Density-Functional-Derived Structures, Spin Properties, and Vibrations for Phenol Radical Cation

Abstract

Phenol radical cation (PhOH•+) structures, proton hyperfine coupling constants, atomic spin densities, vibrational frequencies, and deuterium isotopic vibrational frequency shifts were calculated by using spin-unrestricted Hartree−Fock (UHF), second-order Møller−Plesset perturbation (UMP2), as well as density-functional (DF) and hybrid Hartree−Fock/density-functional (HF/DF) methods. Of the methods tested, the three-parameter, hybrid HF/DF method incorporating the gradient-corrected correlation functional of Lee, Yang, and Parr (B3LYP) appears to yield the most accurate overall results. Calculated CC bond distances for PhOH•+ display pronounced quinoidal features, and the CO bond distance (1.313 Å) implies significant double-bond character. Unscaled vibrational frequencies calculated by using the B3LYP method differ from experiment by an average absolute magnitude of only 15 cm-1 and are therefore more accurate than uniformly scaled UMP2-derived frequencies. Our assignment of the CO stretching mode to a calculated frequency of 1522 cm-1 and the strong mixing between the CO stretching mode 7a and a CH bending mode at lower frequency (19a) suggests that isotopic frequency shifts be measured to distinguish clearly the CO stretching and CH bending modes in radicals related to PhOH•+. PhOH•+ displays a spin density distribution reminiscent of an odd-alternant hydrocarbon, but with large spin density on its oxygen-bearing C1 atom. Of the methods tested, the gradient-corrected BP86 and BLYP and hybrid B3LYP methods yield the most accurate proton hyperfine coupling constants for PhOH•+, whereas calculated spin densities on the carbon and oxygen atoms are much less sensitive to the particular DF method used. The B3LYP method also gives an accurate proton affinity for PhO• (208.5 kcal/mol), within 2.9 kcal/mol of experiment (205.6 ± 0.3 kcal/mol).