The yield of vibrationally excited OH fragments resulting from the vibrationally mediated photodissociation of methyl hydroperoxide (CH3OOH) excited in the vicinity of its 2νOH and 3νOH 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 (2νOH + νOOH). 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 2νOH+νOOH combination band is unexpected given that photodissociation following excitation to the lower-energy 2νOH state produces OH products in v = 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 2νOH+νOOH state to states with torsion excitation compared to the other bands considered in this study.

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.

Hydrogen atom transfer (HAT) reactions are ubiquitous and play a crucial role in chemistries occurring in the atmosphere, biology, and industry. In the atmosphere, the most common and traditional HAT reaction is that associated with the OH radical abstracting a hydrogen atom from the plethora of organic molecules in the troposphere via R–H + OH → R + H2O. This reaction motif involves a single hydrogen transfer. More recently, in the literature, there is an emerging framework for a new class of HAT reactions that involves double hydrogen transfers. These reactions are broadly classified into four categories: (i) addition, (ii) elimination, (iii) substitution, and (iv) rearrangement. Hydration and dehydration are classic examples of addition and elimination reactions, respectively whereas tautomerization or isomerization belongs to a class of rearrangement reactions. Atmospheric acids and water typically mediate these reactions.Organic and inorganic acids are present in appreciable levels in the atmosphere and are capable of facilitating two-point hydrogen bonding interactions with oxygenates possessing an hydroxyl and/or carbonyl-type functionality. As a result, acids influence the reactivity of oxygenates and, thus, the energetics and kinetics of their HAT-based chemistries. The steric and electronic effects of acids play an important role in determining the efficacy of acid catalysis. Acids that reduce the steric strain of 1:1 substrate···acid complex are generally better catalysts. Among a family of monocarboxylic acids, the electronic effects become important; barrier to the catalyzed reaction correlates strongly with the pKa of the acid. Under acid catalysis, the hydration of carbonyl compounds leads to the barrierless formation of diols, which can serve as seed particles for atmospheric aerosol growth. The hydration of sulfur trioxide, which is the principle mechanism for atmospheric sulfuric acid formation, also becomes barrierless under acid catalysis. Rate calculations suggest that such acid catalysis play a key role in the formation of sulfuric acid in the Earth’s stratosphere, Venusian atmosphere, and on heterogeneous surfaces.Over the past few years, theoretical calculations have shown that these acid-mediated double hydrogen atom transfers are important in the chemistry of Earth’s atmosphere as well as that of other planets. This Account reviews and puts into perspective some of these atmospheric HAT reactions and their environmental significance.

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 single 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 below 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.

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 lower 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.

The hydrolysis of glyoxal involving one to three water molecules and also in the presence of a water molecule and formic acid has been investigated. Our results show that glyoxal-diol is the major product of the hydrolysis and that formic acid, through its ability to facilitate intermolecular hydrogen atom transfer, is considerably more efficient than water as a catalyst in the hydrolysis process. Additionally, once the glyoxal-diol is formed, the barrier for further hydrolysis to form the glyoxal-tetrol is effectively reduced to zero in the presence of a single water and formic acid molecule. There are two important implications arising from these findings. First, the results suggest that under the catalytic influence of formic acid, glyoxal hydrolysis can impact the growth of atmospheric aerosols. As a result of enhanced hydrogen bonding, mediated through their polar OH functional groups, the diol and tetrol products are expected to have significantly lower vapor pressure than the parent glyoxal molecule; hence they can more readily partition into the particle phase and contribute to the growth of secondary organic aerosols. In addition, our findings provide insight into how glyoxal-diol and glyoxal-tetrol might be formed under atmospheric conditions associated with water-restricted environments and strongly suggest that the formation of these precursors for secondary organic aerosol growth is not likely restricted solely to the bulk aqueous phase as is currently assumed.

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.

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.

Vapor phase absorption spectra and integrated band intensities of the OH stretching fundamental as well as first and second overtones (2νOH and 3νOH) 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 2νOH and 3νOH 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 2νOH and 3νOH overtone bands, reducing them, respectively, by a factor of 165 and 7020 relative to the OH stretching fundamental.

Computational studies at the B3LYP/6-311++G(3df,3pd) and MP2/6-311++G(3df,3pd) levels are performed to explore the changes in reaction barrier height for the gas phase hydrolysis of SO3 to form H2SO4 in the presence of a single formic acid (FA) molecule. For comparison, we have also performed calculations for the reference reaction involving water assisted hydrolysis of SO3 at the same level. Our results show that the FA assisted hydrolysis of SO3 to form H2SO4 is effectively a barrierless process. The barrier heights for the isomerization of the SO3···H2O···FA prereactive collision complex, which is the rate limiting step in the FA assisted hydrolysis, are found to be respectively 0.59 and 0.08 kcal/mol at the B3LYP/6-311++G(3df,3pd) and MP2/6-311++G(3df,3pd) levels. This is substantially lower than the ∼7 kcal/mol barrier for the corresponding step in the hydrolysis of SO3 by two water molecules—which is currently the accepted mechanism for atmospheric sulfuric acid production. Simple kinetic analysis of the relative rates suggests that the reduction in barrier height facilitated by FA, combined with the greater stability of the prereactive SO3···H2O···FA collision complex compared to SO3···H2O···H2O and the rather plentiful atmospheric abundance of FA, makes the formic acid mediated hydrolysis reaction a potentially important pathway for atmospheric sulfuric acid production.

