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Context: One of the more significant sub-mechanisms of H2/O2 combustion involves the reaction of hydrogen peroxide with hydrogen atoms (H + H2O2), resulting in the production of OH + H2O (R1) and H2 + HO2 (R2) paths. Previous experimental and ab initio calculations reveal some variations in the barrier height for (R1). To improve the energetics of both (R1) and (R2), single reference and multireference ab initio methods are employed, and the rate constants and H/D kinetic isotope effects (KIEs) are calculated as a function of temperature. For (R1), the best results for the barrier height and reaction energies computed with the CASPT2(15,11)/aug-cc-pV6Z are 5.2 and − 70.3 kcal.mol−1, respectively. CCSD(T)/aug-cc-pV5Z + CV (core-valence) calculations for (R2) give 9.7 and − 15.6 kcal.mol−1 to those parameters. The CVT/SCT rate constants of both paths agree well with the fitted rate constants from uncertainty-weighted statistical analysis of the 14-mechanism of H2/O2. The kinetic isotopic effect (kH/kD) for the reaction D + H2O2 → DH + HO2 was found to be 0.47, which is in excellent agreement with the experimental value of 0.43. Methods: The structures of reactants, transition state, and products of (R1) and (R2) are calculated with the aug-cc-pVTZ basis set and M062X DFT, CCSD(T), and CASSCF methods. The barrier heights and reaction energies of (R1) and (R2) are computed using the M06-2X, CCSD(T), MRCI, and CASPT2 methods and various basis sets. The rate constants are calculated with the variational transition state theory including multidimensional tunneling corrections (VTST-MT), with potential energy surfaces built by the M06-2X/aug-cc-pVTZ approach. [ABSTRACT FROM AUTHOR]

Thermochemical and kinetics of sequential hydrogen abstraction reactions from hydrazine by nitrogen atoms were studied. The dehydrogenation was divided in three steps, N2H4 + N, N2H3 + N, and N2H2 + N. The thermal rate constants were calculated within the framework of canonical variational theory, with zero and small curvature multidimensional tunnelling corrections. The reaction paths were computed with the BB1K/aug-cc-pVTZ method and the thermochemical properties were improved with the CCSD(T)/CBS//BB1K/aug-cc-pVTZ approach. The first dehydrogenation step presents the lowest rate constants, equal to 1.22 × 10-20 cm3 molecule-1 s-1 at 298 K. [ABSTRACT FROM AUTHOR]

Accurate potential energy curves and molecular constants for the low-lying electronic states of CuXy (X = B, Al; y = 0, +1) were investigated using the complete active space self-consistent field/multireference configuration interaction (MRCI) methodology with aug-cc-pV5Z basis set. The photoionization spectra of CuX were computed, showing electron detachment in the region of far ultraviolet. The results complement the previous theoretical characterizations and the few experimental studies. A comparative analysis was carried out concerning the different choices of reference configuration state functions in the MRCI calculations with and without the contribution of scalar relativistic effects. The results obtained with a small reference set adequately constructed are competitive to those using a much larger number of configuration state functions, and also the scalar relativistic effects improve significantly the molecular constants in this kind of system containing a 3d metal atom. [ABSTRACT FROM AUTHOR]

The rate constants and H/D kinetic isotope effect for hydrogen abstraction reactions involving isotopomers of methyl formate by methyl radical are computed employing methods of the variational transition state theory (VTST) with multidimensional tunneling corrections. The energy paths were built with a dual‐level method using the moller plesset second‐order perturbation theory (MP2) method as the low‐level and complete basis set (CBS) extrapolation as the high‐level energy method. Benchmark calculations with the CBSD‐T approach give an enthalpy of reaction at 0 K for R1 (−4.5 kcal/mol) and R2 (−4.2 kcal/mol) which are in good agreement with the experiment, that is, −4.0 and − 4.8 kcal/mol. For the reactional paths involving the isotopomers CH3 + CH3OCOH → CH4 + CH3OCO and CH3 + CH3OCOD → CH3D + CH3OCO, the value of kH/kD (T = 455 K) using the canonical VTST/small‐curvature tunneling approximation method is 6.7 in close agreement with experimental value (6.2). © 2019 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

