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Molecular structures of N-Trifluoroacetylpiperidine 1 and 4-Methylpyridine-N-oxide 2 are studied by gas-phase electron diffraction/mass spectrometry (GED / MS) and quantum chemical (QC) calculations. The strong conjugation of the electron lone pair on the nitrogen atom and the double bond of the carbonyl cause molecule 1 to exist solely in the form of structure whereby the trifluoroacetyl group is in an intermediate position. As regards compound 2, the results of quantum chemical calculations and GED showed that molecule 2 has molecular symmetry in the form of CS with the flat pyridine ring. There were no hydrogen atoms on the flat surface of the pyridine ring. Structure parameters obtained from the experiments are appropriate for the parameters from quantum chemical calculation DFT for irreplaceable Noxide molecule. The presence of the CH3 substituent in 2 results in an increase of electron density and a decrease of the ipso angle, and an increase of r (N-O) in comparison with irreplaceable pyridine-N-oxide.