Anion Photoelectron Spectroscopy and Thermochemistry of Deprotonated Benzonitrile Isomers

Abstract

In this study, the negative ion photoelectron spectra of ortho, meta and para deprotonated benzonitrile anionic (o-,m-,p-C6H4(CN)¯) isomers as well as the corresponding thermochemical values of the o-,m-,p-C6H5(CN) isomers are reported. Quantum mechanical calculations based on density functional theory (DFT) with 6-311++G** and the aug-cc-pVQZ basis sets show that the o-,m-,p-C6H4(CN)¯ have electron affinity (EA) values of 1.901, 1.778, and 1.789 eV respectively. The computed Franck-Condon factors obtained using the PESCAL program results in o-,m-,p-C6H4(CN) vibrational structures that have several dominant active vibrational modes: a ring breathing mode around 1600 cm-1, a ring deformation mode at 630 cm-1 and a low frequency butterfly mode at 160 cm-1. Deprotonation at the ortho position gives a calculated gas-phase acidity value of 1600 kJ/mol. The calculated value is in close agreement with the previously reported high-pressure mass spectrometry experimental value of 1603.0 ± 10.0 kJ/mol. The enthalpy of deprotonation of benzonitrile (?acidH298(C6H5CN)), the C-H bond dissociation energy (DH298(H-C6H4CN)), the ionization energy of Hydrogen (IP(H)) and the EAs of the o-,m-,p-C6H4(CN) radicals are related to each other through the negative ion thermochemical cycle:?acidH298(C6H5CN) = DH298(H-C6H4CN)+IP(H)-EA(C6H5CN). The EA values of the o-,m-,p-C6H4(CN) are therefore combined with the ?acidH298(C6H5CN) to obtain values for the C-H bond enthalpy of o-,m-,p-C6H5(CN) at 298K.