Article pubs.acs.org/JPCA
H‑Bonding of Sulfuric Acid with Its Decomposition Products: An Infrared Matrix Isolation and Computational Study of the H2SO4·H2O·SO3 Complex Mark Rozenberg,† Aharon Loewenschuss,*,† and Claus J. Nielsen‡ †
Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem, Israel 919040 Centre for Theoretical and Computational Chemistry, Department of Chemistry, University of Oslo, 1033 Blindern, N-0315 Oslo, Norway
‡
S Supporting Information *
ABSTRACT: The FTIR matrix isolation spectra of H2SO4 vapors show a group of bands with synchronous growth of their relative intensities which is independent of the water species content of the matrix layer. Their frequency positions indicate that the species they represent is H-bonded and composed of all three components (H2SO4, H2O, and SO3) involved in the vapor decomposition equilibrium of the acid molecule. Structure, stabilization energies, and vibrational frequencies of several H-bonded complexes between these components were considered in B3LYP calculations employing Dunning’s correlation-consistent aug-cc-pVTZ basis sets. Correlations between spectral shifts, bond lengths, and H-bond energies were also considered. The best fitting complex is a ring structured 1:1:1 H2SO4·H2O·SO3. The indications are that the complex is formed in the vapor phase and not after deposition. The atmospheric significance may be in its ability to serve as a H-bonding nucleation center even without the presence of additional contaminants.
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INTRODUCTION Sulfuric acid (SA) is a major component of atmospheric aerosols that play an important role as cloud nucleation centers and thus impact climate and rain acidity.1−5 In recent years we have studied the complex formation of sulfuric acid with other small molecules of atmospheric abundance6−12 by matrix isolation infrared spectroscopy accompanied by quantum chemical calculations. These complexes identified were hydrogen- or van der Waals-bonded. Infrared bands not observed in the spectra of the pure parent components were related to complex formation and assigned to computed structures best reproducing the band number and their frequency positions. In all these studies a prominent band was observed at a frequency below the ν(O−H) band positions of H2SO4 and of H2O, but still high enough to be undoubtedly a hydroxyl stretch mode. As some correlation between the intensity of this band and the acid content in the deposited gas mixture was observed, it was assigned as being due to a sulfuric acid dimer.8 However, later computations on other H-bonded complexes9−12 as well as considerations of correlations between calculated H-bond lengths and frequency red shifts13 cast severe doubts on this assignment. In a recent study14 on the matrix isolation spectra of formic acid, a very analogous band was observed. Here again, its wavenumber position below the bands of the matrix isolated acid is still high enough to associate it with a hydroxyl stretch mode. It, along with side bands to other stretching mode peaks, was assigned to complexes formed between formic acid and its © XXXX American Chemical Society
decomposition products, CO and CO2. The obvious analogy led us to reinvestigate the matrix isolation spectra of sulfuric acid vapors with respect to the appearance of this band. The spectral results, which also show the simultaneous growth of several other weak bands as secondary structure to the main sulfuric acid stretching modes, are taken to indicate a vapor phase complexation of the acid molecules with its respective decomposition products. Possible structures of the complexes formed were considered computationally and the best fitting complex conformation is suggested. This triple complex between H2SO4, H2O, and SO3 still has free OH hydroxyl bonds and thus can serve, even in the absence impurities, as center for larger cluster formation by further H-bonding.
