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In the last centuries, human society has strongly modified the biosphere of the planet through consumption of its resources on a highly increasing scale but without planning, causing pollution in various environments through chemical dumping, and the extensive use of fossil fuels in industrialization and transportation, for example. In this last aspect, the use of less polluting alternative fuels for vehicular transport based on the consumption of electricity and hydrogen gas has been suggested. Experimental and theoretical studies are being made to increase the hydrogen storage capacity using porous carbon materials, nanoparticles, and metallic clusters. Most of the electronic and structure calculations of such materials have been carried out with density functional theory (DFT) without gauging the accuracy of the employed DFTs. In this work, we have employed various DFTs to calculate hydrogen adsorption energies of small clusters of BeO and compare with wave function theory methods, in order to evaluate the relative accuracy of these methodologies, since experimental values are not usually available. The hydrogen storage capability of small clusters of (BeO)n (n = 2–7) is studied with various density functionals, MP2, and CCSD(T) methods and benchmarked with the complete basis set (CBS) limit approach. Calculated properties are the equilibrium geometries of isolated and adsorbed clusters with H2, electronic adsorption energy, enthalpy, and Gibbs free energy. Calculations with the CBS method are employed to gauge the accuracy of various methods in calculating structures and cohesive energies of small beryllium clusters. The smallest values of mean unsigned error of the cohesive energies (Eb) calculated with the ωB97X-D, TPPSh, and M06-L DFTs are equal to 0.05, 0.06, and 0.07 eV using as reference the CBSD-T. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Multipath VTST rate constants for methyl formate hydrogen abstraction by deuterium. • The multistructural torsional anharmonicity factor change considerably k (T). • The path-averaged generalized transmission coefficient is dominated by the path 1. • Multidimensional tunneling plays an essential role at low temperatures. Abstract The thermal rate constants for the methyl formate hydrogen abstraction reaction by deuterium was investigated by the multipath canonical variational transition state theory with large curvature tunneling (MP-CVT/LCT). The multistructural torsional anharmonicity factor decreases the thermal rate constants for deuterated reaction R1a and increases considerably for R2a. The path-averaged generalized transmission coefficient is dominated by the characteristics of the lowest-energy paths for both reactions. The MP-CVT/LCT rate constants are in excellent agreement with experimental data (T = 1079–1270 K). [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]

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]

Abstract: Accurate potential energy curves, dissociation energies and spectroscopic constants for several low-lying doublet and quartet electronic states of CaAl were investigated using the CASSCF/MRCI methodology, and the cc-pVQZ basis set. Our results represent an improvement over a previous theoretical description, and also characterizes new higher excited states not previously investigated, thus confirming the assignment of four excited states investigated experimentally. With the theoretical transition moment functions, transition probabilities and radiative lifetimes were estimated via Einstein spontaneous emission coefficients. [Copyright &y& Elsevier]

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]

Abstract: Equilibrium geometries, harmonic vibrational frequencies and the barrier heights of the internal rotation and inversion motion of methylamine (CH3NH2) conformers were calculated with the CCSD(T) method, in combination with the correlation consistent basis sets of Dunning. Valence and all electron correlation calculations were also carried out using the correlation consistent core and valence polarization triple-ζ basis set (cc-pCVTZ). Theoretical equilibrium geometries are in a general good agreement with available microwave and electron diffraction measurements. The best estimate results of the barrier heights for the internal rotation and inversion are, respectively, equal to 1.936 and 5.406kcal/mol. [Copyright &y& Elsevier]

Abstract: High-level multireference calculations are reported for the low-lying electronic states of GaN. Using the CASSCF/MRSDCI approach and the aug-cc-pVQZ basis set a detailed analysis is given for the 22 electronic states that dissociate in the first two channels. The ground state is confirmed as (X3Σ−) but the first excited state ((1)3Π) is found to be very close in energy. A very accurate spectroscopic characterization is made for the lowest-lying triplet and singlet states. The behavior of dipole moment and transition dipole moment with internuclear distance is also described. Relative intensities are predicted using the Einstein coefficients. [Copyright &y& Elsevier]

