Variational Effect

Pressure-dependent reactions are ubiquitous in combustion and atmospheric chemistry mechanisms. Here we employ a new calibration procedure for quantum Rice-Ramsperger-Kassel (QRRK) unimolecular rate theory within a chemical activation mechanism to calculate the pressure-falloff effect of a radical association with an aromatic ring, including anharmonicity. This allows the radical association to be treated conveniently at a level of dynamical theory consistent with the treatment of bimolecular atom transfer reactions. The first application, of the new theoretical framework is a calculation of the kinetics of the reaction of H with toluene, which is a prototypical reaction in the combustion chemistry of aromatic hydrocarbons, which are present in most fuels. Both the hydrogen abstraction reactions and the hydrogen addition reactions are calculated. In our QRRK approach, we adjust QRRK theory with system-specific (SS) parameters to agree with multistructural canonical variational trans...

The abstraction and addition reactions of H with trans-N 2 H 2 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.

We examine the many-body expansion (MBE) for alkaline earth metal clusters, Be n, Mg n, Ca n ( n = 4, 5, 6), at the Møller–Plesset second order perturbation theory, coupled-cluster singles and doubles with perturbative triples, multi-reference perturbation theory, and multi-reference configuration interaction levels of theory. The magnitude of each term in the MBE is evaluated for several geometrical configurations. We find that the behavior of the MBE for these clusters depends strongly on the geometrical arrangement and, to a lesser extent, on the level of theory used. Another factor that affects the MBE is the in situ (ground or excited) electronic state of the individual atoms in the cluster. For most geometries, the three-body term is the largest, followed by a steady decrease in absolute energy for subsequent terms. Though these systems exhibit non-negligible multi-reference effects, there was little qualitative difference in the MBE when employing single vs multi-reference me...

The abstraction and addition reactions of H with trans-N 2 H 2 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.

The abstraction and addition reactions of H with trans-N 2 H 2 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.

The abstraction and addition reactions of H with trans-N 2 H 2 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.

The abstraction and addition reactions of H with trans-N 2 H 2 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.

The abstraction and addition reactions of H with trans-N 2 H 2 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.

Barrier heights of the internal rotation and inversion motions of methylamine, dimethylamine and trimethylamine molecules were calculated with the CCSD(T)//B3LYP methodology in combination with the cc-pVTZ, cc-pVQZ, and cc-pCVTZ basis sets of Dunning. The complete basis set (CBS) extrapolation procedure and core-valence (CV) correlation effects are also examined to the barrier heights. Our best estimate results (CCSD(T)/CBS+CV//B3LYP/cc-pVQZ) are in very good agreement with the experimental data, indicating the use of this methodology to provide accurate predictions of structures and barrier heights for other systems

In the course of our studies on nitrogen trifluoride dissociation, we consider in this paper two different sets of reactive systems: the first one consists of the abstraction and the exchange channels of NF 3 + F and the other is composed of the unimolecular and the abstraction production of N 2 + F. Accurate electronic properties are determined and applied in the scope of the transition state theory (TST) to obtain thermal rate constants for these systems. An extensive investigation in terms of minimum energy paths and intrinsic reaction coordinates was previously carried out in order to ensure the good quality of our TST results. We apply Wigner corrections to consider tunneling effects whenever their importance is numerically verified. The obtained results for the abstraction channel thermal rate constants are in good agreement with experimental data which indicates that this kind of study is of potential use to help to clarify the decomposition mechanism of NF 3 .

In the course of our studies on nitrogen trifluoride dissociation, we consider in this paper two different sets of reactive systems: the first one consists of the abstraction and the exchange channels of NF 3 + F and the other is composed of the unimolecular and the abstraction production of N 2 + F. Accurate electronic properties are determined and applied in the scope of the transition state theory (TST) to obtain thermal rate constants for these systems. An extensive investigation in terms of minimum energy paths and intrinsic reaction coordinates was previously carried out in order to ensure the good quality of our TST results. We apply Wigner corrections to consider tunneling effects whenever their importance is numerically verified. The obtained results for the abstraction channel thermal rate constants are in good agreement with experimental data which indicates that this kind of study is of potential use to help to clarify the decomposition mechanism of NF 3 .

