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The n-propylperoxy radical has been described as a molecule of critical importance to studies of low temperature combustion. Ab initio methods were used to study this three-carbon alkylperoxy radical, normal propylperoxy. Reliable CCSD(T) (coupled-cluster theory, incorporating single, double, and perturbative triple)/ANO0 geometries were predicted for the molecule's five rotamers. For each rotamer, energetic predictions were made using basis sets as large as the cc-pV5Z in conjunction with coupled cluster levels of theory up to CCSDT(Q). Along with the extrapolations, corrections for relativistic effects, zero-point vibrational energies, and diagonal Born-Oppenheimer corrections were used to further refine energies. The results indicate that the lowest conformer is the gauche-gauche (GG) rotamer followed by the gauche-trans (0.12 kcal mol-1 above GG), trans-gauche (0.44 kcal mol-1), gauche'-gauche (0.47 kcal mol-1), and trans-trans (0.57 kcal mol-1). Fundamental vibrational frequencies were obtained using second-order vibrational perturbation theory. This is the first time anharmonic frequencies have been computed for this system. The most intense IR features include all but one of the C-H stretches. The O-O fundamental (1063 cm-1 for the GG structure) also has a significant IR intensity, 19.6 km mol-1. The anharmonicity effects on the potential energy surface were also used to compute vibrationally averaged rg,0K bond lengths, accounting for zero-point vibrations present within the molecule. [ABSTRACT FROM AUTHOR]

Four constitutional isomers of diindium dihydride have been studied utilising rigorous quantum mechanical methods. Geometries were optimised with the CCSD(T) method using the aug-cc-pwCVTZ basis set and a small-core pseudopotential for the indium atoms. Relative energetics were determined at the CCSD(T)/CBS level of theory, and the higher order δT(Q) contributions are also computed. The monobridged and vinylidene-like isomers lie 10.8 and 13.6 kcal mol − 1 above the planar dibridged isomer, respectively. At 0 K, the trans isomer is the least favoured energetically, differing from the dibridged isomer by 16.6 kcal mol − 1 . Anharmonic vibrational frequencies were predicted to provide theoretical insight into future experimental studies. Very detailed comparisons are made to matrix isolation experiments from the group of Downs and the group of Andrews. Numerous agreements and disagreements were found. [ABSTRACT FROM AUTHOR]

Oxiranes are a class of cyclic ethers formed in abundance during low‐temperature combustion of hydrocarbons and biofuels, either via chain‐propagating steps that occur from unimolecular decomposition of β‐hydroperoxyalkyl radicals (β‐̇QOOH) or from reactions of HȮO with alkenes. The cis‐ and trans‐isomers of 2,3‐dimethyloxirane are intermediates of n‐butane oxidation, and while rate coefficients for β‐̇QOOH → 2,3‐dimethyloxirane + ̇OH are reported extensively, subsequent reaction mechanisms of the cyclic ethers are not. As a result, chemical kinetics mechanisms commonly adopt simplified chemistry to describe the consumption of 2,3‐dimethyloxirane by convoluting several elementary reactions into a single step, which may introduce mechanism truncation error—uncertainty derived from missing or incomplete chemistry. The present research examines the isomer dependence of 2,3‐dimethyloxirane reaction mechanisms in support of ongoing efforts to minimize mechanism truncation error. Reaction mechanisms are inferred via the detection of products from Cl‐initiated oxidation of both cis‐2,3‐dimethyloxirane and trans‐2,3‐dimethyloxirane using multiplexed photoionization mass spectrometry (MPIMS). The experiments were conducted at 10 Torr and temperatures of 650 K and 800 K. To complement the experiments, the enthalpies of stationary points on the ̇R + O2 surfaces were computed at the ccCA‐PS3 level of theory. In total, 28 barrier heights were computed on the 2,3‐dimethyloxiranylperoxy surfaces. Two notable aspects are low‐lying pathways that form resonance‐stabilized ketohydroperoxide‐type radicals caused by ̇QOOH ring‐opening when the unpaired electron is localized adjacent to the ether group, and cis‐trans isomerization of ̇R and ̇QOOH radicals, via inversion, which enable reaction pathways otherwise restricted by stereochemistry. Several species were identified in the MPIMS experiments from ring opening of 2,3‐dimethyloxiranyl radicals. Neither of the two conjugate alkene isomers prototypical of ̇R + O2 reactions were detected. Products were also identified from decomposition of ketohydroperoxide‐type radicals. The present work provides the first analysis of 2,3‐dimethyloxirane oxidation chemistry and reveals that consumption pathways are complex and require the expansion of submechanisms in chemical kinetics mechanisms. [ABSTRACT FROM AUTHOR]

