Article pubs.acs.org/JPCA
H‑Bonding of Formic Acid with Its Decomposition Products: A Matrix Isolation and Computational Study of the HCOOH/CO and HCOOH/ CO2 Complexes Mark Rozenberg,† Aharon Loewenschuss,*,† and Claus J. Nielsen‡ †
Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem, 919040, Israel Department of Chemistry, University of Oslo, 1033 Blindern, N-0315 Oslo, Norway
‡
S Supporting Information *
ABSTRACT: The infrared spectra of formic acid/argon matrix layers deposited after flowing over a drying agent show distinct new bands when compared to matrix layers deposited without going through a drying process. The new bands are assigned as due to a formic acid/carbon monoxide H-bonded complex. Several complexes of HCOOH/CO/H2O and HCOOH/CO2 composition have been characterized in B3LYP and MP2 calculations. Comparison with experimental results indicate that the best agreement is obtained for a 1:1 HCOOH−CO hydrogen bonded complex formed between formic acid and CO originating from the decomposition process HCOOH → CO + H2O. Spectral and computational evidence is presented for the formation of HCOOH/CO2 complexes as a result of the HCOOH → CO2 + H2 decomposition path.
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FA and water11 has been studied both experimentally and theoretically. The complexation results between FA and CO2 were also investigated12 as were the interactions of CO and CO2 with H2O.13 In this contribution we shall discuss the complexes between FA and the polar CO molecules, formed during the decomposition process (reaction 2). Under “dry” conditions, when the water molecule concentrations are considerably reduced, the equilibrium is shifted toward the decomposition process, thus increasing the CO concentration and consequently the complex formation with the parent acid molecule. The infrared spectra of solid argon matrix layers containing the parent acid molecules and their decomposition products are then analyzed for the emergence of new vibrational bands due to complex formation. Theoretical calculations of possible stable complexes formed by the FA/H2O/CO, their structures, relative stabilization energies, and calculated vibrational frequencies will also be presented.
INTRODUCTION Formic acid (FA) significantly affects the global atmosphere acidity.1 Its aqueous reactions on aerosols and cloud droplets affect halogen activation and ozone concentrations.2−4 Satellite measurements indicate it to be responsible for ≥50% of rain acidity. A major source of formic acid is fossil oil combustion, in the range of 55 Tg year−1, almost twice than previous estimates.5 In the vapor phase formic acid may decompose by two almost equally probable6 paths: HCOOH → CO2 + H 2
(1)
and HCOOH → CO + H 2O
(2)
A second environmental relevance relates to the development of fuel cells which depends much on relatively safe sources of hydrogen. The first decomposition path of FA into molecular hydrogen (and carbon dioxide) renders it such a potential safe hydrogen source.7 Thus, although formic acid is the simplest organic acid, obtaining its matrix isolation spectra is not straightforward. To obtain a “clean” spectrum of monomeric FA, Reva et al.8 had to resort to specialized evaporation techniques. Spectra produced from the more “natural” evaporation of the acid are complicated by the existence of isomers9 and dimerization.10 Further complications may arise from the interactions of the FA molecules with their decomposition products: water, CO or CO2. The formation of strongly H-bonded complexes between © XXXX American Chemical Society
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EXPERIMENTAL SECTION
Materials. Formic acid (98%, for mass spectrometry) was supplied by Fluka. Argon gas (5.7 purity), supplied by AGA, was used to produce the solid matrix layers. P2O5 (99%, Reagent+) was supplied by Aldrich Received: June 1, 2015 Revised: July 9, 2015
