Radicals as Hydrogen Bond Donors and Acceptors

High-level quantum chemical techniques were used to study the hydrogen bonding interactions in dimers of simple hydrogen bond donors and acceptors. The dimers studied were formed from combinations of CH4, NH3, OH2 with each other and with the (CH3)-C-center dot, (NH2)-N-center dot, and (OH)-O-center dot radicals. It was found that complexes in which a radical serves as a hydrogen bond donor, i.e. (center dot)AH(x)-BHy, are more strongly bound than dimers in which the hydrogen bond donor is the analogous parent molecule, i.e. AH(x+1)-BHy. Complexes in which a radical serves as a hydrogen bond acceptor, i.e. BHy-(center dot)AH(x), are more weakly bound than dimers in which the hydrogen bond acceptor is the analogous parent molecule, i.e. BHy-AH(x+1). The differences in these binding properties are attributable to the facts that, in radicals, the A-H bonds are more polar and the A atoms have less negative partial charges than in molecules. Detailed analyses of spin densities revealed that spin delocalization from a radical to a molecule is negligible. Therefore, spin delocalization plays no role in the binding within the complexes studied in this work. Density functional theory methods were also used to calculate the binding energies of the complexes. It was found that the PBE0 and B971 functionals predict binding energies that are in good agreement with the high-level wavefunction data, whereas the performance of the common B3LYP method is not as good. Correcting the functionals for their ability to treat dispersion interactions in carbon-containing compounds improves the binding energies computed with the B3LYP and PBE0 functionals but results in over-binding with B971.