Oxiranes are a class of cyclic ethers formed in abundance during low‐temperature combustion of hydrocarbons and biofuels, either via chain‐propagating steps that occur from unimolecular decomposition of β‐hydroperoxyalkyl radicals (β‐̇QOOH) or from reactions of HOȮ with alkenes. Ethyloxirane is one of four alkyl‐substituted cyclic ether isomers produced as an intermediate from n‐butane oxidation. While rate coefficients for β‐̇QOOH → ethyloxirane + ȮH are reported extensively, subsequent reaction mechanisms of the cyclic ether are not. As a result, chemical kinetics mechanisms commonly adopt simplified chemistry to describe ethyloxirane consumption by convoluting several elementary reactions into a single step, which may introduce mechanism truncation error—uncertainty derived from missing or incomplete chemistry. The present work provides fundamental insight on reaction mechanisms of ethyloxirane in support of ongoing efforts to minimize mechanism truncation error. Reaction mechanisms are inferred from the detection of products during chlorine atom‐initiated oxidation experiments using multiplexed photoionization mass spectrometry conducted at 10 Torr and temperatures of 650 K and 800 K. To complement the experiments, calculations of stationary point energies were conducted using the ccCA‐PS3 composite method on ̇R + O2 potential energy surfaces for the four ethyloxiranyl radical isomers, which produced barrier heights for 24 reaction pathways. In addition to products from ̇QOOH → cyclic ether + ȮH and ̇R + O2 → conjugate alkene + HOȮ, both of which were significant pathways and are prototypical to alkane oxidation, other species were identified from ring‐opening of both ethyloxiranyl and ̇QOOH radicals. The latter occurs when the unpaired electron is localized on the ether group, causing the initial ̇QOOH structure to ring‐open and form a resonance‐stabilized ketohydroperoxide‐type radical. The present work provides the first analysis of ethyloxirane oxidation chemistry, which reveals that consumption pathways are complex and may require an expansion of submechanisms to increase the fidelity of chemical kinetics mechanisms. [ABSTRACT FROM AUTHOR]

The ethyl radical has been isolated and spectroscopically characterized in 4He nanodroplets. The band origins of the five CH stretch fundamentals are shifted by < 2 cm-1 from those reported for the gas phase species [S. Davis, D. Uy, and D. J. Nesbitt, J. Chem. Phys. 112, 1823 (2000); T. Häber, A. C. Blair, D. J. Nesbitt, and M. D. Schuder, J. Chem. Phys. 124, 054316 (2006)]. The symmetric CH2 stretching band (v1) is rotationally resolved, revealing nuclear spin statistical weights predicted by G12 permutation-inversion group theory. A permanent electric dipole moment of 0.28 (2) D is obtained via the Stark spectrum of the v1 band. The four other CH stretch fundamental bands are significantly broadened in He droplets and lack rotational fine structure. This broadening is attributed to symmetry dependent vibration-to-vibration relaxation facilitated by the He droplet environment. In addition to the five fundamentals, three a1′ overtone/combination bands are observed, and each of these have resolved rotational substructure. These are assigned to the 2v12, v4 + v6, and 2v6 bands through comparisons to anharmonic frequency computations at the CCSD(T)/cc-pVTZ level of theory. [ABSTRACT FROM AUTHOR]