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EXPERIMENTAL AND COMPUTATIONAL METHODS Full experimental details were given before.8 Materials. Sulfuric acid (98%) was supplied by Fluka and argon gas (5.7 purity) by AGA. P2O5 (99%) was supplied by Aldrich. Sample Preparation. Argon was flowed over a drop of acid to get a mixture for deposition. The Ar/H2SO4 ratio was regulated by varying the acid drop temperature. In most Received: January 20, 2016 Revised: April 11, 2016
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DOI: 10.1021/acs.jpca.6b00635 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A experiments, phosphorus pentoxide powder was placed in the vapor path to reduce the water content of the deposited layer. Cooling was provided by an Air Product Displex model 202A helium refrigerator. Spectra were recorded on a Bruker Equinox 55 FTIR spectrometer at a resolution of 0.5 cm−1. DFT calculations employing the B3LYP hybrid functional were carried out with the Gaussian 09 program.15 Dunning’s correlation-consistent aug-cc-pVTZ basis sets16 were employed in all calculations. Detailed computational methods were previously described.12 Anharmonic calculations (as implemented in Gaussian 09, Revision D.01) were carried out for H2SO4, SO3, and H2O and for 1:1:1 complexes of these, employing the aug-cc-pVTZ basis set. The complexes considered are not dominated by dispersion forces and we have previously compared the results from B3LYP and MP2 calculations on various H2SO4 complexes,6−14 showing that the structures compare well, and that the vibrational fundamentals, after suitable scaling, are identical within a few wavenumbers. For large complexes like the ones discussed her, MP2 calculations would consume far too high computer resources and time. Cartesian coordinates of monomeric species and the various complexes 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 Counterpoise (CP) correction. The force fields were scaled according to the procedure, Fi,jscaled = Fi,jcalc αi·αj , where αi and αj are scaling parameters for the valence coordinates i and j, respectively; the scaling parameters are derived from fitting vibrational data of the monomeric compounds.
Figure 1. Matrix isolation spectra (MIS) of H2SO4 vapors in solid argon at 20 K. Spectra were normalized to the 3566.5 cm−1 free H2SO4 band (marked *). Layers differ in impurity concentrations and H2O content. “Wetness” is judged by the (H2O)2 component8 of the 3571.8 cm−1 band and the (H2O)3 shoulder of the 3516.2 cm−1 band (see insert). Note the synchronous growth of the bands traced in red.
on its high frequency side in the water-rich layers (notably trace D). This shoulder corresponds to a band observed and assigned to a water trimeric species.17 In Figure 2 a spectral trace (A) obtained from matrix isolated D2SO4 is compared to a spectral trace (B) recorded from a
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RESULTS AND DISCUSSION Figure 1 reproduces the infrared spectra recorded from several matrix layers of deposited H2SO4/Ar layers. The layers differ in their degree of dryness and in the H2SO4/Ar ratio in the deposited gas. The relative amount of acid or water impurities in the deposited layer could be varied, respectively, by the evaporation temperature of the acid and by placing a drying agent in the vapor path and then monitored spectrally. Absolute matrix/impurity values could, however, not be measured. To facilitate comparison, all spectra were normalized to the band at 3566.5 cm−1 marked by an asterisk (*), corresponding to the free (not involved in any H-bonding)8 ν(OH) stretch of H2SO4. The different degrees of “wetness” of the layers are well expressed by the variations in intensity of the 3582.1 cm−1 band annotated with H2SO4·H2O. This latter band was assigned8 to the bonded ν(OH) band of H2O in the 1:1 H2SO4·H2O complex. This assignment was experimentally based on the band’s intensity dependence on the water content of the matrix layer and its growth upon annealing. A further indication of the matrix “wetness” is the relative intensity of the 3571.8 cm−1 band, marked by #, which is composed of an overlap between the symmetric “free” ν(OH) stretch of H2SO4 and an ν(OH) band of the water dimer.8 Finally, the band at 3516.3 cm−1, which further below will be considered as the major indication for the formation of a not previously reported triple H2SO4· H2O·SO3 complex, shows a shoulder prominently growing in
Figure 2. Effects of deuteration on argon matrix isolated sulfuric acid. (A) Deuterium-enriched sulfuric acid in an argon matrix. (B) Normal sulfuric acid in an argon matrix (cf. trace C in Figure 1).
layer of matrix isolated H2SO4. Due to the rapid exchange with residual water vapors various possible isotopomers may be formed and observed. The most outstanding feature in the spectral traces of Figure 1, is the clear synchronization of relative intensities of the bands traced in red. On the contrary, it is also quite clear that there is no such synchronization between the “red bands” and the “wetness” of the matrix layers. This is most obvious in trace C, where neither the 3582.1 cm−1 band of the 1:1 H2SO4·H2O B
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Table 1. Structural (A), Stabilization Energy (kJ mol−1), and Vibrational Frequency Values (cm−1) for H2SO4/H2O/SO3 Speciesa
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The Journal of Physical Chemistry A Table 1. continued
a
Key: s, symmetric; as, asymmetric; blue bold, anharmonic calculations; green bold, mono deuterated; ip, in phase; oop, out of phase; ctr, center bond. The asterisk (*) identifies noncomplexed HDO.
