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Reaction pathways that bypass the conventional saddle-point transition state (TS) are of considerable interest and importance. An example of such a pathway, termed 'roaming,' has been described in the photodissociation of [H.sub.2]CO. In a combined experimental and theoretical study, we show that roaming pathways are important in the 308-nm photodissociation of C[H.sub.3]CHO to C[H.sub.4] + CO. The C[H.sub.4] product is found to have extreme vibrational excitation, with the vibrational distribution peaked at [approximately equal to] 95% of the total available energy. Quasiclassical trajectory calculations on full-dimensional potential energy surfaces reproduce these results and are used to infer that the major route to C[H.sub.4] + CO products is via a roaming pathway where a C[H.sub.3] fragment abstracts an H from HCO. The conventional saddle-point TS pathway to C[H.sub.4] + CO formation plays only a minor role. H-atom roaming is also observed, but this is also a minor pathway. The dominance of the C[H.sub.3] roaming mechanism is attributed to the fact that the C[H.sub.3] + HCO radical asymptote and the TS saddle-point barrier to C[H.sub.4] + CO are nearly isoenergetic. Roaming dynamics are therefore not restricted to small molecules such as [H.sub.2]CO, nor are they limited to H atoms being the roaming fragment. The observed dominance of the roaming mechanism over the conventional TS mechanism presents a significant challenge to current reaction rate theory. reaction dynamics | roaming mechanisms | photochemistry | quasiclassical trajectories | transition state

The influence of treatment of electron correlation, size of basis set and choice of solvation model on the predicted stabilization of zwitterionic phosphonic acid gamma-aminobutyric acid (GABA) analogues is investigated. Phosphinic and phoshonic acid GABA analogues constrained in a cis-configuration are found to possess only folded, intramolecularly hydrogen bonded stable conformers.

The solution-phase structures of a number of conformationally restricted gamma-aminobutyric acid (GABA) analogues are investigated. The results give a high-quality starting point for subsequent quantitative structure-activity relationship analysis that is expected to provide insight into which conformers of GABA and its conformationally restricted analogues are biologically active.

A study of the vibrational effects on the F-F spin-spin coupling constant ((super 2h)J(sub F-F)) in FHF(super -) and FDF(super -) are evaluated. The results show that in the ground vibrational state, the expectation value of the F-F distance increases and the expectation value of (super 2h)J(sub F-F) decreases to 212.7 Hz. significantly less than the equilibrium value of 254.4 Hz.

An ab initio study of the hydrogen-bonded complexes of the deuterated analogues carried out is reported. The computed results provide insight into the nature of the hydrogen bonds that stabilize these complexes in low-temperature matrixes.

The nuclear vibrational wave function and zero-point vibrational energy of CH5(super +) are calculated using quantum diffusion Monte Carlo techniques. The result showed that the zero-point motion of CH5(super +) renders all five protons equivalent in the ground state and precludes the assignment of a unique structure to the molecule.

The stabilization of Gamma-aminobutyric acid (GABA) zwitterions in aqueous solution is investigated at various levels of theory by considering both short- and long-range solvent interactions. Zwitterionic forms of GABA are expected to exist in various conformations in aqueous solution at 310 K.

Presents a method for expressing the polyatomic molecular potential energy surface by interpolation in internal coordinates. Derivation of local Taylor expansions from ab initio quantum calculations; Use of classical trajectory simulations to provide an iterative scheme; Application of reciprocal interatomic distances as the basic coordinates.

Investigates the structures and infrared spectra of the complexes FH:NH[sub 3], ClH:NH[sub 3], and BrH:NH[sub 3]. Effects of the presence of inert gas atoms on the structures; Characterization of the equilibrium structures of complexes by traditional hydrogen bonds; Stabilization of the proton-shared region of the surface.

Analyzes the use of Cartesian coordinates in expressing molecular potential energy surfaces by interpolation of local Taylor expansions. Implication for the prevention of redundancy problems associated with the use of internal coordinates; Feasibility of the method for bound molecules or clusters; List of problems encountered with transitional modes.

Discusses the construction of a simple model of potential energy surface for use in both quasiclassical trajectory calculations and quantum vibrational close-coupling. Rotational excitation of the p-difluorobenzene molecule; Investigation of vibrational energy transfer channels; Propensity toward the transfer of multiple quanta of vibrational energy.

IR proton-stretching frequencies and NMR properties of hydrogen-bonded complexes, ClH:pyridine are investigated. Results reveal that the zero-field, gas-phase structure of this complex is stabilized by a traditional Cl-H...N hydrogen bond and as the field strength increases, the equilibrium structure exhibit quasi-symmetric structure at a field of 0.0040 au, and then ion pair complex at higher fields.

