MODE-SPECIFIC TUNNELING SPLITTINGS IN 9-HYDROXYPHENALENONE - COMPARISON OF 2 METHODS FOR DIRECT TUNNELING DYNAMICS

A benchmark comparison is presented of two direct dynamics methods for proton tunneling, namely variational transition-state theory with sem iclassical tunneling corrections (VTST/ST) and the instanton method. T he molecules chosen for the comparison are 9-hydroxyphenalenone-d(0) a nd -d(1), which have 64 vibrational degrees of freedom and show large tunneling splittings for the zero-point level and several vibrationall y excited levels of the electronic ground state. Some of the excited-l evel splittings are larger and some smaller than the zero-level splitt ing, illustrating the multidimensional nature of the tunneling. Ab ini tio structure and force field calculations at the Hartree-Fock/6-31G* level are carried out for the two stationary points of the tunneling potential, viz. the equilibrium configuration and the transition state . The VTST/ST calculations are based on both the small- and the large- curvature approximation; the additional quantum-chemical calculations required at intermediate points of the potential are performed at the semiempirical modified neglect of differential overlap (MNDO)/H2 level . The VTST/ST computations use the MORATE 6.5 code developed by Truhla r and co-workers. The instanton dynamics calculations are based on the method we previously developed and applied to tropolone, among others . It uses the transition state rather than the equilibrium configurati on as reference structure and approximates the least action analytical ly. The computations use our ''dynamics of instanton tunneling'' (DOIT ) code. It is found that the large-curvature approximation and the ins tanton method both reproduce the observed zero-level splitting of the do isotopomer if the calculated barrier is reduced by a factor 0.87. W ith this adjusted barrier, the instanton method also reproduces the ze ro-level and excited-level splittings of the dl isotopomer. However, b oth the small- and the large-curvature approximations severely underes timate all these splittings. These methods, which use relatively infle xible trajectories, do not handle the isotope effect well and also are not developed to the point where they can deal satisfactorily with vi bronic level splittings. In addition, there is a striking difference i n efficiency between the two methods: the MORATE 6.5 code took 40 h on an R8000 workstation to perform the dynamics calculations, whereas th e DOIT code took less than 1 min and produced superior results. The ma in reason for this superior performance is ascribed to the effective u se made of the least-action principle by the instanton method and to t he avoidance of the adiabatic approximation, which is not valid for mo des with a frequency equal to or lower than the tunneling-mode frequen cy. (C) 1998 American Institute of Physics.