Sublimation Enthalpies of Substituted Pyridine N-Oxides

The enthalpies of sublimation of five substituted pyridine N- oxides were determined by the Knudsen effusion method with mass spectrometric control of the vapor composition within the framework of the second law of thermodynamics. The sublimation enthalpy of mono-substituted compounds 4-X-PyO depends on the nature of the substituent X and increases in the order CH 3 →NO 2 →OCH 3 . A difference is noted in the nature of dissociative ionization of disubstituted derivatives 2-CH 3 -4-NO 2 PyO and 3-CH 3 -4-NO 2 PyO. The relationship between the packing of molecules in crystals and the Δ H ° subl values is considered.