Ab initio study of OH addition reaction to isoprene

Ab initio molecular orbital calculations have been employed to investigate the structures and energetics of the adduct isomers arising from the additi on reaction of OH to isoprene. Several levels of ab initio theory were eval uated using a set of organic radical species to establish the appropriate l evel of approximation. The method of gradient corrected density functionals (NLDFT) in conjunction with moderate basis sets was found to yield satisfa ctory molecular geometries and vibrational frequencies. Single-point energy calculations were performed using various methods, including MP2, MP4, and CCSD(T). The most energetically favorable isomers are those with OH additi on to the terminal carbon positions. At the CCSD(T)/6-311G** level of theor y corrected with zero-point energy (ZPE), the isomers with OH additions to isoprene at C1 to C4 positions (i.e., isomers I-IV) are 34.8, 24.2, 22.4, a nd 32.3 kcal mol(-1) more stable than the OH and isoprene, respectively. Th e activation energies against OH migration transforming the higher energy i somers into the lower energy ones (i.e., II to I or III to IV) are signific ant (25.5-26.5 kcal mol(-1)), indicating that thermal equilibrium of the OH -isoprene adduct isomers is unlikely to be established. In addition, we hav e developed and validated a computationally efficient method to calculate t he energetics of the OH-isoprene reaction system. (C) 2000 American Institu te of Physics. [S0021-9606(00)31937-7].