Nigel W. Moriarty

Hydrogen migration in polyaromatic growth

New reaction pathways for polyaromatic growth in combustion environments are explored theoretically. The analysis is based on semiempirical quantum calculations of potential energy barriers and vibrational frequencies, followed by standard transition state theory evaluation of reaction rates. The reaction systems considered include formation of five-and six-member aromatic rings, their interconversion, and migration of cyclopenta rings along zigzag edges of aromatic surfaces. All reactions have a distinctive mechanistic feature: the reaction pathways are induced or assisted by hydrogen atom migration. The present calculations show that such migration is rapid and leads to a faster surface growth than reaction pathways previously considered. The results extend the mechanism for the involvement of five-member aromatic rings in surface growth of soot particles. The migrating cyclopenta rings propagate the growth as a continuous replicating front. This is achieved by extending a six-member-ring step upon encounter or converting to a six-member ring at an open end, thus providing a nucleus for the next aromatic layer. Collisions of the propagating fronts may be responsible for creation of sites that cannot be filled by cyclization and thus cannot support further growth. Formation of such surface defects may be responsible for the loss of reactivity of soot particle surfaces to growth, consistent with a prior suggestion.

On unimolecular decomposition of phenyl radical

The unimolecular decomposition of the phenyl radical and ortho -benzyne was examined by ab initio quantum chemical calculations, Rice-Ramsperger-Kassel-Marcus (RRKM) calculations, and numercial simulation of shock tube pyrolysis of phenyl and benzene. The rate constants of C-H fission in the phenyl radical was first determined by re-examining the rates of H-atom production from nitrosobenzenepyrolysis at temperatures from 1450 to 1730 K and pressures from 1.5 to 7 atm. The experimental rate constant was successfully reproduced and extrapolated with RRKM calculations using molecular parameters obtained with the complete active space self-consistent field approach. The theoretical rate constants were then fitted in Troes fall-off expressions for C 6 H 5 (+Ar)→ o -C 6 H 4 +H(+Ar) in the temperature range of 500 to 2500 K, which gives k ∞ ( s −1 )=4.3×10 12 T 0.62 exp(−38,900/ T ) k 0 /[Ar] (cm 3 mol −1 s −1 )=1.0×10 84 T −18.87 exp(−45,340/ T ) F c =(1−0.902) exp(− T /696)+0.902 exp(− T /358)+exp(−3856/ T ) The concerted unimolecular decomposition of ortho -benzyne, o -C 6 H 4 (+Ar)→C 4 H 2 +C 2 H 2 (+Ar) was studied similarly. The high-pressure limit rate parameters were estimated to be k ∞ ( s −1 )=1.2×10 18 T −0.34 exp(−44,200/ T ) It was shown that a revised Bauer-Aten mechanism, featuring H ejection from phenyl followed by the concerted decomposition of o -benzyne, describes very well benzene decomposition in a shock tube.

Geometry optimization of a water molecule in water. A combined quantum chemical and statistical mechanical treatment

A Hartree-Fock optimized geometry for a water molecule in a statistical mechanical Monte Carlo bath of 89 water molecules was determined using a polarization model. Both the O–H bond lengths and ∠HOH bond angle were found to increase from the gas to the liquid phase. The bond length increase is in good agreement with recent neutron diffraction results; liquid water is closer in bond length to gas phase water than ice. In combination with the optimized ∠HOH bond angle approaching tetrahedral, the conclusion is that quadrupole moment dominates the water geometry in the liquid phase.

Propargyl radical: an electron localization function study

Abstract Bonding in the C 3 H 3 radical has been determined using the topological analysis of the electron localization function (ELF) calculated with various wavefunctions (HF, LSDA, MP2, CASSCF, QCISD, BLYP, B3LYP). Not only is ELF independent of quantum chemical approximation, but also produced topologically equivalent molecular partitions. The ELF partition of space into localization domains provides an objective characterization of bonding in C 3 H 3 , supporting a resonance description of almost equal contributions of propargyl and allenyl forms. Moreover, it explains the reported difference between the frequencies of the in-plane and out-of-plane bending modes (∠C (2) C (3) H (3) ) arising from the topology of the C (2) C (3) bonding region.

Tetramethylene: A CASPT2 study

The potential energy surface (PES) of the 1,4-tetramethylene biradical has been reinvestigated at the complete-active-space self-consistent field (CASSCF), complete active space with second-order perturbation theory correction and multi-reference configuration interaction (MRCI) levels of theory. The choice of basis set, the size inconsistency of the MRCI wavefunction and the selection of the active space of the CASSCF wavefunction each have a considerable influence on the shape of the PES. In particular, the relative energy of the so-called GF and TF structures depends significantly on the level of theory employed. The results of the study suggest that the experimentally observed intermediate is more likely to be due to entropic trapping rather than the presence of a transition state.

AB initio study of naphthalene formation by addition of vinylacetylene to phenyl

Reaction pathways leading to the formation of naphthalene by the addition of vinylacetylene to phenyl were examined using density functional theory B3-LYP functionals with the standard triple-zeta basis set, 6-311G(d,p). The chemically activated reaction dynamics were examined employing time-dependent solution of master equations. The addition of phenyl to the triple bond of vinylacetylene was computed to be relatively slow, due to a substantial energy barrier of the intermediate rotation about the double bond. Addition of phenyl to the other end of a vinylacetylene molecule produced equally low rate constants. The most promising pathway appeared to be a two-step reaction sequence via the formation of phenyl-C 4 H 3 molecules. The reaction rate evaluated for this pathway was very close to the value tested in prior flame simulations that demonstrated a dominant character of such a step for naphthalene formation. This indicates that the formation of naphthalene from phenyl and vinylacetylene may play a significant role in flame modeling of aromatic growth, and that the more favorable mechanism of the reaction may be a two-step sequence via the formation of a stable molecular intermediate rather than a “single” chemically activated path. The time-dependent solution of master equations revealed that at flame conditions typical of aromatic growth, the reaction system does not attain a steady state on a timescale of 1 ms, suggesting that dynamics of energy redistribution in such “elementary” reaction systems may need to be treated with inclusion of bimolecular reactions between energized adducts and gaseous partners.

Electric field gradient of a water molecule in water. A combined quantum chemical and statistical mechanical treatment

The electric field gradients for several geometries of a water molecule in liquid water were determined. Hartree-Fock theory was used for one water molecule with the liquid provided by a statistical mechanical Monte Carlo bath of 89 water molecules. Polarization was included in the model. Two facts point to difficulties in the experimental or theoretical methods: the experimental results for Vzz(O) vary by 25% and the calculated Vzz(O) result is outside the experimental range by 15%.