Hydrogen-bonding effects on free-radical properties

Hydrogen-bonding effects on structures, vibrational frequencies, and isotropic hyperfine coupling constants are investigated in semiquinone, phenoxyl, and closely related amine-substituted radicals. Included are the disubstituted isoelectronic series of p-benzosemiquinone anion, p-phenylenediamine cation, and p-aminophenoxyl neutral radicals, as well as the corresponding monosubstituted isoelectronic series of phenoxyl and aniline cation radicals. Computational studies that include all hydrogen-bound first shell water molecules, corresponding to two water molecules coordinated at each oxygen substituent and two coordinated at each amine substituent, give satisfactory agreement with aqueous experimental results in nearly all cases. In those instances where experimental results are also available in non-hydrogen-bonding environments for comparison, the calculations rationalize the signs and most approximate magnitudes of the experimental shifts attributable to hydrogen bonding. Asymmetric solvation structures, corresponding to fewer than the maximum number of hydrogen-bound water molecules, are also investigated. These show that the influences of individual water molecules are often, but not always, additive in determining vibrational frequency shifts. Asymmetric solvation can have a large influence on the spin distribution, where individual effects from each water are often largely canceled out to yield smaller net shifts when the hydrogen-bonding shell is completed.