Examines the vibrational infrared and nuclear magnetic resonance spectroscopy of the hydrogen bond. Details on the anharmonic proton-stretching frequency; Structural information related to the type of hydrogen bond; Analysis on the hydrogen nuclear magnetic resonance chemical shifts and the X-Y spin-spin coupling constant.

Investigates the anharmonicity and matrix effects in hydrogen-bonded complexes of HCl and hydrogen bromide with ammonia and N(CH3) using ab initio calculations. Anharmonic potential surfaces of the complexes; Dependence of structures and proton-stretching vibrational frequencies of complexes between the hydrogen halides and NH3 and N(CH3)3.

Interpolated ab initio potential energy surfaces which describe abstraction and exchange reactions in collisions of hydrogen and water are reported. The electronic structure calculations are performed at the QCISD(T) level of theory, with an additivity approximation. A sufficiently large basis set is required to describe the Rydberg character of the electronic state for molecular configurations which are important for the exchange process. Classical and quantum dynamics calculations on the surfaces are presented. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: The quantitative structure–activity relationship (QSAR) model constructed in this work represents the first quantitative investigation into agonist binding at GABAC receptors. This model is based upon the three-dimensional structures of (γ-aminobutyric acid (GABA) and 12 other biologically active GABA analogues. These structures are obtained by geometry optimization and conformational exploration at MP2/6-31+G* within the conductor-like screening solvation model (COSMO). The biological activity data are obtained from previous two-electrode voltage clamp electrophysiological studies on recombinant GABAC ρ1 receptors expressed in Xenopus laevis oocytes. A QSAR model to predict GABAC agonist binding is constructed from molecular superposition data, generated by least-squares superposition of the stable aqueous phase conformations of GABA and its known biologically active analogues to the bioactive conformation of TACA, the most potent GABAC receptor agonist. A significant relationship is found between the root-mean-squared deviation in atomic position for the stable conformer of each GABA analogue most closely fitting the template TACA structure and the natural log of the normalized biological activity (R 2=0.91, p<0.0001), On the basis of molecular superposition and QSAR results, a pharmacophore model describing three-dimensional features and key interactions at the GABAC receptor binding site is proposed. As there exists no direct experimental knowledge of structure of the GABAC binding site, this approach provides a feasible and reliable alternative to gain insight into its three-dimensional structure. [Copyright &y& Elsevier]

Abstract: The effects of the enantiomers of a number of flexible and cis-constrained GABA analogues were tested on GABAC receptors expressed in Xenopus laevis oocytes using two-electrode voltage-clamp electrophysiology. (1S,2R)-cis-2-Aminomethylcyclopropane-1-carboxylic acid ((+)-CAMP), a potent and full agonist at the ρ1 (EC50 ≈40μM, I max ≈100%) and ρ 2 (EC50 ≈17μM, I max ≈100%) receptor subtypes, was found to be a potent partial agonist at ρ3 (EC50 ≈28μM, I max ≈70%). (1R,2S)-cis-2-Aminomethylcyclopropane-1-carboxylic acid ((−)-CAMP), a weak antagonist at human ρ1 (IC50 ≈890μM) and ρ2 (IC50 ≈400μM) receptor subtypes, was also found to be a moderately potent antagonist at rat ρ3 (IC50 ≈180μM). Similarly, (1R,4S)-4-aminocyclopent-2-ene-1-carboxylic acid ((+)-ACPECA) was a full agonist at ρ1 (EC50 ≈135μM, I max ≈100%) and ρ2 (EC50 ≈60μM, I max ≈100%), but only a partial agonist at ρ3 (EC50 ≈112μM, I max ≈37%), while (1S,4R)-4-aminocyclopent-2-ene-1-carboxylic acid ((−)-ACPECA) was a weak antagonist at all three receptor subtypes (IC50 >>300μM). 4-Amino-(S)-2-methylbutanoic acid ((S)-2MeGABA) and 4-amino-(R)-2-methylbutanoic acid ((R)-2MeGABA) followed the same trend, with (S)-2MeGABA acting as a full agonist at the ρ1 (EC50 ≈65μM, I max ≈100%), and ρ2 (EC50 ≈20μM, I max ≈100%) receptor subtypes, and a partial agonist at ρ3 (EC50 ≈25μM, I max ≈90%). (R)-2MeGABA, however, was a moderately potent antagonist at all three receptor subtypes (IC50 ≈16μM at ρ1, 125μM at ρ2 and 35μM at ρ3). On the basis of these expanded biological activity data and the solution-phase molecular structures obtained at the MP2/6-31+G* level of ab initio theory, a rationale is proposed for the genesis of this stereoselectivity effect. [Copyright &y& Elsevier]