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Five structures of Ge 2 H 2 and Ge 2 H 2 + are investigated in this study. Optimized geometries at the CCSD(T)/cc-pwCVQZ-PP level of theory were obtained. Focal point analyses were performed on these optimized geometries to determine relative energies using the CCSD(T) method with polarized basis sets up to quintuple-zeta. Energy corrections include full T and pertubative (Q) coupled-cluster effects plus anharmonic corrections to the zero-point vibrational energy. Relative ordering in energy from lowest to highest of the five Ge 2 H 2 + structures is butterfly, germylidene, monobridged, trans , then linear. In neutral Ge 2 H 2 , the monobridged structure lies lower in energy than the germylidene structure. Fundamental vibrational frequencies and IR intensities were computed for the minima at the CCSD(T)/cc-pwCVTZ-PP level of theory to compare with experimental research. Partial atomic charges and natural bonding orbital analyses indicated that the positive charge of Ge 2 H 2 + is contained in the region of the Ge-Ge bond.

The cyano radical (CN) is an abundant, open-shell molecule found in a variety of environments, including the atmosphere, the interstellar medium and combustion processes. In these environments, it often reacts with small, closed-shell molecules via hydrogen abstraction. Both carbon and nitrogen atoms of the cyano radical are reactive sites, however the carbon is more reactive with reaction barrier heights generally between 2-15 kcal mol -1 lower than those of the analogous nitrogen. The CN + HX → HCN/HNC + X, with X = H, CH 3 , NH 2 , OH, F, SiH 3 , PH 2 , SH, Cl, C 2 H, CN reactions have been studied at a high-level of theory, including CCSD(T)-F12a. Furthermore, kinetics were obtained over the 100-1000 K temperature range, showing excellent agreement with those rate constants that have been determined experimentally.

Formic acid (FA) is a ubiquitous molecule found in the atmosphere, and is relevant to many important processes. The FA molecule generally exists as the trans isomer, which can decompose into H2O and CO (dehydration). It can also exist in the less favorable cis isomer which can decompose into H2 and CO2 (decarboxylation). Our work examines the complexes formed between each isomer of FA with water. We present geometries and vibrational frequencies obtained at the reliable CCSD(T)/aug-cc-pVTZ level of theory for seven FAwater complexes. We utilize the focal point method to determine CCSDT(Q)/CBS plus corrections binding energies of 7.37, 3.36, and 2.02 kcal mol-1 plus 6.07, 3.79, 2.60, and 2.55 kcal mol-1 for the trans-FAwater and cis-FAwater complexes, respectively. Natural bond orbital analysis is used to further decompose the interactions in each complex and gain insight into their relative strengths. Furthermore, we examine the effect that a single water molecule has on the barrier heights to each decomposition pathway by optimizing the transition states and verifying their connectivity with intrinsic reaction coordinate computations as well as utilizing a kinetic model. Water lowers the barrier to dehydration by at most 15.78 kcal mol-1 and the barrier to decarboxylation by up to 15.90 kcal mol-1. Our research also examines for the first time the effect of one water molecule on the interconversion barrier and we find that the barrier from trans to cis is not catalyzed by water due to the strong FA and water interactions. Our results highlight some instances where different binary complexes result in different decomposition pathways and even a case where one binary complex can form the same decomposition products via two distinct mechanisms. Our results provide a reliable benchmark of the FAH2O system as well as provide insight into future studies of similar atmospheric systems.

Criegee intermediates are important targets for study in atmospheric chemistry because of their capacity to oxidize airborne species. Among these species, ammonia has received critical attention for its presence in polluted agricultural or industrial areas and its role in forming particulate matter and condensation nuclei. Although methylamine has been given less attention than ammonia, both theoretical and experimental studies have demonstrated that the additional methyl substitution on the ammonia derivatives increases the rate constants for some systems. This suggests that the methylamine addition to Criegee intermediates could be more significant to atmospheric processes. In this work, geometries are optimized at the DF-CCSD(T)/ANO1 level for the methylamine addition reactions to the simplest Criegee intermediate and the anti- and syn-methylated Criegee intermediates. Energies for each stationary point were computed at the CCSD(T)/CBS level with corrections from the CCSDT(Q) method. Rate constants are obtained for each reaction using canonical transition state theory. Although methylamine addition proved to be a more favorable reaction relative to ammonia addition, the significantly lower concentration of atmospheric methylamine limits the prevalence of these reactions, even in the most optimal conditions. It is unlikely that the methylamine addition to Criegee intermediates will contribute significantly to the consumption of Criegee intermediates in the atmosphere.