Extensive ab initio calculations have been performed to determine the energy, geometry and vibrational frequencies of all stationary points of the N 2 H 2 ground-state potential energy surface. The geometries of trans-, cis-and iso-minima as well as transition states are reported at the MCSCF/aug-cc-pVQZ level, while the relative energetics is established by single point MRCI/aug-cc-pVQZ calculations including the Davidson size-consistency correction. The data is useful for modeling a single-sheeted global potential energy surface for the title system.

Extensive ab initio calculations have been performed to determine the energy, geometry and vibrational frequencies of all stationary points of the N 2 H 2 ground-state potential energy surface. The geometries of trans-, cis-and iso-minima as well as transition states are reported at the MCSCF/aug-cc-pVQZ level, while the relative energetics is established by single point MRCI/aug-cc-pVQZ calculations including the Davidson size-consistency correction. The data is useful for modeling a single-sheeted global potential energy surface for the title system.

In this paper the first variational transition-state rate constant calculation for the OH + CH 3 COCH 3 → P reaction is presented. The potential energy surface has been described by low level calculations at the B3LYP/6-31G* level combined with higher level calculations using the multilevel CBS-RAD technique. Three different reaction pathways have been found: abstraction of a H atom eclipsed to the carbonyl group of acetone, abstraction of a H atom alternated to the carbonyl group of acetone, and addition of the OH molecule to the carbon atom of the carbonyl group of acetone. To take into account the three different kinetic channels, the competitive canonical unified statistical theory has been used to calculate the global rate constant. However, in practice the global rate constant of the acetone + OH reaction turns out finally to be the sum of the eclipsed and alternated abstraction rate constants, leading to a clearly curved Arrhenius plot. The addition-elimination mechanism has an almost negligible contribution to the global rate constant at whatever temperature. The corresponding branching ratio is at most of ≈2% and attains even smaller values at the lowest temperature range.

In the course of our studies on nitrogen trifluoride dissociation, we consider in this paper two different sets of reactive systems: the first one consists of the abstraction and the exchange channels of NF 3 + F and the other is composed of the unimolecular and the abstraction production of N 2 + F. Accurate electronic properties are determined and applied in the scope of the transition state theory (TST) to obtain thermal rate constants for these systems. An extensive investigation in terms of minimum energy paths and intrinsic reaction coordinates was previously carried out in order to ensure the good quality of our TST results. We apply Wigner corrections to consider tunneling effects whenever their importance is numerically verified. The obtained results for the abstraction channel thermal rate constants are in good agreement with experimental data which indicates that this kind of study is of potential use to help to clarify the decomposition mechanism of NF 3 .

In the course of our studies on nitrogen trifluoride dissociation, we consider in this paper two different sets of reactive systems: the first one consists of the abstraction and the exchange channels of NF 3 + F and the other is composed of the unimolecular and the abstraction production of N 2 + F. Accurate electronic properties are determined and applied in the scope of the transition state theory (TST) to obtain thermal rate constants for these systems. An extensive investigation in terms of minimum energy paths and intrinsic reaction coordinates was previously carried out in order to ensure the good quality of our TST results. We apply Wigner corrections to consider tunneling effects whenever their importance is numerically verified. The obtained results for the abstraction channel thermal rate constants are in good agreement with experimental data which indicates that this kind of study is of potential use to help to clarify the decomposition mechanism of NF 3 .

In the course of our studies on nitrogen trifluoride dissociation, we consider in this paper two different sets of reactive systems: the first one consists of the abstraction and the exchange channels of NF 3 + F and the other is composed of the unimolecular and the abstraction production of N 2 + F. Accurate electronic properties are determined and applied in the scope of the transition state theory (TST) to obtain thermal rate constants for these systems. An extensive investigation in terms of minimum energy paths and intrinsic reaction coordinates was previously carried out in order to ensure the good quality of our TST results. We apply Wigner corrections to consider tunneling effects whenever their importance is numerically verified. The obtained results for the abstraction channel thermal rate constants are in good agreement with experimental data which indicates that this kind of study is of potential use to help to clarify the decomposition mechanism of NF 3 .