complex nor the (H2O)2 dimer component of the 3571.8 cm−1 band is observable, but the “red bands” have, nevertheless, significant intensities. We also note that “red bands” appear next to all major stretching modes of both H2SO4 (892, 887, 834 cm−1) and SO3, (1389 cm−1) strongly indicating that both of these molecules are involved in the formation of the species giving rise to the “red bands” complement. The most prominent “red band” is observed at 3516.3 cm−1. It appears in all matrix isolation experiments involving sulfuric acid.8−12 As mentioned above, its high wavenumber position clearly identifies it as a hydroxyl stretch. It was originally assigned as a ν(OH) band of the (H2SO4)2 dimer as its intensity correlated with the H2SO4 concentration in the deposited matrix mixture. This assignment should be revised: Both our previous studies8−12 of the H-bond strengths in complexes involving the strongly acidic H2SO4 molecule and
the results listed in Table 1 indicate that for a (H2SO4)2 dimer, the calculated stabilization energies, bond lengths, and frequency shifts fit a considerably stronger bonded complex than indicated by the wavenumber position of the 3516.3 cm−1 band. As a result, the calculated positions of the bonded ν(OH) bands of the dimer are considerably further red-shifted from the free H2SO4 bands than the observed 3516.3 cm−1 frequency value. Although discrepancies between calculated and observed band positions and red shifts of H-bonded ν(X−H) frequencies are expected, the discrepancies between calculated and observed frequency shifts are in the opposite direction: the computations underestimate red shifts induced by bonding. As will be shown, the computational results for the dimer place these bands much below the actually recorded 3516.3 cm−1 band, i.e they overestimate, not underestimate, the shift induced by the bonding. D
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The Journal of Physical Chemistry A In a recent FTIR study of matrix isolated formic acid,14 we observed a band at 3470.3 cm−1, in striking analogy to the 3516.3 cm−1 band observed here. Just as in the present case, its position is red-shifted by slightly over 50 cm−1 from the main ν(OH) stretching modes of matrix isolated HCOOH acid molecule bands. Its intensity correlated with the relative intensities of several side bands of lower frequency modes of formic acid as well as with the side bands of CO, an HCOOH decomposition product. It was concluded that this band complement is due to a complex formation between formic acid and its decomposition products. We now suggest that in the case of sulfuric acid we have an analogical complex between H2SO4 and its liquid phase decomposition products according to H 2SO4 → SO3 + H 2O
The two remaining complexes in Table 1 are the closed ring structures WaSoSa_r1 and WaSoSa_r2. Of these, the better agreement between calculated and observed results is obtained with the latter configuration. The bonded ν(OH) stretch of HOH---OSO2 is calculated at 3566 cm−1 (scaled) or 3560 cm−1 (anharmonic) as compared to the experimental 3516.3 cm−1 band position, with the discrepancy here being in the expected direction; namely, the red shift is underestimated by the calculations. There is further support for this assignment from correlation considerations.13 The observed red shift of −185.4 cm−1 of this 3516.3 cm−1 band from the position the uncoupled ν(OH) of the HOD molecule at 3701.7 cm−1 (Figure 2, trace A) correlates with an H-bond length of 2.01 A and H-bond energy of 17.9 kJ mol−1 for the OH--O bond between H2O and SO3. These values agree well with the calculated value of 2.05 Å (marked as R2 in Table 1) and the corresponding correlated energy of 16.8 kJ mol−1. Finally, the calculated wavenumber values for this proposed conformer also reproduce the blue shift induced by H-bonding, observed for the ν(SO) stretches of both the H2SO4 and SO3 moieties of the complex. We looked for further confirmation of these conclusion in spectral data of deuterium-enriched experiments. In trace A of Figure 2, the prominent peak observed at 2594.7 cm−1 may be singled out as a clear counterpart to the 3516.3 cm−1 band discussed above. The resulting observed isotopic frequency ratio ν(H)/ν(D) is 1.355, a value normally