Water Clusters - American Chemical

Article pubs.acs.org/JPCA

Trimethylamine/Sulfuric Acid/Water Clusters: A Matrix Isolation Infrared Study Mark Rozenberg and Aharon Loewenschuss* Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel

Claus J. Nielsen Centre for Theoretical and Computational Chemistry, Department of Chemistry, University of Oslo, 1033 Blindern, N-0315 Oslo, Norway S Supporting Information *

ABSTRACT: In continuation of our studies of sulfuric acid H-bonded complexes of atmospheric relevance we report the infrared spectra of the matrix isolated complexes formed between trimethylamine and sulfuric acid. Evidence for proton transfer was anticipated for the present system, as trimethylamine ((CH3)3N) is of strong basic nature. However, the spectra of this system are complicated by the inevitable presence water in the vapor and in the matrix, resulting in matrix layers containing three species capable of forming H-bonded complexes. The complex formed between trimethylamine and sulfuric acid is of ionic character due to proton transfer of the H+ proton from sulfuric acid to (CH3)3N to form a new N−H bond and the replacement of the intramolecular O−H bond in H2SO4 by a strong intermolecular N−H···O hydrogen bond. The complex is further stabilized by hydration. The skeletal modes show clear bisulfate related bands and are only slightly affected by hydration. The ν(OH) region shows a rich band scheme, best explained by a structure involving (at least) three H2O molecules. A broad spectral feature spanning the 1700−500 cm−1 is assigned, in analogy to previous studies to a double-well potential quasi-symmetric, Zundel-like, ionic species with a (CH3)3−N··· H+···N−(CH3)3 configuration. A band in the skeletal SO stretch spectral region may be assigned to hydrated sulfate as its counterion.

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INTRODUCTION Atmospheric aerosols have important environmental effects and influence on global climate. Sulfuric acid and its bisulfate and sulfate ions, hydrogen bonded to water molecules, form molecular clusters that are notable constituents of aerosols. The significance of these H-bonded clusters for atmospheric chemistry has been addressed by Klemperer and Vaida.1 Quantum mechanical studies of the role of sulfuric acid hydration in atmospheric particle formation have recently been reported by Temelso et al.2 The significance of bisulfate ion and its hydration products in aerosol formation was shown.3,4 However, clusters of sulfuric acid only are not stable enough, so that atmospheric particle formation involves interactions with additional molecular species. Water, ammonia, and stronger bases, such as amines, have been suggested to stabilize sulfuric acid clusters and thus participate in aerosol formation. The importance of amines for atmospheric particle formation has recently become evident from modeling of field observations5 and from controlled experiments in the CLOUD chamber at CERN.6 Sources of atmospheric amines have been discussed by Ge et al.7 © 2014 American Chemical Society

Bisulfate ion and its hydration and their importance for aerosol formation were studied by Husar et al.3 In the liquid phase, Raman frequencies of bisulfate ion in concentrated sulfuric acid have been recorded.8 In the solid, the spectra of ionic RbHSO4 crystal were reported.9 The newer method of gas phase vibrational IRMPD spectra was used for the study of the bisulfate anion/water10 and the bisulfate/sulfuric acid/water11 systems. The structure and spectral properties of the bisulfate ion in water solution were simulated by ab initio methods.4 Spectral properties of strong H-bonded complexes between trimethylamine (TMA) as proton acceptor and substituted phenols and organic acids have been studied12,13 and complexes involving essentially full proton transfer reported.14,15 The new (not observed for the parent moieties), broad bands assigned to the ν(N−H+···X) mode at 2500 cm−1 and to the δ(N−H+···X) deformation modes at 1400−1300 cm−1 along with red shift of the ν(C3N+) bands were interpreted to be the results of TMA Received: November 6, 2013 Revised: January 12, 2014 Published: January 27, 2014 1004

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Figure 1. FTIR spectra of trimethylamine (TMA)/sulfuric acid vapors (and water impurities) matrix isolated in solid argon: the 1300−1000 cm−1 region (skeletal mode region). New complex bands are marked in red. *, see text. (A) (CH3)3N/Ar (approximately 1:200) layer. (B) H2SO4/Ar layer. (C) (CH3)3N/H2SO4/Ar layer, as deposited at 18 K. (D) (CH3)3N/H2SO4/Ar layer, after annealing to 40 K, recooled to 18 K. Insert: Comparison of spectrum D with simulation from calculated band positions and intensities.

