The vibrational assignments of the infrared and Raman spectra of the symmetrically substituted 2,3-diphenylquinoxaline and its N,N′-dioxide: Experimental and computational study

2,3-diphenylquinoxaline (DPQ) and 2,3-diphenylquinoxaline N,N′-dioxide (DPQDO) (also known as 2,3-diphenylquinoxaline 1,4-dioxide) were synthesized, and their infrared and Raman spectra were measured. The structural stability and the vibrational spectra of the two quinoxaline molecules were investigated at the DFT-B3LYP and ab initio MP2 levels of calculation with the 6-311G^** basis set. From the calculations, the two molecules were predicted to exist in a non-planar structure with nearly C₂ symmetry. The quinoxaline and the two phenyl rings of the parent compound were calculated to have tilting angles of about 12, 42 and 42 out of the plane of the molecule at the DFT level as compared to the X-ray angles of 12, 37 and 53. The non-planar structure was predicted to be about 5 and 11 kcal/mol lower in energy than the perpendicular structures in the case of the parent and the dioxide, repectively. Complete vibrational assignments of the infrared and Raman spectra of DPQ and DPQDO molecules were provided by combining experimental and theoretical data.