Physicochemical Properties of Hydrated Complexes in the Earth's Atmosphere

Composition of the Water Dimer and the Heterodimers of Water with N2 and O2 in Earth's Atmosphere

The mole fractions χ and number concentrations n of the water dimer and the heterodimers H2O-N2 and H2O-O2 in Earth's atmosphere are reported up to 20 km. The water dimer data is obtained from published values of the equilibrium constant based on the water equation of state. The mixed equilibrium constants for the heterodimers are obtained from the respective second virial coefficients using an approach introduced by Stogryn and Hirschfelder that extracts the components pertaining to pairwise interactions producing bound and metastable dimers. From these calculations, χ and n for the water dimer and the (H2O)(N2) and (H2O)(O2) heterodimers at standard sea level are 1.79(6) × 10-5, 4.77(12) × 10-5 and 9.90(5) × 10-6 and 4.55(15) × 1014, 1.23(3) × 1016 and 2.56(1) × 1015, respectively. Analytical expressions are provided for these quantities for altitudes between 0-20 km and temperatures from 200-300 K. Sea level values of χ and n are given for two specific locations.

Charge-Shifted Weak Noncovalent Interactions in the Atmospherically Important OCS Microhydrates

Stratospheric aerosol, mainly comprising microhydrated carbonyl sulfide (OCS), is among the primary drivers of climate change. In this study, we investigate the effect of microhydration on the structure, energetics, and vibrational properties of the neutral OCS molecule using ab initio calculation, molecular electrostatic potential (MESP), topological analyses of electron density, and natural bond orbital (NBO) analyses. The complexation energy increases with the cluster size, and the first solvation shell of OCS consists of four water molecules that interact with the OCS moiety preferentially through S_OCS···O_W, O_OCS···O_W, and C_OCS···O_W type of weak noncovalent interaction instead of the typical O_OCS···H-O_W and S_OCS···H-O_W H-bonds. These noncovalent interactions originate due to the electron shift from the water oxygen lone pair to the antibonding orbital of C═S [BD*(C═S)], sometimes via BD*(C═O), which substantially perturbs the bending mode of surrounding water molecules. The present study thus unravels the underlying relationship between the OCS atmospheric hydrolysis and the charge-shifted noncovalent interactions.

Weakly Bound Complex Formation between HCN and CH3Cl: A Matrix-Isolation and Computational Study

The matrix-isolated infrared spectrum of a hydrogen cyanide-methyl chloride complex was investigated in a solid argon matrix. HCN and CH₃Cl were co-condensed onto a substrate held at 10 K with an excess of argon gas, and the infrared spectrum was measured using Fourier-transform infrared spectroscopy. Quantum chemical geometry optimization, harmonic frequency, and natural bonding orbital calculations indicate stabilized hydrogen- and halogen-bonded structures. The two resulting weakly bound complexes are both composed of one CH₃Cl molecule bound to a (HCN)₃ subunit, where the three HCN molecules are bound head-to-tail in a ring formation. Our study suggests that─in the presence of CH₃Cl─the formation of (HCN)₃ is promoted through complexation. Since HCN aggregates are an important precursor to prebiotic monomers (amino acids and nucleobases) and other life-bearing polymers, this study has astrophysical implications toward the search for life in space.

Unimolecular decomposition of acetyl peroxy radical: a potential source of tropospheric ketene

The unimolecular decomposition of acetyl peroxy radicals followed by subsequent nitration is known to lead to the formation of peroxy acetyl nitrate (PAN) in the troposphere. Using high level quantum chemical calculations, we show that the acetyl peroxy radical is a precursor in the formation of tropospheric ketene. The results show that the presence of a single or double water molecule(s) as a catalyst does not influence the decomposition reaction directly to form ketene and hydroperoxy radicals. The electronic excitation of the reactive and product complexes occurs in the wavelength range of ∼1400 nm, suggesting that the complexes undergo photoexcitation in the near IR region. The results ascertain that the dissociation of acetyl peroxy radicals into ketene and hydroperoxy radicals occurs more likely through the excitation route and the corresponding excitation wavelength reveals that the reactions are red-light driven. Three different product complexes, ketene·HO2, ketene·H2O·HO2 and ketene·(H2O)2·HO2, are formed from the reaction. The direct dynamics simulations show that the product complexes are more stable and possess a long lifetime. The calculated temperature dependent equilibrium constant of the product complexes reveals that their atmospheric abundances decrease with increasing altitudes.

