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 ΔH°(subl) values is considered.