with ionic character, involving the appearance of a distinct HSO4− moiety in their structure. We also show some evidence for the formation of a Zundel-like ion with two trimethylamine molecules flanking a proton with hydrated bisulfate as counterion.

protonization. The latter was confirmed by molecular orbital calculations, also showing C−N bonds lengthening.16 Similar band positions were found in (CH3)3N absorbed on zeolites.17 The ν(N−H+) frequency of gas phase and surface attached R3NH+ were computationally predicted as 3300 and 2820 cm−1, respectively.18 In our previous studies we were able to characterize matrix isolated H-bonded systems of sulfuric acid ranging from weakly bonded covalent complexes19,20 to the more strongly bonded complex of sulfuric acid and water21 and to the sulfuric acid/ ammonia system.22 In the latter system, a very strongly H-bonded complex with evidence of its being a complex of essentially ionic character was identified. Even more pronounced evidence for proton transfer was anticipated for the present sulfuric acid/trimethylamine (CH3)3N) system, the latter being of stronger basic nature. However, the study of the present system is complicated by the inevitable presence of water in the vapor (due to chemical equilibrium) and in the matrix, with the result that the matrix layers contained three species capable of forming H-bonded complexes. The ensuing interaction of sulfuric acid, trimethylamine, and water molecules was expected to form even larger complexes, which may be of more significance to atmospheric cluster formation. Our previous matrix isolation study of the binary trimethylamine/water system23 was essential in identifying the new bands assignable to this three component system. These new bands and their bonding and structural implications will be discussed according to the general frequency regions in which they were observed. Given the increasing basicity in going from H2O to NH3, to (CH3)3N, it was expected that the proton donor−proton acceptor interaction will be strong enough to affect an almost full proton transfer, similar to the gas phase ion formation found in the CLOUD chamber experiments.6 It will be shown below that the spectra, backed by quantum mechanical calculations, indeed indicate formation of a complex

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EXPERIMENTAL SECTION Materials. Sulfuric acid (98%) was supplied by Frutarum, Israel. Deuterated sulfuric acid of 98% purity and 99% isotopic enrichment was supplied by Aldrich. Anhydrous TMA (Analytical grade) 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. 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. TMA/Ar mixture in ratios from 1:40 to 1: 400 were prepared by standard manometric techniques. Samples were deposited on a CsI window, with deposition rates ranging from 10 to 1000 mM 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 Au-0.007%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 1005

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Figure 2. Calculated structure (B3LYP-PVTZ) of the (CH3)3N*H2SO4*3H2O complex.

were then applied to the various molecular and ionic complexes; unity scaling factors were applied to all interfragment valence coordinates. Dispersion forces are not described by the B3LYP functional resulting in an underestimation of the interaction between the complexes studied. MP2 theory, however, overestimates the dispersion interaction. Incommensurable results may therefore be expected from the two methods. However, our previous results for various sulfuric acid complexes19−22 show that although the computed interaction energies differ, the scaling method outline above aligns the B3LYP and MP2 calculated vibrational spectra to within a few wavenumbers, which justifies our use of DFT methods for estimating the vibrational spectra of large molecular complexes for which the MP2 method is unrealistic. 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 result is subsequently corrected for the basis set superposition error (BSSE) by the counterpoise (CP) correction.31,32 The values obtained are collected in Table S4 (Supporting Information).

generally coadding 128 scans. With the experimental procedures described, a high level of reproducibility of the spectral results could be attained.

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ELECTRONIC STRUCTURE CALCULATIONS Frozen core MP224 and DFT calculations employing the Becke 3 parameter25 and Lee−Yang−Parr26 (B3LYP) hybrid functional were carried out with the Gaussian 09 program.27 Dunning’s correlation-consistent aug-cc-pVXZ (X = D, T, Q) basis sets28,29 were employed in all calculations. The electronic structure calculations were carried out to assist in the spectral interpretation; the results for all species considered are summarized in the Supporting Information. 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 the procedure, Fscaled = i,j 1/2 where αi and αj are scaling parameters for the Fcalc i,j (αi·αj) 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 spectra19 are reproduced within a few wavenumbers in both B3LYP and MP2 calculations employing six scaling factors (Table S1 in the Supporting Information). Similar agreements are obtained for the (CH3)3N and (CD3)3N gas phase data30 employing five scaling factors (Table S2 in the Supporting Information), and for the H2O matrix isolation data employing two scaling constants (Table S3 in the Supporting Information). These scaling factors

