutions (Russ.). Theor. Experim. Khim. (USSR) 1971, 7, 302−311. (19) Gordon, A. J.; Ford, R. A. The Chemist’s Companion; Wiley: New York, 1972. (20) Barnes, A. J.; Hallam, H. E. Infrared Cryogenic Studies Part 4. Isotopically Substituted Methanols in Argon Matrices. Trans. Faraday Soc. 1970, 66, 1920−1928. (21) Coussan, S.; Roubin, P.; Perchard, J. P. Hydrogen Bonding In ROH:R’OH (R, R′ = H, CH3, C2H5OH) heterodimers: Matrix-
ASSOCIATED CONTENT
S Supporting Information *
Cartesian coordinates for monomers and complexes, results from B3LYP/aug-cc-pVTZ calculations (Table S1). Energies of complexation (Table S2). Observed and calculated wavenumbers of methanol isotopomers (Table S3). Calculated wavenumbers of the methanol dimer (Table S4). Calculated wavenumbers of the methanol/water complexes (Table S5). Calculated wavenumbers of the sulfuric acid/methanol complexes (Table S6). Calculated wavenumbers of the “linear” 1:1:1 CH3OH·H2SO4·H2O complexes (Table S7). Calculated wavenumbers of the “linear” 2:1 CH3OH·H2SO4·CH3OH complexes (Table S8). Calculated wavenumbers of the “cyclic” 1:1:1 H2SO4·CH3OH·H2O complexes (Table S9). Calculated wavenumbers of CH3OS(O)2OH (Table S10). Calculated wavenumbers of CH3OS(O)2OCH3 (Table S11). This material is available free of charge via the Internet at http://pubs.acs.org.
■
Article
AUTHOR INFORMATION
Corresponding Author
*C. J. Nielsen. E-mail: [email protected]. Fax: +47 2285 5441. Tel: +47 2285 5680. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work is supported by the Research Council of Norway through a Centre of Excellence Grant (Grant No. 179568/ V30). I
dx.doi.org/10.1021/jp505965z | J. Phys. Chem. A XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry A
Article
Dependent Structure, and Infrared-Induced Isomerization. J. Phys. Chem. A 2004, 108, 7331−7338. (22) Bakkas, N.; Bouteiller, Y.; Loutellier, A.; Perchard, J. P.; Racine, S. The Water-Methanol Complexes. I. A Matrix Isolation Study and an Ab Initio Calculation on the 1−1 Species. J. Chem. Phys. 1993, 99, 3335−3342. (23) Chaban, G. M.; Jung, J. O.; Gerber, R. B. Anharmonic Vibrational Spectroscopy of Hydrogen-Bonded Systems Directly Computed from ab Initio Potential Surfaces: (H2O)n, n = 2, 3; Cl−(H2O)n, n = 1, 2; H+(H2O)n, n = 1, 2; H2O-CH3OH. J. Phys. Chem. A 2000, 104, 2772−2779. (24) Fileti, E. E.; Canuto, S. Calculated Infrared Spectra of Hydrogen-Bonded Methanol-Water, Water-Methanol, and MethanolMethanol Complexes. Int. J. Quantum Chem. 2005, 104, 808−815. (25) Møller, C.; Plesset, M. S. Note on an Approximate Treatment for Many-Electron Systems. Phys. Rev. 1934, 46, 618−622. (26) Becke, A. D. Density-Functional Thermochemistry 3. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648−5652. (27) Lee, C.; Yang, W.; Parr, R. G. Development of the ColleSalvetty Correlation-Energy Formula into a Functional of the Electron-Density. Phys. Rev. B 1988, 37, 785−789. (28) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09; Gaussian, Inc.: Wallingford, CT, 2009. (29) Dunning, T. H., Jr. Gaussian-Basis Sets for Use in Correlated Molecular Calculations. 1. The Atoms Boron through Neon and Hydrogen. J. Chem. Phys. 1989, 90, 1007. (30) Kendall, R. A.; Dunning, T. H., Jr.; Harrison, R. J. ElectronAffinities of the 1st-Row Atoms Revisited - Systematic Basis-Sets and Wave-Functions. J. Chem. Phys. 1992, 96, 6796−6806. (31) Rozenberg, M.; Loewenschuss, A.; Marcus, Y. An Empirical Correlation between Stretching Vibration Redshift and Hydrogen Bond Length. Phys. Chem. Chem. Phys. 2000, 2699−2702. (32) Luck, W. A. P. The Importance of Cooperativity for the Properties of Liquid Water. J. Mol. Struct. 1998, 448, 131−142. (33) Parra, R. D.; Zeng, X. C. Hydrogen Bonding and Cooperative Effects in Mixed Dimers and Trimers of Methanol and Trifluoromethanol: An Ab Initio Study. J. Chem. Phys. 1999, 110, 6329−6338. (34) Perchard, J. P. Anharmonicity and Hydrogen Bonding. III. Analysis of the Near Infrared Spectrum of Water Trapped in Argon Matrix. Chem. Phys. 2001, 273, 217−233. (35) Borba, A.; Gómez-Zavaglia, A.; Simões, P. N. N. L.; Fausto, R. Matrix-Isolation FT-IR Spectra and Theoretical Study of Dimethyl Sulfate. Spectrochim. Acta, Part A 2005, 61A, 1461−1470.
