THE VIBRATIONAL DEPENDENCE OF THE HYDROGEN AND OXYGEN NUCLEAR MAGNETIC SHIELDING CONSTANTS IN OH- AND OH-CENTER-DOT-H2O

The effect of hydrogen binding and vibrational motions on the oxygen a nd hydrogen nuclear magnetic shielding constants in the OH- and mono-h ydrated OH- ion (H3O2-) is investigated by ab initio calculations. A l arge down-field shift in O-17 shieldings and a small down-field shift in the H-1 sheildings is found for H3O2- relative to OH-. The dependen ce of the nuclear magnetic shielding constants in H3O2- on the strongl y anharmonic symmetric and antisymmetric O ... H ... O stretching moti ons and on the internal rotation motion of the outer hydrogens is stud ied with the non-rigid bender model Hamiltonian [V. Spirko, W. P. Krae mer and A. Cejchan, 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 t he RPA and the second order polarization propagator approximation (SOP PA). Pertinent (analytic) nuclear magnetic shielding functions are obt ained by fitting to the ab initio shielding points and these functions are used to calculate the vibrational averages using the correspondin g vibrational eigenfunctions. The predicted effective shielding consta nts of H3O2- exhibit a sizable and non-monotonic dependence on the str etching vibrational quantum numbers, whereas the dependence on the int ernal rotation is practically negligible. The effective shielding cons tants of OH- show an even larger dependence on the vibrational quantum number. The effect of the end-over-end rotational motion, however, is small.