associated with hydroxyl bonds. Calculated vibrational frequencies for all the possible monodeuterated WaSoSa_r2 complexes are listed as Supporting Information tables. The monodeuterated WaSoSa_r2 complex, with D in the bonded ν(OD) position (as marked by an arrow in Table 1), has a D−O−H moiety bridging the H2SO4 and SO3 molecules. The position of the strongly hydrogen bonded acid ν(O−H) hydroxyl to the H− O−D oxygen is expected to be in the 2100 cm−1 region, where the analogous band of the 1:1 complex HO−SO2−OD·OH2 was observed.8 The less shifted and sharper 2594.7 cm−1 band is therefore assigned to the weaker bonded ν(O−D) stretch of the D−O−H moiety to the SO3 entity. The relevant calculated frequencies (listed in the Supporting Information tables), render an isotopic ratio ν(OH)/ν(OD) = 3560/2716 = 1.364. The spectral trace A of Figure 2 actually provides a comparison to the nonbonded HDO monomer21 for which the observed ν(OH)/ν(OD) = 3701.7/2723.7 = 1.359. The isotopic data may, therefore, be considered to further support the suggested structure of the ternary H2SO4·H2O·SO3 complex and the assignment of the major ν(OH), ν(OD) bands. Returning to Figure 1, another, rather striking spectral feature, is worthy of attention when comparing the spectral traces and especially traces C and D. In trace C the spectral features of the 1:1 H2SO4·H2O complex along with the H2O dimer and trimer bands are, essentially, unobservable but are clearly seen in trace D. Nevertheless, trace C shows prominently the bands assigned to the triple H2SO4·H2O·SO3 complex. This is rather convincing indication that these complexes may be of different origins. The 1:1 H2SO4·H2O complex may be formed both in the vapor phase and in the matrix, the latter evidenced by temperature cycling of the matrix layer, enhancing the mobility of H2O molecules in the more “wet” layers. The triple H2SO4·H2O·SO3 complex probably has its origins only in the vapor phase. We note that the reaction (H2SO4)2 → H2SO4·H2O·SO3 is highly
(1)
This analogy is well supported by the above findings: (a) there being a complement of synchronously growing bands assignable as due to all moieties of all species of the decomposition equilibrium, (b) the “red band” intensities being independent of the amount of free H2O species in the matrix layer, and (c) the position of the main observed 3516.3 cm−1 band is, as usual, red-shifted beyond the value predicted by calculations for the bond ν(OH) band of the H2O moiety in the triple H2SO4·SO3·H2O complex. We compare the described spectral observation with computational results of relevant H-bonded species that may be formed by the partners of the equilibrium (1). Table 1 summarizes the main computation results. Though the monohydrate and dimer species of H2SO4 have been the subject of recent computational studies,18,19 they are presented here to facilitate comparison on the same computational level. More detailed results are given in the Supporting Information. We thus present potential energy minimum structures for several possible configurations, their calculated stabilization energies (also as compared to results of correlation between calculated H-bond lengths and bond enthalpy13), and the scaled wavenumber values of the skeletal stretching modes for several H-bonded (H2SO4)2 dimer configurations, the Hbonded 1:1 H2SO4·H2O complexes8 and for the complexes formed between the H2SO4 sulfuric acid molecule and its decomposition products, SO3 and H2O. For two configurations of the latter we also show the results of anharmonic frequency calculations. For comparison, we also include values of both scaled and anharmonic calculation results for the parent molecules H2SO4, SO3, and H2O. As pointed out above, the spectral evidence indicates a complex involving all three partners of equilibrium 1. Table 1 lists the computation results for four configurations of H2SO4/ SO3/H2O species. Two of these have sequential “linear” bonding and two are of a closed “ring” configuration. The first such linear complex denoted WaSaSo has a central H2SO4 molecule H-bonded to an H2O and to an SO3 molecule to each of its protons, respectively. As was the case with the (H2SO4)2 dimer, the calculated bonded ν(OH) frequencies of the acid molecule are both shifted to significantly lower wavenumber values than actually observed. The next linear configuration, WaSoSa, may be viewed as composed an H2SO4 molecule Hbonded to an SO3·H2O complex.20 Here the calculated bonded ν(OH) band is, again, red-shifted to below the observed 3516.3 cm−1 peak, while the free ν(OH) of H2SO4 is essentially not affected. Also, the bonding between SO3 and H2O is too weak to have a significant effect on the ν(OH) bands. E