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RESULTS AND DISCUSSION Because of the complexity of the system studied, we shall discuss our results in the order of the confidence we attribute to our conclusions, rather than in the order of frequency ranges. The skeletal mode range has the clearest indications of an essentially complete proton transfer. It clearly shows bands assignable to the ionic HSO4− moiety and thus indicating that the proton donor (H2SO4)−proton acceptor ((CH3)3N) interaction is strong enough to affect an almost complete proton transfer. The spectral region, where both the skeletal SO mode stretches and the 1006

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Figure 3. FTIR spectra of trimethylamine (TMA)/sulfuric acid vapors (and water impurities) matrix isolated in solid argon: the ν(O−H/N−H) mode region. (A) (CH3)3N/Ar (approximately 1:200) layer. (B) (CH3)3N/H2O/Ar layer. (C) H2SO4 /Ar layer. (D) (CH3)3N/H2SO4/Ar layer, as deposited at 18 K. (E) (CH3)3N/H2SO4/Ar layer, after annealing to 40 K, recooled to 18 K. Insert: Comparison of spectrum D with simulation from calculated band positions and intensities.

comparison, in our previous study of H2SO4/NH3 complexation,22 an analogous band was recorded at 1359.6 cm−1. We, therefore, assign the 1327 and 1316.2 cm−1 peaks to these antisymmetric SO stretching modes. The much weaker and broader shoulder at 1331.2 cm−1 may be due to effects of additional neighboring H2O molecules. Deuteration of the sulfuric acid removes the mode coupling, producing somewhat shifted bands at 1314.3 and 1268 cm−1, now assigned to the pure contribution of the antisymmetric SO stretches of the DSO4− moiety, which should be almost insensitive to deuteration. The new bands at 1203.8 and 1198.3 cm−1 (essentially unaffected by deuteration) are assigned to the symmetric SO stretch mode strongly coupled to the S−O−H bend. They show a simultaneous growth due to annealing with the 1327.0 cm−1 peak, described above. The observed splitting may, again, be due to interactions with neighboring water molecules. The calculations place this mode at 1199 cm−1 and at lower frequency of 1054.0 cm−1 for the HSO4−*H2O complex, and at 1034 cm−1 for the HSO4−*3H2O anion. For the ionic RbHSO4 crystal, the reported frequency is 1220 cm−1.9 The above assignment is also supported by the analogous band for the NH3*H2SO4 complex, recorded at 1246.5 cm−1.22 A weak band emerging at 1444.4 cm−1 (Figure 1, insert) is assigned to the ν(S−O−H) bending mode in the hydrated bisulphate ionic moiety of the complex shown in Figure 2. Calculations place the corresponding frequency at 1418 cm−1 (scaled).

S−O−H deformation modes are active, is reproduced in Figure 1. The new bands, not observed in the reference matrix spectra of the pure components and regarded as specific to the complexation process, are marked in red. The most prominent new spectral features are the emergence of two new relatively sharp bands at 1327 cm−1 (with a shoulder at 1331 cm−1) and 1316.2 cm−1. They drastically increase in both, intensity and sharpness, with matrix annealing (traces C and D). For a free HSO4− ion, our calculations place bands at 1287 and 1127 cm−1 for the SO antisymmetric stretch modes and rather intense band at 1036 cm−1 representing mainly the S−O−H bend mixed with the symmetric S−O stretch. For the monohydrated bisulfate ion, the corresponding calculated frequencies increase to 1366, 1277, and 1199 cm−1, respectively. For a trihydrated HSO4−*(H2O)3 ion, the calculated frequencies are 1318, 1233, and 1200 cm−1 (all, scaled values). The spectral features in the ν(O−H) region, as discussed below, indicate the formation of a complex cluster as shown in Figure 2. For the antisymmetric SO stretches of this complex, our calculations predict two bands at 1247 cm−1 (1199 cm−1, unscaled) and 1163 cm−1 (1143 cm−1, unscaled). The full calculation results are listed in the Supporting Information. In gas phase IRMPD studies10 of various HSO4− hydrates, a corresponding band was observed at 1321 cm−1 and assigned mainly to the S−O−H deformation. Calculations there indicated that this band fits best a configuration of HSO4− ion surrounded by three H2O molecules, a situation also found in the structure of the (CH3)3N*H2SO4*3H2O in Figure 2 as suggested by us. For 1007

