Quantum state-resolved reactive scattering of F+CH4→HF(v,J)+CH3: Nascent HF(v,J) product state distributions

Reactive scattering dynamics in atom+polyatomic systems: F+C2H6-->HF(v,J)+C2H5

State-to-state scattering dynamics of F+C2H6-->HF(v,J)+C2H5 have been investigated at Ecom=3.2(6) kcalmol under single-collision conditions, via detection of nascent rovibrationally resolved HF(v,J) product states with high-resolution infrared laser absorption methods. State-resolved Doppler absorption profiles are recorded for multiple HF(v,J) transitions originating in the v=0,1,2,3 manifold, analyzed to yield absolute column-integrated densities via known HF transition moments, and converted into nascent probabilities via density-to-flux analysis. The spectral resolution of the probe laser also permits Doppler study of translational energy release into quantum-state-resolved HF fragments, which reveals a remarkable linear correlation between (i) HF(v,J) translational recoil and (ii) the remaining energy available, Eavail=Etot-E(HF(v,J)). The dynamics are interpreted in the context of a simple impulsive model based on conservation of linearangular momentum that yields predictions in good agreement with experiment. Deviations from the model indicate only minor excitation of ethyl vibrations, in contrast with a picture of extensive intramolecular vibrational energy flow but consistent with Franck-Condon excitation of the methylene CH2 bending mode. The results suggest a relatively simple dynamical picture for exothermic atom+polyatomic scattering, i.e., that of early barrier dynamics in atom+diatom systems but modified by impulsive recoil coupling at the transition state between translationalrotational degrees of freedom.

Infrared laser spectroscopy of CH3⋯HF in helium nanodroplets: The exit-channel complex of the F+CH4 reaction

High-resolution infrared laser spectroscopy is used to study the CH3...HF and CD3...HF radical complexes, corresponding to the exit-channel complex in the F + CH4 --> HF + CH3 reaction. The complexes are formed in helium nanodroplets by sequential pickup of a methyl radical and a HF molecule. The rotationally resolved spectra presented here correspond to the fundamental v = 1 <-- 0 H-F vibrational band, the analysis of which reveals a complex with C(3v) symmetry. The vibrational band origin for the CH3...HF complex (3797.00 cm(-1)) is significantly redshifted from that of the HF monomer (3959.19 cm(-1)), consistent with the hydrogen-bonded structure predicted by theory [E. Ya. Misochko et al., J. Am. Chem. Soc. 117, 11997 (1995)] and suggested by previous matrix isolation experiments [M. E. Jacox, Chem. Phys. 42, 133 (1979)]. The permanent electric dipole moment of this complex is experimentally determined by Stark spectroscopy to be 2.4+/-0.3 D. The wide amplitude zero-point bending motion of this complex is revealed by the vibrational dependence of the A rotational constant. A sixfold reduction in the line broadening associated with the H-F vibrational mode is observed in going from CH3...HF to CD3...HF. The results suggest that fast relaxation in the former case results from near-resonant intermolecular vibration-vibration (V-V) energy transfer. Ab initio calculations are also reported (at the MP2 level) for the various stationary points on the F + CH4 surface, including geometry optimizations and vibrational frequency calculations for CH3...HF.

Ab initioand direct quasiclassical-trajectory study of the F+CH4→HF+CH3 reaction

We present an electronic structure and dynamics study of the F+CH4-->HF+CH3 reaction. CCSD(T)/aug-cc-pVDZ geometry optimizations, harmonic-frequency, and energy calculations indicate that the potential-energy surface is remarkably isotropic near the transition state. In addition, while the saddle-point F-H-C angle is 180 degrees using MP2 methods, CCSD(T) geometry optimizations predict a bent transition state, with a 153 degrees F-H-C angle. We use these high-quality ab initio data to reparametrize the parameter-model 3 (PM3) semiempirical Hamiltonian so that calculations with the improved Hamiltonian and employing restricted open-shell wave functions agree with the higher accuracy data. Using this specific-reaction-parameter PM3 semiempirical Hamiltonian (SRP-PM3), we investigate the reaction dynamics by propagating quasiclassical trajectories. The results of our calculations using the SRP-PM3 Hamiltonian are compared with experiments and with the estimates of two recently reported potential-energy surfaces. The trajectory calculations using the SRP-PM3 Hamiltonian reproduce quantitatively the measured HF vibrational distributions. The calculations also agree with the experimental HF rotational distributions and capture the essential features of the excitation function. The results of the SRP semiempirical Hamiltonian developed here clearly improve over those using the two prior potential-energy surfaces and suggest that reparametrization of semiempirical Hamiltonians is a promising strategy to develop accurate potential-energy surfaces for reaction dynamics studies of polyatomic systems.

