The vibrational spectra (100–1500 cm−1) of a series of bicyclo[3.2.1]octanes assigned by means of scaled 3-21G ab initio harmonic force fields

Abstract

The infrared absorption (vapor phase and solution) and Raman (liquid phase) spectra of bicyclo[3.2.1]octane, 8-oxabicyclo[3.2.1]octane, 6-oxabicyclo[3.2.1]octane, 6,8-dioxabicyclo[3.2.1]octane, and the 7,7-dideutero-substituted derivatives of the last two compounds are reported in the region 100–1500 cm⁻¹ for the first time. The vibrational spectra are assigned almost completely with the guidance of ab initio 3-21G geometries and scaled force fields. A total of 14 force-field scale facors are transferred from smaller molecules, predicting the frequencies with an average error of 7.6 cm⁻¹ (1.2%) for 196 assigned transitions. After optimizing the factors in an overlay refinement involving all six molecules, the frequencies are within 5.7 cm⁻¹ (0.75%) of experiment. The ab initio absorption and Raman intensities are calculated with the 3-21G basis set and are demonstrated to be of such accuracy as to be useful for the spectral assignments. These intensities are calculated with uniformly and nonuniformly scaled force fields and compared to the experimental spectra. The intensities derived from the latter force fields are superior, meaning that nonuniform scaling is preferable at this level of theory for both vibrational frequencies and normal mode descriptions.