The abstraction and addition reactions of H with trans-N2H2 are studied by high-level ab initio methods and density functional theory. Rate constants were calculated for these two reactions by multistructural variational transition state theory with multidimensional tunneling and including torsional anharmonicity by the multistructural torsion method. Rate constants of the abstraction reaction show large variational effects, that is, the variational transition state yields a smaller rate constant than the conventional transition state; this results from the fact that the variational transition state has a higher zero-point vibrational energy than the conventional transition state. The addition reaction has a classical barrier height that is about 1 kcal/mol lower than that of the abstraction reaction, but the addition rates are lower than the abstraction rates due to vibrational adiabaticity. The calculated branching ratio of abstraction to addition is 3.5 at 200 K and decreases to 1.2 at 1000 K and 1.06 at 1500 K. [ABSTRACT FROM AUTHOR]

The N[sub 2]O[sub 4]→2 NO[sub 2] dissociation reaction was investigated at a high level of theory using the couple cluster with all single and double excitations and connected triples [CCSD(T)] and complete active space self-consistent field approaches, and the cc-pVDZ, aug-cc-pVDZ, and cc-pVTZ basis sets. Only at the coupled cluster level a first-order saddle point was found connecting reactant and products. Collectively, structural, vibrational, and thermodynamic data for the three stationary points represent the best theoretical description of this reaction system to date, and are in good agreement with available experimental results. Unimolecular transition state theory rate constants (k[sub ∞]) were also evaluated at 250, 298.15, and 350 K. At the CCSD(T)/cc-pVTZ level of calculation these results are 0.62×10[sup 1], 1.90×10[sup 3], and 1.66×10[sup 5] s[sup -1], respectively. Known experimental results at 298 K vary from 1.7×10[sup 5] to 1.0×10[sup 6] s[sup -1]. Including an estimate for basis set superposition error, we predict ΔH[sub 298][sup 0] for the dissociation reaction to be 12.76 kcal/mol (Expt. 13.1–13.7 kcal/mol). © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The density functional approximation M05 with 6-31G** and m-6-31G* basis sets were used to calculate the classical barrier heights, structures, reaction energies, and enthalpies of the hydrogen abstraction reaction paths of 2H + (CH3OH)·Cu4 → 2H2 + (COH2)·Cu4, which is a prototype system for dehydrogenation of methanol catalyzed by copper clusters, consisting of two sequential elementary reactions of the methanol combustion mechanism. Electronic structure and intermolecular/intramolecular bond energies were computed as well as the energy variations for the reaction both free and in the presence of the tetramer copper cluster. It was observed that the first dehydrogenation path, related to the abstraction from hydroxyl group of methanol, shows no classical barrier in this level of calculation. We have demonstrated that the planar rhombus cluster Cu4 has bonding properties which mimic quantitatively catalytic properties for some elementary reactions in the methanol combustion mechanism. © 2012 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

The structures, barrier heights, reaction enthalpies, and thermal rate constants of four reaction paths involving hydrogen abstraction and addition processes between O and CH3SH were computed using the Møller-Plesset perturbation theory at the second order level (MP2) and BB1K methods with the aug-cc-pV(T+d)Z basis set. A more accurate evaluation of the energetic of these reaction systems are performed by extrapolating the single point CCSD(T) energies to the complete basis set (CBS) limit approach using the BB1K and MP2/aug-cc-pV(T+d)Z geometries. The best estimates of the adiabatic barrier heights, determined using the CCSD(T)/CBS//(BB1K or MP2)/aug-cc-pV(T+d)Z method, are between 0.5 and 1.0 kcal mol−1 (R1, CH3S + OH), 5.3 and 5.5 kcal mol−1 (R2, CH2SH + OH), 1.3 and 2.3 kcal mol−1 (R3, CH3 + HSO), and 1.6 and 2.9 kcal mol−1 (R4, CH3SO + H). The thermal rate constants calculated using the dual-level direct dynamics by variational transition state theory with interpolated single-point energy corrections are in good agreement with experimental results. The calculated branching ratios at 300 K are 0.91:0.01:0.05:0.03 for the reactions paths R1-R4, respectively, which indicate that hydrogen abstraction from the SH group is the main reaction path. © 2012 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

The main objective of this study consists in providing reliable structures, harmonic vibrational frequencies, and relative electronic energies of Si, GeSi, GeSi, and Ge clusters using DFT, CCSD(T), CASSCF, and MP2 methods and extended basis sets. The CCSD(T) results are extrapolation to the complete basis set (CBS) limit. For Si cluster, the ground-state is a singlet (A) and the lowest lying electronic state is triplet state ( ) separated by 0.44 kcal mol at the CCSD(T)/(CBS) limit. The ground-states of GeSi, GeSi, and Ge clusters are also singlet state with triplet states very close energetically. Computed equilibrium geometries and vibrational frequencies are compared with previous theoretical and experimental data. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [ABSTRACT FROM AUTHOR]

