Nitroxide/substrate weak hydrogen bonding: Attitude and dynamics of collisions in solution

The study of intermolecular collisions and bonding interactions in solutions is of critical importance in understanding and predicting solute/solvent properties. Previous work has established that stable paramagnetic nitroxide molecules are excellent probes of intermolecular interactions for hydrogen bonding in polar solvents. In this study, H-1, H-2, C-13, N-15 NMR and liquid/liquid intermolecular transfer dynamic nuclear polarization ((LIT)-I-2 DNP) results are obtained for the paramagnetic probe molecule, TEMPO, interacting with the common aprotic and protic polar solvents, CH3CN and CH3CONH2, yielding a profile of both dipolar and scalar interactions. A significant scalar contact hyperfine is observed for the N-O center dot center dot center dot H-C interaction ((CH3)-C-13 hyperfine, a/h = 0.66 MHz) in the CH3CN/TEMPO system, whereas the N-O center dot center dot center dot H-C and N-O center dot center dot center dot H-N interactions for the TEMPO/CH3CONH2 system yield (CH3)-C-13 and N-15 hyperfine couplings of a/h = 0.16 and -0.50 MHz, respectively. The distance and attitude of the scalar interaction for the nitroxide hydrogen bonding at the methyl group in CH3CN and the amino group in CH3CONH2 are computed using density functional theory (DFT), yielding good agreement with the experimental results. These results show that the hyperfine coupling provides a sensitive probe of weak hydrogen-bonding interactions in solution.