Abstract: The CH4 +F(2P)→CH3 +HF abstraction reaction was investigated using the BH&HLYP, MP2, and CCSD(T) approaches, and two series of correlation consistent basis sets. At the CCSD(T)/cc-pVTZ level, the classical barrier height (ΔV #), and the vibrationally adiabatic ground-state barrier height () are equal to 1.8 and 0.5kcal/mol, respectively. CCSD(T)/cc-pVQZ calculations give a value for the enthalpy of reaction at 0K () of −31.5kcal/mol (Expt. −32.1kcal/mol). Rate constants calculated with the zero-order interpolated variational transition state (IVTST-0) method are in agreement with experiment. The activation energy computed as local two-points slopes of Arrhenius plots in the temperature range of 200–400K, equal to 0.59kcal/mol, is also in good agreement with experiment (0.53kcal/mol). [Copyright &y& Elsevier]

Accurate equilibrium geometries, harmonic vibrational frequencies, and equilibrium rotational constants of the methyl radical were computed using coupled-cluster theory in combination with the cc-pVnZ and cc-pCVnZ basis sets of Dunning. Valence and all electron calculations were carried out. Using all electron correlations and CBS extrapolation, the re value is equal to 1.0758 Å, which is 0.0032 Å shorter than the experimental value of r0 (1.079 Å). The harmonic (ω1, ω2, ω3, and ω4) frequencies calculated using the CBS extrapolation differ from the experimental fundamental frequencies, respectively, by +142, +125, +21, and –82 cm-1. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [ABSTRACT FROM AUTHOR]

Abstract: Kinetic isotope effects (KIE) for hydrogen abstraction from CH4, CD4, and 13CH4 by fluorine was studied by the variational transition state theory with multidimensional tunneling contributions (VTST/MT). A low-level potential energy surface is obtained with the AM1SRP method. High-level energies, geometries and frequencies of the reactants, transition state, and products were computed using MP2 and CCSD(T) methods. Using the CVT/μOMT method, the kH/kD kinetic isotope effect value is 1.52 (298K), in excellent agreement with experimental values (1.4–1.5). All computed values of k12C/k13C KIE over a large range of temperatures (200–2000K) show normal values (KIE>1). [Copyright &y& Elsevier]

The low-lying electronic states of PCl are investigated at a high level of correlation treatment using the CASSCF/MRSDCI approach, and the aug-cc-pVQZ-g basis set augmented with diffuse functions. The potential energy curves characterize unambiguously the complex structure of the electronic states, identifying avoided crossing regions among the 3Π states of relevance to experimentalists. Besides improving the accuracy of spectroscopic data upon previous theoretical studies, it unequivocally determines the dissociation energy of the ground state. The behavior of the dipole moment, transition dipole moment, and spin–orbit coupling constants with internuclear distance is also described. Relative intensities are predicted by the Einstein coefficients. [Copyright &y& Elsevier]

Accurate potential energy curves, transition moments, spectroscopic constants, radiative transition probabilities, and lifetimes for the states X 3Σ-, (1) 3Π, and (2) 3Σ- of BeC, MgC, and CaC are reported using high level calculations. Diagonal transitions dominate for the (2) 3Σ- → X 3Σ- system, with band origins falling at 745, 648, and 770 nm, and an intensity ratio of 6,3,1, respectively for BeC, MgC, and CaC. Radiative lifetimes for v′=0 are equal to 0.48, 1.45, and 3.29 μs for the state (2) 3Σ-, and equal to 13.57, 10.44, and 9.62 μs for the state (1) 3Π for BeC, MgC, and CaC. [Copyright &y& Elsevier]

Potential energy curves, dipole and transition moment functions, spectroscopy constants, radiative transition probabilities, and lifetimes for the two lower-lying doublet states (X2Π and A2Σ+) of the BeB, MgB, and CaB molecules are reported with multireference singles and doubles configuration interaction methodology. Radiative lifetimes for ν' = 0 are predicted to be 158 μs, 281 μs, and 87 μs for BeB, MgB, and CaB, respectively. The strongest transitions from ν' = 0 should occur to vibrational levels ν" = 0 and 1 for BeB, to ν" = 2 and 3, for MgB and to ν" = 3 and 4 for CaB. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 205–212, 2003 [ABSTRACT FROM AUTHOR]

