Negative Ion Photoelectron Spectra of Deprotonated Benzonitrile Isomers via Computation of Franck-Condon Factors.

Negative ion photoelectron spectra of ortho ( o -), meta ( m -), and para ( p -) deprotonated benzonitrile ( o -, m -, p -C 6 H 4 (CN) - ) isomers as well as the associated thermochemical values corresponding to deprotonation at o -, m -, and p -positions in C 6 H 5 (CN) are presented. Quantum mechanical results based on the density functional theory (DFT) utilizing the aug-cc-pVQZ basis set indicate that the o -, m -, p -C 6 H 4 (CN) ● radicals have electron affinity values (EAs) of 1.901, 1.778, and 1.789 eV, respectively. The computed Franck-Condon (FC) factors give rise to o -, m -, and p -C 6 H 4 (CN) - negative ion spectra with FC active ring distortion vibrational modes with harmonic vibrational frequencies of ∼450, 760, and 1000 cm -1 as the dominant vibrational progressions. Deprotonation at the o -, m -, and p -positions in C 6 H 5 (CN) results in calculated gas-phase acidity values (Δ acid H 298K o ) of 383.9, 385.7, and 385.3 kcal mol -1 , respectively. The calculated Δ acid H 298K o is in close agreement with the previously reported high-pressure mass spectrometry experimental value of 383.4.0 ± 4.4 kcal mol -1 . The computed Δ acid H 298K o and EAs are utilized to estimate the bond dissociation energy ( DH 298 (H-C 6 H 4 CN)) associated with the formation o -, m -, and p -C 6 H 4 (CN) ● using the negative ion thermochemical cycle: DH 298 (C 6 H 5 CN) = Δ acid H 298K o (H-C 6 H 4 (CN) + EA (C 6 H 5 CN) ● - IP(H). The respective values of DH 298 (H-C 6 H 4 CN) corresponding to the formation of ortho , meta , and para C 6 H 4 (CN) radicals are 114.15, 113.11, and 113.51 kcal mol -1 .