Ab initio coupled cluster determination of the equilibrium structures of cis- and trans-1,2-difluoroethylene and 1,1-difluoroethylene

The equilibrium structures of cis- and trans-1,2-difluoroethylene and 1,1-difluoroethylene, C(2)H(2)F(2), have been determined with high-level coupled cluster techniques combined with large basis sets, explicit consideration of core/valence, and scalar relativistic and higher order correlation effects. Excellent agreement was found with new semiexperimental structures, increasing the level of confidence in both approaches. Differences in bond lengths among ethylene and the fluoroethylenes are discussed.     

Ab initio thermochemistry and kinetics for carbon-centered radical addition and beta-scission reactions

A quantitative comparison of ab initio calculated rate coefficients using five computational methods and five different approaches of treating hindered internal rotation and tunneling with experimental values of rate coefficients for nine carbon-centered radical additions/beta scissions at 300, 600, and 1000 K is performed. The high-accuracy compound methods, CBS-QB3 and G3B3, and the density functionals, MPW1PW91, BB1K, and BMK, have been evaluated using the following approaches: (i) the harmonic oscillator approximation; (ii) the hindered internal rotor approximation for the internal rotation about the forming/breaking bond in the transition state and product; and the hindered internal rotation approximation combined with (iii) Wigner, (iv) Skodje and Truhlar, and (v) Eckart zero-curvature tunneling corrections. The density functional theory (DFT) based values for beta-scission rate coefficients deviate significantly from the experimental ones at 300 K, and the DFT methods do not accurately predict the equilibrium coefficient. The hindered rotor approximation offers a significant improvement in the agreement with experimental rate coefficients as compared to the harmonic oscillator treatment, especially at higher temperatures. Tunneling correction factors are smaller than 1.40 at 300 K and 1.03 at 1000 K. For both the CBS-QB3 method, including the hindered rotor treatment but excluding tunneling corrections, and the G3B3 method, including hindered rotor and Eckart tunneling corrections, a mean factor of deviation with experimentally observed values of 3 is found.     

Ab initio study of substituent effect on the addition of hydrogen fluoride to fluoroethylenes

An ab initio 3-21G study of the direct addition of HF to C₂H_nF_(4–n), with n = 0 to 4, has been performed to investigate the effect of the substituent on the reaction. Geometry optimization of all charge-transfer complexes and transition states has been done. Standard analysis of activation energies of addition reactions, vibrational and thermodynamical analysis, as well as Morokuma energy decomposition, BSSE correction, PMO analysis, and Pauling bond orders were used to explain the results. A subset of the reactions, including that of C₂H₄ as reference one and the two most favorable cases, was also studied at the MP2/6–31G(d,p)//HF/6–31G(d,p) level. The barriers so obtained are in agreement with the indirectly found from experimental data. It was found that the effect of the substituent is not monotonic for the additions. Decomposition of the interaction energy is shown to be adequate to explain this nonmonotonic behavior. The implications for laser chemistry of the addition of hydrogen halides to fluorosubstituted olefins is briefly discussed.     

Ab initio study of the cyclooctatetraenyl radical

Ab initio calculations have been carried out on the 1,3,5,7- and 1,2,4,7-tetraene configurations of the cyclooctatetraenyl radical at UHF, ROHF, MCSCF, ROCISD, QCISD, and CCSD(T) levels of theory with 6-311G(d,p) and cc-pVDZ basis sets. Although spin contamination is present, the ROCISD calculations support the energies obtained from less intensive, UHF-based coupled cluster calculations over the energies obtained from MCSCF analysis of the pi-electron orbitals. The 1,3,5,7-form is a local minimum at the coupled cluster levels, higher in energy than the resonance-stabilized 1,2,4,7-form by 10-13 kJ/mol, but bounded by a barrier of less than 0.5 kJ/mol. The isomerization surface connecting these two structures is described and results reported from integration of the vibrational Schr?dinger equation on that surface. Excited vibrational states at energies just above the isomerization barrier are dominated by the character of the 1,3,5,7-tetraenyl radical, which suggests that chemistry involving this intermediate at typical combustion temperatures may branch at this juncture.     