The catalytic ability of water, formic acid, and sulfuric acid to facilitate the isomerization of the CH3O radical to CH2OH has been studied. It is shown that the activation energies for isomerization are 30.2, 25.7, 4.2, and 2.3 kcal mol−1, respectively, when the reaction is carried out in isolation and with water, formic acid, or sulfuric acid as a catalyst. The formation of a doubly hydrogen bonded transition state is central to lowering the activation energy and facilitating the intramolecular hydrogen atom transfer that is required for isomerization. The changes in the rate constant for the CH3O-to-CH2OH isomerization with acid catalysis have also been calculated at 298 K. The largest enhancement in the rate, by over 12 orders of magnitude, is with sulfuric acid. The results of the present study demonstrate the feasibility of acid catalysis of a gas-phase radical isomerization reaction that would otherwise be forbidden.

The gas phase spectra of several vibrational bands of peroxyformic acid (PFA), an atmospheric molecule exhibiting intramolecular hydrogen bonding, are presented. In the fundamental region, Fourier transform infrared (FT-IR) spectroscopy is used to probe the C–O, O–H and C–H stretching vibrations, while in the region of the first and second OH-stretching overtones (2νOH and 3νOH) photoacoustic spectroscopy is used. Integrated absorption cross sections for the PFA vibrational bands are determined by comparing their respective peak areas with that for the OH-stretching bands of n-propanol for which the absorption cross section is known. The measured integrated intensities of the OH stretching bands are then compared with a local mode model using a one-dimensional dipole moment function in conjunction with the OH stretching potential computed at both the MP2/aug-cc-pVDZ and CCSD(T)/aug-cc-pVDZ levels. The data allow us to investigate changes in the OH stretch band position and intensity as a function of overtone order arising from the influence of hydrogen bonding. Furthermore, calculations at the MP2/aug-cc-pVDZ level show that there are three stable conformers of PFA with relative energies of 0, 13.54, and 13.76 kJ/mol, respectively. In the room temperature spectra, however, we see evidence for transitions from only the lowest energy conformer. The geometrical parameters and vibrational frequencies of the most stable conformer are presented.

State-selected photodissociation is used to record the partially rotationally resolved action spectra of CH3OOH in the region of its first and second OH-stretching overtones (2νOH and 3νOH) under free-jet expansion conditions. From an analysis of the rotational band contours for the OH-stretching states and their corresponding COOH torsion combination bands, effective rotational constants and transition dipole moment orientations are determined for the vibrational eigenstates. The level splitting between the lowest symmetric and antisymmetric pair of COOH torsion levels, 0+ and 0−, associated with the 2νOH overtone state is found to be ∼3.9 cm−1. Comparison of spectra in the region of the 2νOH and 2νOH + νCOOH bands in CH3OOH and CD3OOH reveals that the spectral features in CH3OOH are substantially more perturbed compared to those of its deuterated counterpart, suggesting that modes involving the methyl rotor contribute significantly to promoting intramolecular vibrational energy redistribution (IVR) in CH3OOH. Furthermore, a comparison of the average rotational line widths in both CH3OOH and CD3OOH for the 2νOH and 2νOH + νCOOH bands appears to suggest that at these energies, adding one quanta of low-frequency COOH torsional motion does not enhance the IVR rate relative to that of the pure OH-stretching overtone.

Abstract

Carr et al. failed to detect hydroxyl radical formation from the reaction of excited state nitrogen dioxide with water, contrary to our findings. We present several reasons, based on energetic and spectroscopic considerations, why the OH radicals we observed from this reaction are not likely to be due to multiphoton excitation as they suggest.

Abstract

Hydroxyl radicals are often called the “detergent” of the atmosphere because they control the atmosphere's capacity to cleanse itself of pollutants. Here, we show that the reaction of electronically excited nitrogen dioxide with water can be an important source of tropospheric hydroxyl radicals. Using measured rate data, along with available solar flux and atmospheric mixing ratios, we demonstrate that the tropospheric hydroxyl contribution from this source can be a substantial fraction (50%) of that from the traditional O(¹D) + H₂O reaction in the boundary-layer region for high solar zenith angles. Inclusion of this chemistry is expected to affect modeling of urban air quality, where the interactions of sunlight with emitted NO_x species, volatile organic compounds, and hydroxyl radicals are central in determining the rate of ozone formation.

Atmospheric field measurement and modeling studies have long noted discrepancies between observation and predictions of OH and HO2 concentrations in the atmosphere. Novel photochemical mechanisms have been proposed to explain these differences. Although inclusion of these additional sources improves agreement, they are unable to fully account for the observations. We report and demonstrate the importance of weak electronic absorption features, normally ignored or not measured, in contributing to significant OH radical production. Experiments on methyl hydroperoxide, a prototypical organic peroxide in large abundance in the troposphere, highlights how photochemistry in the neglected electronic absorption tail makes an important addition to the tropospheric OH budget. The present results underscore the need to measure absorption cross sections for atmospheric molecules over a wider dynamic range, especially over the wavelength regions where the solar flux is high, to fully quantitate their contributions to atmospheric photochemistry.