Role of the Solvent in Computing the 1,4-Benzosemiquinone g-Tensor by the Coupled-Perturbed Kohn−Sham Hybrid Density Functional Method

The 1,4-benzosemiquinone (BzSQ) g-tensor components, in the gas phase and in methanol, are computed. Neese's coupled-perturbed Kohn−Sham method is used, in conjunction with the UB1LYP and UPBE0 exchange-correlation hybrid density functionals. The computed g-tensor elements are analyzed and broken down into their individual first- and second-order contributions. The inclusion of electron correlation, via the UB1LYP and UPBE0 hybrid density functionals, improves these calculations over those carried out at the Hartree−Fock level. Further improvement is obtained if the SCF reaction field method (SCRF) of Tomasi, as implemented by Barone, is employed to mimic the solvent effects. The best agreement with experiment is achieved using a model in which the BzSQ hydrogen bonds to four methanol solvent molecules to form a supermolecule. This occurs because the main contribution to the g-tensor components arises from the net spin densities on the carbonyl oxygen atoms, which are highly influenced by hydrogen bonding with the solvent. Thus, provided the solvent effects are explicitly included, the calculated g-tensor components are in very good agreement with the corresponding experimental values. BzSQ in the gas phase has D2h symmetry and a 2B2g ground state with no off-diagonal g-tensor components. Hydrogen bonding to the four methanol molecules reduces its symmetry to D2. However, a group theoretical analysis reveals that there should be no shift in the directions of the g-tensor principal axes due to hydrogen bonding. Finally, the isotropic 1H hyperfine coupling constants, obtained by these calculations, are also in excellent agreement with their experimental counterparts.