J
dx.doi.org/10.1021/jp505965z | J. Phys. Chem. A XXXX, XXX, XXX−XXX
Hydrogen Bonding in the Sulfuric Acid−Methanol−Water System: A Matrix Isolation and Computational Study Mark Rozenberg,† Aharon Loewenschuss,† and Claus J. Nielsen*,‡ †
Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem, Israel 919040 Department of Chemistry, University of Oslo, 1033 Blindern, N-0315 Oslo, Norway
‡
S Supporting Information *
ABSTRACT: Frozen core MP2 and DFT computations were carried out on possible configurations of 1:1 H2SO4·CH3OH and 1:1:1 H2SO4·CH3OH·H2O complexes. Minimum energy structures, stabilization energies, H-bond lengths and vibrational frequencies were calculated. The latter complex can exist in either sequential “linear” configurations involving four Hbonds or “cyclic” structures involving three H-bonds only. However, there is little difference in the energy of stabilization between these two possible forms, indicating a “cooperative effect” between the H-bonds in the latter. This effect is also evidenced by the calculated H-bond lengths. In the cyclic complex, the hydroxyl of either CH3OH or H2O may be the proton donor to the H-bond between them. Argon matrix isolation FTIR spectra of layers with various concentration ratios were recorded. In the hydroxyl stretch wavenumber regions several weak new bands were observed. Their position was found to fit best the cyclic structures. The observed red shifts exceed the corresponding calculated values. Together with the considerable observed bandwidths they are further manifestations of the cooperative effect between the H-bonds. The lower skeletal mode wavenumber regions show a number of sharper bands compatible with those previously reported for dimethyl sulfate and hydrogen methyl sulfate, indicating their formation in the vapor mixing region or at the solid matrix layer interface.
■
INTRODUCTION Sulfuric acid and methanol are both substances of significant atmospheric importance. Sulfuric acid is an important component of atmospheric aerosols which in turn impacts the environment. The major sources of atmospheric sulfuric acid are volcanic SO2 emanations and industrial activities. Atmospheric methanol is formed predominantly by the decay of organic matter. Its atmospheric impact is primarily due to its interaction with sulfuric acid and the consequent formation of sulfate grains, which in turn serve as nucleation centers for cloud formation.1−4 The atmospheric complexation processes between atmospheric contaminations such as sulfuric acid, water and others, including hydrogen bonding formation, have been recently reviewed.5 The matrix isolation studies of complexes formed between sulfuric acid and several molecules of atmospheric interest have been reported by us.6−11 Here we study the interactions in the H2SO4/CH3OH/H2O system by matrix isolation spectroscopic methods and theoretical computations. The observed complexes may be precursors in the atmospheric sulfate and bisulfate cluster formation. From a more theoretical point of view, the interest in the interaction of sulfuric acid and methanol is due to the similarities between the latter and water in their similar tendency to form H-bonds. In the liquid phase of both, © XXXX American Chemical Society
hydrogen bonds have an essential role in the intermolecular forces. The vaporization enthalpy of water, with its four Hbonds per molecule in the liquid phase, is 40.65 kJ mol−1 (373 K).12 In comparison, for methanol the vaporization enthalpy is 35.21 kJ mol−1 (337.7 K),13 with two H-bonds per molecule in the liquid. The H-bond energies of water dimer and methanol dimer are 15.014 and 24.8 kJ mol−1 (estimated from the dissociation energy of the methanol tetramer15), respectively. Both liquids are also similar in their acid/base properties, as shown by various acidity scales. For example, the covalent proton donor parameters are, respectively, 1.15 and 0.86 and the proton acceptor parameters are 0.78 and 0.74.16 Similarly, in the compilation of Kamlet17 of hydrogen bonding donor acidities, water is slightly more acidic than methanol, whereas according to Iogansen18 the opposite order is suggested. Likewise, the pKa values of both are very close 15.7 and 16.0,19 with methanol thus being slightly more acidic. We reported the matrix isolation spectra of the H-bonded sulfuric acid/water monohydrated complex8 and have also Special Issue: Markku Räsänen Festschrift Received: June 16, 2014 Revised: September 10, 2014
A
dx.doi.org/10.1021/jp505965z | J. Phys. Chem. A XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry A
Article
Table 1. Calculated Structures, H-Bond Lengths, and Stabilization Energies (and Enthalpies, kJ mol−1) for Complexes Related to the H2SO4/CH3OH/H2O System
B
dx.doi.org/10.1021/jp505965z | J. Phys. Chem. A XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry A
Article
Table 1. continued
C
dx.doi.org/10.1021/jp505965z | J. Phys. Chem. A XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry A
Article
Table 1. continued
a From B3LYP/aug-cc-pVTZ calculations. calculations. dReference 31.
b
In angstroms. cComplexation energies (kcal mol−1) from BSSE corrected MP2/aug-cc-pVTZ
The water content of the matrix depends on the degree of predrying (P2O5) and prepumping and is not fully reproducible. It could, however, be monitored by the intensity of the pure water bands. Argon was introduced by flowing it over the acid sample. To achieve the desired vapor pressure, the sample was heated by irradiating it with a small electrical bulb. The degree of heating was determined by monitoring the recorded spectral intensities. Methanol/Ar mixtures in ratios from 1:40 to 1:200 were prepared by standard manometric techniques. Samples were deposited on a CsI window, with deposition rates ranging from 10 to 1000 mmol of Ar/h and deposition window temperatures in the 17−24 K range. Cooling was provided by an Air Product Displex model 202A closed cycle helium refrigerator. To protect the window, a pure Ar layer was deposited for several minutes before the first warming of the acid drop. Deposition temperatures were monitored by an Au0.007%Fe/Chromel thermocouple and controlled with an APDE temperature controller. Deposited samples were temperature cycled to up to 40 K. Spectra were recorded on a Bruker Equinox 55 FTIR spectrometer with a DTGS detector at a resolution of 0.5 cm−1 and generally coadding 128 scans.
