THEORETICAL-STUDY ON THE STRUCTURES, FORCE-FIELD, AND VIBRATIONAL-SPECTRA OF CYCLOOCTATETRAENE AND CYCLOOCTATETRAENE-D(8)

The structures and force field of 1,3,5,7-cyclooctatetraene (COT) have been studied using ab initio theory at the SCF level with the 4-21G b asis set. The quadratic force field of the D2d structure obtained by s ystematic scaling of the ab initio force constants successfully reprod uces the observed frequencies of COT and COT-d8 with a mean deviation of less than 10 cm-1 for non-CH stretching modes. On the basis of the calculated results, assignments of the fundamental vibrations are exam ined. The normal mode upsilon5 is reassigned to a weak band at 758 cm- 1 in the Raman spectrum of COT and to a weak band at 591 cm-1 in the R aman spectrum of COT-d8. The calculations favor the assignment Of upsi lon26 given by LIPPINCOTT et al. [J. Am. Chem. Soc. 73, 3370 (1951)] o ver the revised assignment Of PEREC [Spectrochim. Acta 47A, 799 (1991) ]. The calculations also furnish reliable prediction for the inactive A2 fundamentals of COT and COT-d8. The fundamental frequencies and IR and Raman intensities of (CC7H8)-C-13, which constitutes about 9% of C OT in natural abundance, are also calculated. Only v10 (calculated at 908 cm-1) of the formal inactive A2 modes has appreciable Raman intens ity (0.23 angstrom4/amu). A spectral feature due to this fundametal is identified in the liquid Raman spectrum of TABACIK and BLAISE [C R. A cad. Sci. Ser. II 303, 539 (1986)] as a weak peak at 908 cm-1.