AB-INITIO STUDY OF THE 1,3-BUTADIENYL RADICAL ISOMERS

Ab initio calculations at the B3LYP, QCISD, and MCSCF levels of theory and using the 6-311G(d,p) basis set were carried out on the ground st ate of the 1,3-butadiene derived radicals. The 1,3-butadien-1-yl and 1 ,3-butadien-2-yl radicals are obtained from 1,3-butadiene by abstracti on of a hydrogen atom from a primary and secondary carbon, respectivel y. The 1,2-butadien-4-yl radical was also studied to examine the possi bility of the relocalization of the unpaired electron from 1,3-butadie n-2-yl. 1,2-Butadien-4-yl was consistently found to be the most stable isomer. The MCSCF relative energies are 29 kJ mol(-1) for 1,3-butadie n-2-yl and 35 kJ mol(-1) for the most stable of the 1,3-butadien-1-yl configurational isomers. The 1,3-butadien-2-yl structure is found to b e a local minimum in MCSCF calculations, but with an isomerization bar rier of less than 1 kJ mol(-1), and deforms to the 1,2-butadien-4-yl i somer at all other levels of theory used. The energy of the most stabl e 1,3-butadien-1-yl isomer relative to 1,3-butadien-2-yl ranges from 6 to 18 kJ mol(-1) across all levels of theory used, substantially lowe r than previous predictions by ab initio and semiempirical means. Opti mized geometries, relative energies, permanent dipole moments, Fermi c ontact terms and harmonic vibrational frequencies are reported.