shown that water may be involved in other hydrogen bonded complexes of sulfuric acid.10 In the present study we extend the investigation to the methanol involvement in complex formation with sulfuric acid along with the involvement of water molecules in this process. Similarly to water, methanol tends to form dimers in its vapor phase. In a mixed vapor phase both homogeneous and heterogeneous dimers may be formed due to the similarity in the proton donation properties of both molecules. In the water−methanol 1:1 mixed dimers both molecules can, in principle, assume the role of proton acceptor or proton donor. Both methanol−methanol and methanol−water dimers have been investigated experimentally20−22 and theoretically.23,24 These studies are relevant to the present work because, as we shall show, the mixed 1:1 methanol−water dimer is involved in the complex formation with sulfuric acid. We present computational results of the various conformations of the 1:1 sulfuric acid−methanol and the 1:1:1 sulfuric acid− methanol−water complexes along with recalculations of the two possible 1:1 methanol−water dimers so as to have all results on the same computational level and ensure valid comparisons. Experimentally, we observed several new bands not recorded for either of the matrix isolated pure compounds or in the spectra of matrix isolated “wet” sulfuric acid.8 Of these, the bands indicating the formation of new hydrogen bonded species will be discussed and related to the computational results. In addition, spectral features associated with the results of a chemical reaction of the vapors will be indicated and discussed.
■
ELECTRONIC STRUCTURE CALCULATIONS Frozen core MP225 and DFT calculations employing the Becke three-parameter26 and Lee−Yang−Parr27 (B3LYP) hybrid functional were carried out with the Gaussian 09 program.28 Dunning’s correlation-consistent aug-cc-pVXZ (X = D, T, Q) basis sets29,30 were employed in all calculations. The electronic structure calculations were carried out to obtain possible structures and configurations of the complexes and to assist in the spectral interpretation; the structures of all species considered are illustrated in Table 1; the Cartesian coordinates obtained in B3LYP/aug-cc-pVTZ calculations are presented in Table S1 in the Supporting Information. Complexation energies have been estimated as the energy of the complex minus the monomer energies, ΔEcomplex = E(A··· B···C) − E(A) − E(B) − E(C). The results are subsequently corrected for the basis set superposition error (BSSE) by the
■
EXPERIMENTAL SECTION Materials. Sulfuric acid (98%) was supplied by Frutarum, Israel. Analytical grade methanol was supplied by Aldrich. Argon gas (5.7 purity), supplied by AGA was used to produce the solid matrix layers. Sample Preparation. An acid drop was placed in a glass tube ending with a 2 mm pinhole and pumped for up to 100 h. Additional drying was attained by placing phosphorus pentoxide powder in the vapor path in front of the acid drop. D
dx.doi.org/10.1021/jp505965z | J. Phys. Chem. A XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry A
Article
counterpoise (CP) correction. The values obtained are also included in Table 1; energies obtained in B3LYP/aug-cc-pVTZ and MP2/aug-cc-pVTZ calculations are collected in Table S2 in the Supporting Information. It is noted that the counterpoise corrections are quite substantial in the MP2 calculations and that aug-cc-pVQZ basis set calculations are needed to bring the BSSE energies below 4 kJ mol−1. Vibrational wavenumbers and infrared intensities were obtained in the harmonic approximation and additional anharmonic vibrational wavenumbers were obtained directly from numerically derived cubic and quartic force constants as implemented in G09. The force fields were scaled according to 1/2 the procedure, Fscaled = Fcalc where αi and αj are scaling i,j i,j ·(αi·αj) parameters for the valence coordinates i and j, respectively. The scaling parameters are derived from fitting vibrational data of the monomeric compounds. The described frequency scaling has the advantage of being more mode specific than the application of a uniform scaling factor. The H2SO4 and D2SO4 matrix isolation spectra6 are reproduced within a few wavenumbers in both B3LYP and MP2 calculations employing six scaling factors. Similar agreements are obtained for the matrix isolation data for eight methanol isotopomers20 employing six scaling factors (Table S3 in the Supporting Information), and for the H2O matrix isolation data employing two scaling constants.11 These scaling factors were then applied to the various molecular complexes; unity scaling factors were applied to all interfragment valence coordinates. The harmonic and scaled wavenumbers of the complexes are collected in Tables S4−S9 in the Supporting Information. Dispersion forces are not described by the B3LYP functional, resulting in an underestimation of the interaction between the complexes studied. MP2 theory, on the other hand, overestimates the dispersion interaction. Incommensurable results may therefore be expected from the two methods. However, our previous results for various sulfuric acid complexes8−11 show that although the computed interaction energies differ, the scaling method outlined above aligns the B3LYP and MP2 calculated vibrational spectra to within a few wave numbers, which justifies our use of DFT methods for estimating the vibrational spectra of large molecular complexes for which MP2 calculations are unrealistic.
Figure 1. CH3OH·H2SO4 complex potential energy curve and barrier height of interconfigurational conversion.
■
Figure 2. 3520−3550 cm−1 range. New complex ν(OH) stretching mode bands are marked in red: A, a sulfuric acid/argon matrix layer; B, a methanol/argon (1:100) matrix layer; C, D, and E, matrix layers of H2SO4/CH3OH/H2O with successively increasing H2O contents. (Note growth of CH3OH-H2O dimer band, marked in cyan.)