Accurate potential energy curves, dipole moment functions, dissociation energies, and molecular constants for several low-lying singlet and triplet electronic states of BeS were investigated using the CASSCF/MRCI methodology, and the cc-pV5Z basis set for beryllium, and the aug-cc-pV(5+d)Z set for sulfur. Besides presenting improved results for the three lowest lying states, this study presents the first theoretical characterization of another set of nine excited states so far unknown experimentally. Our results are sufficiently accurate to reliably guide the experimental search and characterization of these states, and also to confirm the experimental assignment of the BΣ excited state. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [ABSTRACT FROM AUTHOR]

DFT methods are used in calculations of geometries, energies, and frequencies for O (3P) + CH3OH reactions. CCSD(T) single-point calculations are carried out followed by extrapolation to the complete basis set (CBS) limit and inclusion of core correlation. The best estimate results of the reaction enthalpies (ΔH00) for the first (-7.1 kcal/mol), second (2.0 kcal/mol), and the third (25.8 kcal/mol) reactions path are in excellent agreement with the experimental values, i.e., -7.2 ± 0.2, 1.2 ± 1.0, and 26.0 ± 1.2 kcal/mol, respectively. Rate constants and activation energies in the range of 300–2,000 K calculated with the variational transition state theory are in good agreement with previous studies. H/D and 12C/13C kinetic isotope effects are also calculated in good agreement with available experimental data. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [ABSTRACT FROM AUTHOR]

In this work, a comparative study of the ground electronic state spectroscopic properties of diatomic molecules formed by the cooper (Cu) atom with the second period elements (B, C, N, O, and F) was performed. The CCSD(T) approach and DFT using GGA (BP86 and BPW91) and H-GGA (B3LYP) methods have been applied, using Wachters + f basis sets for the cooper atom and aug-cc-pVTZ for the second period elements. The results are in good agreement with available theoretical and experimental data available. The GGA functionals have good performance to describe bond lengths and harmonic vibrational frequencies, whereas B3LYP method presents better performance to describe dissociation energies. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [ABSTRACT FROM AUTHOR]

Density functional theory (DFT) geometries, vibrational frequencies, barrier heights, and reaction energies are computed for the first reactive channels of reactions involving the hydrogen atoms with CH3OH and C2H5OH. For both reactions, the density functional BB1K specially fitted to study hydrogen abstraction reactions was able to give barrier heights and reaction enthalpies at 0 K with accuracy close to 1.0 kcal/mol. The B3LYP systematically underestimate the classical barrier heights and predict reasonable values for the geometries and frequencies of CH3OH and C2H5OH. The results show that the studied DFTs have strengths and weaknesses which are somewhat complementary. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [ABSTRACT FROM AUTHOR]

Abstract: Geometries, energies, frequencies, and rate constants of the reaction F (2P)+C2H6 →C2H5 +HF were computed using the MP2 and CCSD(T) approaches, correlation-consistent basis sets, and an extrapolation scheme to assess the complete basis set (CBS) limit energies. Values of the enthalpy of reaction at 0K calculated with the CCSD(T)/cc-pVQZ method and extrapolated to the CBS limit are equal to −35.2 and −36.7kcal/mol, respectively (Exp. −35.3±0.5kcal/mol). Rate constants calculated with the zero-order interpolated variational transition state (IVTST-0) method are in good agreement with experiment. [Copyright &y& Elsevier]

Abstract: Geometries, frequencies, energies, and rate constants of the reaction N(4S)+CH4 →NH+CH3 were computed using coupled-cluster theory, correlation-consistent basis sets, and extrapolations schemes to assess the complete basis set limit energies. Our best calculation gives a classical barrier height and reaction energy of 32.9 and 29.3kcal/mol, respectively, and 24.6kcal/mol for the reaction enthalpy (. Thermal (200–2000K) rate constants were calculated with the canonical variational transition state theory, and indicate a very slow reaction when compared with the ones with N(2D) and N(2P). Using the CVT/μOMT method, the value of H/D KIE is 6.3 (300K). [Copyright &y& Elsevier]