MODE-SPECIFIC HYDROGEN TUNNELING IN TROPOLONE - AN INSTANTON APPROACH

Calculations are reported of hydrogen and deuterium tunneling splittin gs in the ground state S-0 ((X) over tilde, (1)A(1)) and the first exc ited singlet state S-1 ((A) over tilde,B-1(2)) of tropolone-d(0) and - d(1). The main focus of the calculations is on the splittings observed in vibrationally excited levels of S-1, some of which are larger whil e others are smaller than the zero-point splitting. To account for the se observations, a potential-energy surface is constructed by standard quantum-chemical methods and the dynamics on this surface is treated by a method derived from the instanton approach. The potential-energy surface is a complete multidimensional surface resulting from the comb ination of a potential-energy curve along the tunneling coordinate wit h a harmonic force field calculated at the stationary points. The leve l of calculation adopted is HF/6-31G* for S-0 and CIS/6-31G** for S-1 . A few other, nominally more accurate, methods were tried but proved to be unsatisfactory. To deal with the dynamics, the instanton method, used previously for the calculation of zero-point level splittings, i s modified so as to make it applicable to excited levels. As expected, it is found that excitation of the tunneling mode strongly promotes h ydrogen transfer. The effects of exciting modes that are symmetric or antisymmetric with respect to the symmetric transition state are evalu ated for all such modes with assigned splittings by a straightforward generalization of the correction terms previously derived for zero-poi nt splittings. Of special interest are out-of-plane modes, some of whi ch show up as overtones with splittings smaller than the zero-point sp litting, despite the fact that there is no linear coupling between the se modes and the tunneling mode. The effect is ascribed to anharmonic coupling and an effort is made to calculate the required anharmoniciti es quantum-chemically. In general the agreement between theory and exp eriment is satisfactory for modes that are linearly coupled while the situation is less clear for anharmonically coupled modes. (C) 1996 Ame rican Institute of Physics.