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-hexamethylhexahydroimidazo[1,2-a]pyrazine-3,6-dione were computed by the HF, B3LYP, B3P86 and B3PW91 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 observed experimentally decreases in the series HF, B3LYP, B3PW91 and B3P86; for 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 spectrum. The correlation between the calculated and experimental vibration frequencies is characterized by the coefficients higher than 0.9999 for all three DFT methods; for HF it is slightly lower than 0.9995. The calculated absolute band intensities satisfactory match the observed relative intensities as well. Optimal uniform scaling factors calculated are 0.8973, 0.9593, 0.9540 and 0.9552 for HF, B3LYP, B3P86 and B3PW91, respectively.
Basiuk
V A
VA
Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de Mexico, DF, Mexico. basiuk@nuclecu.unam.mx
D007093:Imidazoles / Q000737:chemistry*; D008432:Mathematical Computing; D008958:Models, Molecular; D015394:Molecular Structure; D011719:Pyrazines / Q000737:chemistry*; D013055:Spectrophotometry, Infrared / Q000379:methods
C441230:2,2,5,5,8,8-hexamethylhexahydroimidazo(1,2-a)-pyrazine-3,6-dione; D007093:Imidazoles; D011719:Pyrazines