From Benzonitrile to Dicyanobenzenes: The Effect of an Additional CN Group on the Thermochemistry and Negative Ion Photoelectron Spectra of Dicyanobenzene Radical Anions.

The negative ion photoelectron spectra of 1,2-dicyanobenzene ( o -DCNB), 1,3-dicyanobenzene ( m -DCNB), and 1,4-dicyanobenzene ( p -DCNB) radical anions (DCNB· - ), acquired through the computation of Frack-Condon (FC) factors, are presented. The FC calculations utilize harmonic frequencies and normal mode vectors derived from density functional theory at the B3LYP/aug-cc-pVQZ basis set. All the totally symmetric vibrational modes are treated with Duschinsky rotations to yield neutral DCNBs in their singlet (S o ) and lowest triplet (T 1 ) states, following an electron removal from the doublet anionic ground state. For the S o state, the adiabatic electron affinities (EAs) for o- , m- , and p- DCNB are 1.179, 1.103, and 1.348 eV. The EAs for the lowest T 1 state in o- , m- , and p- DCNB are 4.151, 4.185, and 4.208 eV, resulting in an S o -T 1 energy difference (Δ E ST ) of 2.973, 3.082, and 2.860 eV. A vibrational analysis reveals evidence of FC activity involving ring distortion, C-N bending, and ring C═C stretching vibrational progressions in both the S o and T 1 states. With the detection of cyanonaphthalene (C 10 H 7 CN) and cyanoindene (C 9 H 7 CN) in the interstellar medium (ISM), our results highlight the extent to which replacing a single hydrogen on an aromatic molecule with a cyano group, C≡N, can alter the vibrational structure of the molecule/radical anion. As such, dicyano-polyaromatic hydrocarbons may be reasonably robust in the ISM, making it appealing to search for them in future interstellar detection missions.