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(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) Rozenberg, M.; Loewenschuss, A.; Nielsen, C. J. Hydrogen Bonding in the Sulfuric Acid−Methanol−Water System: A Matrix Isolation and Computational Study. J. Phys. Chem. A 2015, 119, 2271− 2280. (13) Rozenberg, M.; Loewenschuss, A.; Marcus, Y. An Empirical Correlation between Stretching Vibration Redshift and Hydrogen Bond Length. Phys. Chem. Chem. Phys. 2000, 2, 2699−2702. (14) Rozenberg, M.; Loewenschuss, A.; Nielsen, C. J. H-Bonding of Formic Acid with Its Decomposition Products: A Matrix Isolation and Computational Study of the HCOOH/CO and HCOOH/CO2 Complexes. J. Phys. Chem. A 2015, 119 (31), 8497−8502. (15) 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. (16) 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. (17) Keutsch, F. N.; Cruzan, J. D.; Saykally, R. J. The Water Trimer. Chem. Rev. 2003, 103 (7), 2533−2577. (18) Temelso, B.; Phan, T. N.; Shields, G. C. Computational Study of the,Hydration of Sulfuric Acid Dimers. Implication for Acid Dissociation and Aerosol Formation. J. Phys. Chem. A 2012, 116, 9745−9758. (19) Partanen, L.; Hanninen, V.; Halonen, L. Ab Initio Structural and Vibrational Investigation of Sulfuric Acid Monohydrate. J. Phys. Chem. A 2012, 116, 2867−2879. (20) Givan, A.; Loewenschuss, A.; Nielsen, C. J.; Rozenberg, M. FTIR and Computational Studies of Pure and Water Containing SO3 Species in Solid Argon Matrices. J. Mol. Struct. 2007, 830, 21−34. (21) Engdahl, A.; Nelander, B. Water in Krypton Matrices. J. Mol. Struct. 1989, 193, 101−109.
endothermic and that the triple complex therefore cannot be formed in such a process. In summary, by FTIR matrix isolation and computational studies we found indication for the existence of a cyclic H2SO4· SO3·H2O complex formed in the vapor phase of sulfuric acid between the H2SO4 acid molecule and its decomposition products, SO 3 and H 2SO 4 . This complex may be of atmospheric significance as a nucleation center (even without further contaminants present) by its free O−H bonds attaching to acid, water, or sulfur trioxide molecules.
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ASSOCIATED CONTENT
S Supporting Information *
This material is available free of charge on the World Wide Web at The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.jpca.6b00635. Cartesian coordinates for individual molecules and complexes. Electronic energies, zero-point energies, and energies of complexation. Results from B3LYP/aug-ccpVTZ calculations (Table S1). Observed and calculated wavenumbers of SO3. Results from scaling of B3LYP/ aug-cc-pVTZ harmonic force field (Table S2). Calculated wavenumbers and infrared intensities of H2SO4·H2SO4 complexes. Results from B3LYP/aug-cc-pVTZ calculations (Table S3). Calculated wavenumbers and infrared intensities of H2SO4·H2O complexes. Results from B3LYP/aug-cc-pVTZ calculations (Table S4). Calculated wavenumbers and infrared intensities of H2SO4·(H2O)2 complexes. Results from B3LYP/aug-cc-pVTZ calculations (Table S5). Calculated wavenumbers and infrared intensities of the H2SO4·SO3 complex. Results from B3LYP/aug-cc-pVTZ calculations (Table S6). Calculated wavenumbers and infrared intensities of H2SO4·H2O· SO3 complexes. Results from B3LYP/aug-cc-pVTZ calculations (Table S7). Calculated wavenumbers and infrared intensities of the monodeuterated H2SO4·H2O· SO3 “r2” complexes. Results from B3LYP/aug-cc-pVTZ calculations (Table S8). (PDF)
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AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work has received support from the Research Council of Norway through a Centre of Excellence Grant (Grant No. 179568/V30).
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REFERENCES
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