The atmospheric reaction of H 2 S with Cl has been reinvestigated to check if, as previously suggested, only explicit dynamical computations can lead to an accurate evaluation of the reaction rate because of strong recrossing effects and the breakdown of the variational extension of transition state theory. For this reason, the corresponding potential energy surface has been thoroughly investigated, thus leading to an accurate characterization of all stationary points, whose energetics has been computed at the state of the art. To this end, coupled-cluster theory including up to quadruple excitations has been employed, together with the extrapolation to the complete basis set limit and also incorporating core−valence correlation, spin−orbit, and scalar relativistic effects as well as diagonal Born−Oppenheimer corrections. This highly accurate composite scheme has also been paralleled by less expensive yet promising computational approaches. Moving to kinetics, variational transition state theory and its variable reaction coordinate extension for barrierless steps have been exploited, thus obtaining a reaction rate constant (8.16 × 10 −11 cm 3 molecule −1 s −1 at 300 K and 1 atm) in remarkable agreement with the experimental counterpart. Therefore, contrary to previous claims, there is no need to invoke any failure of the transition state theory, provided that sufficiently accurate quantum-chemical computations are performed. The investigation of the puzzling case of the H 2 S + Cl system allowed us to present a robust approach for disclosing the thermochemistry and kinetics of reactions of atmospheric and astrophysical interest.

The endothermic reaction N2++N2→N3++N has been investigated both experimentally and by ab initio calculations. Integral cross sections are presented as a function of collision energy. For excited N2+ ions the reaction onset shifts towards lower energies, indicating that the internal energy of the reactant ions promotes the reaction. Analysis of the energy dependence of the cross section does not allow an unequivocal determination of the reaction endothermicity. However, calculations of the ground doublet surface indicate that the minimum energy path from reactants to products proceeds through strongly bent geometries, with an endothermicity equivalent to the thermodynamic value of about 5 eV. For linear geometries an additional barrier of about 1 eV is found in the product channel. The structure of N4+ in its first excited quartet state has been also calculated by ab initio methods

© Springer-Verlag Berlin Heidelberg (outside the uSA) 2014 2 , T 1 , D 1 , and %TAe[(T)] diagnostics, we assess the Hartree-Fock HOMO-luMO gap, maximum occupation number defect, first vertical excitation energy, a direct estimate of multireference effects, and combinations of diagnostics as indicators of errant thermochemistry.

We have theoretically studied the NF 3 ) NF 2 + F, NF 2 + F ) NF + F 2 , and NF 2 + F ) NF 2 + F reactive processes. More precisely, we have evaluated the thermal rate constants (TRC), with the Wigner and Eckart tunneling corrections, minimum energy path, and the intrinsic reaction coordinates of these systems. The NF 3 ) NF 2 + F conventional and Wigner TRCs agree very well with experimental data available in the literature for a wide range of temperatures. This study gives a first step to understand and determine the correct decomposition path of nitrogen trifluoride (NF 3 ).

The reaction between molecular oxygen and two nitric oxide(II) molecules is studied with high-level ab initio wave function methods, including geometry optimizations with coupled cluster (CCSD(T,full)/cc-pCVTZ) and complete active space with second order perturbation theory levels (CASPT2/cc-pVDZ). The energy at the critical points was refined by calculations at the CCSD(T,full)/aug-cc-pCVTZ level. The controversies found in the previous theoretical studies are critically discussed and resolved. The best estimate of the activation energy is 6.47 kJ/mol.

MP2 and CCSD(T) geometry optimization and harmonic vibrational frequency calculations were performed on the X 1 ϩ states of HBS and FBS and the X 2 and A 2 ϩ states of their cations with basis sets as large as 6-311G(2df,2p). Franck-Condon analyses and spectral simulation were carried out on the first two He I photoelectron bands of these species. The theoretical spectra obtained by employing the ab initio geometries, vibrational frequencies and force constants agree well with the observed ones. ᭧

A practical and accurate semiclassical method for calculating the tunneling splitting of the ground state in polyatomic molecules is presented based on a recent version of the instanton theory ͓J. Chem. Phys. 115, 6881 ͑2001͔͒. The method uses ab initio quantum chemical data for the potential energy surface without any concomitant extrapolation and requires only a small number of ab initio data points to get convergence even for large molecules. This enables one to use an advanced level of electronic structure theory and achieve a high accuracy of the result. The method is applied to the 9-atomic malonaldehyde molecule by making use of the potential energy surface at the level of CCSD͑T͒ with the hybrid basis set of aug-cc-pVTZ ͑for oxygen atoms and the transferred hydrogen atom͒ and cc-pVTZ ͑for other atoms͒.

