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 state 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 abstraction of a hydrogen atom from a primary and secondary carbon, respectively. The 1,2-butadien-4-yl radical was also studied to examine the possibility of the relocalization of the unpaired electron from 1,3-butadien-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-butadien-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 be a local minimum in MCSCF calculations, but with an isomerization barrier of less than 1 kJ mol-1, and deforms to the 1,2-butadien-4-yl isomer at all other levels of theory used. The energy of the most stable 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 lower than previous predictions by ab initio and semiempirical means. Optimized geometries, relative energies, permanent dipole moments, Fermi contact terms and harmonic vibrational frequencies are reported.