This research presents an ab initio characterization of the potential energy surface for the methylamine plus 1 D oxygen atom reaction, which may be relevant to interstellar chemistry. Geometries and harmonic vibrational frequencies were determined for all stationary points at the CCSD(T)/aug-cc-pVTZ level of theory. The focal point method along with several additive corrections was used to obtain reliable CCSDT(Q)/CBS potential energy surface features. Extensive conformational analysis and intrinsic reaction coordinate computations were performed to ensure accurate chemical connectivity of the stationary points. Five minima were determined to be possible products of this reaction and three novel transition states were found that were previously unreported or mislabeled in the literature. The pathways we present can be used to guide further searches for NH 2 containing species in the interstellar medium.

Bimolecular reactions involving stabilized Criegee intermediates (SCI) have been the target of many studies due to the role these molecules play in atmospheric chemistry. Recently, kinetic rates for the addition reaction of the simplest SCI (formaldehyde oxide) and its methylated analogue (acetone oxide) with methanol were reported both experimentally and theoretically. We re-examine the energy profile of these reactions by employing rigorous ab initio methods. Optimized CCSD(T)/ANO1 geometries are reported for the stationary points along the reaction path. Energies are obtained at the CCSD(T)/CBS level of theory. Contributions of full triple and quadruple excitations are computed to assess the convergence of this method. Rate constants are obtained using conventional canonical transition state theory under the rigid rotor harmonic oscillator approximation and with the inclusion of a one-dimensional hindered rotor treatment. These corrections for internal rotations have a significant impact on computed kinetic rate constants. With this approach, we compute rate constants for the addition of methanol to formaldehyde oxide (H 2 COO) and acetone oxide [(CH 3 ) 2 COO] at 298.15 K as (1.2 ± 0.8) × 10 -13 and (2.8 ± 1.3) × 10 -15 cm 3 s -1 , respectively. Additionally, we investigate the temperature dependence of the rate constant, concluding that the transition state barrier height and tunneling contributions shape the qualitative behaviour of these reactions.

Hypohalous acids (HOX) are a class of molecules that play a key role in the atmospheric seasonal depletion of ozone and have the ability to form both hydrogen and halogen bonds. The interactions between the HOX monomers (X = F, Cl, Br) and water have been studied at the CCSD(T)/aug-cc-pVTZ level of theory with the spin free X2C-1e method to account for scalar relativistic effects. Focal point analysis was used to determine CCSDT(Q)/CBS dissociation energies. The anti hydrogen bonded dimers were found with interaction energies of -5.62 kcal mol-1, -5.56 kcal mol-1, and -4.97 kcal mol-1 for X = F, Cl, and Br, respectively. The weaker halogen bonded dimers were found to have interaction energies of -1.71 kcal mol-1 and -3.03 kcal mol-1 for X = Cl and Br, respectively. Natural bond orbital analysis and symmetry adapted perturbation theory were used to discern the nature of the halogen and hydrogen bonds and trends due to halogen substitution. The halogen bonds were determined to be weaker than the analogous hydrogen bonds in all cases but close enough in energy to be relevant, significantly more so with increasing halogen size.

Despite the interest in sulfur monoxide (SO) among astrochemists, spectroscopists, inorganic chemists, and organic chemists, its interaction with water remains largely unexplored. We report the first high level theoretical geometries for the two minimum energy complexes formed by sulfur monoxide and water, and we report energies using basis sets as large as aug-cc-pV(Q+d)Z and correlation effects through perturbative quadruple excitations. One structure of SOH2O is hydrogen bonded and the other chalcogen bonded. The hydrogen bonded complex has an electronic energy of -2.71 kcal mol-1 and a zero kelvin enthalpy of -1.67 kcal mol-1, while the chalcogen bonded complex has an electronic energy of -2.64 kcal mol-1 and a zero kelvin enthalpy of -2.00 kcal mol-1. We also report the transition state between the two structures, which lies below the SOH2O dissociation limit, with an electronic energy of -1.26 kcal mol-1 and an enthalpy of -0.81 kcal mol-1. These features are much sharper than for the isovalent complex of O2 and H2O, which only possesses one weakly bound minimum, so we further analyze the structures with open-shell SAPT0. We find that the interactions between O2 and H2O are uniformly weak, but the SOH2O complex surface is governed by the superior polarity and polarizability of SO, as well as the diffuse electron density provided by sulfur's extra valence shell.

