Ortho-, Meta- , and Para- Cyanophenol Radical Anions: Voltammetric Characterization, Insights from Computed Negative Ion Photoelectron Spectra, and Computational Thermochemistry of the Three Cyanophenol Isomers

The electrochemical reductions of ortho-, meta-, and para-cyanophenols (o-, m-, and p-CNPs) in N,N-dimethylformamide (DMF) have revealed that the initially formed cyanophenol radical anions (CNP•–) are rapidly followed by protonation, where H+ is transferred from parent CNP molecules to form cyanophenoxide anions (CNP–). In anodic oxidation, the initial one-electron, one-proton oxidation steps are rapidly followed by deprotonation to form o-, m-, and p-cyanophenoxyl radicals (CNP•), generating irreversible anodic waves. The m-CNP isomer exhibits a slightly lower oxidation peak potential (Epa) than the ortho and para, leading to a deviation from the expected positive linear correlation between Epa and the adiabatic electron affinities (EAs) of the CNP•. In the second part of this study, computed photodetachment spectra for the CNP•– are utilized to shed light on the nature of the vibrational structures of the parent CNP molecules after an electron is detached from CNP•– in a one-electron-transfer process. The spectra are obtained via the calculation of Franck–Condon (FC) factors that are based on a double harmonic oscillator model that utilizes the density functional theory (DFT) outputs at the B3LYP/Aug-cc-pVQZ level of theory while considering the Duschinsky rotation effects. The calculated adiabatic EAs for the ground electronic singlet states (S0) in o-, m-, and p-CNP are found to be 0.3106, 0.3282, and 0.1674 eV, with respective singlet–triplet energy gaps (ΔEST) of 3.1264, 3.0268, and 3.1186 eV. The presence of the CN group decreases the gas-phase deprotonation enthalpies for the three isomers to 1395 kJ/mol (ortho); 1399 kJ/mol (meta); and 1380 kJ/mol (para), relative to that of the bare phenol at 1458 kJ/mol.