Calculated gas-phase infrared spectra of 2,2,5,5,8,8-hexamethylhexahydroimidazo[1,2-a]pyrazine-3,6-dione: relative performance of Hartree-Fock and hybrid density functional theory methods

Geometric parameters and IR spectra of 2,2,5, 5,8,8-hexamethylhexahydroimid azo[1,2-a]pyrazine-3,6-dione were computed by the HF, B3LYP, B3P86 and B3PW 91 methods with the 6-31G(d) basis set. All the computation methods produce a boat conformation of the six-membered ring with the orientation opposite to the one experimentally observed, that is obviously due to incapability of such calculations to take into account mutual influence of the molecules in a crystal package. Quality of predicting the interatomic distances obse rved experimentally decreases in the series HF, B3LYP, B3PW91 and B3P86; fo r bond angles, in the series B3LYP, P3PW91, B3P86 and HF. At the same time, the IR spectra computed are in a good agreement with the observed FTIR spe ctrum. The correlation between the calculated and experimental vibration fr equencies is characterized by the coefficients higher than 0.9999 for all t hree DFT methods; for HF it is slightly lower than 0.9995. The calculated a bsolute band intensities satisfactory match the observed relative intensiti es as well. Optimal uniform scaling factors calculated are 0.8973, 0.9593, 0.9540 and 0.9552 for HF, B3LYP, B3P86 and B3PW91, respectively. (C) 2001 E lsevier Science B.V. All rights reserved.