THE PUCKERING INVERSION BARRIER AND VIBRATIONAL-SPECTRUM OF CYCLOPENTENE - A SCALED QUANTUM-MECHANICAL FORCE-FIELD ALGORITHM

High-level ab initio quantum chemical studies have been performed to ascertain the equilibrium molecular structure and ring-puckering inversion barrier of cyclopentene, as well as the complete infrared and Raman spectra of cyclopentene-d0, -1-d1, -1,2,3,3-d4, and -d8. The planar (C2v) and puckered (C(s)) structures of cyclopentene have been optimized at the DZ(d) SCF and MP2 levels, and reasonable agreement with available experimental data is found. The DZ(d) MP2 prediction for the puckering angle is 23.4-degrees, which compares favorably with several experimental values in the range 22-26-degrees. High-level electron correlation treatments with extended basis sets are found to be necessary to converge the predicted puckering inversion barrier. In accord with experimental values, a final theoretical proposal of 235 +/- 20 cm-1 is derived for the vibrationless inversion barrier from a combination of TZ(d,p) MP4 and PZ(3d2f,2p1d) MP2 results. The fundamental vibrational frequencies (including intensity and polarization data) of cyclopentene isotopomers have been obtained from DZ(d) SCF force constants by using the scaled quantum mechanical (SQM) force field procedure. An improved SQM algorithm has been formulated which allows facile optimization of empirical scale factors to determine optimal quadratic force fields. Numerous reassignments are proposed among the 128 observed fundamental frequencies of the four isotopomers considered. It is shown that zero-point vibrational effects engender a substantial contribution to the empirically derived puckering inversion barrier.