Impact of water on the BrO + HO2 gas-phase reaction: mechanism, kinetics and products

The BrO + HO2 reaction, which participates in the cycle of ozone removal via BrOH formation, was explored both in the absence and in the presence of water using ab initio calculations. Two main sets of products, (i) HBr + O3 and (ii) BrOH + O2, are formed regardless of the presence of water, following a hydrogen abstraction mechanism. The HBr + O3 products are formed from the intermediate BrOOOH adduct, whereas BrOH + O2 are formed either from the intermediate OBrOOH adduct or via a barrierless hydrogen transfer from HO2 to BrO. Owing to the formation of molecular oxygen that can bear different spin configurations, the formation of BrOH + O2 products was examined both on the singlet and the triplet surfaces. Under relevant atmospheric temperatures and pressure, the formation of products (i) is energetically and kinetically less favorable than that of products (ii). The rate coefficient at 298 K for the HBr + O3 formation was determined to be 2.00 × 10-20 cm3 molecule-1 s-1, and found to decrease by 1-2 orders of magnitude when one or both reactants are clustered with water. For the formation of BrOH + O2, a rate coefficient of 2.21 × 10-11 cm3 molecule-1 s-1 is determined on both singlet and triplet surfaces in the absence of water. Though this rate coefficient slightly decreases for the hydrated reactions, the fractions of the reactants that are effectively complexed with water are not high enough to shift the overall BrOH + O2 formation rate. The current study further indicates that humidity plays a negligible role in ozone removal via the BrO + HO2 reaction.

Role of (H2O) n (n = 1-2) in the Gas-Phase Reaction of Ethanol with Hydroxyl Radical: Mechanism, Kinetics, and Products

The effect of water on the hydrogen abstraction mechanism and product branching ratio of CH₃CH₂OH + ^•OH reaction has been investigated at the CCSD(T)/aug-cc-pVTZ//BH&HLYP/aug-cc-pVTZ level of theory, coupled with the reaction kinetics calculations, implying the harmonic transition-state theory. Depending on the hydrogen sites in CH₃CH₂OH, the bared reaction proceeds through three elementary paths, producing CH₂CH₂OH, CH₃CH₂O, and CH₃CHOH and releasing a water molecule. Thermodynamic and kinetic results indicate that the formation of CH₃CHOH is favored over the temperature range of 216.7-425.0 K. With the inclusion of water, the reaction becomes quite complex, yielding five paths initiated by three channels. The products do not change compared with the bared reaction, but the preference for forming CH₃CHOH drops by up to 2%. In the absence of water, the room temperature rate coefficients for the formation of CH₂CH₂OH, CH₃CH₂O, and CH₃CHOH are computed to be 5.2 × 10^-13, 8.6 × 10^-14, and 9.0 × 10^-11 cm³ molecule^-1 s^-1, respectively. The effective rate coefficients of corresponding monohydrated and dihydrated reactions are 3-5 and 6-8 orders of magnitude lower than those of the unhydrated reaction, indicating that water has a decelerating effect on the studied reaction. Overall, the characterized effects of water on the thermodynamics, kinetics, and products of the CH₃CH₂OH + ^•OH reaction will facilitate the understanding of the fate of ethanol and secondary pollutants derived from it.

Far-IR and UV spectral signatures of controlled complexation and microhydration of the polycyclic aromatic hydrocarbon acenaphthene

UV and IR spectroscopic study of the controlled complexation and microhydration of a polycyclic aromatic hydrocarbon under isolated conditions using free electron laser FELIX.

Hydrogen bond docking preference in furans: OH⋯π vs. OH⋯O

The docking sites of hydrogen bonds in complexes formed between 2,2,2-trifluoroethanol (TFE), furan (Fu), and 2-methyl furan (MF) have been investigated. Using density functional theory (DFT) calculations, gas phase and matrix isolation FTIR spectroscopies, the strengths of OH⋯O and OH⋯π hydrogen bonds in the complexes were compared to find the docking preference. Calculations suggest that the hydrogen bond donor, TFE, is more likely to dock onto the oxygen atom of the aromatic furans ring, and consequently, the OH⋯O type hydrogen bond is relatively stronger than the OH⋯π type. The FTIR spectrum in the OH-stretching fundamental range obtained at room temperatures has been compared with that obtained at extremely low temperatures in the matrix. The fundamental and the red shifts of OH-stretching vibrations were observed in both FTIR spectra, confirming the formation of hydrogen bonded complexes. By assessing the ability of furan and MF to participate in the formation of OH⋯O hydrogen bond, the effect of ring methylation has been highlighted. From the calculated geometric and thermodynamic parameters as well as the frequency shift of the OH-stretching vibrations in complexes, TFE-MF is found to be more stable than TFE-Fu, which suggests that the strength of the OH⋯O hydrogen bond in TFE-MF originates from the high activity of the furan molecule caused by the methylation of the aromatic ring. The present study furthers the knowledge of docking preference in heteroaromatic molecules and is helpful to understand the nature of intermolecular interactions between hydrogen bond donors and acceptors, including both electron-deficient atoms and π cloud.

