SE836: Fully anharmonic IR spectra of neutral polycyclic aromatic hydrocarbons with up to 38 carbons

Interstellar aromatic infrared band (AIB) spectra are showing strong agreement with presently-computed, anharmonic spectra for various types of polycyclic aromatic hydrocarbons (PAH) determined using a novel semi-empirical quantum chemical method. Since the AIB spectrum is a blend of emission spectra from a vast number of different PAHs, likely with an array of different molecular structures, the spectra for a sample set of 31 PAHs ranging in size from naphthalene (C10H8) up to circumbiphenyl (C38H16) are combined giving consideration to almost all PAH structural motifs. The computed, numerical quartic force fields coupled to second-order vibrational perturbation theory clearly confirm that PAH C-H stretches are responsible for the features close to 3000 cm-1 (3.3 um). The shoulder at 3080 cm-1 (3.24 um) of this feature likely also arises from similar motions, notably in small PAHs. Additionally, the present work highlights that the mid-IR features of neutral PAH combination bands could be responsible for the bulk of the plateaus underlying the strong AIBs in the 2000-1000 and the 980-590 cm-1 (5-10 and 11-15 um) ranges. Finally, the 5.25 um feature is confirmed to arise from intense, lower-frequency combination bands as hypothesized nearly 40 years ago. PAHs of all classes are likely to contribute to different portions of interstellar spectra, but more anharmonic PAH spectra are needed to delineate such fully.