Quasiclassical Trajectory Study of the F + CH4 Reaction Dynamics on a Dual-Level Interpolated Potential Energy Surface

An ab initio interpolated potential energy surface (PES) for the F + CH4 reactive system has been constructed using the interpolation method of Collins and co-workers. The ab initio calculations have been performed using second-order Möller-Plesset (MP2) perturbation theory to build the initial PES. Scaling all correlation (SAC) methodology has been employed to improve the ab initio calculations and to construct a dual-level PES. Using this PES, a detailed quasiclassical trajectory study of integral and differential cross sections, product rovibrational populations and internal energy distributions has been carried out for the F + CH4 and F + CD4 reactions and the theoretical results have been compared with the available experimental data.

Potential Energy Surface for the F(2P3/2,2P1/2) + CH4 Hydrogen Abstraction Reaction. Kinetics and Dynamics Study

A modified and recalibrated potential energy surface (PES) is reported for the gas-phase F(2P(3/2),2P(1/2)) + CH4 reaction and its deuterated analogue. This semiempirical surface is completely symmetric with respect to the permutation of the four methane hydrogen atoms and is calibrated with respect to the updated experimental and theoretical stationary point properties and experimental thermal rate constants. To take into account the two spin-orbit electronic states of the fluorine atom, two versions of the surface were constructed, the PES-SO and PES-NOSO surfaces, which differ in the choice of the zero reference level of the reactants. On both surfaces, the thermal rate constants were calculated using variational transition-state theory with semiclassical transmission coefficients over a wide temperature range, 180-500 K. While the PES-SO surface overestimates the experimental rate constants, the PES-NOSO surface shows a better agreement, reproducing the experimental variation with temperature. The influence of the tunneling factor is negligible, due to the flattening of the surface in the entrance valley, and we found a direct dependence on temperature, and therefore positive and small activation energies, in agreement with experiment. The kinetic isotope effects calculated showed good agreement with the sparse experimental data at 283 and 298 K. Finally, on the PES-NOSO surface, other dynamical features, such as the coupling between the reaction coordinate and the vibrational modes, were analyzed. It was found qualitatively that the FH stretching and the CH3 umbrella bending modes in the products appear vibrationally excited. These kinetics and dynamics results seem to indicate that a single, adiabatic PES is adequate to describe this reaction.

Molecular beam studies of the F atom reaction with propyne: Site specific reactivity

The dynamics of the F atom reaction with propyne (CH(3)CCH) has been investigated using a universal crossed molecular beam apparatus. Two reaction channels have been clearly observed: H+C(3)H(3)F and HF+C(3)H(3). The substitution of F for H occurs mainly via a complex formation mechanism, producing reaction products with some contribution from a direct reaction mechanism. The HF product, however, appears to be dominantly forward scattered relative to the F atom beam direction, suggesting that the HF formation occurs via a direct abstraction mechanism. Branching ratios for the two observed reaction channels are also determined. The H formation channel is found to be the major reaction pathway, while the HF formation channel is also significant. From the measurements of DF versus HF products from the F atom reaction with deuterated propyne, the H atom picked up by the F atom in the reaction with normal propyne seems to come mostly from the CH(3) group. In addition, the H atom produced in the H atom formation channel appears to be mostly from the CH(3) group with some contribution from the CCH group.