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DOI: 10.1021/acs.jpca.5b05214 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A Sample Preparation. Argon/FA mixtures in 300:1 to 4000:1 ratios were prepared by manometric techniques without further purification. To obtain the “dry” mixtures, phosphorus pentoxide powder was placed in the vapor path. Actual water content of the matrix depended on the degree of gas mixture predeposition drying (by P2O5) and prepumping and, while not fully reproducible, could readily be monitored by the characteristic water bands in the resulting spectra. Cooling was provided by an Air Product Displex model 202A closed cycle helium refrigerator. Samples were deposited on a CsI window, with deposition rates ranging from 10 to 1000 mmol h−1 of (Ar + FA) and deposition window temperatures in the 19 ± 2 K range as monitored by an Au−0.007% Fe/ Chromel thermocouple and controlled with an APDE temperature controller. To protect the window from direct acid damage, a pure Ar layer was deposited for several minutes before the first mixture deposition. Spectra were recorded on a Bruker Equinox 55 FTIR spectrometer with a DTGS detector at a resolution of 0.5 cm−1 and coadding 128 scans. Electronic Structure Calculations. Frozen core MP214 and DFT calculations employing the Becke 3 parameter15,16 (B3LYP) hybrid functional were carried out with the Gaussian 09 program.17 Dunning’s correlation-consistent aug-cc-pVXZ (X = D, T, Q) basis sets18,19 were employed in all calculations. Additional anharmonic calculations (as implemented in Gaussian 09, revision D.01) were carried out for the CO complexes employing the aug-cc-pVTZ basis set. The electronic structure calculations were carried out to obtain possible structures and configurations of the complexes and to assist in the spectral interpretation. 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 counterpoise (CP) correction. 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. The force fields 1/2 were scaled according to the procedure, Fscaled = Fcalc i,j i,j (αiαj) 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. The described frequency scaling has the advantage of being more mode specific than the application of a uniform scaling factor. The HCOOH and DCOOH matrix isolation spectra8 are reproduced within a few wave numbers in both B3LYP and MP2 calculations employing nine scaling factors (Table S3 in the Supporting Information). Similar agreements are obtained for the matrix isolation data for CO and CO2 and for the H2O matrix isolation data employing two scaling constants.20 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−S12 in the Supporting Information. Tables S4 and S5 also include the results from anharmonic calculations.
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 complexes20−23 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 wave numbers, which justifies our use of DFT methods for estimating the vibrational spectra of large molecular complexes for which MP2 calculations are unrealistic. The anharmonic contributions to the calculated integrated harmonic intensities, however, cannot readily be obtained through simple scaling of the transition moments or dipole moment derivatives in the same manner. The most obvious difference is the intensity of the symmetric OH stretch in water. Only recently have more reliable procedures been implemented in quantum chemistry packages (e.g., see Bloino and Barone, ref 24). However, anharmonic vibrational frequency calculations are by no means yet a standard black box procedure (note that the anharmonic calculations by necessity were carried out employing very tight convergence criteria and a ultrafine integration grid). For very anharmonic systems, such as the HOH−OC complex, the calculations clearly fail for the intermolecular modes (Table S4).
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RESULTS AND DISCUSSION Figure 1 shows recorded spectral traces of matrix isolated formic acid in the >2000 cm−1 region. All traces show a group of intense bands in the 3510−3560 cm−1 range with varying relative intensities of its components and due to various formic acid and formic acid/water species: They include the peaks for
Figure 1. The 3800−2000 cm−1 range of “wet”, “almost “dry”, and “dry” HCOOH/Ar matrix layers deposited at 20 K, showing the emergence of HCOOH·CO complex bands (traced in red) and HCOOH·CO2, HCOOH(CO2)2 (traced in violet). All spectra are normalized to the ν(C−H) stretch mode band at 2952.5 cm−1. (A) 1:900 HCOOH/Ar ratio, “wet”: deposited as is. (B) 1:1000 HCOOH/Ar ratio, “almost dry”: deposited after passing over P2O5 drying agent. (C) 1:3700 HCOOH/Ar ratio, “dry”: deposited after passing over P2O5 drying agent. B