Symmetry-adapted perturbation theory (SAPT) provides a means of probing the fundamental nature of intermolecular interactions. Low-orders of SAPT (here, SAPT0) are especially attractive since they provide qualitative (sometimes quantitative) results while remaining tractable for large systems. The application of density fitting and Laplace transformation techniques to SAPT0 can significantly reduce the expense associated with these computations and make even larger systems accessible. We present new factorizations of the SAPT0 equations with density-fitted two-electron integrals and the first application of Laplace transformations of energy denominators to SAPT. The improved scalability of the DF-SAPT0 implementation allows it to be applied to systems with more than 200 atoms and 2800 basis functions. The Laplace-transformed energy denominators are compared to analogous partial Cholesky decompositions of the energy denominator tensor. Application of our new DF-SAPT0 program to the intercalation of DNA by proflavine has allowed us to determine the nature of the proflavine-DNA interaction. Overall, the proflavine-DNA interaction contains important contributions from both electrostatics and dispersion. The energetics of the intercalator interaction are are dominated by the stacking interactions (two-thirds of the total), but contain important contributions from the intercalator-backbone interactions. It is hypothesized that the geometry of the complex will be determined by the interactions of the intercalator with the backbone, because by shifting toward one side of the backbone, the intercalator can form two long hydrogen-bonding type interactions. The long-range interactions between the intercalator and the next-nearest base pairs appear to be negligible, justifying the use of truncated DNA models in computational studies of intercalation interaction energies. [ABSTRACT FROM AUTHOR]

Using a Lagrangian-based approach, we present a more elegant derivation of the equations necessary for the variational optimization of the molecular orbitals (MOs) for the coupled-cluster doubles (CCD) method and second-order Mo\ller-Plesset perturbation theory (MP2). These orbital-optimized theories are referred to as OO-CCD and OO-MP2 (or simply 'OD' and 'OMP2' for short), respectively. We also present an improved algorithm for orbital optimization in these methods. Explicit equations for response density matrices, the MO gradient, and the MO Hessian are reported both in spin-orbital and closed-shell spin-adapted forms. The Newton-Raphson algorithm is used for the optimization procedure using the MO gradient and Hessian. Further, orbital stability analyses are also carried out at correlated levels. The OD and OMP2 approaches are compared with the standard MP2, CCD, CCSD, and CCSD(T) methods. All these methods are applied to H2O, three diatomics, and the O4+ molecule. Results demonstrate that the CCSD and OD methods give nearly identical results for H2O and diatomics; however, in symmetry-breaking problems as exemplified by O4+, the OD method provides better results for vibrational frequencies. The OD method has further advantages over CCSD: its analytic gradients are easier to compute since there is no need to solve the coupled-perturbed equations for the orbital response, the computation of one-electron properties are easier because there is no response contribution to the particle density matrices, the variational optimized orbitals can be readily extended to allow inactive orbitals, it avoids spurious second-order poles in its response function, and its transition dipole moments are gauge invariant. The OMP2 has these same advantages over canonical MP2, making it promising for excited state properties via linear response theory. The quadratically convergent orbital-optimization procedure converges quickly for OMP2, and provides molecular properties that are somewhat different than those of MP2 for most of the test cases considered (although they are similar for H2O). Bond lengths are somewhat longer, and vibrational frequencies somewhat smaller, for OMP2 compared to MP2. In the difficult case of O4+, results for several vibrational frequencies are significantly improved in going from MP2 to OMP2. [ABSTRACT FROM AUTHOR]

