The molecular structure of 4-methylpyridine-N-oxide: Gas-phase electron diffraction and quantum chemical calculations

Abstract

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.

Molecular structure and electron distribution of 4-nitropyridine N-oxide: Experimental and theoretical study of substituent effects

“…Table 4 compares the structure of pyridine-N-oxide and its two substituted obtained by GED, X-ray and DFT. Taking into account that the structural parameters obtained by different methods have different physical meaning, we can note a good agreement of the values obtained in our research (therein and [24]) with the theoretical results (therein and [21]) and, also, with the parameters for the crystals [46,47] in contrast to the values recommended by Chiang J. F. et al [20]. Apparently, some increase of the semipolar N/O bond lengths in a crystal phase compared to the free molecules is due to intermolecular hydrogen bonding.…”

“…This allows us to determine the structural parameters more precisely compared to the studies performed in 1982. The experimental structural data for 4-methylpyridine-Noxide obtained in our study [24] are in a good agreement with the theoretical parameters as well as the experimental X-ray data.…”

“…Unfortunately, large uncertainties in GED parameters for PyO [19] do not allow a comparison of the experimental structures. Thus, we can compare only the values obtained in our GED study (therein and [24]) and the calculated values. The structural data for free molecules in Table 4 show that the insertion of the methyl or nitro group in para e position to the PyO has no significant effect on the structural parameters of the pyridine ring, excluding only the ipsoangle : C2C1C5, which is changes from 117.7 in PyO to 115.9 in 4-Me-PyO and 119.9 in 4-NO 2 -PyO (B3LYP/aug-cc-pVTZ).…”

“…et al [19,20] and other methods motivated us to perform a new gas-electron diffraction study of 4substituted-N-oxide. The first GED reinvestigation was performed for 4-methylpyridine-N-oxide [24] because of the largest structural contradictions for this molecule. Now more sophisticated methods for the analysis of GED data are available, including the possibility of quantum chemical results to be used in the interpretation of the experimental data.…”

Sublimation Enthalpies of Substituted Pyridine N-Oxides

“…Our quantum-chemical study of a number of N- oxides [ 17 ] made it possible to detect the effect of donor and acceptor substituents on some characteristics of the N→O bond, whereas the electron diffraction study of 4-CH 3 PyO 1 [ 18 ] and 4-NO 2 PyO 2 [ 19 ] molecules helped to refine their structure (Fig. 1 ).…”