SPIN-COUPLED STUDY OF THE ELECTRONIC-STRUCTURE OF POLYENYL RADICALS C3H5-C9H11

Spin-coupled (SC) theory is applied to the description of the electronic structure of polyenyl radicals allyl (C3H5), C5H7, C7H9, and C9H11. A SC wave function treating all seven electrons involved in the carbon-carbon bonds in C3H5 as active is found to reproduce the correct C-2 upsilon geometry for allyl in its ground state, as well as the proper spatial and spin symmetry of this state ((2)A(2)). Although no sigma-pi separation is imposed a priori upon the seven-electron active space, the optimized SC orbitals display this symmetry and exhibit no tendency toward bent-bond formation and/or symmetry breaking. The most important correlation effects are shown to be included in the pi space: The energy of the pi-only three-electron SC wave function for C3H5 coincides with that of the corresponding ''3 in 3'' complete-active-space self-consistent-field (CAS SCF) wave function. The SC model for the a electrons in allyl confirms the utility of the antipair concept introduced with the SC treatment of antiaromaticsystems. The SC approach is demonstrated to yield highly correlated valence-bond-style pi-active-space wave functions of the appropriate spatial and spin symmetry ((2)A(2) and B-2(1)) for the subseries of polyenyl radicals, involving 4n + 3 and 4n + 1 carbon atoms, respectively. Except for allyl, the a electrons are accommodated in tightly localized SC orbitals, which resemble distorted C(2p(pi)) atomic orbitals. [4n + 3] chains (with the exception of the C3H5) are best described by an antiresonance between equivalent Kekule structures. The SC wave functions for the other ([4n + 1]) series are dominated by a single symmetry-adapted Kekule structure which is in conventional resonance with less important pairs of equivalent structures.