Accurate potential energy curves, dipole moment functions, dissociation energies and spectroscopic constants for three quartet electronic states (X4Σ−, A4Π and B4Σ−) of the CaN molecule are reported with the multireference singles and doubles configuration interaction methodology. Also, are present radiative transition probabilities and lifetimes for the A4Π–X4Σ−, B4Σ−–X4Σ− and B4Σ−–A4Π band systems. The ground state (X4Σ−) has an equilibrium distance equal to 4.410a0 and the dissociation energy is 0.85 eV. The A4Π has a shorter equilibrium distance equal to 4.142a0 and the B4Σ− has a much longer equilibrium distance, equal to 5.426a0. Radiative lifetimes for v′=0 are predicted to be 27 000, 0.256 and 565 μs for the A4Π–X4Σ−, B4Σ−–X4Σ− and B4Σ−–A4Π band systems, respectively. [Copyright &y& Elsevier]

The equilibrium geometries, harmonic vibration frequencies, and the rotational barrier of C2H6 have been calculated using CCSD(T) level of theory in combination with correlation-consistent basis sets of Dunning. The best estimate values of re(CH), re(CC) and θe(HCH) are 1.0895, 1.5240 A˚ and 107.66°, which are in close agreement with the experimental equilibrium geometry of 1.089±0.005 A˚ for re(CH), 1.524±0.003 A˚ for re(CC), and 106.9±0.5° for θe(HCH). The structure and harmonic vibrational frequencies for the CH4 molecule have also been analyzed. [Copyright &y& Elsevier]

Electronic energies, geometries, and harmonic vibration frequencies for the reactants, products, and transition state for the Cl(3P)+C2H6→C2H5+HCl abstraction reaction were evaluated at the HF and MP2 levels using several correlation consistent polarized-valence basis sets. Single-point calculations at PMP2, MP4, QCISD(T), and CCSD(T) levels were also carried out. The values of the forward activation energies obtained at the MP4/cc-pVTZ, QCISD(T)/cc-pVTZ, and CCSD(T)/cc-pVTZ levels using the MP2/cc-pVTZ structures are equal to −0.1, −0.4, and −0.3 kcal/mol, respectively. The experimental value is equal to 0.3±0.2 kcal/mol. We found that the MP2/aug-cc-pVTZ adiabatic vibration energy for the reaction (−2.4 kcal/mol) agrees well with the experimental value −(2.2–2.6) kcal/mol. Rate constants calculated with the zeroth-order interpolated variational transition state (IVTST-0) method are in good agreement with experiment. In general, the theoretical rate constants differ from experiment by, at most, a factor of 2.6. [Copyright &y& Elsevier]

Accurate optimized equilibrium geometries, harmonic frequencies and rotational constants of the gauche conform (C2 symmetry) of hydrazine (N2H4) have been calculated employing a large series of basis sets and several ab initio methods. Our best estimate computed equilibrium geometries are rNN=1.434 A˚, rNHi=1.013 A˚, rNHo=1.010 A˚, θNNHi=111.3°, θNNHo=106.8° and θHiNNHo=89.7°. These predictions were used as reference in the analysis of the experimental and theoretical data, and we expected that the present results could guide future investigations in this direction. [Copyright &y& Elsevier]

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]

Presents corrections to the article, entitled `Energetic and structural features of the CH[sub 4]+O([sup 3]P) CH[sub 3]+OH abstraction reaction: Does perturbation theory from a multiconfiguration reference state (finally) provide a balanced treatment of transition states?,' published in the volume 111 issue of the `Journal of Chemical Physics'.

