THE TORSIONAL BARRIER OF CLOOCL

Ab initio internal rotation barrier heights V-cis and V-trans for the peroxide form of Cl2O2 have been calculated using shape-consistent eff ective core potentials (ECP) and triple-zeta valence-only basis sets s pecially optimized for the ECP recently developed by us (Pacios, L, F. ; Gomez, P. C. Int. J. Quantum Chem. 1994, 49, 817), These basis sets are augmented with standard polarization functions, and correlation is accounted for at the MPn (n = 2, 4), CCSD, and CCSD(T) levels of theo ry. All calculations consistently produce a cis barrier higher than th e trans one, being our highest level results (CCSD(T)): V-cis = 3538 c m(-1) and V-trans = 1890 cm(-1). This disagrees with previous experime ntal estimates, and the discrepancy is discussed here. Internal rotati on potential and torsional constants g(beta beta)(beta) are calculated for a grid of points at the MP2 level allowing for full relaxation of the geometry, fitted to a Fourier series, and used to calculate some of the lowest torsional energy levels, A single-point calculation pote ntial at the CCSD(T) level has also been calculated and used for this purpose. The torsional levels are presented, and the torsional fundame ntal transition is found to be in good agreement with the experimental values. Optimized geometry, rotational constants, harmonic frequencie s, and dipole moments are presented as a test of the reliability of ou r calculations for future studies on other halogen peroxides.