Mean pollen production and mean nearest neighbor distance were recorded for several populations of Taxus canadensis and correlated with the proportion of ovules pol~in~ted_ and s~ed. set. Distance and pollen production together explained 86% of the va~at~on m polhnatwn success, each variable significantly adding to the regression when adJustmg for the other. Seed set was correlated significantly with pollen production and nea~est neighbor d_ist~nce separately, but the multiple regression including the latter two vanables was not significant. Seed set was correlated most strongly with pollination success and me~n ovule production (R 2 = 0.71), suggesting that variation in seed set among Taxus populatwns was a combination of differences in pollen and resource availability..

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This paper presents the first test of the popular trajectory surface-hopping (TSH) method against accurate three-dimensional quantum mechanics for a reactive system. The system considered is a model system in which an excited atom with an excitation energy of 0.76 eV reacts with or is quenched by the H 2 molecule. The electronically nonadiabatic collisions occur primarily near a conical intersection of an exciplex with a repulsive ground state. The accurate quantal results are calculated using the outgoing wave variational principle in an electronically diabatic representation. Four variants of the TSH method are tested, differing in the criteria for hopping and the component of momentum that is adjusted in order to conserve energy when a hop occurs. Coupling between the ground and excited surface occurs primarily in the vicinity of a conical intersection and is mediated by an exciplex found on the upper surface. We find that the overall TSH quenching probabilities are in good agreement with quantum mechanical results, but the branching ratios between reactive and nonreactive trajectories and many of the state-selected results are poorly reproduced by trajectory calculations. The agreement between trajectory surface hopping and quantal results is on average worse for the relatively more "quantum mechanical" j ) 0 initial state and M + H 2 quenching process and better for the relatively more "classical" j ) 2 initial state and MH + H′ reactive process. We also perform a statistical calculation of overall quenching probability and unimolecular rate of the nonadiabatic decay of the exciplex. We find that only about 10 % of trajectories can be described as "statistical" and that statistical calculation overestimates the total quenching rate significantly.

transition-state theory, trajectory calculations, reduced-dimet~sionality QM studies, variational transition-state theory with multidimensional tunneling, and approximate QM scattering calculations (17). In 1991, converged QM scattering calculations for the C1 + H2 reaction on the G Q Q surface were reported (1 8).In this report, we present the results of a CMB investigation of reaction 1. We obtained differential cross sections (DCSs) for E,,, = 5.85 kcal/mol for Angular distributions and time-of-flight spectra for the reaction CI + H, + HCI t H obtained from a high-resolution, crossed-molecular beam experiment were compared to differential cross sections calculated by both converged quantum mechanical scattering and quasi-classical trajectory methods. Good agreement was found between the experimental results and each theoretical prediction. The results demonstrate that excellent agreement can be obtained between state-of-the-art simulations and experiments for the detailed dynamical properties of this prototype chlorine atom reaction.T h e fundamental goal of chemical reacolaved a central role in f~~ndamental chem-tion dynamics is to learn about the forces that control chemical reactivity, that is, for each chemical reaction to learn about the nature of the potential energy surface (PES) that governs the nuclear motions. Our goal is to model the PES well enough to predict the detailed dvAamics of reactions. TheZcal kinetics and has served as a critical test case for bimolecular reaction rate theory, especially transition-state and kinetic isotope effect (KIE) theories (7). This reaction is also a prototype for a host of C1 reactions where u (u') and j ( j ' ) are reactant (product) vibrational and rotational quantum numbers, by measuring angular and velocity distributions of the HCl(w' = 0) product.that are important in atmospheric chemistry and photochemical air pollution.Experimental work on the rate constants of reaction 1 and its H isotowic variants was physical observables most sensitive to the nature of the PES are reaction cross sections, especially differential cross sections(1, 2). If converged quantum mechanical (QM) scattering calculations on an assumed PES can be performed, a comparison of calculated and experimental cross sec-recently summarized (8). However, cross sections have not been determined to date. The HC1 product from reaction 1 can only be formed in the ground vibrational level for low relative translational energies Ere, (Fig. 1A); this limitation rules out the application of infrared chemilurninescence (9) to detect products. The products can be detected with a mass spectrometer in a crossed rnolec~~lar tions provides a sensitive test of our ability to predict detailed dyna~nical attributes of reactions. To date, this kind of test has only been applied to the D + H2 (3), H + D2 (4), and F + HI reactions (5). In this report we extend this fundamental comparison to Reaction coordinate beam (CMB) arrangement, but this procedure is very challenging because of a high a third chemical reacti...

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We present a new potential energy surface (called G3) for the chemical reaction Cl + H2 → HCl + H. The new surface is based on a previous potential surface called GQQ, and it incorporates an improved bending potential that is fit to the results of ab initio electronic structure calculations. Calculations based on variational transition state theory with semiclassical transmission coefficients corresponding to an optimized multidimensional tunneling treatment (VTST/OMT, in particular improved canonical variational theory with least-action ground-state transmission coefficients) are carried out for nine different isotopomeric versions of the abstraction reaction and six different isotopomeric versions of the exchange reaction involving the H, D, and T isotopes of hydrogen, and the new surface is tested by comparing these calculations to available experimental data. The theoretical data are also used to investigate the equilibrium constant and the branching ratio for the reverse reaction, and calculations of these quantities are compared to the available experimental and theoretical data. Disciplines Chemistry CommentsReprinted (adapted) ReceiVed: March 13, 1996; In Final Form: May 15, 1996 X We present a new potential energy surface (called G3) for the chemical reaction Cl + H 2 f HCl + H. The new surface is based on a previous potential surface called GQQ, and it incorporates an improved bending potential that is fit to the results of ab initio electronic structure calculations. Calculations based on variational transition state theory with semiclassical transmission coefficients corresponding to an optimized multidimensional tunneling treatment (VTST/OMT, in particular improved canonical variational theory with leastaction ground-state transmission coefficients) are carried out for nine different isotopomeric versions of the abstraction reaction and six different isotopomeric versions of the exchange reaction involving the H, D, and T isotopes of hydrogen, and the new surface is tested by comparing these calculations to available experimental data. The theoretical data are also used to investigate the equilibrium constant and the branching ratio for the reverse reaction, and calculations of these quantities are compared to the available experimental and theoretical data.

We present a systematic test of four general semiclassical procedures for the theoretical treatment of multistate molecular processes such as electronically nonadiabatic photochemical reactions. The methods are tested by comparing their predictions to accurate quantal results for three two-state model reactions involving conical intersections. The four methods tested are Tully's fewest-switches version of trajectory surface hopping ͑1990͒, the Blais-Truhlar trajectory surface hopping method ͑1983͒, the Ehrenfest scheme ͑1975-1979͒, and the Meyer-Miller method ͑1979͒.We test the ability of the classical path methods to predict both electronic probabilities and product rovibrational distributions. For each of the four basic approaches we test six options for extracting final-state information from the calculated dynamics. We find that, although in most cases there is qualitative agreement between average quantum mechanical and trajectory results, the overall average error is about 50% for Tully's fewest-switches method, the Ehrenfest method, and the Meyer-Miller method, and even higher, about 60%, for the Blais-Truhlar method. These values do not include additional errors in the below-threshold regions, which are especially large for the Meyer-Miller method because of the electronic zero-point energy in the Meyer-Miller classical analog Hamiltonian.