RESULTS AND DISCUSSION The simplest sulfuric acid−methanol complex is of the 1:1 H2SO4·CH3OH configuration. Our calculations indicate two possible conformers, as shown in Table 1, with a minimal energy difference (less than 0.5 kJ mol−1) between them. Both involve two H-bonds in an analogous manner to the bonding in the H2SO4·H2O complex.8 The main H-bonding is generated by the “acidic” H atom of H2SO4 to the free electron pair of CH3OH, whereas a secondary hydrogen-bond is the result of the interaction between the hydroxyl H atom of CH3OH and the free electron pair of a sulfuric acid double-bonded oxygen. The barrier for conformational interconversion is relatively low, around 2.5 kJ mol−1; the potential is shown in Figure 1. It is reasonable to assume that similar potential barriers will result from scans of the potential surfaces of the other complexes discussed here. As the complexes studied involve two hydrophilic substances, on the one hand, and due to the natural presence of H2O in the sulfuric acid decomposition equilibrium, on the other, H2O molecules may be expected to get involved in the complex structures. Such has also been our previous experience.8 In the
present case, the simplest 1:1:1 sulfuric acid−methanol−water complex may assume two major configurationsa “linear” one, denoted as CH3OH·H2SO4·H2O or Me·Sa·Wa and two cyclic forms, denoted as H2SO4·CH3OH·H2O or Sa·Me·Wa and H2SO4·H2O·CH3OH or Sa·Wa·Me, respectively. The two cyclic forms have either the H2O or the CH3OH molecules as the proton acceptor from sulfuric acid. This bonding scheme also determines whether CH3OH or H2O is the proton donor to the H-bond between them, in the CH3OH·OH2 or HOH· O(H)CH3 moieties, respectively. This is very similar to the two bonding possibilities encountered for the 1:1 CH3OH·H2O complex.21 The possible structures described here are all shown in Table 1 along with their calculated H-bond lengths. While the four “linear” 1:1:1 complexes Me·Sa·Wa(n) have complexation energies spread of over 11 kJ mol−1, the spread for the “cyclic” configurations Sa·Wa·Me(n) is about 5 kJ mol−1. E
dx.doi.org/10.1021/jp505965z | J. Phys. Chem. A XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry A
Article
In spite of the different number of H-bonds involved in the cyclic and linear configurations, respectively, both forms are of similar stabilization energy values. This seeming discrepancy may be explained by a “cooperative effect” between the Hbonds in the cyclic configuration, resulting in shorter H-bond lengths and higher stabilization energy per bond. However, it is evident from a comparison between the experimentally observed “new” complex band positions and the calculated frequencies of both configurations, that the better agreement is obtained for the “cyclic” complexes. A possible explanation is that the formation of the “cyclic” complex occurs between a H2SO4 molecule and a CH3OH·H2O dimer (existing in the vapor) whereas the “linear” complex needs to be formed by two separate CH3OH and H2O molecules getting into the vicinity of the same H2SO4 molecule. From the experimental spectroscopic aspect, the H2SO4/ CH3OH/H2O ternary system poses a challenge where the identification of the complex bands is concerned. Even separately, all parent molecules produce a rich matrix isolation spectrum and therefore the spectral characterization of the complex species formed must rely on the appearance of new bands of low peak intensities, due to both the relatively low abundance of the complex and their greater bandwidths. These impediments also dictate the deposition of rather concentrated matrix layers and longer deposition times. Actually, an identifying feature of the bands assignable to complex species is their intensity dependence (relative to those of the parent molecules bands) on the deposition time and on the concentration ratios in the deposition mixtures (Figure 3). As may be expected, the frequency regions most indicative of new species formation are those where the hydroxyl groups of the parent molecules are active spectroscopically. Figure 2 compares five spectra of the 3520−3550 cm−1 wavenumber range. Curve A is the spectrum of a sulfuric acid/argon matrix layer, curve B is the spectrum of a methanol/argon (1:100) matrix layer, whereas curves C, D, and E show spectra of matrix layers of H2SO4/CH3OH/H2O with successively increasing H2O contents. Two new bands are identified in this region, which is dominated by bands of the bonded ν(OH) of the methanol dimer20,21 and of the CH3OH·H2O complex.22 The 3546 cm−1 band is rather prominent in its relative growth, whereas the lower frequency 3537 cm−1 band is almost overshadowed by the 3535 cm−1 band of the CH3OH·H2O dimer. Figure 3 illustrates the growth of bands attributable to OH stretching modes by deposition time. Trace A in Figure 3 is the spectrum of a sulfuric acid/argon matrix layer, trace B is the spectrum of a methanol/argon (1:100) matrix layer, whereas traces C and D show spectra of matrix layers of H2SO4/ CH3OH/H2O obtained after successively increasing deposition times. We consider the rather broad (∼50 cm−1) band, marked red in Figure 3 and centered at about 3020 cm−1, to represent a single spectral feature related to a strongly H-bonded hydroxyl, as further discussed in the text related to assignments. The effect of deposition time on the shape of this broad spectral feature supports the assertion that the complexes are formed at the vapor/solid interface. The thicker matrix layer may be of slightly higher temperature and allow more mobility of the molecules before their being frozen into the matrix. The relative growth of the sulfate bands in trace D (discussed below) is further evidence of this effect. The third spectral region of 2600−2770 cm−1 in which bands related to H-bonded hydroxyl groups were discerned, is shown
Figure 3. 2970−3055 cm−1 range: effect of increasing deposition times. New complex ν(OH) stretching mode bands are marked in red: A, a sulfuric acid/argon matrix layer; B, a methanol/argon (1:100) matrix layer; C, a matrix layer of H2SO4/CH3OH/H2O (18 min deposition time); D, a matrix layer of H2SO4/CH3OH/H2O (45 min deposition time).
