A density functional theory analysis of Raman and IR spectra of 2-adamantanone

Abstract

The vibrational force fields for cage-like molecules 2-adamantanone (2-A) and adamantane (A) were calculated by employing density functional theory (DFT) using 6-31G(d,p) basis set with two non-local exchange-correlation functionals BLYP (Becke–Lee–Young–Parr) and the Becke's three-parameter compound functional B3LYP. DFT infrared intensities and the Raman scattering activities were also calculated to simulate the vibrational spectra. The analysis of the two chosen models show that they are comparable in predicting geometry, vibrational frequencies, IR and Raman intensities. The agreement between the predicted and observed IR and Raman spectra is not satisfactory in the region 1500–1200 cm −1 for either of them. The overestimation of the computed intensity connected to the motion of methylene hydrogen atoms is ascribed to the systematic shortcoming of the computational theory. The approach to assignment of the vibrational spectra that combines the comparison of not only calculated and observed frequencies but also of IR and Raman intensities is tested on A and applied to reassignment of the normal modes of 2-A. The scale factors for the force constants of the CO group were derived. The scaled BLYP and B3LYP force fields reproduced the experimental frequencies of 2-A with an average error of 12 and 10 cm −1, respectively. The error in reproducing the experimental frequencies of A is 13 cm −1 (BLYP).