Experimental and Theoretical IR Spectra of 2-Nitrosopyridines

Previously unknown IR spectra of 2-nitrosopyridines 1-3 were recorded in the solid phase after cryogenic photolysis of their dimers, and identified by the comparison with the corresponding theoretical spectra. Optimal results were obtained with double-hybrid (DH) density functionals and def2-TZVP basis set, the best being DSD-PBEP86-D3(BJ), B2GP-PLYP-D3(BJ) and mPW2-PLYP-D2. Experimental frequencies were obtained by scaling harmonic frequencies with appropriate scaling factors, either taken from the literature or calculated using modified HFREQ2014 dataset. In the case of DH functionals, anharmonic frequencies do not offer any advantage in comparison with the scaled harmonic ones. Unexpectedly, all non-DH density functionals, including highly praised omega B97X-V and omega B97M-V, have serious problems with normal modes containing N=O bond stretching vibration. It was concluded that the origin of the problem is in the nitroso group itself, most likely caused by its multireference character. The best results among non-DH functionals were achieved at TPSSh-D3(BJ)/def2-TZVP level of theory.