Water catalysis of the reaction between hydroxyl radicals and linear saturated alcohols (ethanol and n-propanol) at 294 K

Water accelerates the title reaction by lowering the energy barrier and increasing the dipole moments of the reactants.

Matrix isolation FTIR study of hydrogen-bonded complexes of methanol with heterocyclic organic compounds

Hydrogen bonded complexes of heterocyclic compounds with methanol were studied using matrix isolation FTIR spectroscopy and theoretical calculations.

Structure and Abundance of Nitrous Oxide Complexes in Earth's Atmosphere

We have investigated the lowest energy structures and binding energies of a series of atmospherically relevant nitrous oxide (N2O) complexes using explicitly correlated coupled cluster theory. Specifically, we have considered complexes with nitrogen (N2-N2O), oxygen (O2-N2O), argon (Ar-N2O), and water (H2O-N2O). We have calculated rotational constants and harmonic vibrational frequencies for the complexes and the constituent monomers. Statistical mechanics was used to determine the thermodynamic parameters for complex formation as a function of temperature and pressure. These results, in combination with relevant atmospheric data, were used to estimate the abundance of N2O complexes in Earth's atmosphere as a function of altitude. We find that the abundance of N2O complexes in Earth's atmosphere is small but non-negligible, and we suggest that N2O complexes may contribute to absorption of terrestrial radiation and be relevant for understanding the atmospheric fate of N2O.

Accurate thermodynamic properties of gas phase hydrogen bonded complexes

The increase in temperature causes a decrease in equilibrium constant, which makes accurate determination of the enthalpy of complex formation possible.

The benzoic acid-water complex: a potential atmospheric nucleation precursor studied using microwave spectroscopy and ab initio calculations

The pure rotational, high-resolution spectrum of the benzoic acid-water complex was measured in the range of 4-14 GHz, using a cavity-based molecular beam Fourier-transform microwave spectrometer. In all, 40 a-type transitions and 2 b-type transitions were measured for benzoic acid-water, and 12 a-type transitions were measured for benzoic acid-D2O. The equilibrium geometry of benzoic acid-water was determined with ab initio calculations, at the B3LYP, M06-2X, and MP2 levels of theory, with the 6-311++G(2df,2pd) basis set. The experimental rotational spectrum is most consistent with the B3LYP-predicted geometry. Narrow splittings were observed in the b-type transitions, and possible tunnelling motions were investigated using the B3LYP/6-311++G(d,p) level of theory. Rotation of the water moiety about the lone electron pair hydrogen-bonded to benzoic acid, across a barrier of 7.0 kJ mol(-1), is the most likely cause for the splitting. Wagging of the unbound hydrogen atom of water is barrier-less, and this large amplitude motion results in the absence of c-type transitions. The interaction and spectroscopic dissociation energies calculated using B3LYP and MP2 are in good agreement, but those calculated using M06-2X indicate excess stabilization, possibly due to dispersive interactions being over-estimated. The equilibrium constant of hydration was calculated by statistical thermodynamics, using ab initio results and the experimental rotational constants. This allowed us to estimate the changes in percentage of hydrated benzoic acid with variations in the altitude, region, and season. Using monitoring data from Calgary, Alberta, and the MP2-predicted dissociation energy, a yearly average of 1% of benzoic acid is expected to be present in the form of benzoic acid-water. However, this percentage depends sensitively on the dissociation energy. For example, when using the M06-2X-predicted dissociation energy, we find it increases to 18%.

The effect of fluorine substitution in alcohol–amine complexes

The effect of fluorine substitution on the hydrogen bond strength in alcohol–amine molecular complexes was investigated, with a combination of vapour phase infrared and near infrared spectroscopy and theoretical calculations.

The gas-phase reaction of methane sulfonic acid with the hydroxyl radical without and with water vapor

The gas phase reaction between methane sulfonic acid (CH3SO3H; MSA) and the hydroxyl radical (HO), without and with a water molecule, was investigated with DFT-B3LYP and CCSD(T)-F12 methods. For the bare reaction we have found two reaction mechanisms, involving proton coupled electron transfer and hydrogen atom transfer processes that produce CH3SO3 and H2O. We also found a third reaction mechanism involving the double proton transfer process, where the products and reactants are identical. The computed rate constant for the oxidation process is 8.3 × 10(-15) cm(3) s(-1) molecule(-1). CH3SO3H forms two very stable complexes with water with computed binding energies of about 10 kcal mol(-1). The presence of a single water molecule makes the reaction between CH3SO3H and HO much more complex, introducing a new reaction that consists in the interchange of H2O between HO and CH3SO3H. Our kinetic calculations show that 99.5% of the reaction involves this interchange of the water molecule and, consequently, water vapor does not play any role in the oxidation reaction of methane sulfonic acid by the hydroxyl radical.