Understanding the structural and dynamical conformational properties of molecules is of fundamental and practical value in chemistry. One of the most fascinating issues in this respect is the folding of proteins, which represents a very important but simultaneously extremely complicated and multidimensional problem. The most detailed information can be obtained by studying small molecules. In this respect, formic acid (FA), being the simplest organic acid, represents a good model system for studying the conformational properties of carboxylic acids and the carboxylic group in general. It is known that interconversion of different conformers can be induced in the solid state by vibrational excitation. 1 Coupling of the initially excited state with other modes of the isolated molecule and with lattice phonons leads to the flow of energy into the reaction coordinate, finally promoting the change of the geometry of the molecule from one conformation to another. 1,2 Usually, it is natural to assume that to induce the conformational conversion, the initially excited state should lie well above the reaction barrier. 2,3 In this communication, we show that in FA, due to tunneling, the conversion from the lower energy trans to the higher energy cis form can be induced with high efficiency at energies well below the reaction barrier.

The double proton transfer in the formamide dimer is characterized computationally by combining density functional theory and ab initio methods. The intrinsic reaction coordinate (IRC) is obtained at the B3LYP level of theory. Energies of several points along the IRC are treated by the more rigorous focal point method to test the validity of the B3LYP functional. The reaction mechanism is examined in terms of the energy profile, the reaction force, the chemical potential, and the reaction electronic flux. The energy profile for the activation process of the formamide dimer to the imino ether product obtained with the B3LYP functional is in agreement with the results of the focal point method. Together with the reaction force analysis and the reaction electronic flux a precise assignment of the structural and electronic contributions to the activation barrier becomes possible. The results show that the reaction starts with a structural rearrangement, where the two dimers approach each other, and is followed by electronic changes before the system reaches the transition state. This electronic contribution to the activation barrier steers the activation process. After the transition state is reached, deviations of the B3LYP functional from the more accurate focal point energies become apparent, where the errors may be rationalized in terms of the treatment of exchange. The inconsistency could be assigned to the incapacity of the functional to describe delocalization effects over the whole system.

Using systematic sequences of the newly developed correlation consistent core-valence basis sets from cc-pCVDZ through cc-pCV6Z, the spectroscopic constants of the homonuclear diatomic molecules containing ®rst row atoms, B±F, are calculated both with and without inclusion of 1s correlation. Internally contracted multireference con®guration interaction (IC-MRCI) and singles and doubles coupled cluster (CCSD) theory with a perturbational estimate of connected triple excitations, CCSD(T), have been investigated. By exploiting the convergence of the correlation consistent basis sets, complete basis set (CBS) limits have been estimated for total energies, dissociation energies, equilibrium geometries, and harmonic frequencies.

Based on accurate electronic structure calculations, a new analytical potential energy surface (PES) was fitted to simultaneously describe the hydrogen abstraction reaction from ammonia by a hydrogen atom, and the ammonia inversion. Using a wide spectrum of properties of the reactive system (equilibrium geometries, vibrational frequencies, and relative energies of the stationary points, topology of the reaction paths, and points on the reaction swaths) as reference, the resulting analytical PES reproduces reasonably well the input ab initio information obtained at the CCSD(T)/cc-pVTZ level, which represents a severe test for the new surface. As a first application, on this analytical PES we perform an extensive kinetics study using variational transition-state theory with semiclassical transmission coefficients over a wide temperature range, 200-2000 K. For the hydrogen abstraction reaction, the forward rate constants reproduce the experimental measurements, while the reverse ones are slightly underestimated. Another severe test of the new surface is the analysis of the kinetic isotope effects (KIEs). The KIEs between unsubstituted and all deuterated reactions agree with experiment in the common temperature range. For the ammonia inversion reaction, the splitting of the degenerate vibrational levels of the double well due to the tunneling contribution, which is very important in this reaction representing 93% of the reactivity at 200 K, was calculated for the NH 3 and ND 3 species. The values found were 3.6 and 0.37 cm -1 , respectively, which although higher than experimental values, reproduce the experimental behavior on isotopic substitution.