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Table 1. Calculated (B3LYP/aug-cc-pVTZ, scaled) vs. Experimental Frequencies [cm−1] of Trimethyl Amine+ and Hydrated Bisulfate Ion and Their Complexes ν(O−H)

species HSO4

−

HSO4−*H2O

HSO4−*(H2O)3

(CH3)3NH+ H2SO4*(CH3)3N [(CH3)3NH+ HSO4−]*[H2O] [(CH3)3NH+ HSO4−]*[H2O]3

[(CH3)3NH+ HSO4−]*[H2O]3 exptl

a

3612 (26) 3693(34) 3504 (271) 3142 (890) 3417 (534)

3606 (97) 3660 (83) 3377 (613) 3193 (649) 3709 (14) 3703 (121) 3697 (89) 3432 (299) 3417 (638) 3392 (597) 3153 (853) 3593.5 3560.7 (4) w 3545.8 (3) w 3537.0 (4) w 3524.5 (4) vw 3465.5 w 3461.6 vw

ν(H2O)free ν(HSO4−)free ν(H2O)bond ν(HSO4−)bond

ν(H2O)free ν(H2O)bond ν(HSO4−)bond ν(H2O)free ν(H2O)free ν(H2O)free ν(H2O)bond ν(H2O)bond ν(H2O)bond ν(HSO4−)bond

νas(SO + S−O−Hdef)

νs(SO + S−OHdef)

1287 (339) 1127 (107) 1336 (113) 1277a (293)

1036 (84) 730 (274) 1199 (370) 1054 (154)

1318 (183) 1233a (321) 1200 (381)

1034 (86)

1351 (360) 1377 (155) 1244a (319) 1143 (399)

1157 (295) 988 (316) 822 (166)

1418 (151) 1247 (430) 1163 (303)

1331.0 w 1327.0 s 1316.2 w 1309.0 w

1045 (197) 1418 (151) (S−OH)

1203.8 1198.3 m 1012 m

ν(N−H+)

ν(N−H···O)def

ν(S−O−H)oop 59 (?) (66) 466 (49)

589 (55)

3287 (57) 2172 (2618) 2155 (2777)

1585 (20) (N−H···O)ip 1066 (48) (N−H···O)oop

669 (241)

1489.5 w 1012.4 i.p. 1006.4 o.o.p.

1444.4 594.2

2621 (1561)

2769.3

1314.3 m (D) 1268 w (D)

Observed band at 1241 cm−1. (italics): calculated intensities. (D): with deuterated sulfuric acid

frequencies of R3NH+ in gas phase and on the surface were predicted at 3300 and 2820 cm−1, respectively.18 Similarly, the N−H stretching frequency for NH4Cl in nujol was found at 2735 cm−1.34 The proton transfer also produces new N−H···O bending modes. A new band emerging with annealing at 1489.5 cm−1 is assigned to the in-plane deformational motion, which is coupled with the CH3 umbrella bending modes. It is absent in the deuterated (CH3)3N/D2SO4/(CH3)3N layer. The scaled calculated frequency 1513 cm−1 (1527 cm−1, unscaled) is assigned to this mode. A second bending mode is calculated at 1585 cm−1 (1540 cm−1, unscaled). No corresponding feature was found in deuterated sample spectra. The formation of the new N−H bond is also correlated with the appearance of a new band in the spectrum at 1012.0 cm−1. The corresponding calculated value is 1066 cm−1 (1079 cm−1, unscaled) for the out-of-plane N−H···O deformation coupled with the N−CH3 librations. A new deuteration sensitive band recorded at 594.2 cm−1 is related by us to the deformation mode of the hydroxyl SO−H of the HSO4− moiety coupled to libration of the water hydrogen bonded to it. The calculation predicts the corresponding scaled frequency to be at 669 cm−1 (654 cm−1, unscaled). The skeletal modes of (CH3)3N are also affected by the complexation with sulfuric acid. In the ν(C−N) mode region the new band, which as expected, is not affected by sulfuric acid deuteration and is observed (CH3)3N at 1038.7 cm−1 next to the ν(C−N) free mode.35 This band shifted to 1032.2 cm−1 is assigned to the asymmetric stretch ν(C−N) in the complex.