Figure 4. 2570−2770 cm−1 range: effect of increasing amount of water in the matrix. New complex ν(OH) stretching mode bands are marked in red: A, a methanol/argon (1:100) matrix layer; B, a sulfuric acid/ H2O/argon matrix layer; C, a matrix layer of H2SO4/CH3OH/H2O; D, a matrix layer of H2SO4/CH3OH/H2O with higher H2O content.
The ternary complex “linear” bonding scheme H2O·H2SO4· CH3OH, has four possible configurations, which involve four H-bonds each. In comparison, the cyclic configurations have only three H-bonds each. Table 1 also shows the calculated stabilization energies for all considered structures. Stabilization enthalpies may also be estimated from their correlation with Hbond lengths.31 The results of this correlation, as applied to the calculated bond lengths, are also listed in Table 1 (the latter values are evaluated for room temperature). For the enthalpies obtained by correlation, the spread of values is in 0.5−5 kJ mol−1. F
dx.doi.org/10.1021/jp505965z | J. Phys. Chem. A XXXX, XXX, XXX−XXX
a
G
(111) (108) (112) (110)
(105) (103) (114) (104)
3575 3577 3577 3578
3575 3576 3582 3576
See Table 1 for structure nomenclature.
Sa·Me·Wa1 Sa·Me·Wa2 Sa·Me·Wa3 Sa·Me·Wa4 observed Sa·Wa·Me1 Sa·Wa ·Me2 Sa·Wa·Me3 Sa·Wa·Me4 observed
Me·Sa·Me3
Me·Sa·Me2
3577 (117) 3577 (121)
Sa·Me1 Sa·Me2 Me·Sa·Wa1 Me·Sa·Wa2 Me·Sa·Wa3 Me·Sa·Wa4 Me·Sa·Me1
3606 (138) 3613 (149)
3684 (44)
Me·Wa1 Me·Wa2
ν(OH)free (CH3OH)
3683 (43)
ν(OH)free (H2SO4)
Me·Me
structurea 3530 (517) 3535.2, 3540.9
ν(MeOH···O(Me)
3634 (7)s 3728 (83)as
ν(OH) (H2O)
3689 3695 3692 3692
3701 3706 3704 3700
3699 3699 3698 3698
(87) (78) (80) (83)
(108) (111) (113) (115)
(111) (112) (117) (118)
3707 (83)
ν(OH)free (H2O)
(1024) (1039) (834) (1062)
2572 (2094) 2560 (2175) 2520 (2294) 2540 (2192) 2744
3008 3006 3014 3012
ν(SOH···OH2)
2820 (1822) 2814 (1814) 2848 (1990) 2842 (2062) 2909 (1698) 2839 (2088) 2884 (240)s 2855 (3260)as 2884 (154)s 2854 (3446)as 2884 (35)s 2853 (3640)as 2344 (2743) 2393 (2647) 2395 (2708) 2358 (2688) 2675, 2615
ν(SOH···O(Me))
3545 (128) 3548 (127) 3543 (132) 3545 (134) 3589 (192)s 3587 (150)as 3588 (152)s 3585 (200)as 3588 (79)s 3568 (286)as 3468 (522) 3477 (515) 3472 (530) 3474 (522) 3537 3527 (586) 3528 (595) 3521 (606) 3527 (583) 3546
ν(OH···OS)
Table 2. Scaled Calculated (MP2/aug-cc-pVTZ) and Observed Frequencies for Complexes Related to the H2SO4/CH3OH/H2O System
3131 (990) 3169 (913) 3162 (917) 3132 (992) 2996.0
3492 (426)
ν(OH)H2O···O(Me)
3254 (840) 3281 (788) 3276 (806) 3255 (831) 3016.6
3560 (421) 3535.2, 3537.1
ν(OH)CH3OH···OH2
The Journal of Physical Chemistry A Article
dx.doi.org/10.1021/jp505965z | J. Phys. Chem. A XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry A
Article
position of the four Sa·Me·Wa complexes is 2371 cm−1. The difference of 177 cm−1 is not far from the wavenumber difference between the observed bands at 2615 and 2675 cm−1 and the band at 2744 cm−1 (Table 2). Although it is rather clear that all three bands are due to the H-bonded hydroxyl of sulfuric acid, their similar wavenumber values do not allow a confident assignment to a certain configuration of the cyclic 1:1:1 sulfuric acid/methanol/water complex. Table 2 also lists the calculated frequencies for the Me·Me dimer and Me·Wa heterodimer. Their calculated ν(OH) wavenumber values are very similar. The observed values are similarly close, both in this and in previous studies21 and thus do not allow a confident distinction in their assignment. The second strongest H-bonded hydroxyl group is between the two possible configurations of the CH3OH/H2O hydrogen bonded moieties, namely CH3OH·OH2 and H2O·O(H)CH3 in the respective 1:1:1 H2SO4·CH3OH·H2O and H2SO4·H2O· CH3OH complexes. As seen in Table 2 and Figure 3, this spectral feature centered at 3016 cm−1, is shifted to lower frequencies than those predicted by the calculations, as discussed in the computational section. We have previously encountered such underestimations of the H-bonding induced red shifts by theoretical calculations.8−11 The observed red shift of this broad band places it also at a considerably lower frequency than the analogous bands in the 1:1 methanol/water complexes at 3537 cm−1 (Figure 2) as assigned.21,22 This additional red shift is attributed to the “cooperative effect”32,33 between the three H-bonds of these closed cycle configurations. The actual width of this band is difficult to estimate, as its position coincides with that of the very strong ν(CH3) modes of the parent methanol molecules. However, it is of considerable width, which may also be due to the cooperative effect. The weakest and least shifted ν(O−H) mode bands are those between “free” oxygen electrons of a sulfuric acid SO bond and a hydroxyl hydrogen of either methanol or water, in the H2SO4·H2O·CH3OH and H2SO4·CH3OH·H2O complexes, respectively The two bands, at 3536 and 3547 cm−1, marked in red in Figure 2, are assigned to this mode. The 3536 cm−1 band is shifted by 130 cm−1 from the ν(OH) band of free matrix isolated methanol at 3666 cm−1 (this work). The 3547 cm−1 band is red-shifted by 140 cm−1 from the mean position of the ν(OH) modes of matrix isolated H2O molecules at 3687 cm−1.34 This latter shift is