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The ring closure reaction of C4H3 with acetylene to give phenyl radical is one proposed mechanism for the formation of the first aromatic ring in hydrocarbon combustion. There are two low-lying isomers of C4H3; 1-dehydro-buta-1-ene-3-yne (n-C4H3) and 2-dehydro-buta-1-ene-3-yne (iso-C4H3). It has been proposed that only n-C4H3 reacts with acetylene to give phenyl radical, and since iso-C4H3 is more stable than n-C4H3, formation of phenyl radical by this mechanism is unlikely. We report restricted Hartree–Fock (RHF) plus singles and doubles configuration interaction calculations with a Davidson’s correction (RHF+1+2+Q) using the Dunning correlation consistent polarized valence double zeta basis set (cc-pVDZ) for stationary point structures along the reaction pathway for the reactions of n-C4H3 and iso-C4H3 with acetylene. n-C4H3 plus acetylene (9.4) has a small entrance channel barrier (17.7) (all energetics in parentheses are in kcal/mol with respect to iso-C4H3 plus acetylene) and the subsequent closure steps leading to phenyl radical (-91.9) are downhill with respect to the entrance channel barrier. Iso-C4H3 plus acetylene also has an entrance channel barrier (14.9) and there is a downhill pathway to 1-dehydro-fulvene (-55.0). 1-dehydro-fulvene can rearrange to 6-dehydro-fulvene (-60.3) by a 1,3-hydrogen shift over a barrier (4.0), which is still below the entrance channel barrier, from which rearrangement to phenyl radical can occur by a downhill pathway. Thus, both n-C4H3 and iso-C4H3 can react with acetylene to give phenyl radical with small barriers. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The potential energy surface (PES) for NH+NO has been characterized using complete active space self-consistent field (CASSCF) gradient calculations to determine the stationary point geometries and frequencies followed by CASSCF/internally contracted configuration interaction (CCI) calculations to refine the energetics. The present results are in qualitative accord with the BAC-MP4 calculations of Melius and Binkley, but there are differences as large as 8 kcal/mol in the detailed energetics. Addition of NH to NO on a 2A’ surface, which correlates with N2+OH or H+N2O products, involves barriers of 3.2 kcal/mol (trans) and 6.3 kcal/mol (cis). Experimental evidence for these barriers is found in the work of Böhmer et al. The 2A‘ surface has no barrier to addition, but does not correlate with ground state products. Surface crossings between the barrierless 2A‘ surface and the 2A’ surface may be important. Production of N2+OH products is predicted to occur via a planar saddle point of 2A’ symmetry. This is in accord with the preferential formation of Π(A’) Λ doublet levels of OH in the experiments of Patel-Misra and Dagdigian. Addition of NH 1Δ to NO is found to occur on an excited state surface and is predicted to lead to N2O product as observed by Yamasaki et al. [ABSTRACT FROM AUTHOR]

In a previous paper in this series [J. Chem. Phys. 93, 2384 (1990)] a global potential energy surface (PES) was presented for H+N2→HN2. In this paper, we report additional calculations which characterize PES’s for the excited 2A‘ state, for a bound 2 2A’ state, for HN+2, and for the Rydberg states associated with HN+2. It is anticipated that these excited state PES’s will be important in interpreting and designing experiments to characterize the ground state HN2 species via neutralized ion beam techniques. [ABSTRACT FROM AUTHOR]

The reactant, product, and saddle point regions of the 5Π and 3Π potential energy surfaces for the reaction NH+O→N+OH have been characterized using complete active space self consistent field/externally contracted configuration interaction (CASSCF/CCI) calculations with large atomic natural orbital (ANO) basis sets. The computed barrier heights are 5.6 and 11.7 kcal/mol on the 5Π and 3Π surfaces, respectively. Transition state theory with an Eckart tunneling correction is used to estimate the rate constant on the 5Π surface. [ABSTRACT FROM AUTHOR]

The potential energy surface for the reaction H+O2→HO[ATOTHER]@B|[/ATOTHER]2 →HO+O has been characterized in the vicinity of the minimum energy path using CASSCF/contracted CI calculations with a basis set which is triple zeta valence quality plus three sets of polarization functions. CASSCF/CI calculations were carried out along the CCI minimum energy path. The latter calculation shows essentially no barrier for addition of an H atom to O2, in agreement with predictions made in earlier studies. The potential surface for recombination of OH and O is complicated by a crossing, at rOO approx. 5.5a0, between the surface for electrostatic (OH dipole–O quadrupole) interaction and that for the formation of an O–O chemical bond. This surface crossing results in a small (approx. 0.5 kcal/mol) barrier. [ABSTRACT FROM AUTHOR]

Complete active space SCF/contracted CI (CASSCF/CCI) calculations, using large Gaussian basis sets, are presented for selected portions of the potential surfaces for the reactions in the Zeldovich mechanism for the conversion of N2 to NO. The N+O2 reaction is exoergic by 32 kcal/mol and is computed to have an early barrier of 10.2 kcal/mol for the 2A’ surface and 18.0 kcal/mol for the 4A’ surface. The O+N2 reaction is endoergic by 75 kcal/mol. The 3A‘ surface is calculated to have a late barrier of 0.5 kcal/mol, while the 3A’ surface is calculated to have a late barrier of 14.4 kcal/mol relative to NO+N. [ABSTRACT FROM AUTHOR]

We present a quasiclassical trajectory study of the NH+NO reaction using a global potential energy surface that is capable of describing branching to the H+N2O and OH+N2 products after initial formation of a HNNO intermediate complex. The surface is based on a many-body expansion wherein fragment potentials for the species N2H, HNO, and N2O are incorporated, using either previously developed potentials, or in the case of N2O, a newly developed potential. The three-body parts of these fragment potentials are damped in the four-body region to provide a zeroth order four-body surface, and then additional four-body terms and mapping transformations are applied to make the final four-body potential match the results of ab initio calculations for eight important HNNO stationary points (minima and saddle points) and for several reaction paths. In addition to this ‘‘best fit’’ surface (surface I), a second surface (surface II) is developed in which the ordering of the saddle points leading to formation of H+N2O and OH+N2 is reversed, and the energy release during 1,3 hydrogen migration is modified so that the N–N stretch experiences smaller distortions from N2 equilibrium during the reaction leading to OH+N2. Quasiclassical trajectory results on surface I show generally good correspondence with experiment, with a branching fraction of 13±3% for the formation of OH+N2 at 300 K, and relatively low OH and N2 vibration/rotation excitation. The results on surface II are similar with respect to both branching and energy partitioning, indicating relatively weak sensitivity of the results of key features of the surface. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have studied the reaction of N and O atoms with a (9,0) carbon nanotube using clusters and the ONIOM method with the B3LYP/6-31G* level of theory. For both N and O atoms in their ground state, addition to the CNT requires a surface crossing and presumably involves a barrier, while we show that addition of the excited state of N(2D) has no barrier, and we believe this also to be true for O(1D). Both N and O atoms bond in a bridged structure and have bond strengths of 25.0 and 67.9 kcal/mol (estimated), respectively, with respect to CNT plus the ground state of the N or O atom. [Copyright &y& Elsevier]