An ab initio and DFT study of radical addition reactions of imidoyl and thioyl radicals to methanimine

Ab initio and DFT calculations reveal that both imidoyl and thioyl radicals add to the nitrogen end of methanimine through simultaneous SOMO-π*(imine), SOMO-π(imine), SOMO-LP(N) and π*(radical)-LP(N) interactions between the radical and the imine. At the CCSD(T)/cc-pVDZ//BHandHLYP/cc-pVTZ level of theory, barriers of 13.8 and 26.1 kJ mol(-1) are calculated for the attack of the methylimidoyl radical at the carbon- and nitrogen- end of methanimine, respectively, indicating that the imidoyl radial has a preference for addition to the nitrogen end of imine. On the other hand, barriers of 25.1 and 13.4 kJ mol(-1) are calculated at the same level of theory for the addition reaction of the methanethioyl radical at the carbon- and nitrogen- end of methanimine, respectively. Natural bond orbital (NBO) analysis at the BHandHLYP/6-311G** level of theory reveals that SOMO-π*(imine), SOMO-π(imine), SOMO-LP(N) and π*(radical)-LP(N) interactions are worth 111, 89, 115 and 17 kJ mol(-1), respectively, in the transition state (4) for the reaction of methylimidoyl radical at the nitrogen end of methanimine; similar interactions are observed for the chemistry involving all the radicals studied here. These multi-component interactions are responsible for the unusual motion vectors associated with the transition states involved in these reactions.     

Ab initio study of 1,4-pentadienyl electrocyclic reactions

The thermochemistry and transition states of the electrocyclic ring closures of the resonance-stabilized 1,4-pentadienyl radical to cyclopenten-3-yl, cyclobut-2-enylmethyl, and 2-vinylcyclopropyl are investigated at Hartree-Fock and coupled-cluster levels of theory. The CCSD(T)//QCISD/cc-pVDZ calculations predict activation barriers of 130, 169, and 236 kJ/mol, respectively, and DeltaH values of -60, 115, and 155 kJ/mol. Experimental evidence for the appearance of vinylcyclopropyl following photolytic generation of pentadienyl is more likely the result of a distinct electrocyclic reaction than quenching of a two-step mechanism for formation of cyclopentenyl. Higher energy pathways for formation of polycyclic structures are also briefly examined.     

Kinetics of the hydrogen abstraction reactions of 1,1‐ and 1,2‐difluoroethane with hydroxyl radical: an ab initio study

The hydrogen abstraction reactions of 1,1‐ and 1,2‐difluoroethane with the OH radical have been investigated by the ab initio molecular orbital theory. The geometries of the reactants, products, and transition states have been optimized at the (U)MP2=full level of theory in conjunction with 6‐311G(d,p) basis functions. Single‐point (U)MP2=full with larger basis set, such as 6‐311G(3d,2p), and QCISD(T)=full/6‐311G(d,p) calculations have also been carried out to observe the effects of basis sets utilized and higher order electron correlation. Three and four reaction channels have been identified for 1,1‐ and 1,2‐difluoroethane, respectively. In the case of 1,1‐difluoroethane, hydrogen abstraction from the α‐carbon has been found to be easier than that from the β‐carbon. The barriers of the four reaction channels for 1,2‐difluoroethane are close to each other. Weak hydrogen bonding interactions have been observed between hydroxyl hydrogen and a fluorine atom in the transition states. Rate constants for the reactions of 1,1‐ and 1,2‐difluoroethane with the OH radical have been calculated using the standard transition state theory and found to be in good agreement with the experimental results. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 1305–1318, 2000     

Ab initio direct dynamics study of cyclopropyl radical ring-opening

Quasiclassical direct dynamics simulations, at the CASSCF(3,3)/6-31G(d) level of theory, are used to study the stereochemistry of the electrocyclic ring-opening reaction of the cyclopropyl radical. The trajectories are initiated at the reaction's transition state (TS), with their initial conditions sampled from the TS's 174 degrees C Boltzmann distribution. Intrinsic reaction coordinate calculations predict the overall reaction to have disrotatory stereochemistry. Though this is the preferred initial reaction stereochemistry in the trajectories, 43% of the trajectories follow the conrotatory path. Four unique trajectory types are observed during 200 fs dynamics of the product allyl radical. Intramolecular vibrational energy redistribution and internal rotation are incomplete on this time scale, and a statistical distribution of the allyl isomers is not observed.     