Spectral support for the essentially complete proton transfer suggested should be the expected emergence of a band assignable to the ν(N−H) stretch in the positively charged (CH3)3N−H+ counterion. For the free cation, this mode is predicted by B3LYP/aug-cc-pVQZ level calculations at 3287 cm−1 (scaled), within the typical frequency span of N−H stretch frequencies. However, we did not observe any new spectral feature in this frequency range. For the 1:1 (CH3)3N−H*HSO4− complex, this calculated value shifts drastically to 2172 cm−1. For the monohydrated complex (CH3)3N−H*HSO4−*H2O, the corresponding value is 2155 cm−1. For the trihydrated complex of Figure 2, the calculated frequency is 2621 cm−1. An observed new band, emerging with annealing (Figure 3) at 2769.3 cm−1, is now assigned to this new N−H bond of the N−H···O−S scheme. This red shift of intermediate magnitude from the free ion ν(N−H) frequency indicates that the ions are not completely separated and remain strongly H-bonded. The full width characteristic of such strongly H-bonded bands (judging by the extent of the red shift), may be partly obscured by the neighboring intensive (CH3)3N bands. The calculated H-bond length for this complex is 1.608 A. The correlation between H-bond length and red-shift33 places this band at about 2640 cm−1. There is further support for this assignment: TMA served as a probe of acidity of zeolites with corresponding bands in the 2500 to 3000 cm−1 range assigned to this R3NH3+ mode, its position depending on the zeolite acidity.17 In a computational study of the interaction of amines with H-type mordenite, a proton transfer to amine was indicated with a stabilization of the R3NH+ moiety by hydrogen bonding between the negatively charged zeolite framework and the N−H bonds. The N−H stretching 1008

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Figure 4. Right panel: Comparison of broad backgrounds representing the Zundel-ion and Zundel-like ion bands. (A) (CH3)3N/H2SO4/Ar layer, after annealing to 40 K, recooled to 18 K. (B) (CH3)3N/D2SO4/Ar layer, after annealing to 40 K, recooled to 18 K. (C) H3N···H+···NH3/Ar gas phase IRMPD spectrum.44 (D) H2O···H+···OH2/Ar gas phase IRMPD spectrum43 (red trace); black trace from ref 42, as shown in ref 43. Left panel: Computed structure of the (CH3)3N−H···N(CH3)3 ion. (top) Calculated structure (B3LYP-PVTZ) of (CH3)3N···H+···N(CH3)3 Zundel-like ion. (bottom) Double-well potential for (CH3)3N···H+···N(CH3)3 Zundel-like ion scanned from a (B3LYP-PVDZ) energy surface.

The corresponding calculated frequency values are 991 and 980 cm−1 (scaled). The observed band pair at 808.5 and 800.0 cm−1 is assigned to the symmetric ν(N−C) stretch (the pair structure of the bands is due to an internal coupling with the N−CH3 librational modes). Our calculations have these peaks at 868 and 816 cm−1 (scaled). Similar frequencies and frequency shifts have been observed for the complexation of TMA with acetic and fluoroeacetic acids.13,14 The corresponding values for the free (CH3)3N modes are 1038 cm−1 (asymmetric C−N stretch) and 824 cm−1 (symmetric C−N stretch). These shifts may be related to a C−N bond lengthening at protonation, as predicted by molecular orbital calculations.16 In our previous studies21,23 the most indicative spectral range for the structure and nature of bonding in H-bonded complexes is the high frequency range of 3700−3400 cm−1. However, the number and position of the ν(O−H) bands in this range proved to be the most complicated spectral region, in the present case. It is also the most difficult for interpretation, mainly due to the possible involvement of a variety of nonmonomeric water species in the complexation process. We observed up to five bands specific to complex formation, and even if of low intensity, they are clearly present in the initial deposition layers and exhibit marked intensity gains upon annealing, synchronous with the

growth of the previously described skeletal bands assigned to the complex. The relevant spectral region is reproduced in Figure 3, with the new complex-assigned bands shown in red. As H2O molecules are the only species that are assumed to migrate in the matrix layer, (even more so its monomeric species), these bands show a characteristic intensity gain upon matrix annealing (and subsequent recooling). They are also sensitive to deuteration by having pronounced counterparts in the corresponding ν(O−D) region with the characteristic isotopic shift ratios of about 1.36. It is therefore to be concluded that the complex, or complexes formed, must involve H2O moieties in their structure. Of the large variety of possible configurations calculated for the triple (CH3)3N/H2SO4/H2O system (detailed in the Supporting Information), the best fit of the number and position of calculated ν(O−H) bands was obtained for the molecular model reproduced in Figure 2. It involves two H2O molecules back-H-bonded to the SO bonds and a third H2O molecule Hbonded to the SO−H bond, in analogy to the H-bonding in H2SO4.21 Its stabilization energy is −163 kJ mol−1 (B3LYP/augcc-pVTZ, BSSE corrected), which is lower by about −90 kJ mol−1 than the stabilization energy of the nonhydrated 1:1 (CH3)3N*H2SO4 complex and significantly lower than the values derived for the other species considered (as detailed in Table S4 of the Supporting Information). 1009

