Influence of Intramolecular Hydrogen Bonding on OH-Stretching Overtone Intensities and Band Positions in Peroxyacetic Acid

Vapor phase absorption spectra and integrated band intensities of the OH stretching fundamental as well as first and second overtones (2ν_OH and 3ν_OH) in peroxyacetic acid (PAA) have been measured using a combination of FT-IR and photoacoustic spectroscopy. In addition, ab initio calculations have been carried out to examine the low energy stable conformers of the molecule. Spectral assignment of the primary features appearing in the region of the 2ν_OH and 3ν_OH overtone bands are made with the aid of isotopic substitution and anharmonic vibrational frequency calculations carried out at the MP2/aug-cc-pVDZ level. Apart from features associated with the zeroth-order OH stretch, the overtone spectra are dominated by features assigned to combination bands composed of the respective OH stretching overtone and vibrations involving the collective motion of several atoms in the molecule resulting from excitation of the internal hydrogen bonding coordinate. Integrated absorption cross section measurements reveal that internal hydrogen bonding, the strength of which is estimated to be ∼20 kJ/mol in PAA, does not result in a enhanced oscillator strength for the OH stretching fundamental of the molecule, as is often expected for hydrogen bonded systems, but does cause a precipitous drop in the oscillator strength of its 2ν_OH and 3ν_OH overtone bands, reducing them, respectively, by a factor of 165 and 7020 relative to the OH stretching fundamental

Gas Phase Hydrolysis of Formaldehyde To Form Methanediol: Impact of Formic Acid Catalysis

We find that formic acid (FA) is very effective at facilitating diol formation through its ability to reduce the barrier for the formaldehyde (HCHO) hydrolysis reaction. The rate limiting step in the mechanism involves the isomerization of a prereactive collision complex formed through either the HCHO···H₂O + FA and/or HCHO + FA···H₂O pathways. The present study finds that the effective barrier height, defined as the difference between the zero-point vibrational energy (ZPE) corrected energy of the transition state (TS) and the HCHO···H₂O + FA and HCHO + FA···H₂O starting reagents, are respectively only ∼1 and ∼4 kcal/mol. These barriers are substantially lower than the ∼17 kcal/mol barrier associated with the corresponding step in the hydrolysis of HCHO catalyzed by a single water molecule (HCHO + H₂O + H₂O). The significantly lower barrier heights for the formic acid catalyzed pathway reveal a new important role that organic acids play in the gas phase hydrolysis of atmospheric carbonyl compounds

UV Photochemistry of Peroxyformic Acid (HC(O)OOH): An Experimental and Computational Study Investigating 355 nm Photolysis

The photochemistry of peroxyformic acid (PFA), a molecule of atmospheric interest exhibiting internal hydrogen bonding, is examined by exciting the molecule at 355 nm and detecting the nascent OH fragments using laser-induced fluorescence. The OH radicals are found to be formed in their ground electronic state with the vast majority of available energy appearing in fragment translation. The OH fragments are vibrationally cold (v″ = 0) with only modest rotational excitation. The average rotational energy is determined to be 0.35 kcal/mol. Further, the degree of OH rotational excitation from PFA is found to be significantly less than that arising from the dissociation of H₂O₂ as well as other hydroperoxides over the same wavelength. Ab initio calculation at the EOM-CCSD level is used to investigate the first few electronic excited states of PFA. Differences in the computed torsional potential between PFA and H₂O₂ help rationalize the observed variation in their respective OH fragment rotational excitation. The calculations also establish that the electronic excited state of PFA accessed in the near UV is of ¹A″ symmetry and involves a σ*_(O–O) ← n_(O) excitation. Additionally, the UV absorption cross section of PFA at 355 and 282 nm is estimated by comparing the yield of OH from PFA at these wavelengths to that from hydrogen peroxide for which the absorption cross sections is known

Carboxylic Acid Catalyzed Hydration of Acetaldehyde

Electronic structure calculations of the pertinent stationary points on the potential energy surface show that carboxylic acids can act effectively as catalysts in the hydration of acetaldehyde. Barriers to this catalyzed process correlate strongly with the p<i>K</i>_a of the acid, providing the potential to provide the predictive capacity of the effectiveness of carboxylic acid catalysts. Transition states for the acid-catalyzed systems take the form of pseudo-six-membered rings through the linear nature of their hydrogen bonds, which accounts for their relative stability compared to the more strained direct and water-catalyzed systems. When considered as a stepwise reaction of a dimerization followed by reaction/complexation, it is likely that collisional stabilization of the prereactive complex is more likely than reaction in the free gas phase, although the catalyzed hydration does retain the potential to proceed on water surfaces or in droplets. Lastly, it is observed that postreactive diol–acid complexes are significantly stable (∼12–17 kcal/mol) relative to isolated products, suggesting the possibility of long-lived hygroscopic species that could act as a seed molecule for condensation of secondary organic aerosols

Organic Acid Formation from the Atmospheric Oxidation of Gem Diols: Reaction Mechanism, Energetics, and Rates