Ab initio study of hydrogen migration across n-alkyl radicals

A thorough ab initio investigation is conducted on all possible hydrogen migration pathways for the 1-ethyl, 1-propyl, 1-butyl, 1-pentyl, 1-hexyl, 1-heptyl, and 1-octyl radicals in order to determine underlying trends in reaction enthalpies, activation energies, Arrhenius A-factors, tunneling, and rate coefficients. The G4, G2, and CBS-Q composite methods are used to determine the enthalpy of reaction and activation energy barrier for each reaction. Each method shows excellent agreement with eight experimental enthalpy of reaction values, with root mean squared values of 0.8, 0.9, and 0.6 kcal mol(-1) for CBS-Q, G2, and G4, respectively. Differences in barrier heights, A-factors, tunneling, and rate coefficients are observed for axial and equatorial arrangements as well as between secondary hydrogen migration sites, depending on the location of the secondary site relative to the terminal carbon. The validity of using cycloalkane model systems to estimate rate parameters is also assessed. The failure of two key assumptions inherent to the cycloalkane models, resulting in a breakdown in the accuracy of these methods for larger transition states, is discussed. This study has significant ramifications for future theoretical, experimental, and modeling studies involving the decomposition of n-alkanes.     

Ab initio study of oxygen atom transfer from hydrogen peroxide to trimethylamine

Unlike what has been theoretically proposed for ammonia oxidation with hydrogen peroxide, trimethylamine oxidation occurs with a concerted mechanism, which is favored even when an explicit water molecule is added or continuum solvent (water) is simulated.     

Addition of atomic hydrogen to acetylene. Chain reactions of the vinyl radical

The main reaction products resulting from the addition of atomic hydrogen to acetylene are shown to be ethylene, 1,3-butadiene, and benzene. The mechanism involves chain reactions of the vinyl and butadienyl radicals, which regenerate atomic hydrogen. Some of the rate parameters are estimated.     

Ab initio study of the thermodynamic and kinetic stability of the ammonium radical

An increasing interest in the possible existence of the NH₄ radical has emerged in recent years. In this paper we report an ab initio UHF CI study of the ammonium radical, an investigation of parts of the energy surface around NH₄ and a theoretical prediction of the kinetic parameters of the radical formation and dissociation reactions within the framework of the TST theory. The ground state of the ammonium radical appears to be of the Rydberg type. Its ionization potential is found to be 4.29 eV. The NH₄ formation reaction from NH₂ + H₂ is very slightly exothermic whereas the reaction from NH₃ + H is slightly endothermic. We find a transition state of C_3v symmetry for the dissociation of NH₄ into NH₃ + H. The insertion of H₂ into NH₂ occurs according to a two-step mechanism whose determining step corresponds to the crossing of a saddle point with C_s symmetry previously obtained in the study of the reaction NH₂ + H₂ → NH₃ + H. Finally, we predict for NH₄ and ND₄ lifetimes of 0.1 and 1.4 μs respectively.     

Ab initio calculations on the barrier height for the hydrogen addition to ethylene and formaldehyde. The importance of spin projection

Heats of reaction and barrier heights have been computed for H + CH₂CH₂ → C₂H₅, H + CH₂O → CH₃O, and H + CH₂O → CH₂OH using unrestricted Hartree-Fock and Møller–Plesset perturbation theory up to fourth order (with and without spin annihilation), using single-reference configuration interaction, and using multiconfiguration self-consistent field methods with 3-21G, 6-31G(d), 6-31G(d,p), and 6-311G(d,p) basis sets. The barrier height in all three reactions appears to be relatively insensitive to the basis sets, but the heats of reaction are affected by p-type polarization functions on hydrogen. Computation of the harmonic vibrational frequencies and infrared intensities with two sets of polarization functions on heavy atoms [6-31G(2d)] improves the agreement with experiment. The experimental barrier height for H + C₂H₄ (2.04 ± 0.08 kcal/mol) is overestimated by 7?9 kcal/mol at the MP2, MP3, and MP4 levels. MCSCF and CISD calculations lower the barrier height by approximately 4 kcal/mol relative to the MP4 calculations but are still almost 4 kcal/mol too high compared to experiment. Annihilation of the largest spin contaminant lowers the MP4SDTQ computed barrier height by 8?9 kcal/mol. For the hydrogen addition to formaldehyde, the same trends are observed. The overestimation of the barrier height with Møller-Plesset perdicted barrier heights for H + C₂H₄ → C₂H₅, H + CH₂O → CH₃O, and H + CH₂O → CH₂OH at the MP4SDTQ /6-31G(d) after spin annihilation are respectively 1.8, 4.6, and 10.5 kcal/mol.     

