Vibrational carbon character analysis of astronomical PAHs

Emission features of astronomical spectra typically attributed to aromatic and aliphatic portions of polycyclic aromatic hydrocarbons (PAHs) should not be so clearly binned as previously ascribed. Vibrational carbon character analysis (VCCA) herein shows that the 3.4 μm aromatic infrared emission band (AIB), typically labeled as “aliphatic,” possesses more than 45% non-aliphatic (i.e., aromatic) character for the test case of indene, the smallest astronomically detected PAH. Significant non-aliphatic character is seen as well for the 6.85 and 7.25 μm emission bands of indene, which are also considered to be markers of aliphaticity. VCCA predicts similar behavior for all three emission bands in the case of 2-ethynyltoluene, the proposed interstellar precursor of indene. Although the typical aromatic emission bands (i.e., 3.3 and 6.2 μm bands) exhibit slight aliphatic character for both indene and 2-ethynyltoluene, these bands are primarily aromatic. While these are only two example molecules, if significant character mixing occurs for the emission bands of most potential AIB carriers, current aliphatic/aromatic AIB attributions must be reconsidered.