Although never spectroscopically identified in the laboratory, hydrogenated nitrogen (HN2) is thought to be an important species in combustion chemistry. The classical barrier height (10.6±0.2 kcal mol-1) and exothermicity (3.6±0.2 kcal mol-1) for the HN2→N2+H reaction are predicted by high level ab initio quantum mechanical methods [up to CCSDT(Q)]. Total energies are extrapolated to the complete basis set limit applying the focal point analysis. Zero-point vibrational energies are computed using fundamental (anharmonic) frequencies obtained from a quartic force field. Relativistic and diagonal Born–Oppenheimer corrections are also taken into account. The quantum mechanical barrier with these corrections is predicted to be 6.4±0.2 kcal mol-1 and the reaction exothermicity to be 8.8±0.2 kcal mol-1. The importance of these parameters for the thermal NOx decomposition (De-NOx) process is discussed. The unimolecular rate constant for dissociation of the HN2 molecule and its lifetime are estimated by canonical transition-state theory and Rice–Ramsperger–Kassel–Marcus theory. The lifetime of the HN2 molecule is here estimated to be 2.8×10-10 s at room temperature. Our result is in marginal agreement with the latest experimental kinetic modeling studies (τ=1.5×10-8 s), albeit consistent with the very rough experimental upper limit (τ<0.5 μs). For the dissociation reaction, kinetic isotope effects are investigated. Our analysis demonstrates that the DN2 molecule has a longer lifetime than the HN2 molecule. Thus, DN2 might be more readily identified experimentally. The ionization potential of the HN2 molecule is determined by analogous high level ab initio methods and focal point analysis. The adiabatic IP of HN2 is predicted to be 8.19±0.05 eV, in only fair agreement with the experimental upper limit of 7.92 eV deduced from sychrothon-radiation-based photoionization mass spectrometry. [ABSTRACT FROM AUTHOR]

Although never spectroscopically identified in the laboratory, hydrogenated nitrogen (HN2) is thought to be an important species in combustion chemistry. The classical barrier height (10.6±0.2 kcal mol-1) and exothermicity (3.6±0.2 kcal mol-1) for the HN2→N2+H reaction are predicted by high level ab initio quantum mechanical methods [up to CCSDT(Q)]. Total energies are extrapolated to the complete basis set limit applying the focal point analysis. Zero-point vibrational energies are computed using fundamental (anharmonic) frequencies obtained from a quartic force field. Relativistic and diagonal Born–Oppenheimer corrections are also taken into account. The quantum mechanical barrier with these corrections is predicted to be 6.4±0.2 kcal mol-1 and the reaction exothermicity to be 8.8±0.2 kcal mol-1. The importance of these parameters for the thermal NOx decomposition (De-NOx) process is discussed. The unimolecular rate constant for dissociation of the HN2 molecule and its lifetime are estimated by canonical transition-state theory and Rice–Ramsperger–Kassel–Marcus theory. The lifetime of the HN2 molecule is here estimated to be 2.8×10-10 s at room temperature. Our result is in marginal agreement with the latest experimental kinetic modeling studies (τ=1.5×10-8 s), albeit consistent with the very rough experimental upper limit (τ<0.5 μs). For the dissociation reaction, kinetic isotope effects are investigated. Our analysis demonstrates that the DN2 molecule has a longer lifetime than the HN2 molecule. Thus, DN2 might be more readily identified experimentally. The ionization potential of the HN2 molecule is determined by analogous high level ab initio methods and focal point analysis. The adiabatic IP of HN2 is predicted to be 8.19±0.05 eV, in only fair agreement with the experimental upper limit of 7.92 eV deduced from sychrothon-radiation-based photoionization mass spectrometry. [ABSTRACT FROM AUTHOR]

