Kinetics and mechanism of the β-alanine + OH gas phase reaction: A quantum mechanical approach

Abstract

The OH hydrogen abstraction reaction from β-alanine has been studied using the BHandHLYP hybrid HF–density functional and 6-311G(d,p) basis sets. The energies have been improved by single point calculations at the CCSD(T)/6-311G(d,p) level of theory. The structures of the different stationary points are discussed. Reaction profiles are modeled including the formation of pre-reactive and product complexes. Negative net activation energy is obtained for the overall reaction. A complex mechanism is proposed, and the rate coefficients are calculated using transition state theory over the temperature range of 250–400 K. The rate coefficients are proposed for the first time and it was found that in the gas phase the hydrogen abstraction occurs mainly from the CH₂ group next to the amino end. The following expressions, in cm³ mol⁻¹ s⁻¹, are obtained for the overall rate constants, at 250–400 and 290–310 K, respectively: k_250–400 = 2.36 × 10⁻¹² exp(340/T), and k_290–310 = 1.296 × 10⁻¹² exp(743/T). The three parameter expression that best describes the studied reaction is k_250–400 = 1.01 × 10⁻²¹T^3.09 exp(1374/T). The β-alanine + OH reaction was found to be 1.5 times faster than the α-alanine + OH reaction.