SPECTRA AND STRUCTURE OF SMALL RING COMPOUNDS .56. RAMAN AND FAR-INFRARED SPECTRA, CONFORMATIONAL STABILITY VIBRATIONAL ASSIGNMENTS, NORMAL COORDINATE ANALYSIS AND ABINITIO CALCULATIONS OF CHLOROCYCLOBUTANE

The Raman (3100 to 10 cm−1) and far‐infrared (374 to 74 cm−1) spectra have been recorded of gaseous chlorocyclobutane. A series of Q‐branches observed in both of these spectra beginning at 158 cm−1 with successive transition falling to lower frequencies have been assigned to the ring puckering vibrations of both the low‐energy equatorial and high‐energy axial conformers. These data have been fitted to an asymmetric potential function of the form V (cm−1) = (5.13 ± 0.34) × 105X4 − (3.35 ± 0.14) × 104X2 + (3.79 ± 0.30) × 104X3 with a reduced mass of 198 amu. Utilizing this potential, the difference between the puckering angles for the two conformers was calculated to be 5° with the equatorial conformer having the larger value of 22°. This potential function is consistent with an energy difference between the equatorial and axial forms of 449 cm−1 (1.28 kcal mol−1) and a barrier to ring inversion of 827 cm−1 (2.36 kcal mol−1). Experimental values for the enthalpy difference between the two conformers have been determined for both the liquid and gas from relative intensities of a pair of Raman lines. The conformational stability, barrier to ring inversion, ro structural parameters, and fundamental vibrational frequencies, which have been determined experimentally, are compared with those obtained from ab initio Hartree‐Fock calculations employing both the 3–21G* basis sets, and to the corresponding quantities obtained for some similar molecules. The ab initio results indicate that the puckering angle for the equatorial conformers should be about 30°, with that for the axial conformer being about 8° smaller. Copyright © 1990 John Wiley & Sons, Ltd.