Benchmark DFT studies on C-CN homolytic cleavage and screening the substitution effect on bond dissociation energy.

Nitriles are important chemical species in organic transformations, material chemistry, and environmental sciences. Nitriles are used as cyanating reagents in many organic reactions, where the C-CN bond dissociation has an important role. The reactivity of nitriles can be better understood by studying the bond dissociation energy (BDE) of the C-CN bond. In this benchmark study, homolytic cleavage of the C-CN bond in 12 nitrile compounds is studied. Thirty-one functionals from eight different DFT classes along with three types of basis sets are employed. Theoretical results are compared with the available experimental data. Based on statistical outcomes, the CAM-B3LYP functional of the range separated hybrid GGA class with Pople 6-311G(d,p) basis set provides the most accurate results for calculating the BDE of the C-CN bond. The mean absolute error (MAE) value is 0.06 kcal mol, whereas standard deviation (SD) and Pearson's correlation (R) are 2.79 kcal mol and 0.96, respectively, when compared with experimental data. The substitutional effect on the homolytic cleavage (BDE) of respective bonds in differently substituted nitriles is also investigated. The BDE results indicate that electron withdrawing groups (EWGs) lower the BDE, while electron donating groups (EDGs) increase the BDE of the C-CN bond. The NBO and HOMO-LUMO orbitals analyses are also performed to further elaborate the variational BDE patterns of C-CN bond cleavage. Graphical Abstract Benchmark DFT studies on C-CN homolytic cleavage.