Vibrational study of 4‐nitropyridine N‐oxide. III—The charge‐transfer phenomenon as observed in solution

The IR and Raman spectra of various saturated solutions of 4-nitropyridine N-oxide (NPO) and NPO-d4 were recorded in the range 4000–350 cm−1. They were assigned according to previous results for the crystal. A strong Fermi resonance is observed in the νas(NO2) region for solutions of NPO in CCI4, CS2 and C2Br2F4. The hydrogen bonds between a proton of a water or a methanol molecule and the N-oxide group of NPO are strong. These hydrogen bonds favour one of the two resonance forms of NPO and modify the spectra of the saturated solutions of NPO in donor solvents. Comparison with the vibrational frequencies of the crystal also shows that the main component of the crystalline field is a dipol- diple interaction, the strength of which is approximately that of a hydrogen bond between NPO and a methanol or a water molecule.