Fundamental and overtone vibrational spectroscopy, enthalpy of hydrogen bond formation and equilibrium constant determination of the methanol-dimethylamine complex

We have measured gas phase vibrational spectra of the bimolecular complex formed between methanol (MeOH) and dimethylamine (DMA) up to about 9800 cm(-1). In addition to the strong fundamental OH-stretching transition we have also detected the weak second overtone NH-stretching transition. The spectra of the complex are obtained by spectral subtraction of the monomer spectra from spectra recorded for the mixture. For comparison, we also measured the fundamental OH-stretching transition in the bimolecular complex between MeOH and trimethylamine (TMA). The enthalpies of hydrogen bond formation (ΔH) for the MeOH-DMA and MeOH-TMA complexes have been determined by measurements of the fundamental OH-stretching transition in the temperature range from 298 to 358 K. The enthalpy of formation is found to be -35.8 ± 3.9 and -38.2 ± 3.3 kJ mol(-1) for MeOH-DMA and MeOH-TMA, respectively, in the 298 to 358 K region. The equilibrium constant (Kp) for the formation of the MeOH-DMA complex has been determined from the measured and calculated transition intensities of the OH-stretching fundamental transition and the NH-stretching second overtone transition. The transition intensities were calculated using an anharmonic oscillator local mode model with dipole moment and potential energy curves calculated using explicitly correlated coupled cluster methods. The equilibrium constant for formation of the MeOH-DMA complex was determined to be 0.2 ± 0.1 atm(-1), corresponding to a ΔG value of about 4.0 kJ mol(-1).

Identification of the dimethylamine-trimethylamine complex in the gas phase

We have identified the dimethylamine-trimethylamine complex (DMA-TMA) at room temperature in the gas phase. The Fourier transform infrared (FTIR) spectrum of DMA-TMA in the NH-stretching fundamental region was obtained by spectral subtraction of spectra of each monomer. Explicitly correlated coupled cluster calculations were used to determine the minimum energy structure and interaction energy of DMA-TMA. Frequencies and intensities of NH-stretching transitions were also calculated at this level of theory with an anharmonic oscillator local mode model. The fundamental NH-stretching intensity in DMA-TMA is calculated to be approximately 700 times larger than that of the DMA monomer. The measured and calculated intensity is used to determine a room temperature equilibrium constant of DMA-TMA of 1.7 × 10(-3) atm(-1) at 298 K.

Infrared shifts of the water dimer from the fully flexible ab initio HBB2 potential

We report calculations of the infrared shifts for the water dimer, as obtained from the recent ab initio fully flexible HBB2 potential of Bowman and co-workers. The rovibrational calculations, which formally are 12-dimensional plus overall rotation, were performed within the [6+6]d adiabatic separation which decouples the 'fast' intramolecular modes from the 'slow' intermolecular ones. Apart from this decoupling, each set of modes is treated in a fully variational approach. The intramolecular motion was described in terms of Radau coordinates, using the f-embedding formulation of Wei & Carrington, and neglecting the rovibrational Coriolis coupling terms. Within this adiabatic approximation, the intermolecular motion is handled in a similar way as for rigid monomers, except for the rotational constants B's, averaged over intramolecular modes that depend now on the intermolecular geometry. Comparison with experimental data shows an excellent overall agreement.

Fourier transform infrared spectroscopy and theoretical study of dimethylamine dimer in the gas phase

Dimethylamine (DMA) has been studied by gas-phase Fourier transform infrared (FTIR) spectroscopy. We have identified a spectral transition that is assigned to the DMA dimer. The IR spectra of the dimer in the gas phase are obtained by spectral subtraction of spectra recorded at different pressures. The enthalpy of hydrogen bond formation was obtained for the DMA dimer by temperature-dependence measurements. We complement the experimental results with ab initio and anharmonic local mode model calculations of monomer and dimer. Compared to the monomer, our calculations show that in the dimer the N-H bond is elongated, and the NH-stretching fundamental shifts to a lower wavenumber. More importantly, the weak NH-stretching fundamental transition has a pronounced intensity increase upon complexation. However, the first NH-stretching overtone transition is not favored by the same intensity enhancement, and we do not observe the first NH-stretching overtone of the dimer. On the basis of the measured and calculated intensity of the NH-stretching transition of the dimer, the equilibrium constant for dimerization at room temperature was determined.