STRUCTURE OF 6,7-DIMETHYL-2,3-DIPHENYLQUINOXALINE

C22H18N2, M(r) = 310.4, orthohombic, Pbca, a = 8.013 (2), b = 20.003 (4), c = 20.846 (4) angstrom, V = 3341 angstrom 3, Z = 8, D(x) = 1.234 g cm-3, lambda-(Mo K-alpha) = 0.71069 angstrom, mu = 0.677 cm-1, F(000) = 1312, T = 291 K, R = 0.0667 for 1855 unique reflections. The molecule has dihedral angles of 22.2 (1) and 48.1 (1)-degrees between the best planes of the phenyl rings and the quinoxaline parent. Energy calculations, taking into account the mutual dependence of the twist out of the quinoxaline plane by the two phenyl groups, give the optimal conformation when both groups have torsion angles that are 60-degrees. The twists from this calculated conformation to the experimental one incur about 4.0 kcal mol-1. They are a result of packing forces which are usually estimated to 2 kcal mol-1. (1 kcal mol-1 = 4.2 kJ mol-1.)