also considerably higher than that of the analogous band in 1:1 H2SO4·H2O complex at 3582 cm−1.8 This larger shift is, again, attributed to the “cooperative effect” with the other H-bonds of this configuration. Table 2 shows the calculated frequency values for these modes and, again, the computed values underestimate the experimental red shift. However, the calculated frequencies retain the order of the above band assignments. In addition to the bands attributed to hydrogen bonded complex formation in the ν(OH) region, the skeletal mode range shows additional new bands. This is illustrated in Figure 5, in which trace A is the spectrum of methanol/argon (1:100) matrix layer, and trace B is the spectrum of sulfuric acid/argon matrix layer. Trace C shows the spectrum of a matrix layer of H2SO4/CH3OH/H2O, in which reaction product bands marked with wavenumber values. These were noted to increase at long deposition time as compared to the ν(OH) complexation related bands and even relative to the pure methanol bands. These bands fit well the positions of bands of matrix
Figure 5. 780−1480 cm−1 skeletal mode range: A, a methanol/argon (1:100) matrix layer (red curve); B, a sulfuric acid/argon matrix layer (blue curve); C, a matrix layer of H2SO4/CH3OH/H2O (balck curve). Reaction product bands are marked with wavenumber values.
Table 3. Matrix Isolation Bands Resulting from H2SO4/ CH3OH Reactions band due to H2SO4/CH3OH/Ar depositiona
dimethyl sulfate bandsb
methyl hydrogen sulfate 3614.4c
2973.2 1427.7 1212.3 1017.7 855.5 785.6 581.1
2968.0 1415.5 1216.0 1025.1 821.9 755.8 597.9
1020d 790d
assignments ν(OH)c ν(CH3)b ν(SO)asb ν(SO)sb ν(CO)s ν(CO)asb ν(SO)sb γ(OSO)b
a
This work. New bands, not observed for parent molecules. bAr matrix. Reference 35. cAr matrix. Reference 9. dSulfuric acid solution. Reference 3.
in Figure 4, illustrating the effect of increasing the amount of water in the matrix. Trace A is the spectrum of a methanol/ argon (1:100) matrix layer, trace B is the spectrum of sulfuric acid/H2O/argon matrix layer. Trace C is the spectrum of a matrix layer of H2SO4/CH3OH/H2O, and the spectrum shown in trace D is obtained with higher water content. The most prominent bands at 2722 and 2593 cm−1, clearly visible in trace B of Figure 4, coincide with the position of the highly redshifted ν(O−H) bands of sulfuric acid, H-bonded to the oxygen of H2O, in the H2SO4·H2O monohydrate complex.8 In addition, three new peaks of intermediate width (10−15 cm−1) are identified at 2744, 2675, and 2615 cm−1. An additional band at 2632 cm−1 is too sharp to be assigned to a fundamental of an H-bonded moiety and is probably due to an overtone or combination band of methanol or dimethyl sulfate, also formed during deposition and further discussed below. Three observed peaks, marked in red in traces C and D of Figure 4, are therefore also assigned to the ν(OH) mode of H2SO4, H-bonded to the hydroxyl oxygen of either CH3OH or H2O in the cyclic ternary sulfuric acid/methanol/water complex. Possibly the highest wavenumber band in this region (2744 cm−1) is due to the SO−H···OH2 stretch of the 1:1:1 H2SO4·H2O·CH3OH and the lowest one at 2615 cm−1 is due to the H-bonding of the sulfuric acid hydroxyl to the somewhat more acidic methanol in the H2SO4·CH3OH·H2O complexes. The average of the calculated frequencies of the four Sa·Wa·Me complexes is 2548 cm−1. Similarly, the average calculated H
dx.doi.org/10.1021/jp505965z | J. Phys. Chem. A XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry A
■
isolated dimethyl sulfate, as reported by Borba et al.,35 Table 3 (calculated wavenumbers given in Table S11 in the Supporting Information). Several of them are also close to the position of some bisulfate bands,2,3 but the ν(OH) band of the latter was not observed in our experiments. Often the H-bonded species are formed by migration in the matrix of the lighter parent molecules. The H2SO4/CH3OH reaction involves high molecular mass molecules, unlikely to diffuse in the cold matrix layer. It must be concluded that here this reaction occurs in the vapor phase or at the interface between the deposition vapor and the solid layer, where the gas jets mix just before being frozen into the matrix. The formation of methyl hydrogen sulfate as a result of the reaction of sulfuric acid methanol was indeed observed in condensed phases and at the interface of vapor and liquid phases.2−4 Dimethyl sulfate is considerably less volatile than either methanol or hydrogen methyl sulfate and its formation here may indicate that it is also formed under atmospheric conditions and thus playing the role of nucleation centers in cloud formation. In summary, we have shown that matrix isolation layers produced from water/sulfuric acid/Ar and CH3OH/H2O mixtures show new bands in the ν(OH) region assignable to a 1:1:1 H2SO4/CH3OH/H2O complex of cyclic configuration. The red shifts and calculated stabilization energies and bond lengths indicate a cooperative effect between the H-bonds of this complex. In addition, we observed skeletal mode bands assignable to dimethyl sulfate (and possibly hydrogen methyl sulfate) in the vapor phase or at its interface with the matrix layer.