Ab initio study of hydrogen migration in 1-alkylperoxy radicals

Alkylperoxy and hydroperoxyalkyl radicals are key reactive intermediates in hydrocarbon oxidation mechanisms. An understanding of the interconversion of these two species via a hydrogen migration reaction is of fundamental importance to the prediction of chain branching reactions and end product composition. An extensive ab initio investigation of the hydrogen migration reaction in 1-ethyl, 1-propyl, 1-butyl, 1-pentyl, and 1-hexylperoxy radicals is conducted to assess the validity of using cycloalkanes to model the ring strain of their transition states as well as the effect of both location of the migrating hydrogen and directionality of the remaining alkyl chain in the transition state of the reaction involving a secondary hydrogen. The G2 and CBS-Q composite methods are used to determine the activation energy and enthalpy of reaction relative to the alkylperoxy radical. Both methods show good agreement with five experimentally determined reaction enthalpies, having root mean squared deviations of 0.7 and 1.3 kcal mol(-1) for the CBS-Q and G2 methods, respectively. The effect of hydrogen abstraction site and transition state geometry, particularly axial and equatorial geometries of the remaining alkyl chain, on the activation energy, Arrhenius A-factor, tunneling, and rate coefficient are discussed. Differences between terminal adjacent and nonterminal adjacent secondary sites result in small but consistent differences in barrier height. Failure of key assumptions within the cycloalkane based estimation method leads to the break down in the accuracy for both small and large transition states. For large transition states, the breakdown of these assumptions also results in the failure of the current cycloalkane method as a conceptual model. Of great interest is the observed alteration in the preferred H-migration from the 1,5 to the 1,6 H-migration within the temperature region where these reactions are particularly important to the combustion mechanism.     

Ab initio study of C + H3+ reactions

The reaction C + H3+ --> CH(+) + H2 is frequently used in models of dense interstellar cloud chemistry with the assumption that it is fast, i.e. there are no potential energy barriers inhibiting it. Ab initio molecular orbital study of the triplet CH3+ potential energy surface (triplet because the reactant carbon atom is a ground state triplet) supports this hypothesis. The reaction product is 3 pi CH+; the reaction is to exothermic even though the product is not in its electronic ground state. No path has been found on the potential energy surface for C + H3+ --> CH2(+) + H reaction.     

Ab initio study of the magnetic behavior of four dithiadiazolyl radical compounds

We have studied, by means of ab initio calculations, the magnetic interaction mechanisms in four radical crystals, X–C₆F₄–CNSSN (X = O₂N, α-NC, β-NC, Br), which has allowed us to explain their different magnetic behaviour (ferromagnetism, antiferromagnetism, paramagnetism, spin frustration, etc.). First, we have identified the magnetic exchange pathways considering those with distances between two atoms of different dithiadiazolyl rings shorter than 7 Å and those with an intermolecular distance between an atom of the heterocyclic ring and an atom in a neighbouring radical shorter than 4 Å. Second, the calculations have been carried out in the framework of the DFT Broken Symmetry. Following this procedure we have determined the magnitude and the sign of the relevant coupling constants for the X–C₆F₄–CNSSN (X = O₂N, α-NC, β-NC, Br) radicals. In the cases where the radicals order magnetically, ordering temperatures determined with our ab initio calculations agree very well with the experimental ones. Thus, in the case of the O₂N derivative ferromagnetic ordering is observed below 1.3 K, in very good agreement with an ordering temperature around 1.6 K predicted from our calculated exchange constants and using a mean field approximation.