Infrared Spectroscopy of Matrix-Isolated Polycyclic Aromatic Compounds and Their Ions. 6. Polycyclic Aromatic Nitrogen Heterocycles

Abstract

The matrix-isolation technique has been employed to measure the mid-infrared spectra of several polycyclic aromatic nitrogen heterocycles in both neutral and cationic forms. The species studied include: 7,8-benzoquinoline (C 13 H 9 N), 2-azapyrene (C 15 H 9 N), 1- and 2- azabenz[ a ]anthracene (C 17 H 11 N), and 1-, 2-, and 4-azachrysene (also C 17 H 11 N). The experimentally measured band frequencies and intensities for each molecule are tabulated and compared with their calculated values computed using density functional theory at the B3LYP/4-31G level. The overall agreement between experiment and theory is good, in keeping with previous investigations involving the parent aromatic hydrocarbons. Several interesting spectroscopic trends are found to accompany nitrogen substitution into the aromatic framework of these compounds. For the neutral species, the nitrogen atom produces a significant increase in the total integrated infrared intensity across the 1600−1100 cm -1 region and plays an essential role in the molecular vibration that underlies an uncharacteristically intense, discrete feature that is observed near 1400 cm -1 in the spectra of 7,8-benzoquinoline, 1-azabenz[ a ]anthracene, and 4-azachrysene. The origin of this enhanced infrared activity and the nature of the 1400 cm -1 vibrational mode are explored. As a secondary result of the computations, the computed dipole moments and rotational constants for the species under study are reported. The dipole moments calculated are significantly stronger than those of the parent PAH and in principle could facilitate interstellar detection of these PAH related compounds.