Since the discovery of ozone depletion, the doublet electronic states of the ozone radical cation have received much attention in experimental and theoretical investigations, while the low-lying quartet states have not. In the present research, viable pathways to the quartet states from the lowest three triplet states of ozone, 3A2, 3B2, and 3B1, and excitations from the 2A1 and 2B2 states of the ozone radical cation have been studied in detail. The potential energy surfaces, structural optimizations, and vibrational frequencies for several states of ozone and its radical cation have been thoroughly investigated using the complete active space self-consistent field, unrestricted coupled cluster theory from a restricted open-shell Hartree-Fock reference including all single and double excitations (UCCSD), UCCSD method with the effects of connected triple excitations included perturbatively, and unrestricted coupled cluster including all single, double, and triple excitations with the effects of connected quadruple excitations included perturbatively. These methods used Dunning’s correlation-consistent polarized core-valence basis sets, cc-pCVXZ (X=D, T, Q, and 5). The most feasible pathways (symmetry and spin allowed transitions) to the quartet states are 4A1←3A2, 4A2←3A2, 4A1←3B2, 4A2←3B1, 4B2←3B1, 4A2←1A1, 4B2←1A1, and 4A1←1A1 with vertical ionization potentials of 12.46, 12.85, 12.82, 12.46, 12.65, 13.43, 13.93, and 14.90 eV, respectively. [ABSTRACT FROM AUTHOR]

The singlet ground (X 1Σ+) and excited (1Σ-,1Δ) states of HCP and HPC have been systematically investigated using ab initio molecular electronic structure theory. For the ground state, geometries of the two linear stationary points have been optimized and physical properties have been predicted utilizing restricted self-consistent field theory, coupled cluster theory with single and double excitations (CCSD), CCSD with perturbative triple corrections [CCSD(T)], and CCSD with partial iterative triple excitations (CCSDT-3 and CC3). Physical properties computed for the global minimum (X 1Σ+HCP) include harmonic vibrational frequencies with the cc-pV5Z CCSD(T) method of ω1=3344 cm-1, ω2=689 cm-1, and ω3=1298 cm-1. Linear HPC, a stationary point of Hessian index 2, is predicted to lie 75.2 kcal mol-1 above the global minimum HCP. The dissociation energy D0[HCP(X 1Σ+)→H(2S)+CP(X 2Σ+)] of HCP is predicted to be 119.0 kcal mol-1, which is very close to the experimental lower limit of 119.1 kcal mol-1. Eight singlet excited states were examined and their physical properties were determined employing three equation-of-motion coupled cluster methods (EOM-CCSD, EOM-CCSDT-3, and EOM-CC3). Four stationary points were located on the lowest-lying excited state potential energy surface, 1Σ-→1A″, with excitation energies Te of 101.4 kcal mol-1(1A″ HCP), 104.6 kcal mol-1(1Σ- HCP), 122.3 kcal mol-1(1A″ HPC), and 171.6 kcal mol-1(1Σ- HPC) at the cc-pVQZ EOM-CCSDT-3 level of theory. The physical properties of the 1A″ state with a predicted bond angle of 129.5° compare well with the experimentally reported first singlet state (Ã 1A″). The excitation energy predicted for this excitation is T0=99.4 kcal mol-1(34 800 cm-1,4.31 eV), in essentially perfect agreement with the experimental value of T0=99.3 kcal mol-1(34 746 cm-1,4.308 eV). For the second lowest-lying excited singlet surface, 1Δ→1A′, four stationary points were found with Te values of 111.2 kcal mol-1 (21A′ HCP), 112.4 kcal mol-1 (1Δ HPC), 125.6 kcal mol-1(2 1A′ HCP), and 177.8 kcal mol-1(1Δ HPC). The predicted CP bond length and frequencies of the 2 1A′ state with a bond angle of 89.8° (1.707 Å, 666 and 979 cm-1) compare reasonably well with those for the experimentally reported C 1A′ state (1.69 Å, 615 and 969 cm-1). However, the excitation energy and bond angle do not agree well: theoretical values of 108.7 kcal mol-1 and 89.8° versus experimental values of 115.1 kcal mol-1 and 113°. [ABSTRACT FROM AUTHOR]

