The vibrational dependence of the hydrogen and oxygen nuclear magnetic shielding constants in OH− and OH− · H2O Original Research Article

The effect of hydrogen binding and vibrational motions on the oxygen and hydrogen nuclear magnetic shielding constants in the OH− and mono-hydrated OH− and mono-hydrated OH− ion (H3O2−) is investigated by ab initio calculations. A large down-field shift in 17O shieldings and a small down-field shift in the 1H shieldings is found for H3O2− relative to OH−. The dependence of the nuclear magnetic shielding constants in H3O2 on the strongly anharmonic symmetric and antisymmetric O···H···O stretching motions and on the internal rotation motion of the outer hydrogens is studied with the non-rigid bender model Hamiltonian [V. Špirko, W. P. Kraemer and A. Čejchan, J. Mol. Spectrosc. 136 (1989) 340] at the level of the random phase approximation (RPA). The dependence of the shielding constants in OH− on the bond length is investigated at the level of the RPA and the second order polarization propagator approximation (SOPPA). Pertinent (analytic) nuclear magnetic shielding functions are obtained by fitting to the ab initio shielding points and these functions are used to calculate the vibrational averages using the corresponding vibrational eigenfunctions. The predicted effective shielding constants of H3O2− exhibit a sizable and non-monotonic dependence on the stretching vibrational quantum numbers, whereas the dependence on the internal rotation is practically negligible. The effective shielding constants of OH− show an even larger dependence on the vibrational quantum number. The effect of the end-over-end rotational motion, however, is small.