pi-donor substituent effects on calculated structures, spin properties, and vibrations of radical anions of p-chloranil, p-fluoranil, and p-benzoquinone

Vibrational and electronic properties of p-benzosemiquinone radical anions are important indicators of charge transfer in ion pairs, charge-transfer salts, and electron-transfer reactions. To provide missing data for radical anions of p-chloranil, p-fluoranil, and p-benzoquinone, and to test the capabilities of various computational methods, calculated structures, spin properties, and harmonic vibrational frequencies and mode assignments are reported based on molecular orbital, density functional, and hybrid Hartree-Fock/density functional computations. Qualitative structural changes upon reducing the neutral quinones to their semiquinone radical anions are predictable from the nodal structure of p-benzoquinone's LUMO. Furthermore, trends in calculated CO and C=C bond distance changes for the three molecules follow a pattern expected from the pi-bonding abilities of the substituents. The most accurate calculated spin densities and isotropic hyperfine coupling constants show qualitative agreement with;experimentally derived values for p-benzosemiquinone radical anion, whereas predicted spin densities and hyperfine coupling constants are reported for the other two species. Although calculated vibrational frequencies display small average absolute differences from experimentally measured frequencies (24 cm(-1) for p-chloranil's radical anion), for p-chloranil's radical anion calculations generally fail to reproduce several important details of the vibrational spectra.