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 kinetics of the reaction of N2H4 with oxygen depends sensitively on the initial conditions used. In oxygen-rich systems, the rate constant shows a conventional positive temperature dependence, while in hydrazine-rich setups the dependence is negative in certain temperature ranges. In this study, a theoretical model is presented that adequately reproduces the experimental results trend and values for hydrazine-rich environment, consisting of the hydrogen abstraction from the hydrazine (N2H4) dimer by an oxygen atom. The thermochemical properties of the reaction were computed using two quantum chemical approaches, the coupled cluster theory with single, double, and noniterative triple excitations (CCSD(T)) and the M06-2X DFT approach with the aug-cc-pVTZ and the maug-cc-pVTZ basis sets, respectively. The kinetic data were calculated with the improved canonical variational theory (ICVT) using a dual-level methodology to build the reaction path. The tunneling effects were considered by means of the small curvature tunneling (SCT) approximation. Potential wells on both sides of the reaction ((N2H4)2 + O → N2H4···N2H3 + OH) were determined. A reaction path with a negative activation energy was found leading, in the temperature range of 250-423 K, to a negative dependence of the rate constant on the temperature, which is in good agreement with the experimental measurements. Therefore, the consideration of the hydrazine dimer model provides significantly improved agreement with the experimental data and should be included in the mechanism of the global N2H4 combustion process, as it can be particularly important in hydrazine-rich systems. [ABSTRACT FROM AUTHOR]

Highlights: [•] Thermochemical and kinetics parameters for the H abstraction reaction S(3P)+CH4 →CH3 +SH(2Π). [•] CCSD(T) and DFT geometries, energies, and vibrational frequencies of the stationary states. [•] Reactional paths of the S(3P)+CH4 reaction calculated with M05/MG3S method. [•] Thermal rate constants (200–2000K) using VTST/MT theory. [ABSTRACT FROM AUTHOR]

Abstract: Accurate potential energy curves, dissociation energies and molecular constants for several low-lying singlet and triplet electronic states of MgS were investigated using the CASSCF/MRCI methodology, and the cc-pVQZ basis set. Our results represent an improvement over previous theoretical description for the lowest lying states, characterize some higher excited states not previously investigated, and also confirm the assignment of the B1Σ+ excited state investigated experimentally. Transition probabilities and radiative lifetimes were also predicted using Einstein spontaneous emission coefficients. [Copyright &y& Elsevier]

Abstract: Equilibrium geometries, harmonic vibrational frequencies of the C2h and D2h conformers of the hydrazine ion (N2H4 +) were calculated with the CCSD(T) method, in combination with the correlation consistent basis sets of Dunning. Valence and all electron correlation calculations were also carried out using the correlation consistent core and valence polarization triple-zeta basis set (cc-pCVTZ). The adiabatic ionization potential of hydrazine were also calculated using CCSD(T) and CASSCF/MRCI with the CBS extrapolation with the best results respectively equal to 8.07 and 7.81eV, which are closer to experimental data, equal to 8.1±0.15 eV. [Copyright &y& Elsevier]

Abstract: In this work is presented, for the first time, an accurate spectroscopic characterization of the lowest-lying doublet and quartet states of BeAl molecule. We used a high level CASSCF/MRCI correlation methodology and the aug-cc-pVQZ basis set. The behavior of the dipole moment and the transition dipole moment functions are described, and transition probabilities and radiative lifetimes are also predicted using the Einstein coefficients. [Copyright &y& Elsevier]

Abstract: The low-lying electronic states of MgAl are investigated at a high level of electron correlation treatment using the CASSCF/MRCI approach, and the aug-cc-pVQZ basis set. Besides the confirmation of the ground state (X 2Π) and the first excited state ((1) 2Σ+), this study also provides a very accurate spectroscopic characterization of the lowest-lying doublet and quartet states. The behavior of the dipole moment and the transition dipole moment functions with internuclear distance are also described. Relative intensities for the (2) 2Π→X 2Π, (3) 2Π→X 2Π and (1) 2Δ→X 2Π band systems are predicted by the Einstein coefficients. [Copyright &y& Elsevier]