■
REFERENCES
(1) Schobesberger, S.; Junninen, H.; Bianchi, F.; Lönn, G.; Ehn, M.; Lehtipalo, K.; Dommen, J.; Ehrhart, I.; Ortega, K.; Franchin, A.; et al. Molecular Understanding of Atmospheric Particle Formation from Sulfuric Acid and Large Oxidized Organic Molecules. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 17223−17228. (2) Van Loon, L. L.; Allen, H. C. Methanol Reaction with Sulfuric Acid: A Vibrational Spectroscopic Study. J. Phys. Chem. A 2004, 108, 17666−17674. (3) Van Loon, L. L.; Allen, H. C. Uptake and Surface Reaction of Methanol by Sulfuric Acid Solutions Investigated by Vibrational Sum Frequency Generation and Raman Spectroscopies. J. Phys. Chem. A 2008, 112, 7873−7880. (4) Van Loon, L. L.; Allen, H. C. Effective Diffusion Coefficients for Methanol in Sulfuric Acid Solutions Measured by Raman Spectroscopy. J. Phys. Chem. A 2008, 112, 10758−10763. (5) Klemperer, W.; Vaida, V. Molecular Complexes in Close and Far Away. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 10584−10588. (6) Givan, A.; Larsen, L. A.; Loewenschuss, A.; Nielsen, C. J. Matrix Isolation Mid- and Far-Infrared Spectra of Sulfuric Acid and Deuterated Sulfuric Acid Vapors. J. Mol. Struct. 1999, 509, 35−47. (7) Givan, A.; Larsen, L. A.; Loewenschuss, A.; Nielsen, C. J. Matrix Isolation Mid- and Far-Infrared Spectra of Sulfuric Acid and Deuterated Sulfuric Acid Vapors. J. Chem. Soc. Faraday Trans. 1998, 94, 827−835. (8) Rozenberg, M.; Loewenschuss, A. Matrix Isolation Infrared Spectrum of the Sulfuric Acid-Monohydrate Complex: New Assignments and Resolution of the “Missing H-Bonded n(OH) Band” Issue. J. Phys. Chem. A 2009, 113, 4963−4971. (9) Rozenberg, M.; Loewenschuss, A.; Nielsen, C. J. Complexes of Molecular and Ionic Character in the Same Matrix Layer: Infrared Studies of the Sulfuric Acid/Ammonia System. J. Phys. Chem. A 2011, 115, 5759−5766. (10) Rozenberg, M.; Loewenschuss, A.; Nielsen, C. J. H-Bonded Clusters in the Trimethylamine/Water System: A Matrix Isolation and Computational Study. J. Phys. Chem. A 2012, 116, 4089−4096. (11) Rozenberg, M.; Loewenschuss, A.; Nielsen, C. J. Trimethylamine/Sulfuric Acid/Water Clusters: A Matrix Isolation Infrared Study. J. Phys. Chem. A 2014, 118, 1004−1011. (12) Marsh, K. N., Ed. Recommended Reference Materials for the Realization of Physicochemical Properties; Blackwell: Oxford, U.K., 1987. (13) Majer, V.; Svoboda, V. Enthalpies of Vaporization of Organic Compounds: A Critical Review and Data Compilation; Blackwell Scientific Publications: Oxford, U.K., 1985. (14) Curtiss, L. A.; Frurip, D. J.; Blander, M. Studies of Molecular Association in H2O and D2O Vapors by Measurement of Thermal Conductivity. J. Chem. Phys. 1979, 71, 2703−2711. (15) Renner, T. A.; Kucera, G. H.; Blander, M. A Study of Hydrogen Bonding in Methanol Vapor by Measurement of Thermal Conductivity. J. Chem. Phys. 1977, 66, 177−184. (16) Joerg, S.; Drago, R. S.; Adams, J. Donor-acceptor and Polarity Parameters for Hydrogen Bonding Solvents. J. Chem. Soc., Perkin Trans. 2 1997, 2431−2438. (17) Kamlet, M. J.; Abboud, J-L. M.; Abraham, M. H.; Taft, R. W. Linear Solvation Energy Relationships. 23. A Comprehensive Collection of the Solvatochromic Parameters, π*, a and b, and Some Methods for Simplifying the Generalized Solvatochromic Equation. J. Org. Chem. 1983, 48, 2884. (18) Iogansen, A. V. Rule of Products of Acid-Base Functions of Molecules at their Association by H-bonds in CCl4 Solutions (Russ.). Theor. Experim. Khim. (USSR) 1971, 7, 302−311. (19) Gordon, A. J.; Ford, R. A. The Chemist’s Companion; Wiley: New York, 1972. (20) Barnes, A. J.; Hallam, H. E. Infrared Cryogenic Studies Part 4. Isotopically Substituted Methanols in Argon Matrices. Trans. Faraday Soc. 1970, 66, 1920−1928. (21) Coussan, S.; Roubin, P.; Perchard, J. P. Hydrogen Bonding In ROH:R’OH (R, R′ = H, CH3, C2H5OH) heterodimers: Matrix-