Computational chemistry is used to investigate the gas phase reaction of several gem diols in the presence of OH radical and molecular oxygen (³O₂) as would occur in the Earth’s troposphere. Four gem diols, represented generically as R–HC­(OH)₂, with R being either −H, −CH₃, −HC­(O), and −CH₃C­(O) are investigated. We find that after the abstraction of the hydrogen atom from the C–H moiety of the diol by atmospheric OH, molecular oxygen quickly adds onto the resulting radicals leading to the formation of a geminal diol peroxy adduct (R–C­(OO)­(OH)₂), which is the key intermediate in the oxidation process. Unimolecular reaction of this R–C­(OO)­(OH)₂ radical adduct, occurs via a proton-coupled electron transfer (PCET) mechanism and leads to the formation of an organic acid and a HO₂ radical. Further, the barrier for the unimolecular reaction step decreases along the R substitution series: −H, −CH₃, −HC­(O), −CH₃C­(O); this trend most likely arises from increased internal hydrogen bonding along the series. The reaction where the R group is CH₃C­(O), associated with methylglyoxal diol, has the lowest barrier with its transition state being ∼4.3 kcal/mol above the potential energy well of the corresponding CH₃C­(O)-C­(OO)­(OH)₂ peroxy adduct. The rate constants for the four diol oxidation reactions were investigated using the MESMER master equation solver kinetics code over the temperature range between 200 and 300 K. The calculations suggest that once formed, gem diol radicals react rapidly with O₂ in the atmosphere to produce organic acids and HO₂ with an effective gas phase bimolecular rate constant of ∼1 × 10^–11 cm³/molecule s at 300 K

Role of Torsion-Vibration Coupling in the Overtone Spectrum and Vibrationally Mediated Photochemistry of CH₃OOH and HOOH

The yield of vibrationally excited OH fragments resulting from the vibrationally mediated photodissociation of methyl hydroperoxide (CH₃OOH) excited in the vicinity of its 2ν_OH and 3ν_OH stretching overtones is compared with that resulting from excitation of the molecule to states with three quanta in the CH stretches and to the state with two quanta in the OH stretch and one in the OOH bend (2ν_OH + ν_OOH). We find that the OH fragment vibrational state distribution depends strongly on the vibrational state of CH₃OOH prior to photodissociation. Specifically, dissociation from the CH stretch overtones and the stretch/bend combination band involving the OH stretch and OOH bend produced significantly less vibrationally excited OH fragments compared to that produced following excitation of CH₃OOH to an overtone in the OH stretch. While the absence of vibrationally excited OH photoproducts following excitation of the CH overtone is not surprising, the lack of vibrationally excited OH following excitation to the 2ν_OH+ν_OOH combination band is unexpected given that photodissociation following excitation to the lower-energy 2ν_OH state produces OH products in <i>v</i> = 1 as well as in its ground state. This trend persists even when the electronic photodissociation wavelength is changed from 532 to 355 nm and thus suggests that the observed disparity arises from differences in the nature of the initially populated vibrational states. This lack of vibrationally excited OH products likely reflects the enhanced intramolecular vibrational energy redistribution associated with the stretch/bend combination level compared to the pure OH stretch overtone. Consistent with this hypothesis, photodissociation from the stretch/bend combination level of the smaller HOOH molecule produces more vibrationally excited OH fragments compared to that resulting from the corresponding state of CH₃OOH. These results are investigated using second-order vibrational perturbation theory based on an internal coordinate representation of the normal modes. Consistent with the observations, the first-order correction to the wave function shows stronger coupling of the 2ν_OH+ν_OOH state to states with torsion excitation compared to the other bands considered in this study

Oxygenate-Induced Tuning of Aldehyde-Amine Reactivity and Its Atmospheric Implications

Atmospheric aerosols often contain a significant fraction of carbon–nitrogen functionality, which makes gas-phase aldehyde-amine chemistries an important source of nitrogen containing compounds in aerosols. Here we use high-level ab initio calculations to examine the key determinants of amine (ammonia, methylamine, and dimethylamine) addition onto three different aldehydes (acetaldehyde, glycolaldehyde, and 2-hydroperoxy acetaldehyde), with each reaction being catalyzed by a single water molecule. The model aldehydes reflect different degrees of oxygenation at a site adjacent to the carbonyl moiety, the α-site, and represent typical oxygenates that can arise from atmospheric oxidation especially under conditions where the concentration of NO is low. Our results show that the reaction barrier is influenced not only by the nature of the amine but also by the nature of the aldehyde. We find that, for a given amine, the reaction barrier decreases with increasing oxygenation of the aldehyde. This observed trend in barrier height can be explained through a distortion/interaction analysis, which reveals a gradual increase in internal hydrogen bonding interactions upon increased oxygenation, which, in turn, impacts the reaction barrier. Further, the calculations reveal that the reactions of methylamine and dimethylamine with the oxygenated aldehydes are barrierless under catalysis by a single water molecule. As a result, we expect these addition reactions to be energetically feasible under atmospheric conditions. The present findings have important implications for atmospheric chemistry as amine-aldehyde addition reactions can facilitate aerosol growth by providing low-energy neutral pathways for the formation of larger, less volatile compounds, from readily available smaller components

