Theoretical study on the structures, force field, and vibrational spectra of cyclooctatetraene and cyclooctatetraene-d8

Abstract

Abstract 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 basis set. The quadratic force field of the D 2 d structure obtained by systematic scaling of the ab initio force constants successfully reproduces the observed frequencies of COT and COT- d 8 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 examined. The normal mode υ 5 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 Raman spectrum of COT- d 8 . The calculations favor the assignment of υ 26 given by Lippincott et al . [J. Am. Chem. Soc. 73, 3370 (1951)] over the revised assignment of Perec [Spectrochim. Acta 47A, 799 (1991)]. The calculations also furnish reliable prediction for the inactive A 2 fundamentals of COT and COT- d 8 . The fundamental frequencies and IR and Raman intensities of 13 CC 7 H 8 , which constitutes about 9% of COT in natural abundance, are also calculated. Only ν 10 (calculated at 908 cm −1 ) of the formal inactive A 2 modes has appreciable Raman intensity (0.23 A 4 /amu). A spectral feature due to this fundametal is identified in the liquid Raman spectrum of Tabacik and Blaise [C. R. Acad. Sci. Ser. II 303, 539 (1986)] as a weak peak at 908 cm −1 .