Crystal, Molecular Structure and DFT Study of 2,6,7-Trimethyl-3-p-tolylquinoxaline and 2-(4-Methoxyphenyl)-3,6,7-trimethylquinoxaline

Abstract

The molecular structure of 2,6,7-trimethyl-3-p-tolylquinoxaline (hereafter denoted I) was confirmed by single crystal X-ray structure analysis. In the title compound, C18H18N2, I, the quinoxaline ring system is essentially planar. The phenyl ring at the 2-position makes a dihedral angle of 40.23° with the quinoxaline mean plane. In the crystal, molecules are linked into layers mainly by H2C–H···π and (π)C–H···π interactions. Stacking π···π interactions are absent in the structure. The experimental and theoretical structural aspects of I and the recently reported methoxy derivative, 2-(4-methoxyphenyl)-3,6,7-trimethylquinoxaline, II, have been investigated and compared. Experimental geometric parameters (bond lengths and bond angles and dihedral angles) were compared with computed values from density functional theory with B3LY6−31G(d,p) basis set. Results show that values are in good agreement. The highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) energies have been also calculated and used to determine the charge transfer within the molecule. MEP surfaces analyses were performed by the DFT method. From the MEP map, negative region is mainly localized over the electronegative nitrogen atoms (in I), and the nitrogen and oxygen atoms (in II) and the maximum positive region is localized on the methyl groups. These surfaces seem to correlate well with the intermolecular interactions in the lattice. Time-dependent density functional theory has been carried out to study the UV spectra of both molecules in gas phase and in methanol using polarizable continuum model and the results are presented and discussed. Crystal, molecular structure, crystal supramolecularity and DFT calculations of 2,6,7-Trimethyl-3-p-tolylquinoxaline and 2-(4-Methoxyphenyl)-3,6,7-trimethylquinoxaline are reported.