a b s t r a c tThe molecular structure of 4-methylpiridine-N-oxide, 4-MePyO, has been studied by gas-phase electron diffraction monitored by mass spectrometry (GED/MS) and quantum chemical (DFT) calculations. Both, quantum chemistry and GED analyses resulted in C S molecular symmetry with the planar pyridine ring. Obtained molecular parameters confirm the hyperconjugation in the pyridine ring and the sp 2 hybridization concept of the nitrogen and carbon atoms in the ring. The experimental geometric parameters are in a good agreement with the parameters for non-substituted N-oxide and reproduced very closely by DFT calculations. The presence of the electron-donating CH 3 substituent in 4-MePyO leads to a decrease of the ipso-angle and to an increase of r(N/O) in comparison with the non-substituted PyO. Electron density distribution analysis has been performed in terms of natural bond orbitals (NBO) scheme. The nature of the semipolar N/O bond is discussed.

The molecular structure of 4-nitropyridine N-oxide, 4-NO 2 -PyO, has been determined by gas-phase electron diffraction monitored by mass spectrometry (GED/MS) and by quantum chemical calculations (DFT and MP2). Comparison of these results with those for non-substituted pyridine N-oxide and 4methylpyridine N-oxide CH 3 -PyO, demonstrate strong substitution effects on structural parameters and electron density distribution. The presence of the electron-withdrawing eNO 2 group in para-position of 4-NO 2 -PyO results in an increase of the ipso-angle and a decrease of the semipolar bond length r(N/O) in comparison to the non-substituted PyO. The presence of the electron-donating eCH 3 group in 4-CH 3 -PyO leads to opposite structural changes. Electron density distribution in pyridine-N-oxide and its two substituted compounds are discussed in terms of natural bond orbitals (NBO) and quantum theory atoms in molecule (QTAIM).