Based on accurate electronic structure calculations, a new analytical potential energy surface (PES) was fitted to simultaneously describe the hydrogen abstraction reaction from ammonia by a hydrogen atom, and the ammonia inversion. Using a wide spectrum of properties of the reactive system (equilibrium geometries, vibrational frequencies, and relative energies of the stationary points, topology of the reaction paths, and points on the reaction swaths) as reference, the resulting analytical PES reproduces reasonably well the input ab initio information obtained at the CCSD(T)/cc-pVTZ level, which represents a severe test for the new surface. As a first application, on this analytical PES we perform an extensive kinetics study using variational transition-state theory with semiclassical transmission coefficients over a wide temperature range, 200-2000 K. For the hydrogen abstraction reaction, the forward rate constants reproduce the experimental measurements, while the reverse ones are slightly underestimated. Another severe test of the new surface is the analysis of the kinetic isotope effects (KIEs). The KIEs between unsubstituted and all deuterated reactions agree with experiment in the common temperature range. For the ammonia inversion reaction, the splitting of the degenerate vibrational levels of the double well due to the tunneling contribution, which is very important in this reaction representing 93% of the reactivity at 200 K, was calculated for the NH 3 and ND 3 species. The values found were 3.6 and 0.37 cm -1 , respectively, which although higher than experimental values, reproduce the experimental behavior on isotopic substitution.

The potential-energy hypersurface of the addition reaction OHϩC 2 H 4 was partially explored following two different approaches. First, the stationary points were located at the MP2͑FULL͒/ 6-31G͑d,p͒ level and then the minimum energy path ͑MEP͒ was built starting from the MP2 saddle-point geometry. In order to improve the energetics along the MEP, single-point calculations were carried out at several higher levels, in particular, PMP2, MP4sdtq, PMP4sdtq, and QCIsd͑t͒. In a different approach, the C-O bond length was assumed to provide an accurate parametrization of the reaction path in the vicinity of the transition state. The minimum energy structures at the MP4sdq/6-311ϩG͑d,p͒ level for 16 points along the R C-O coordinate have been calculated, followed by a generalized normal-mode analysis at the MP2͑FULL͒/6-311ϩG͑d,p͒ level for each point. The initial potential information from both approaches was used to calculate canonical variational transition state ͑CVT͒ association rate constants for the temperature range 200-1000 K. Our calculations at the PMP4sdtq/6-311ϩG͑d,p͒//MP4sdq/6-311ϩG͑d,p͓͒MP2͑FULL͒/6-311 ϩG͑d,p͔͒ level reproduce the inverse dependence of the rate constant with temperature at T Ͻ565 K, in agreement with the experimental evidence that this reaction has a negative activation energy at room temperature. The analysis of the enthalpic and entropic contributions to the Gibbs free-energy profile has allowed us to understand those negative values of the activation energy.

Benchmark dynamics calculations have played a critical role in assessing the validity of approximation schemes. A benchmark dynamics calculation is one in which the nuclearmotion Schrödinger equation is solved accurately ͑to within some specific numerical tolerance͒ for a given potential energy surface ͑or surfaces, if non-Born-Oppenheimer effects are involved͒. The potential energy surface should be realistic, although it need not be quantitatively accurate. By carrying out calculations employing an approximate dynamical scheme with the same potential energy surface, one can test the dynamical scheme itself more definitively than by comparison to experiment, where uncertainities in the potential energy surface usually cloud the interpretation.

In this work we analyzed the multiple channels of the reaction NF+F through the evaluation of thermal rate constants with both Wigner and Eckart tunneling corrections. Minimum energy paths and intrinsic reaction coordinates of the systems were obtained and accurately studied in order to ensure the consistency of our results. Specifically, we investigated the NF + F = N + F 2 , NF + F = NF + F, and NF 2 = NF + F, reactive systems. As experimental data are available for the latter reaction, we were able to conclude that our thermal rate constants are in agreement for a wide range of temperatures. The here performed study is relevant to the understanding of the decomposition process of nitrogen trifluoride (NF 3 ).

