An ab initio study of hydrogen bonding effects on the 15N and 1H chemical shielding tensors in the Watson-Crick base pairs

Density functional theory is applied to explore changes upon hydrogen bonding in the N-15 and H-1 nuclear magnetic resonance (NMR) chemical shielding tensors of the imino group in the Watson-Crick nucleic acid base pairs. The dependence of results on the quantum chemical method used, on the basis set superposition error, and on the effect of the relaxation of the base pair geometry with changing hydrogen bond length is addressed. The systematic variation with changing separation between bases in calculated shielding data and resulting auto- and cross-corelation chemical shift anisotropy parameters is documented. Possible implications for NMR studies of the dynamics of N-15-H-1 bond vectors and for transverse relaxation-optimized spectroscopy experiments on imino protons are discussed.