6-DIMENSIONAL QUANTUM CALCULATIONS OF VIBRATION-ROTATION-TUNNELING LEVELS OF NU(1) AND NU(2) HCL-STRETCHING EXCITED (HCL)(2)

Results of the first full-dimensional (6D) quantum calculations of the vibrational levels of the nu(1) and nu(2) HCl-stretch excited (HCl)(2 ), for total angular momentum J = 0, are presented. Three 6D potential energy surfaces (PESs) were employed. Two widely used PESs, the ab in itio PES of Bunker and co-workers and the semiempirical PES by Elrod a nd Saykally, are found to give negligible tunneling splittings (less t han or equal to 5x10(-2) cm(-1)) for the vibrational eigenstates of th e nu(1)/nu(2) excited (HCl)(2), in sharp disagreement with the experim ental tunneling splittings in the nu(1) and nu(2) fundamentals, -3.32 and 3.18 cm(-1). In an effort to overcome this problem, a 6D electrost atic interaction potential is constructed and added to the ES1 PES; th e resulting 6D PES is denoted ES1-EL. Quantum 6D calculations on the E S1-EL PES yield greatly improved tunneling splittings for nu(1) (-2.31 cm(-1)) and nu(2) (2.45 cm(-1)), which are 70% and 77%, respectively, of the corresponding experimental values. The nu(1) and nu(2) fundame ntal HCl-stretching frequencies calculated on the ES1-EL PES are only 5.9 cm(-1) lower and 2.9 cm(-1) higher, respectively, than their exper imental counterparts. In addition. the quantum 6D calculations on the ES1-EL PES provide a comprehensive characterization of the nu(1)/nu(2) supported vibrational eigenstates of (HCl)(2), including their energi es, assignments, and tunneling splittings. The vibration-rotation-tunn eling dynamics of (HCl)(2) in the nu(1) and nu(2) excited states which emerged from our calculations differs substantially from that observe d for the HF-stretch excited (HF)(2). (C) 1998 American Institute of P hysics.