A Computational Study Investigating the Energetics and Kinetics of the HNCO + (CH₃)₂NH Reaction Catalyzed by a Single Water Molecule

High-level ab initio calculations are used to explore the energetics and kinetics for the formation of 1,1-dimethyl urea via the reaction of isocyanic acid (HNCO) with dimethyl amine (DMA) catalyzed by a single water molecule. Compared to the uncatalyzed HNCO + DMA reaction, the presence of a water molecule lowers the reaction barrier, defined here as the energy difference between the separated HNCO + DMA + H₂O reactants and the transition state (TS), by ∼26 kcal/mol. In addition to the HNCO + DMA + H₂O reaction, the energetics of the analogous reactions involving, respectively, ammonia and methyl amine were also investigated. Comparing the barriers for these three amine addition reactions, which can be represented as HNCO + R-NH-R′ + H₂O with R and R′ being either −CH₃ or −H, we find that the reaction barrier decreases with the degree of methylation on the amine nitrogen atom. The effective rate constants for the bimolecular reaction pathways HNCO··H₂O + DMA and HNCO··DMA + H₂O were calculated using canonical variational TS theory coupled with both small curvature and zero-curvature tunneling corrections over the 200–300 K temperature range. For comparison, we also calculated the rate constant for the HNCO + OH reaction. Our results suggest that the HNCO + H₂O + DMA reaction can make a non-negligible contribution to the gas-phase removal of atmospheric HNCO under conditions where the HNCO and water concentrations are high and the temperature is low

Hydrolysis of Ketene Catalyzed by Formic Acid: Modification of Reaction Mechanism, Energetics, and Kinetics with Organic Acid Catalysis

The hydrolysis of ketene (H₂CCO) to form acetic acid involving two water molecules and also separately in the presence of one to two water molecules and formic acid (FA) was investigated. Our results show that, while the currently accepted indirect mechanism, involving addition of water across the carbonyl CO bond of ketene to form an ene–diol followed by tautomerization of the ene–diol to form acetic acid, is the preferred pathway when water alone is present, with formic acid as catalyst, addition of water across the ketene CC double bond to directly produce acetic acid becomes the kinetically favored pathway for temperatures below 400 K. We find not only that the overall barrier for ketene hydrolysis involving one water molecule and formic acid (H₂C₂O + H₂O + FA) is significantly lower than that involving two water molecules (H₂C₂O + 2H₂O) but also that FA is able to reduce the barrier height for the direct path, involving addition of water across the CC double bond, so that it is essentially identical with (6.4 kcal/mol) that for the indirect ene–diol formation path involving addition of water across the CO bond. For the case of ketene hydrolysis involving two water molecules and formic acid (H₂C₂O + 2H₂O + FA), the barrier for the direct addition of water across the CC double bond is reduced even further and is 2.5 kcal/mol <i>lower</i> relative to the ene–diol path involving addition of water across the CO bond. In fact, the hydrolysis barrier for the H₂C₂O + 2H₂O + FA reaction through the direct path is sufficiently low (2.5 kcal/mol) for it to be an energetically accessible pathway for acetic acid formation under atmospheric conditions. Given the structural similarity between acetic and formic acid, our results also have potential implications for aqueous-phase chemistry. Thus, in an aqueous environment, even in the absence of formic acid, though the initial mechanism for ketene hydrolysis is expected to involve addition of water across the carbonyl bond as is currently accepted, the production and accumulation of acetic acid will likely alter the preferred pathway to one involving addition of water across the ketene CC double bond as the reaction proceeds

Dimethylamine Addition to Formaldehyde Catalyzed by a Single Water Molecule: A Facile Route for Atmospheric Carbinolamine Formation and Potential Promoter of Aerosol Growth

We use ab initio calculations to investigate the energetics and kinetics associated with carbinolamine formation resulting from the addition of dimethylamine to formaldehyde catalyzed by a <i>single</i> water molecule. Further, we compare the energetics for this reaction with that for the analogous reactions involving methylamine and ammonia separately. We find that the reaction barrier for the addition of these nitrogen-containing molecules onto formaldehyde decreases along the series ammonia, methylamine, and dimethylamine. Hence, starting with ammonia, the reaction barrier can be “tuned” by the substitution of an alkyl group in place of a hydrogen atom. The reaction involving dimethylamine has the lowest barrier with the transition state being 5.4 kcal/mol <i>below</i> the (CH₃)₂NH + H₂CO + H₂O separated reactants. This activation energy is significantly lower than that for the bare reaction occurring without water, H₂CO + (CH₃)₂NH, which has a barrier of 20.1 kcal/mol. The negative barrier associated with the single-water molecule catalyzed reaction of dimethylamine with H₂CO to form the carbinolamine (CH₃)₂NCH₂OH suggests that this reaction should be energetically feasible under atmospheric conditions. This is confirmed by rate calculations which suggest that the reaction will be facile even in the gas phase. As amines and oxidized organics containing carbonyl functional groups are common components of secondary organic aerosols, the present finding has important implications for understanding how larger, less volatile organic compounds can be generated in the atmosphere by combining readily available smaller components as required for promoting aerosol growth