Pinpointing the CN Stretch Frequency of Neutral Cyano-Polycyclic Aromatic Hydrocarbons: A Laboratory and Quantum Chemical Spectroscopic Study of 9-Cyanoanthracene

The CN stretch frequency of neutral, gas-phase 9-cyanoanthracene is 2207 cm^–1 (4.531 μm) based on high-resolution infrared absorption experiments coupled with a new hybrid anharmonic quantum chemical methodology. A broad band (full-width at half-maximum of 47 cm^–1) is observed and assigned to multiple transitions, including the CN stretch fundamental and various combination bands that gather intensity from strong anharmonic coupling with the bright CN stretch. The new hybrid computational approach utilizes the harmonic force constants from the double-hybrid rev-DSDPBEP86 functional that includes MP2 electron correlation, and the cubic and quartic force constants from the B3LYP density functional. In combination, this method computes a band center of 2207 cm^–1 for 9-cyanoanthracene, a direct match with experiment. Further, the hybrid method produces a difference of less than 1 cm^–1 for the two isomers of cyanonaphthalene and cyanobenzene. As shown from comparison with CCSD(T)-F12b anharmonic frequency computations of cyanobenzene, inclusion of electron correlation is required to properly characterize the electronic structure of the highly electron withdrawing CN group on polycyclic aromatic hydrocarbons. In agreement with earlier studies, computation of the CN stretch of 14 small CN-PAHs produces a narrow (∼20 cm^–1) band from 2207–2229 cm^–1 (4.53–4.48 μm). The remainder of the spectrum below 2000 cm^–1 and from 3000–3120 cm^–1 shows good agreement between experiment and the hybrid theory with a mean absolute error of 16 and 14 cm^–1, respectively.