Direct-dynamics approaches to proton tunneling rate constants.: A comparative test for molecular inversions and an application to 7-azaindole tautomerization

Tunneling rate constants as a function of temperature are calculated for (a) three medium-size molecules. undergoing inversion: oxiranyl, dioxolanyl, and aziridine, together with the monodeutero isotopomers, and (b) two larger species undergoing tautomerization excited 7-azaindole and its 1:1 complex :with water. The former calculations allow a comparison of two direct-dynamics methods for multidimensional tunneling: transition-state theory with semiclassical tunneling corrections: (TST/ST) and the instanton approach. The latter calculations illustrate the possibility of applying the instanton approached biological model systems. The potential-energy surfaces for the inversions are evaluated on the basis of density functional theory at the level (U)B3LYP/6-31G*,which for the present problem is shown to yield results. similar to:UQCISD/6-311+G**. All vibrational degrees of freedom are included in the calculation and, except for minor adjustments; of:barrier heights, all parameters are used as calculated. The TST/ST calculations are performed with;the GAUSSRATE 7;2 program in the small-curvature approximation and-the instanton calculations with the DOIT 1.1 program. For the same input parameters the two methods produce very similar results, which are in excellent agreement with the observed rate constants; (except for the well-known artifact that leads GAUSSRATE converge to the wrong low-temperature limit). These results clearly show that, even ii the case of inversions characterized by low tunneling frequencies and barriers,, tunneling dominates the dynamics well beyond room temperature, implying that for all reactions in which a proton is exchanged between two heavier atoms, tunneling transfer will tend to remain dominant at even higher temperatures, While both methods support this conclusion and are capable; of yielding reliable tunneling rate constants, they differ:greatly in efficiency, the;instanton method being cconsiderably faster; and therefore able to deal:with much larger system This is demonstrated ; On 7-azaindole, which, together with its 1:1 complex with water,has been used as a biological model system. Comparison df the calculated rate constants of tautomerization by proton transfer i;i the-excited state with the available data sheds new light on the proposed transfer mechanism in protic Sol solutions. The instanton calculations : are based pn the same method and program as those for the inversions; except for a lower level of quantum, chemistry.