DOI: 10.1021/acs.jpca.5b05214 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A free trans acid stretching ν(OH) mode8,11,25 the ν(OH) stretch of water OH in the FA/H2O complex, and other less intensive bands due to the 2ν(CO) overtone. In the “dry” layers, spectra B and C, we note the absence of the free ν(OH) band of cis-FA at about 3600 cm−1.25 All shown spectra were normalized to the intensity of the ν(C−H) band at 2952.2 cm−1, marked with asterisks, as this mode is expected to be the least affected by any complexation process (although the calculation results shown below indicate small shifts in its band positions and some slight variations in intensity). As comparisons of the wavenumber range, characteristic of the H2O bond stretching modes26 of 3700−3800 cm−1, indicate that the water species content is the highest in matrix layer A, is then greatly reduced in layer B, and is essentially reduced to nonexistent in layer C. Along with the reduction of the water bands intensities for the dryer matrix layers B and C, a new band gradually grows in at 3470.3 cm−1 (traced in red), well separated by a red shift of over 70 cm −1 from the multicomponent ν(OH) mode of formic acid around 3540 cm−1. The wavenumber position of the 3470.3 cm−1 band places it in the hydroxyl stretching mode range, and its intensity is clearly not correlated with either the intensity of the ν(C−H) stretching mode or the “pure” FA ν(OH) bands.This band was observed by Lundell et al.27 in matrix isolation experiments of photochemical decomposition of the FA anhydride and assigned to an FA·CO complex. This band must therefore be associated with a newly formed species between formic acid and its decomposition products. Its intensity growth in the dryer matrix layer is most probably due to the increase in the CO content of the deposited gas mixture, owing to a shift to the right of the equilibrium of the decomposition process (reaction 2), when H2O molecules are removed by the P2O5 drying agent. This contention is strongly supported by the bands recorded in the CO stretching mode region around 2150 cm−1, also delineated in red in Figure 1. A strong and sharp band grows in for the dryer matrix layers at 2158.4 cm−1. Its intensity correlates well with the intensity of the new band at 3470.3 cm−1, indicating a common molecular origin. Two weaker bands at 2149.0 and 2141.8 cm−1, with a shoulder at 2137.5 cm−1, exhibit variations in relative intensities. Peaks of very similar position were assigned by Dubost et al.28,29 to a CO· H2O complex, CO aggregates, and monomeric matrix isolated CO, respectively. The slight discrepancy in wavenumber position is probably due to the earlier work not having the advantage of FTIR accuracy. Newer, very similar experimental values are summarized and compared to calculated frequencies in a more recent study.30 In Figure 2 we show again, for comparison, the ν(X−H) stretching mode bands at 3470.3 and 3540 cm−1, discussed above. The right-hand side of Figure 2 shows the lower 1800− 1115 cm−1 wavenumber range of a “wet” layer (A) and of a “dry” layer (B). Two distinctly new bands (outlined in red) appear in trace B of the “dry” matrix layer at 1713 and 1142.9 cm−1, respectively. The wavenumber positions of the new bands place them clearly in the range of the ν(CO) and ν(C−O) stretching mode, respectively. They show intensity growths which are well synchronized with those of the high frequency comparison peaks at the left-hand side of the figure. We note an additional, rather drastic change between the “wet” and “dry” layers for these two new bands: as the new 1713 and 1142 cm−1 peaks gain intensity, the strong 1736.3 cm−1 band (marked in blue) of the “wet” layer A essentially
Figure 2. Right hand side: The 1775−1090 cm−1 range of “wet” and “dry” HCOOH/Ar matrix layers deposited at 20 K, showing the emergence of HCOOH·CO complex bands (traced in red) and almost complete disappearance of H2O·CO band (traced in blue). Left hand side: the ν(X−H) range shown for comparison. Spectra are normalized to the ν(C−H) stretch mode band at 2952.5 cm−1. (A) 1:2400 HCOOH/Ar ratio, “wet”: deposited as is. (B) 1:900 HCOOH/Ar ratio, “dry”: deposited after passing over P2O5 drying agent.
