Journal of Chemical Physics vol:116 issue:9 pages:3700-3709
In this work, a pulsed laser photolysis/chemiluminescence (PLP/CL) technique was used to measure absolute rate coefficients for the reaction of C2H+H-2-->products over the temperature range 295-666 K. Ethynyl radicals were produced pulsewise by excimer laser photolysis of acetylene at 193 nm and real-time pseudo-first-order decays of C2H were monitored by the CH(A (2)Delta-->X (2)Pi) chemiluminescence resulting from their reaction with O-2. Over the experimental temperature range, the results indicate that the rate coefficient exhibits a non-Arrhenius behavior in line with theoretical predictions, k(hydrogen)(T)=3.92x10(-19) T2.57+/-0.30 exp[-(130+/-140) K/T] cm(3) molecule(-1) s(-1). Experiments were supplemented by ab initio molecular orbital calculations up to the coupled-cluster theory including all single and double excitations plus perturbative corrections for the triples, UCCSD(T), with the 6-311++G(d,p) basis set for geometry optimizations and the aug-cc-pVTZ for electronic energy single points, revealing that the direct hydrogen abstraction yielding HCdropCH+H is the only product channel of any importance. There is also no important crossing between the doublet and quartet energy surfaces. Finally, geometry optimizations at the UCCSD(T)/6-311++G(2df,2p) level have shown that the transition structure for H-abstraction is linear; harmonic vibration frequencies at this level, and single-point UCCSD(T)/aug-cc-pVTZ energies for these geometries result in an adiabatic barrier height for H-abstraction, including harmonic vibration zero point energies, of 12.8 kJ/mol, while the classical potential energy barrier is 9.2 kJ/mol. (C) 2002 American Institute of Physics.