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ionic species involving a (CH3)3−N···H+···N−(CH3)3 configuration. Other experimental results also place the A···H···A bands with similar considerable width in this spectral range. Its negatively charged counter-ion is probably a hydrated bisulphate ion, for which only the strongest band, the antisymmetric SO stretch, is observed.

The relevant bond lengths calculated at the B3LYP/aug-ccpVTZ level are marked in Figure 2. As confirmed by the spectral bands, this calculated structure involves a proton transfer from H2SO4, to form a new N−H bond in a (CH3)3N−H+ ionic moiety as well as an H···O hydrogen bond, replacing the original O−H bond of H2SO4. The observed and calculated frequencies are listed in Table 1. In most spectra, especially in the more concentrated layers, we observe a very broad background band in the 1000−1500 cm−1 region. In matrix layers involving deuterated sulfuric acid, this background, only slightly red-shifted, is also evident (Figure 4). In our previous study of the H2SO4/NH3 system an absorption band, similar in position and width, was observed. The position of this spectral feature (center of gravity about 1200 cm−1) is characteristic of band positions of Zundel ion-like species involving a central proton symmetrically H-bonded between to proton acceptor moieties, similar to the protonated water dimer.36,37 Their bonding nature has been investigated by Roscioli et al.38−40 The large width has been attributed (in part, at least) to anharmonicity induced coupling to low frequency intermolecular and intramolecular modes.41 These infrared bands bear a general resemblance to IRMPD spectra observed by Asmis42 and Fridgen,43 which, in turn, may be noted to be similar in position but differ from each other in the detailed intensity distribution. We therefore attribute this background band, which is more prominent in H2SO4 richer layers, to a Zundel-like ion of (CH3)3−N···H+···N−(CH3)3 configuration. This assignment is also in good analogy to the band observed by us in our H2SO4/ NH3 matrix isolation study.22 The counterion for this H-bonded positively charged ionic species is most likely a neighboring bare hydrated bisulfate anion. Experimentally, the spectral signature of the latter is observed as a band at 1241 cm−1. For the trihydrated bisulfate ion, this band position is calculated at 1233 and 1200 cm−1, with similar frequencies for the monohydrated bisulfate ion (see corresponding Supporting Information tables) . In Figure 4 we also show a calculated configuration of this ion along with a double-well potential characteristic of such quasisymmetric ion species. By scanning the calculated B3LYP/augcc-pVDZ energy surface, the potential curve V(x) = −81x2 + 1029x4 kJ mol−1 was obtained. More tentatively, the rather broad bands at 947.8 and 928.6 cm−1 are attributed to the in-plane N···H+···N deformation of this Zundel-like ion, similar to the 1012.2 and 1006.4 cm−1 deformations of the complex in Figure 2 assigned above. In conclusion, the main finding of this study is that the highly basic trimethylamine forms a hydrated cluster complex with sulfuric acid. This complex is of high ionic character as evident by its sharp and rather intense bands in the skeletal stretching mode region of the bisulfate moiety. Because of proton transfer of the H+ proton from sulfuric acid to (CH3)3N, a new N−H bond of length typical of N−H bonds is formed and one of the original O−H bonds in H2SO4 is replaced by a strong NH···O hydrogen bond. The (CH3)3N−H+*HSO4− complex is further stabilized by hydration. The bands in the ν(O−H) stretching mode region are best explained by the structure shown in Figure 2, involving (at least) three H2O molecules, attached to the SO oxygens and to the SO−H bond (in a manner similar to the H2SO4*H2O complex).21 As an additional finding, the very broad spectral background feature, spanning the frequency range of 1700−500 cm−1 is attributed to a quasi-symmetric (double-well potential) Zundel-like

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ASSOCIATED CONTENT

S Supporting Information *

Tables of calculation results of the various ions and their complexes we considered for interpretation. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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REFERENCES

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