Ab initio molecular orbital theory on intramolecular charge polarization: Effect of hydrogen abstraction on the charge sensitivity of aromatic and nonaromatic species

We performed ab initio molecular orbital (MO) calculations of the response kernel (δQ(a)/δV(b)), which represents the response of the intramolecular charge polarization by external electrostatic field, on the basis of the coupled perturbed Hartree-Fock equation. The response kernels of some organic molecules including pyrazine, pyrazinyl radical, acetone, and 2- hydroxypropyl radical were calculated along the present formulation. The results revealed that the hydrogen abstraction of pyrazine causes the product radical to be remarkably deformable in the partial charge distribution, while the hydrogen abstraction of acetone does not induce such enhancement of the charge sensitivity. The augmented sensitivity does not appear in the usual polarizability for a uniform field but emerges for a local fluctuated field. To elucidate the remarkable difference, we performed the normal mode analysis and decomposition based on the intrinsic soft MO pairs or localized orbitals. As a result, the enhancement in the aromatic species is attributed to the softest normal mode due to the π-σ mixing that facilitates the deformation of the π-electron orbitals. In the nonaromatic species, on the other hand, this effect is not dominant and is canceled by the breakdown of hyperconjugation. We suggest that the particular sensitivity of aromatic radicals is the origin of anomalously slow diffusion in solution.