In the course of our studies on nitrogen trifluoride dissociation, we consider in this paper two different sets of reactive systems: the first one consists of the abstraction and the exchange channels of NF 3 + F and the other is composed of the unimolecular and the abstraction production of N 2 + F. Accurate electronic properties are determined and applied in the scope of the transition state theory (TST) to obtain thermal rate constants for these systems. An extensive investigation in terms of minimum energy paths and intrinsic reaction coordinates was previously carried out in order to ensure the good quality of our TST results. We apply Wigner corrections to consider tunneling effects whenever their importance is numerically verified. The obtained results for the abstraction channel thermal rate constants are in good agreement with experimental data which indicates that this kind of study is of potential use to help to clarify the decomposition mechanism of NF 3 .

A complete-active-space (US) multiconfguration self-consistent tield (MCSCF) wavefunction with a polarized correlationconsistent basis set was used to determine the stationary points on the OH+HNO potential energy surface. The single-point energies were determined with a multireference confguration interaction (MRCI ) wavefunction consisting of single and double excitations from selected configurations chosen from the reference wavefunction. Saddle points were confirmed at the Bveelectron, tive-active-orbital CASSCF level by calculating analytical second derivatives. Segments of the minimum-energy paths (MEPs) for the hydrogen abstraction reaction and for the radical addition to nitrogen were calculated. It was found that the zero-point energy contribution for the abstraction pathway increases as one moves away from the MCSCF saddle point toward the reactants.

The major atmospheric sink for the greenhouse gas methane is reaction with the OH radical. Computer simulations that use the 13 C isotopic composition of atmospheric methane as a constraint have been performed to quantify the global distribution of methane sources and sinks; the output from these simulations is found 1 to be sensitive to the input 12 C/ 13 C kinetic isotope effect (KIE) for the reaction OH + CH 4 f H 2 O + CH 3 . Such a situation calls for accurate knowledge of the 12 C/ 13 C KIE and its temperature dependence in the whole atmospherically important temperature range. Unfortunately the temperature dependence is smaller than the experimental error bars. Theoretical investigations 4-7 on the 12 C/ 13 C KIE for have also been carried out, but the results show large variations in the magnitude of the KIE and even larger variations in its temperature dependence. For example, the calculations by Lasaga and Gibbs 4 and by Gupta et al. 6 were based on conventional transition state theory (TST) with one-dimensional tunneling corrections, and they both predicted very flat temperature dependence (see ). In this work, we address this problem by direct dynamics calculations based on variational transition state theory with multidimensional tunneling contributions (VTST/MT). 8-10 The potential energy surfaces (PESs) were generated at different levels of electronic structure theory, in particular, the hybrid density functional theory (HDFT) method MPW1K 11,12 using the MG3S 12 basis set and the recently developed doubly hybrid density functional theory 13 (DHDFT), in particular the MC3MPW and MC3BB models. Comparisons of our calculated KIEs with experimental data and theoretical values in the literature reveal the critical contributions of multidimensional tunneling and torsional anharmonicity.

Systematic quantum chemical calculations have been performed to obtain precise estimates of the equilibrium and vibrationally averaged molecular structure and electric dipole moment of vinylacetylene (VA, 1-buten-3-yne). Anharmonic (cubic and semi-diagonal quartic) MP2/cc-pVTZ force fields in normal coordinates were computed to account for anharmonic vibrational effects, including zero-point contributions to the rotational constants and the electric dipole moment. A simultaneous weighted least-squares structural refinement was performed, resulting in the best semispectroscopic estimate of the r e structure of VA. The refinement was based on experimentally measured ground-state rotational constants of two isotopologs of VA corrected to equilibrium values using MP2/cc-pVTZ vibration-rotation interaction constants and all-electron CCSD(T)/ aug-cc-pVTZ structural constraints. The semispectroscopic r e structure of VA agrees excellently with the high-level CCSD(T)/aug-cc-pVTZ ab initio structure. The most dependable, CCSD(T)/cc-pVQZ//CCSD(T)/ aug-cc-pVTZ equilibrium electric dipole moment of VA, in D, is µ a ) 0.4088, µ b ) 0.0004, and µ c ) 0. The vibrationally corrected a-component of 0.4214 D is in excellent agreement with one of the available experimental values. The present analysis shows that µ b is negligible even after vibrational correction. A simple quantitative structure-property relationship (QSPR) model resulted in a highly similar estimate, 0.45 D, for the electric dipole moment of VA.

