A COMPARISON OF 2 METHODS FOR DIRECT TUNNELING DYNAMICS - HYDROGEN-EXCHANGE IN THE GLYCOLATE ANION AS A TEST-CASE

Two methods for studying tunneling dynamics are compared, namely the i nstanton model and the approach of Truhlar and co-workers, which are b ased on the direct output of electronic structure calculations and thu s are parameter free. They are employed to evaluate the zero-level tun neling splitting due to intramolecular hydrogen exchange in the glycol ate anion. The first method was developed in a series of recent studie s and presents a combination of the instanton theory with quantum-chem ically computed potentials and force fields. For the compound at hand, which has 21 internal degrees of freedom, a complete potential-energy surface is generated in terms of the normal modes of the transition-s tate configuration. It is made up of the potential-energy curve along the tunneling coordinate and harmonic force fields at the stationary p oints. The level of theory used is HF/6-31++G*. AU modes that are dis placed between the equilibrium configuration and the transition state are linearly coupled to the tunneling mode, the couplings being propor tional to the displacements in dimensionless units. These couplings af fect the instanton trajectory profoundly and, depending on the symmetr y of the skeletal modes, can enhance or suppress the tunneling. In the glycolate anion all modes have such displacements and thus are includ ed in the calculation Based on the similarity with malonaldehyde, it i s argued that tunneling prevails in the studied process, and the zero- level tunneling splitting is predicted. The latter is found within the computational scheme developed earlier, which avoids explicit evaluat ion of the instanton path and thus greatly simplifies the tunneling dy namics. These results are tested by the method of large-curvature tunn eling of Truhlar and co-workers implemented in a dual-level scheme. Th e potential energy surface needed for the dynamics calculations is gen erated at the semiempirical PM3 level of theory and then corrected by interpolation with high-level HF/6-31++G* results for the stationary points. The code corresponding to this approximation is in the package MORATE 6.5. The tunneling splittings found by the two approaches are in quantitative agreement. We have found that the computational scheme based on the instanton model is much less time consuming both in the static and dynamics part. This computational efficiency, also demonstr ated in a number of earlier studies, merits future application of the method to fairly large systems of practical interest, such as clusters and organic compounds with excited-state proton transfer. (C) 1997 Am erican Institute of Physics.