In this report, the O(1D)+CH4 reaction has been reinvestigated using universal crossed molecular beam methods. Angular resolved time-of-flight spectra have been measured for various reaction channels of the title reaction: OH+CH3, H+H2COH/H3CO, and H2+HCOH/H2CO. Different product angular distributions have been observed for these product channels, indicating that these reaction channels occur via distinctive dynamical pathways. This study provides an excellent example of multiple dynamical pathways in a single chemical reaction, which opens enormous opportunities in investigating the dynamics of complicated chemical reactions that are important in combustion and atmospheric chemistry, and also provides a link between kinetics studies and dynamical research.

The chemical reaction dynamics to form cyanoacetylene, HCCCN (X 1Σ+), via the radical–neutral reaction of cyano radicals, CN(X 2Σ+;ν=0), with acetylene, C2H2(X 1Σg+), are unraveled in crossed molecular beam experiments at two collision energies of 21.1 and 27.0 kJ mol−1. Laboratory angular distributions and time-of-flight spectra of the HCCCN product are recorded at m/e=51 and 50. Experiments were supplemented by electronic structure calculations on the doublet C3H2N potential energy surface and RRKM investigations. Forward-convolution fitting of the crossed beam data combined with our theoretical investigations shows that the reaction has no entrance barrier and is initiated by an attack of the CN radical to the π electron density of the acetylene molecule to form a doublet cis/trans HCCHCN collision complex on the A′2 surface via indirect reactive scattering dynamics. Here 85% of the collision complexes undergo C–H bond rupture through a tight transition state located 22 kJ mol−1 above the cyanoacetylene, HCCCN (X 1Σ+) and H(2S1/2) products (microchannel 1). To a minor amount (15%) trans HCCHCN shows a 1,2-H shift via a 177 kJ mol−1 barrier to form a doublet H2CCCN radical, which is 46 kJ mol−1 more stable than the initial reaction intermediate (microchannel 2). The H2CCCN complex decomposes via a rather loose exit transition state situated only 7 kJ mol−1 above the reaction products HCCCN (X 1Σ+) and H(2S1/2). In both cases the geometry of the exit transition states is reflected in the observed center-of-mass angular distributions showing a mild forward/sideways peaking. The explicit identification of the cyanoacetylene as the only reaction product represents a solid background for the title reaction to be included in reaction networks modeling the chemistry in dark, molecular clouds, outflow of dying carbon stars, hot molecular cores, as well as the atmosphere of hydrocarbon rich planets and satellites such as the Saturnian moon Titan.

The optimized structures and harmonic frequencies for the transition states and intermediates on the ground state potential energy surfaces of ethylenes, including C 2 H 4 , C 2 D 4 , D 2 CCH 2 , and cis-and trans-HDCCDH, related to the molecular and atomic hydrogen elimination channels of photodissociation in VUV were characterized at the B3LYP/6-311G͑d,p͒ level. The coupled cluster method, CCSD͑T͒/6-311ϩG͑3df,2p͒, was employed to calculate the corresponding energies with the zero-point energy corrections by the B3LYP/6-311G͑d,p͒ approach. Ethylidene was found to be an intermediate in the 1,2-H 2 elimination channel. The barrier for the 1,1-H 2 elimination was computed to be the lowest ͑4.10-4.16 eV͒, while the 1,2-H 2 elimination and H loss channels have barriers of a similar height ͑4.70-4.80 eV͒. The rate constant for each elementary step of ethylene photodissociation at 193 and 157 nm was calculated according to the RRKM theory based on the ab initio surfaces. The rate equations were subsequently solved, and thus the concentration of each species was obtained as a function of time. The concentrations at t→ϱ were taken for calculating branching ratios or yields. In accord with previous experimental findings, the calculated branching ratio for the 1,1-H 2 elimination process is higher than that for the 1,2-H 2 elimination, and the atomic elimination channel is predicted to be favored at increasing excitation energy when competing with the molecular elimination. The significant discrepancies between theoretical and experimental results in the magnitude of the yields and their dependence on the wavelength for the molecular elimination channels suggest the dynamics of either 1,2-H 2 , or 1,1-H 2 elimination, or both channels may be nonstatistical in nature.

The neutral–neutral reaction of the cyano radical, CN(X 2Σ+), with ethylene, C2H4(X 1Ag), has been performed in a crossed molecular beams setup at two collision energies of 15.3 and 21.0 kJ mol−1 to investigate the chemical reaction dynamics to form vinylcyanide, C2H3CN(X 1A′) under single collision conditions. Time-of-flight spectra and the laboratory angular distributions of the C3H3N products have been recorded at mass-to-charge ratios 53−50. Forward-convolution fitting of the data combined with ab initio calculations show that the reaction has no entrance barrier, is indirect (complex forming reaction dynamics), and initiated by addition of CN(X 2Σ+) to the π electron density of the olefin to give a long-lived CH2CH2CN intermediate. This collision complex fragments through a tight exit transition state located 16 kJ mol−1 above the products via H atom elimination to vinylcyanide. In a second microchannel, CH2CH2CN undergoes a 1,2 H shift to form a CH3CHCN intermediate prior to a H atom emission via a loose exit transition state located only 3 kJ mol−1 above the separated products. The experimentally observed mild “sideways scattering” at lower collision energy verifies the electronic structure calculations depicting a hydrogen atom loss in both exit transition states almost parallel to the total angular momentum vector J and nearly perpendicular to the C2H3CN molecular plane. Since the reaction has no entrance barrier, is exothermic, and all the involved transition states are located well below the energy of the separated reactants, the assignment of the vinylcyanide reaction product soundly implies that the title reaction can form vinylcyanide, C2H3CN, as observed in the atmosphere of Saturn’s moon Titan and toward dark, molecular clouds holding temperatures as low as 10 K. In strong agreement with our theoretical calculations, the formation of the C2H3NC isomer was not observed.