Rate constant for the reaction C2H5 + HBr → C2H6 + Br.

RRKM theory has been employed to analyze the kinetics of the title reaction, in particular, the once-controversial negative activation energy. Stationary points along the reaction coordinate were characterized with coupled cluster theory combined with basis set extrapolation to the complete basis set limit. A shallow minimum, bound by 9.7 kJ mol(-1) relative to C(2)H(5) + HBr, was located, with a very small energy barrier to dissociation to Br + C(2)H(6). The transition state is tight compared to the adduct. The influence of vibrational anharmonicity on the kinetics and thermochemistry of the title reaction were explored quantitatively. With adjustment of the adduct binding energy by ∼4 kJ mol(-1), the computed rate constants may be brought into agreement with most experimental data in the literature, including new room-temperature results described here. There are indications that at temperatures above those studied experimentally, the activation energy may switch from negative to positive.

Reaction kinetics of the addition of atomic sulfur to nitric oxide.

The reaction of S((3)P(J)) with NO ((2)Pi) in an Ar bath gas has been studied by the laser photolysis-resonance fluorescence technique over 300-810 K at pressures from 60 to 800 mbar. The observed second-order rate constants are close to the low-pressure limit. Fitting of Troe's formalism to experiment, with an estimated F(cent) = 0.78 exp(-T/7445) and k(infinity) given subsequently, yields k(0) = (6.2+/-0.6) x 10(-33) exp(+ (940+/-40)/T) cm(6) molecule(-2) s(-1). Error limits are +/-25%. A theoretical analysis of this value suggests that the average energy transferred during collisions between Ar and the excited intermediate is DeltaE = -360(-160) (+90) cm(-1). Over 300-800 K, the high-pressure limit is predicted to be k(infinity) = 2.2 x 10(-10) (T/300)(0.24) cm(3) molecule(-1) s(-1). Doublet and quartet adducts between S and NO were characterized via CBS-QB3 theory. The kinetic data can be rationalized with SNO ((2)A(')) as the major product, and an ab initio estimate of Delta(f)H(298) for SNO is 176+/-8 kJ mol(-1).