Conformational stabilization of phthalan: physical origin of tiny ring-puckering barrier

The conformational property of phthalan has been investigated using ab initio calculation and natural bond orbital (NBO) analysis methods. Geometry optimizations for the planar (C-2v) and puckered (C-s) conformers have been carried out using the HF, B3LYP, and MP methods, and the results indicate that this molecule has a tiny ring-puckering barrier. This barrier appears to be in good agreement with the previous experimental result. NBO analysis shows that the tiny ring-puckering barrier is closely related to the molecular orbital interactions around the C-O bonds of the five-membered ring. The gas-phase infra-red and liquid-phase Raman spectra of phthalan and 1,3-benzodioxole have been recorded and analyzed in terms of C-2v symmetry. Vibrational frequency calculations using the B3LYP method have also been performed to compare with the spectroscopic data. The B3LYP frequency calculations do a reasonable job of estimating the frequencies. (C) 2002 Elsevier Science B.V. All rights reserved.