Accurate potential energy curves, dipole moment functions, dissociation energies and spectroscopic constants for six electronic states (3Σ−,3Π,5Σ−,1Δ,1Π,1Σ+) of the CaC molecule are reported with the multireference singles and doubles configuration interaction methodology. The ground state has symmetry 3Σ−, with a dissociation energy (D0) equal to 1.94 eV. The 5Σ− state is the first excited state lying 695 cm−1 above the 3Σ− ground state. The 1Δ and 3Π states are the second and third excited states separated, respectively, by 10 763 and 12 167 cm−1 from the 3Σ− ground state. [Copyright &y& Elsevier]

• Identification and quantification of soot precursors by quantum chemistry approach. • Sub-mechanism containing hydrogen abstraction and beta-scission reactions. • Kinetic parameters for several alkane elementary reactions. • Farnesane produces ten times lower soot precursors than n -dodecane. The worldwide increase concern about the use of fossil fuels is causing a running for the development of green fuels, trying to slow down the emissions of soot and greenhouse gases. Recent studies are pointing up that branched carbon chain structures of alkanes, terpenoids, and sesquiterpenes produce less soot during combustion, they have been employed as an additive in diesel/kerosene engines. In this work, a decomposition sub-mechanism of farnesane and n -dodecane fuels was studied, using the calculated Arrhenius parameters for several hydrogen abstractions and β -scission elementary reactions to compare the preceding species of the soot precursors. The calculations were carried out using density functional theory to obtain the thermochemical properties of all molecules and transition state theory to calculate the thermal rate coefficients. The results showed the importance of the thermochemical analysis in choosing the appropriate temperature range for the selected reactions, to obtain a qualitatively appropriate set of kinetic parameters. The molar fraction comparison resulting from the sub-mechanism of the two fuels revealed that farnesane yields a smaller amount of species necessary to produce soot precursors due to the position of the branched carbons. The methodology used here allows the construction of additive mechanisms and can be used to aid the design of new fuels that could be more efficient and/or less polluting. By investigating, for example, the first steps of a fuel decomposition in a reduced mechanism, avoiding the complexity of a complete mechanism, while also generating fairly accurate kinetic parameters for elementary reactions. [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 reaction system formed by the methanethiol molecule (CH3SH) and a hydrogen atom was studied via three elementary reactions, two hydrogen abstractions and the C-S bond cleavage (CH3SH + H → CH3S + H2 (R1); → CH2SH + H2 (R2); → CH3 + H2S (R3)). The stable structures were optimized with various methodologies of the density functional theory and the MP2 method. Two minimum energy paths for each elementary reaction were built using the BB1K and MP2 methodologies, and the electronic properties on the reactants, products, and saddle points were improved with coupled cluster theory with single, double, and connected triple excitations (CCSD(T)) calculations. The sensitivity of coupling the low and high-level methods to calculate the thermochemical and rate constants were analyzed. The thermal rate constants were obtained by means of the improved canonical variational theory (ICVT) and the tunneling corrections were included with the small curvature tunneling (SCT) approach. Our results are in agreement with the previous experimental measurements and the calculated branching ratio for R1:R2:R3 is equal to 0.96:0:0.04, with kR1 = 9.64 × 10-13 cm³ molecule-1 s-1 at 298 K. [ABSTRACT FROM AUTHOR]

This work presents a comprehensive DFT study on the interaction between hydrazine derivatives with a platinum catalyst surface, which is represented by a tetrahedral Pt4 cluster model. Three separate reaction pathways were investigated; two of which are related to possible pathways of NH3 formation. The first pathway describes the intramolecular transfer of one hydrogen atom in the hydrazine molecule forming the NHNH3 intermediate, then dissociating into NH and NH3. The second describes the addition of one external hydrogen atom to hydrazine forming N2H5, followed by its dissociation to NH2 and NH3. The third reaction pathway involves the formation of N2H3 by means of hydrogen abstraction by an external hydrogen. The reactions were studied in both the absence and the presence of a Pt4 cluster. We find that the assistance of the Pt4 cluster lacks a systematic effect on the reactions barrier heights. It is also shown that the ammonia formation can possibly proceed through the formation of the N2H5 intermediate, leading to more exothermic intermediate steps in the presence of the Pt4 cluster. [ABSTRACT FROM AUTHOR]