Electronic structure calculations using various methods, up to the coupled-cluster CCSD(T) level, in conjunction with the aug-cc-pVnZ basis sets with n ) D, T, and Q, extrapolated to the complete basis set limit, show that the borane molecule (BH 3 ) can act as an efficient bifunctional acid-base catalyst in the H 2 elimination reactions of XH n YH n systems (X, Y ) C, B, N). Such a catalyst is needed as the generation of H 2 from isoelectronic ethane and borane amine compounds proceeds with an energy barrier much higher than that of the X-Y bond energy. The asymptotic energy barrier for H 2 release is reduced from 36.4 kcal/mol in BH 3 NH 3 to 6.0 kcal/mol with the presence of BH 3 relative to the molecular asymptote. The NH 3 molecule can also participate in a similar catalytic process but induces a smaller reduction of the energy barrier. The kinetics of these processes was analyzed by both transition-state and RRKM theory. The catalytic effect of BH 3 has also been probed by an analysis of the electronic densities of the transition structures using the atom-in-molecule (AIM) and electron localization function (ELF) approaches.

The catechol functionality present in the catechins is responsible for the protective effects exerted by green tea against a wide range of human diseases. High-level electronic structure calculations and canonical variational transition-state theory including multidimensional tunneling corrections have allowed us to understand the key factors of the antioxidant effectiveness of the catechol group. This catechol group forms two hydrogen bonds with the two oxygen atoms of the lipid peroxyl radical, leading to a very compact reactant complex. This fact produces an extremely narrow adiabatic potential-energy profile corresponding to the hydrogen abstraction by the peroxyl radical, which makes it possible for a huge tunneling contribution to take place. So, quantum-mechanical tunneling highly increases the corresponding rate constant value, in such a way that catechins become able to trap the lipid peroxyl radicals in a dominant competition with the very damaging free-radical chain-lipid peroxidation reaction.

The present work aims to elucidate the mechanism of the oxidation reaction of lithium atoms with nitrous oxide based on the complete-active-space-self-consistent-field plus second-order perturbation theory (CASSCF-MP2(11e/ 12o)) ab initio calculations. The title reaction is found to occur via two lower-lying channels with the energy barriers of 4.5 and 6.0 kcal/mol. Both barriers originate as a result of an avoided crossing of the two lowest 2 A 0 potential energy surfaces (PES), corresponding to the neutral and ionic reactant states. Due to a large energy separation between surfaces in the transition regions, the reaction likely occurs on the lowest adiabatic PES. Earlier photoelectron spectroscopic experiments related to the ionization of LiO are also discussed within the present model. Ó

The CH 4 + HO 2 • reaction is studied by using explicitly correlated coupled-cluster theory with singles and doubles (CCSD-R12) in a large 19s14p8d6f4g3h basis (9s6p4d3f for H) to approach the basis-set limit at the coupled-cluster singles-doubles level. A correction for connected triple excitations is obtained from the conventional CCSD(T) coupled-cluster approach in the correlation-consistent quintuple-basis (cc-pV5Z). The highly accurate results for the methane reaction are used to calibrate the calculations of the hydroperoxylradical hydrogen abstraction from other alkanes. For the alkanes C n H 2n+2 with n ) 2 f 4, the reactions are investigated at the CCSD(T) level in the correlation-consistent triple-(cc-pVTZ) basis. The results are adjusted to the benchmark methane reaction and compared with those from other approaches that are commonly used in the field such as CBS-QB3, CBS-APNO, and density functional theory. Rate constants are computed in the framework of transition state theory, and the results are compared with previous values available.