The spectroscopic identification of Bi4 has been very elusive. Two constitutional Bi4 isomers of Td and C2v symmetry are investigated and each is found to be a local energetic minimum. The optimized geometries and vibrational frequencies of these two isomers are obtained at the CCSD(T)/cc-pVQZ-PP level of theory, utilizing the Stoll, Metz, and Dolg 60-electron effective core potential. The fundamental frequencies of the Td isomer are obtained at the same level of theory. The focal point analysis method, from a maximum basis set of cc-pV5Z-PP, and proceeding to a maximum correlation method of CCSDTQ, was employed to determine the dissociation energy of Bi4 (Td) into two Bi2 and the adiabatic energy difference between the C2v and Td isomers of Bi4. These quantities are predicted to be +65 kcal mol-1 and +39 kcal mol-1, respectively. Two electron vertical excitation energies between the Td and C2v electronic configurations are computed to be 156 kcal mol-1 for the Td isomer and 9 kcal mol-1 for the C2v isomer. The most probable approach to laboratory spectroscopic identification of Bi4 is via an infrared spectrum. The predicted fundamentals (cm-1) with harmonic IR intensities in parentheses (km mol-1) are 94(0), 123(0.23), and 167(0) for the Td isomer. The moderate IR intensity for the only allowed fundamental may explain why Bi4 has yet to be observed. Through natural bond orbital analysis, the C2v isomer of Bi4 was discovered to exhibit "long-bonding" between the furthest apart 'wing' atoms. This long-bonding is postulated to be facilitated by the σ-bonding orbital between the 'spine' atoms of the C2v isomer.

Vinyl alcohol and acetaldehyde are isoelectronic products of incomplete butanol combustion. Along with the radicals resulting from the removal of atomic hydrogen or the hydroxyl radical, these species are studied here using ab initio methods as complete as coupled cluster theory with single, double, triple, and perturbative quadruple excitations [CCSDT(Q)], with basis sets as large as cc-pV5Z. The relative energies provided herein are further refined by including corrections for relativistic effects, the frozen core approximation, and the Born-Oppenheimer approximation. The effects of anharmonic zero-point vibrational energies are also treated. The syn conformer of vinyl alcohol is predicted to be lower in energy than the anti conformer by 1.1 kcal mol -1 . The alcoholic hydrogen of syn-vinyl alcohol is found to be the easiest to remove, requiring 84.4 kcal mol -1 . Five other radicals are also carefully considered, with four conformers investigated for the 1-hydroxyvinyl radical. Beyond energetics, we have conducted an overhaul of the spectroscopic literature for these species. Our results also provide predictions for fundamental modes yet to be reported experimentally. To our knowledge, the ν 3 (3076 cm -1 ) and ν 4 (2999 cm -1 ) C-H stretches for syn-vinyl alcohol and all but one of the vibrational modes for anti-vinyl alcohol (ν 1 -ν 14 ) are yet to be observed experimentally. For the acetyl radical, ν 6 (1035 cm -1 ), ν 11 (944 cm -1 ), ν 12 (97 cm -1 ), and accounting for our changes to the assignment of the 1419.9 cm -1 experimental mode, ν 10 (1441 cm -1 ), are yet to be observed. We have predicted these unobserved fundamentals and reassigned the experimental 1419.9 cm -1 frequency in the acetyl radical to ν 4 rather than to ν 10 . Our work also strongly supports reassignment of the ν 10 and ν 11 fundamentals of the vinoxy radical. We suggest that the bands assigned to the overtones of these fundamentals were in fact combination bands. Our findings may be useful in constructing improved combustion models of butanol and in spectroscopically characterizing these molecules further.

Interest in peroxy radicals derives from their central role in tropospheric and low-temperature combustion processes; however, their transient nature limits the scope of possible experimental characterization. As a result, theoretical methods (notably, coupled-cluster theory) have become indispensable in the reliable prediction of properties of such ephemeral open-shell systems. Herein, the X[combining tilde] and à state conformers of ethylperoxy radical (C 2 H 5 O 2 ) have been structurally optimized at the CCSD(T)/ANO2 level of theory. Relative enthalpies at 0 K [including à ← X[combining tilde] transition origins (T 0 )] are reported, incorporating CCSD(T) electronic energies extrapolated to the complete basis set limit via the focal point approach. Higher-level computations, employing basis sets as large as cc-pV5Z and post-HF methods up to CCSDT(Q), prove essential in achieving predictions to within 10 cm -1 for experimental T 0 ; we predict 7363 and 7583 cm -1 for the trans and gauche conformers, respectively. Furthermore, predictions of X[combining tilde] state fundamental transitions incorporating CCSD(T)/ANO0 anharmonic contributions are given. For each conformer, all 21 modes were characterized, improving upon the 16 modes reported in the experimental literature, and providing predictions for the 5 remaining modes.