The existence or nonexistence of GaH5 has been widely discussed [N. M. Mitzel, Angew. Chem. Int. Ed. 42, 3856 (2003)]. Seven possible structures for gallium pentahydride have been systematically investigated using ab initio electronic structure theory. Structures and vibrational frequencies have been determined employing self-consistent field, coupled cluster including all single and double excitations (CCSD), and CCSD with perturbative triples levels of theory, with at least three correlation-consistent polarized-valence-(cc-pVXZ and aug-cc-pVXZ) type basis sets. The X 1A′ state for GaH5 is predicted to be weakly bound complex 1 between gallane and molecular hydrogen, with Cs symmetry. The dissociation energy corresponding to GaH5→GaH3+H2 is predicted to be De=2.05 kcal mol-1. The H–H stretching fundamental is predicted to be v=4060 cm-1, compared to the tentatively assigned experimental feature of Wang and Andrews [J. Phys. Chem. A 107, 11371 (2003)] at 4087 cm-1. A second Cs structure 2 with nearly equal energy is predicted to be a transition state, corresponding to a 90° rotation of the H2 bond. Thus the rotation of the hydrogen molecule is essentially free. However, hydrogen scrambling through the C2v structure 3 seems unlikely, as the activation barrier for scrambling is at least 30 kcal mol-1 higher in energy than that for the dissociation of GaH5 to GaH3 and H2. Two additional structures consisting of GaH3 with a dihydrogen bond perpendicular to gallane (C3v structure 4) and an in-plane dihydrogen bond [Cs(III) structure 5] were also examined. A C3v symmetry second-order saddle point has nearly the same energy as the GaH3+H2 dissociation limit, while the Cs(III) structure 5 is a transition structure to the C3v structure. The C4v structure 6 and the D3h structure 7 are much higher in energy than GaH3+H2 by 88 and 103 kcal mol-1, respectively. [ABSTRACT FROM AUTHOR]

The singlet electronic ground state isomers, X 1Σg+ (AlOAl D∞h) and X 1Σ+ (AlAlO C∞ν), of dialuminum monoxide have been systematically investigated using ab initio electronic structure theory. The equilibrium structures and physical properties for the two molecules have been predicted employing self-consistent field (SCF) configuration interaction with single and double excitations (CISD), multireference CISD (MRCISD), coupled cluster with single and double excitations (CCSD), CCSD with perturbative triples [CCSD(T)], CCSD with iterative partial triple excitations (CCSDT-3 and CC3), and full triples (CCSDT) coupled cluster methods. Four correlation consistent polarized valence (cc-pVXZ) type basis sets were used. The AlAlO system is rather challenging theoretically. The two isomers are confirmed to have linear structures at all levels of theory. The symmetric isomer AlOAl is predicted to lie 81.9 kcal mol-1 below the asymmetric isomer AlAlO at the cc-pV(Q+d)Z CCSD(T) level of theory. The predicted harmonic vibrational frequencies for the X 1Σg+ AlOAl molecule, ω1=517 cm-1, ω2=95 cm-1, and ω3=1014 cm-1, are in good agreement with experimental values. The harmonic vibrational frequencies for the X 1Σ+ AlAlO structure, ω1=1042 cm-1, ω2=73 cm-1, and ω3=253 cm-1, presently have no experimental values with which to be compared. With the same methods the barrier heights for the isomerization AlOAl→AlAlO and AlAlO→AlOAl reactions were predicted to be 84.3 and 2.4 kcal mol-1, respectively. The dissociation energies D0 for AlOAl (X 1Σg+) and AlAlO (X 1Σ+)→AlO (X 2Σ+)+Al (2P) were determined to be 130.8 and 48.9 kcal mol-1, respectively. Thus, both symmetric AlOAl (X 1Σg+) and asymmetric AlAlO (X 1Σ+) isomers are expected to be thermodynamically stable with respect to the dissociation into AlO (X 2Σ+) + Al (2P) and kinetically stable for the isomerization reaction (AlAlO→AlOAl) at sufficiently low temperatures. [ABSTRACT FROM AUTHOR]

This research presents an ab initio characterization of the potential energy surface for the methylamine plus 1D 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 NH2 containing species in the interstellar medium. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 SO…H2O 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 SO…H2O 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 SO…H2O 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]