Infrared spectroscopy of matrix-isolated neutral polycyclic aromatic nitrogen heterocycles: The acridine series

The matrix-isolated, mid-infrared spectra of seven acridine-based polycyclic aromatic nitrogen heterocycles (PANHs) have been measured and compared to their non-nitrogen containing parent molecule. The acridine species investigated include acridine, benz[a]acridine, benz[c]acridine, dibenz[a,j]acridine, dibenz[c,h]acridine, dibenz[a,h]acridine and dibenz[a,c]acridine. The previously reported results for 1 and 2-azabenz[a]anthracenes are included for comparison. The experimentally determined band frequencies and intensities are compared with their B3LYP/6-31G(d) values. The overall agreement between experimental and theoretical values is good and in line with our previous investigations. Shifts, typically to the blue, are noted for the C-H out-of-plane (CH_oop) motions upon insertion of a nitrogen atom. The formation of a bay region upon addition of additional benzene rings to the anthracene/acridine structure splits the solo hydrogen motions into a bay region solo and an external solo hydrogen, with the bay region solo hydrogen coupling to the quartet hydrogen motions and the external solo hydrogen coupling with the duo hydrogen motions resulting in an extreme decrease in intensity for the CH_oop solo hydrogen band when the external hydrogen is replaced by a nitrogen atom. The C-C and C-H in-plane region of this acridine series exhibits the characteristic two fold increase in intensity, noted previously for PANHs. The strong ≈1400cm^-1 band, which was identified in the previous PANH study, is noted in several molecular species as well as another strong PANH feature between 1480 and 1515cm^-1 for several molecules. The presence of these strong bands appear to be primarily responsible for the two-fold increase in the C-H in-plane region's (1100-1600cm^-1) intensity. The C-H stretching region can be characterized by contributions from the solo (bay or external), duo and quartet hydrogens, similar to what was observed in the dibenzopolyacene compounds.