Dynamics of the F atom reaction with propene

The F+C2H3CH3 reaction has been investigated using the crossed molecular beam technique. Three reaction channels have been observed in this reaction: H+C3H5F, CH3+C2H3F, and HF+C3H5. Time-of-flight spectra as well as product laboratory angular distributions have been measured for the HF, C2H3F, and C3H5F products from these three channels. Relative branching ratios of the three observed reaction channels have also been estimated. Experimental results indicate that these different channels exhibit significantly different reaction dynamics.

Ab initiopotential energy surface, variational transition state theory, and quasiclassical trajectory studies of the F+CH4→HF+CH3 reaction

In this work we present a study of the F+CH(4)-->HF+CH(3) reaction (DeltaHdegrees(298 K)=-32.0 kcal mol(-1)) using different methods of the chemical reaction theory. The ground potential energy surface (PES) is characterized using several ab initio methods. Full-dimensional rate constants have been calculated employing the variational transition state theory and using directly ab initio data. A triatomic analytical representation of the ground PES was derived from ab initio points calculated at the second- and fourth-order Møller-Plesset levels with the 6-311+G(2df,2pd) basis set, assuming the CH(3) fragment to be a 15 a.m.u. pseudoatom in the fitting process. This is suggested from experiments that indicate that the methyl group is uncoupled to the reaction coordinate. A dynamics study by means of the quasiclassical trajectory (QCT) method and employing this analytical surface was also carried out. The experimental data available on the HF internal states distributions are reproduced by the QCT results. Very recent experimental information about the reaction stereodynamics is also borne out by our QCT calculations. Comparisons with the benchmark F+H(2) and analogous Cl+CH(4) reactions are established throughout.

State-to-state dynamics of the Cl+CH3OH→HCl+CH2OH reaction

Molecular chlorine, methanol, and helium are co-expanded into a vacuum chamber using a custom designed "late-mixing" nozzle. The title reaction is initiated by photolysis of Cl2 at 355 nm, which generates monoenergetic Cl atoms that react with CH3OH at a collision energy of 1960 +/- 170 cm(-1) (0.24 +/- 0.02 eV). Rovibrational state distributions of the nascent HCl products are obtained via 2 + 1 resonance enhanced multiphoton ionization, center-of-mass scattering distributions are measured by the core-extraction technique, and the average internal energy of the CH3OH co-products is deduced by measuring the spatial anisotropy of the HCl products. The majority (84 +/- 7%) of the HCl reaction products are formed in HCl(v = 0) with an average rotational energy of [Erot] = 390 +/- 70 cm(-1). The remaining 16 +/- 7% are formed in HCl(v = 1) and have an average rotational energy of [Erot] = 190 +/- 30 cm(-1). The HCl(v = 1) products are primarily forward scattered, and they are formed in coincidence with CH2OH products that have little internal energy. In contrast, the HCl(v = 0) products are formed in coincidence with CH2OH products that have significant internal energy. These results indicate that two or more different mechanisms are responsible for the dynamics in the Cl + CH3OH reaction. We suggest that (1) the HCl(v = 1) products are formed primarily from collisions at high impact parameter via a stripping mechanism in which the CH2OH co-products act as spectators, and (2) the HCl(v = 0) products are formed from collisions over a wide range of impact parameters, resulting in both a stripping mechanism and a rebound mechanism in which the CH2OH co-products are active participants. In all cases, the reaction of fast Cl atoms with CH3OH is with the hydrogen atoms on the methyl group, not the hydrogen on the hydroxyl group.

Imaging the pair-correlated excitation function: The F+CH4→HF(v′)+CH3(ν=0) reaction

The velocity map ion imaging technique was applied to measure the reaction excitation function for the first time. It was found that the "raw" excitation function was significantly distorted by the density-to-flux transformation of the title reaction. Through a systematic investigation, possible reasons for such a dramatic effect are outlined. In addition, the state-resolved, pair-correlated excitation functions and branching ratios are presented. Effects of imperfect time slicing in the time-sliced velocity imaging technique in general are also discussed.