Theoretical prediction of gas-phase infrared spectra of imidazo[1,2-a]pyrazinediones and imidazo[1,2-a]imidazo[1,2-d]pyrazinediones derived from glycine.
Imidazo[1,2-a]pyrazine-3,6-diones and imidazo[1,2-a]imidazo[1,2-d]pyrazine-3,8-diones can be produced by pyrolysis of simple amino acids. While such bicyclic and tricyclic amidines were detected and characterized by IR spectroscopy for some alpha-substituted amino acids, the parent systems composed of glycine fragments are unknown up to now. IR spectra for five amidines derived from glycine were calculated by using different semi-empirical (PM3, AM1, MNDO and MINDO/3), HF, and hybrid DFT (B3LYP, B3P86 and B3PW91) methods in conjunction with 6-31G(d) basis set (for HF and DFT). Vibration frequencies in the experimental IR spectra were predicted based upon the B3LYP data, by correcting the calculated wavenumbers by a scaling factor of 0.959. The behavior of most characteristic bands (nu(CX), nu(NH), etc.) and their shifts with respect to such bands in the spectra of alanine and alpha-aminoisobutyric acid derivatives studied before, are discussed. Performance of the semi-empirical methods was tested, bearing in mind possible future needs for IR spectra predictions for larger molecular systems of similar chemical nature; the use of MINDO/3 and MNDO is recommended. A basis set effect on the B3LYP fundamental vibration frequencies for hexahydroimidazo[1,2-a]pyrazine-3,6-dione was studied by varying Pople basis sets from minimal STO-3G to 6-311++G(d, p). No significant improvements were found beyond the 6-31G(d) basis set, which thus can be recommended to predict IR spectra for the amidines and similar molecules.
Contreras-Torres
Flavio F
FF
Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, Circuito Exterior C.U., A. Postal 70-543, 04510 México D.F., Mexico.
Basiuk
Vladimir A
VA
D000578:Amidines / Q000737:chemistry*; D003500:Cyclization; D005740:Gases / Q000737:chemistry*; D005998:Glycine / Q000737:chemistry*; D006859:Hydrogen / Q000737:chemistry; D007093:Imidazoles / Q000737:chemistry*; D015394:Molecular Structure; D044367:Phase Transition; D013055:Spectrophotometry, Infrared; D014732:Vibration
D000578:Amidines; D005740:Gases; D007093:Imidazoles; D006859:Hydrogen; D005998:Glycine