Accurate potential energy curves, dissociation energies, and molecular constants for several low-lying singlet and triplet electronic states of AsCl were investigated using the CASSCF/MRCI methodology, including the spin–orbit splitting with the aug-cc-pV5Z-DK basis set. Our results represent an improvement over previous the theoretical descriptions for the lowest lying states, as well as characterize another higher excited state not previously investigated. Transition probabilities and radiative lifetimes for the (1) 3 Π → X 3 Σ − band system were also predicted using Einstein spontaneous emission coefficients. [ABSTRACT FROM AUTHOR]

Thermochemicaland kinetics properties of the hydrogen abstractionfrom the hydrazine molecule (N2H4) by an oxygenatom were computed using high-level ab initio methods and the M06-2XDFT functional with aug-cc-pVXZ (X = T, Q) and maug-cc-pVTZ basissets, respectively. The properties along the reaction path were obtainedusing the dual-level methodology to build the minimum energy pathwith the potential energy surface obtained with the M06-2X methodand thermochemical properties corrected with the CCSD(T)/CBS//M06-2X/maug-cc-pVTZresults. The thermal rate constants were calculated in the frameworkof variational transition-state theory. Wells on both sides of thereaction (reactants and products) were found and considered in thechemical kinetics calculations. Additionally, the product yields wereinvestigated by means of a study of the triplet and singlet surfacesof the N2H4+ O → N2H2+ H2O reaction. [ABSTRACT FROM AUTHOR]

Highlights: [•] Low-lying electronic states of the GeB molecule. [•] Spin–orbit states. [•] Molecular constants. [•] Transition probabilities. [ABSTRACT FROM AUTHOR]

Highlights: [•] Accurate structures of aluminum clusters: Al n x (n =1–9, x =0, ±1). [•] Electron affinities and ionization potential. [•] DFT and CCSD(T) electronic calculations. [ABSTRACT FROM AUTHOR]

Abstract: Thermochemical and kinetics properties of the hydrogen abstraction and addition processes of the reaction were computed using high-level ab initio and DFT approximation methods with aug-cc-pVXZ (X=T,Q) basis set. The CCSD (T)/CBS//BB1K/aug-cc-pVTZ results for classical barrier height are 13.1 and 15.0kcal/mol for the abstraction and addition reactions, respectively. The thermal rate constants were calculated using the dual-level direct dynamics by variational transition state theory with the BB1K potential energy surface and thermochemical properties corrected with the CCSD (T)/CBS//BB1K/aug-cc-pVTZ results. The rate constants calculated show that the variational and tunneling effects play a relevant role only for the abstraction reaction. [Copyright &y& Elsevier]

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]

Abstract: The lowest singlet and triplet states of AlP3, GaP3 and BP3 molecules with Cs, C2v and C3v symmetries were characterized using the B3LYP functional and the aug-cc-pVTZ and aug-cc-pVQZ correlated consistent basis sets. Geometrical parameters and vibrational frequencies were calculated and compared to existent experimental and theoretical data. Relative energies were obtained with single point CCSD(T) calculations using the aug-cc-pVTZ, aug-cc-pVQZ and aug-cc-pV5Z basis sets, and then extrapolating to the complete basis set (CBS) limit. [Copyright &y& Elsevier]

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]

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: Barrier heights, structures, reaction energies, and rate constants are calculated with the DFT, MP2, and CCSD(T) methods for the first two channels of the H+CH3OH reactions. The best estimate results based on CCSD(T) calculations give reaction enthalpies () for the first (−8.4kcal/mol) and second (0.8kcal/mol) reactive channels, which are comparable to the experimental values, −8.8±0.9 and −0.3±0.9kcal/mol, respectively. Rate constants and activation energies calculated with the IVTST-0 method using CCSD(T)/cc-pVTZ geometries and frequencies are also in excellent agreement with experiment data. [Copyright &y& Elsevier]