ASSOCIATED CONTENT
S Supporting Information *
Cartesian coordinates for monomers and complexes, results from B3LYP/aug-cc-pVTZ calculations (Table S1). Energies of complexation (Table S2). Observed and calculated wavenumbers of methanol isotopomers (Table S3). Calculated wavenumbers of the methanol dimer (Table S4). Calculated wavenumbers of the methanol/water complexes (Table S5). Calculated wavenumbers of the sulfuric acid/methanol complexes (Table S6). Calculated wavenumbers of the “linear” 1:1:1 CH3OH·H2SO4·H2O complexes (Table S7). Calculated wavenumbers of the “linear” 2:1 CH3OH·H2SO4·CH3OH complexes (Table S8). Calculated wavenumbers of the “cyclic” 1:1:1 H2SO4·CH3OH·H2O complexes (Table S9). Calculated wavenumbers of CH3OS(O)2OH (Table S10). Calculated wavenumbers of CH3OS(O)2OCH3 (Table S11). This material is available free of charge via the Internet at http://pubs.acs.org.
■
Article
AUTHOR INFORMATION
Corresponding Author
*C. J. Nielsen. E-mail: [email protected]. Fax: +47 2285 5441. Tel: +47 2285 5680. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work is supported by the Research Council of Norway through a Centre of Excellence Grant (Grant No. 179568/ V30). I
dx.doi.org/10.1021/jp505965z | J. Phys. Chem. A XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry A
Article
Dependent Structure, and Infrared-Induced Isomerization. J. Phys. Chem. A 2004, 108, 7331−7338. (22) Bakkas, N.; Bouteiller, Y.; Loutellier, A.; Perchard, J. P.; Racine, S. The Water-Methanol Complexes. I. A Matrix Isolation Study and an Ab Initio Calculation on the 1−1 Species. J. Chem. Phys. 1993, 99, 3335−3342. (23) Chaban, G. M.; Jung, J. O.; Gerber, R. B. Anharmonic Vibrational Spectroscopy of Hydrogen-Bonded Systems Directly Computed from ab Initio Potential Surfaces: (H2O)n, n = 2, 3; Cl−(H2O)n, n = 1, 2; H+(H2O)n, n = 1, 2; H2O-CH3OH. J. Phys. Chem. A 2000, 104, 2772−2779. (24) Fileti, E. E.; Canuto, S. Calculated Infrared Spectra of Hydrogen-Bonded Methanol-Water, Water-Methanol, and MethanolMethanol Complexes. Int. J. Quantum Chem. 2005, 104, 808−815. (25) Møller, C.; Plesset, M. S. Note on an Approximate Treatment for Many-Electron Systems. Phys. Rev. 1934, 46, 618−622. (26) Becke, A. D. Density-Functional Thermochemistry 3. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648−5652. (27) Lee, C.; Yang, W.; Parr, R. G. Development of the ColleSalvetty Correlation-Energy Formula into a Functional of the Electron-Density. Phys. Rev. B 1988, 37, 785−789. (28) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09; Gaussian, Inc.: Wallingford, CT, 2009. (29) Dunning, T. H., Jr. Gaussian-Basis Sets for Use in Correlated Molecular Calculations. 1. The Atoms Boron through Neon and Hydrogen. J. Chem. Phys. 1989, 90, 1007. (30) Kendall, R. A.; Dunning, T. H., Jr.; Harrison, R. J. ElectronAffinities of the 1st-Row Atoms Revisited - Systematic Basis-Sets and Wave-Functions. J. Chem. Phys. 1992, 96, 6796−6806. (31) Rozenberg, M.; Loewenschuss, A.; Marcus, Y. An Empirical Correlation between Stretching Vibration Redshift and Hydrogen Bond Length. Phys. Chem. Chem. Phys. 2000, 2699−2702. (32) Luck, W. A. P. The Importance of Cooperativity for the Properties of Liquid Water. J. Mol. Struct. 1998, 448, 131−142. (33) Parra, R. D.; Zeng, X. C. Hydrogen Bonding and Cooperative Effects in Mixed Dimers and Trimers of Methanol and Trifluoromethanol: An Ab Initio Study. J. Chem. Phys. 1999, 110, 6329−6338. (34) Perchard, J. P. Anharmonicity and Hydrogen Bonding. III. Analysis of the Near Infrared Spectrum of Water Trapped in Argon Matrix. Chem. Phys. 2001, 273, 217−233. (35) Borba, A.; Gómez-Zavaglia, A.; Simões, P. N. N. L.; Fausto, R. Matrix-Isolation FT-IR Spectra and Theoretical Study of Dimethyl Sulfate. Spectrochim. Acta, Part A 2005, 61A, 1461−1470.
J
dx.doi.org/10.1021/jp505965z | J. Phys. Chem. A XXXX, XXX, XXX−XXX