Antonio Fernández-Ramos was born in Ourense (Galicia), Spain in 1970. He received his BA in Chemistry in 1993 and his Ph.D. in 1998 from the Universidade de Santiago de Compostela, the latter under the direction of Miguel A. Ríos and Jesús Rodríguez. He ...

We have used correlated electronic structure calculations and direct dynamics methods, including variational transition state theory with multidimensional semiclassical methods for calculating tunneling probabilities, to predict kinetic isotope effects (KIEs) and activation energies for hydride transfer in a polycyclic hydroxy ketone in the gas phase and aqueous solution. The need to include tunneling in the calculation of the KIEs has been shown, and the enhanced tunneling arising from a multidimensional model, compared to the one-dimensional Wigner treatment, has been demonstrated. The effect of solvent on the multidimensional tunneling contributions to the KIEs is significant and partly cancels the other contributions of the solvent to the KIEs. We also examined structural features on the kinetics for three other polycyclic hydroxy ketones by using conventional transition state theory and a Wigner tunneling approximation.

Electronic structure calculations using various methods, up to the coupled-cluster CCSD(T) level, in conjunction with the aug-cc-pVnZ basis sets with n ) D, T, and Q, extrapolated to the complete basis set limit, show that the borane molecule (BH 3 ) can act as an efficient bifunctional acid-base catalyst in the H 2 elimination reactions of XH n YH n systems (X, Y ) C, B, N). Such a catalyst is needed as the generation of H 2 from isoelectronic ethane and borane amine compounds proceeds with an energy barrier much higher than that of the X-Y bond energy. The asymptotic energy barrier for H 2 release is reduced from 36.4 kcal/mol in BH 3 NH 3 to 6.0 kcal/mol with the presence of BH 3 relative to the molecular asymptote. The NH 3 molecule can also participate in a similar catalytic process but induces a smaller reduction of the energy barrier. The kinetics of these processes was analyzed by both transition-state and RRKM theory. The catalytic effect of BH 3 has also been probed by an analysis of the electronic densities of the transition structures using the atom-in-molecule (AIM) and electron localization function (ELF) approaches.

Analytical formulae for the threshold region of the reaction probability curve for a nearly degenerate H-atom transfer XC @en and shown to yield good agreement with e.\act results for a t\\o-state model system. Application to H and H+ transfer in complcl systems is discussed.

Electronic structure calculations using various methods, up to the coupled-cluster CCSD(T) level, in conjunction with the aug-cc-pVnZ basis sets with n ) D, T, and Q, extrapolated to the complete basis set limit, show that the borane molecule (BH 3 ) can act as an efficient bifunctional acid-base catalyst in the H 2 elimination reactions of XH n YH n systems (X, Y ) C, B, N). Such a catalyst is needed as the generation of H 2 from isoelectronic ethane and borane amine compounds proceeds with an energy barrier much higher than that of the X-Y bond energy. The asymptotic energy barrier for H 2 release is reduced from 36.4 kcal/mol in BH 3 NH 3 to 6.0 kcal/mol with the presence of BH 3 relative to the molecular asymptote. The NH 3 molecule can also participate in a similar catalytic process but induces a smaller reduction of the energy barrier. The kinetics of these processes was analyzed by both transition-state and RRKM theory. The catalytic effect of BH 3 has also been probed by an analysis of the electronic densities of the transition structures using the atom-in-molecule (AIM) and electron localization function (ELF) approaches.

A complete-active-space (US) multiconfguration self-consistent tield (MCSCF) wavefunction with a polarized correlationconsistent basis set was used to determine the stationary points on the OH+HNO potential energy surface. The single-point energies were determined with a multireference confguration interaction (MRCI ) wavefunction consisting of single and double excitations from selected configurations chosen from the reference wavefunction. Saddle points were confirmed at the Bveelectron, tive-active-orbital CASSCF level by calculating analytical second derivatives. Segments of the minimum-energy paths (MEPs) for the hydrogen abstraction reaction and for the radical addition to nitrogen were calculated. It was found that the zero-point energy contribution for the abstraction pathway increases as one moves away from the MCSCF saddle point toward the reactants.