Locally aromatic polycyclic hydrocarbons as potential carriers of infrared emission features

We report B- LYP/ 6- 31G* and B3- LYP/ 6- 31G* density functional theory calculations on a set of polycyclic hydrocarbons, ranging in size from C19H22 to C36H32, combining aromatic ( unsaturated) and aliphatic ( saturated, sp(3)- hybridized carbon) ring systems. These locally aromatic polycyclic hydrocarbons ( LAPHs), generally exhibiting large deviations from planarity, may be considered as intermediate structures between polycyclic aromatic hydrocarbons ( PAHs) and nanodiamonds. Calculated infrared vibrational frequencies are found to be similar to those observed experimentally in spectra of hydrogenated amorphous carbon ( HAC) and other carbonaceous solids. In the C - H stretching region ( similar to 3.1 - 3.6 mum) these species are characterized by strong absorption/ emission within both the aliphatic and aromatic C - H bands. They also show spectral features associated with tertiary C - H. Similar features are evident in calculated spectra of the corresponding ions, which we have characterized in some cases. Ionization results in the particular enhancement of a spectral feature typically seen at similar to 6.4 mum, in the aromatic C - C stretching region. In keeping with previous experimental and theoretical studies on the spectra of neutral and cationic PAHs, we find that the influence of ionization on the relative intensities of C - C and C - H stretching features is much greater than the influence of molecular structure. We suggest that LAPHs may be significant contributors to emission in Type B unidentified infrared emission sources.