disappears in the “dry” matrix of trace B. This spectral peak has been assigned by George et al.11 to the νCO stretching mode of the 1:1 HCOOH·H2O complex. The intensity reduction of this band indicates the scarce formation of its origin species, the HCOOH·H2O complex, in the dry matrix layer. The spectral data described above convincingly indicate the emergence of complexation species between the parent formic acid molecules and its decomposition product CO. The emergence of the complex associated bands in the dried matrix layers along with the simultaneous drastic reduction in the H2O·CO band points to the formed complex not involving H2O molecules. To support these contentions, we performed a series of calculations of structure, stabilization energies, and calculated vibrational frequencies. These are summarized in Table 1. Previous computations may be found in ref 26 for comparison. When comparing calculated and experimental vibrational data, it is important to consider not only the absolute wavenumber value but also the shifts induced by the complexation. To this end, the table also includes “reference” monomeric and dimeric molecules which although studied computationally before, are listed here to facilitate comparisons on the same computational level. This is especially true for the new band in the ν(OH) region at 3470.5 cm−1. The conclusion that the new complex formed is of HCOOH/CO composition may lead to the question of which of the two atoms of carbon monoxide is bonded to the hydroxyl group of FA. For the analogous H2O·CO complex the study by Dubost et al. identified the bands assigned for the complex but did not specify which of the two structures for the H2O·CO complex prevails. A later study by Tso et al.13 interpreted their results in terms of an HOH−CO H-bond. For the present case, from the values listed in Table 1, the HCOOH−CO complex emerges as significantly more stable than the HCOOH−OC alternative structure. Consequently, the red shift calculated for the ν(OH) is also much larger for C
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The Journal of Physical Chemistry A
Table 1. Computational and Experimental Structures and Vibrational Frequencies for Several HCOOH/CO/H2O Complexesa
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The Journal of Physical Chemistry A Table 1. continued
a Energies and (scaled) frequencies: normal font: B3LYP; italic, MP2; bold/bold: anharmonic (shift): from calculated free HCOOH values. ∗: from free FA in Ar matrix values.8
interactions. The calculations also indicate that addition of another CO2 molecule to form a FA·(CO2)2 complex has only a small effect on the H-bonding of the complex. This result is well in line with the assignment of the higher frequency band of the doublet at 3515.4 cm−1 to the FA·(CO2)2 complex. The structure and calculation results are shown in Table 1 and in greater detail in Table S12 of the Supporting Information. We also note that in several experiments we observed a group of unresolved weak bands instead of the clear doublet seen in Figure 1. Most probably these are due to the formation of higher FA·(CO)n complexes. In summary, we have shown the existence of FA·CO and FA· CO2 H-bonded complexes, the formations of which are due to the partial decomposition of formic acid. The HCOOH·CO emergence process is strongly enhanced by drying of the deposited gas mixture, thus shifting the equilibrium of reaction 1 in the direction of product formation. It may also be concluded that the complex formations occur in the streaming gas phase and not in the matrix layer.
the former configuration. Thus, the comparison with experiment clearly indicates that the better fit is achieved with the 1:1 HCOOH·CO H-bonded complex, the structure of which is given in Figure 3.
Figure 3. Calculated (B3LYP/aug-cc-pVTZ) structure of the HCOOH−CO complex.
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The HCOOH decomposition process (reaction 1) may also lead to complex formations. However, their spectral signatures are less prominent, and hence, their assignments are presented with less confidence. Two very weak bands are recorded at 3515.4 and 3505.5 cm−1, traced in violet in Figure 1. The latter band was identified by Tsuge et al.12 as ν(OH) band of the rather weakly bonded complex formed between formic acid and carbon dioxide FA*CO2. Our calculations (Table 1) confirm this assignment. The second higher wavenumber band at 3515.4 cm−1 is assigned to the HCOOH·CO2·CO2. The smaller red shifts of these bands from the monomeric FA bands, as compared to the red shift of the ν(OH) band of the HCOOH·CO complex, are evidence of weaker complexation
ASSOCIATED CONTENT
S Supporting Information *
Cartesian coordinates for monomers and complexes, results from B3LYP/aug-cc-pVTZ and MP2/aug-cc-pVTZ calculations (Table S1); energies of complexation (Table S2); observed and calculated wavenumbers of formic acid isotopomers (Table S3); calculated wavenumbers of the water/CO complexes (Table S4); calculated wavenumbers of the formic acid/CO complexes (Table S5); calculated wavenumbers of the formic acid/water complex (Table S6); calculated wavenumbers of the formic acid/CO2 complex (Table S7); calculated wavenumbers E
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The Journal of Physical Chemistry A
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of formic acid/water/CO complexes (Tables S8−S10); calculated wavenumbers of the formic acid/water/CO2 complex (Table S11); calculated wavenumbers of the 1:2 formic acid/CO2 complex (Table S12). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b05214.
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AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work has received support from the Norwegian Supercomputing Program (NOTUR) through a grant of computer time (Grant NN4654K).
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REFERENCES
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