Theoretical study of the contribution of vibrational motions to nuclear shielding constants

Contributions of the vibrational motions to the NMR shielding constants are calculated and studied theoretically for the isolated H2, HF and H2O molecules in the gas phase. Intramolecular potential energy surfaces (PES) and their corresponding wave functions have been calculated using B3LYP/6-311++G** method. Shielding constant surfaces for the 1H, 17O and 19F nuclei in these molecules have been calculated over their PES's using CSGT method. Distribution of the shielding constants and their corresponding averages are calculated for each vibrational state. Combination effects of the populated vibrational levels (and different vibrational modes in the case of H2O molecule) to the NMR shielding constants are also evaluated. From the results obtained for H2O, it can be concluded that for polyatomic molecules different vibrational modes may have opposite effects on the nuclear shielding constants and thus may partially cancel vibrational contributions to shielding constants. The zero-point vibrational contributions to the NMR shielding constant of the nuclei of the HF and H2O molecules calculated via our approach are comparable with those reported by Ruud et al. [K. Ruud, P.-O. Åstrand, P.R. Taylor, J. Chem. Phys. 112 (2000) 2668, K. Ruud, P.-O. Åstrand, P.R. Taylor, J. Am. Chem. Soc. 123 (2001) 4826].