We reanalysed the 14N hyperfine structure in the rotational spectra of pyridine and [4-D]-pyridine with higher precision and determined the D quadrupole coupling. It is intended to provide a reference for comparison with substituted pyridines.We investigated by microwave Fourier transform (MWFT) [1] spectroscopy the rotational spectra of a number of substituted pyridines. The aim was a study of substitutional effects reflected by the I4N quadrupole coupling. Therefore, high precision was necessary to analyse the hyperfine structure (hfs) in duced by the 14N nucleus and other nuclei with quadrupole moments.To base the investigation on comparable experi mental methods and analyses we remeasured as reference pyridine, C5H6N, itself, which has been the subject of many investigations, the last by So rensen et al. [2,3], In this publication we present the results for pyridine and [4-D]pyridine [3,4], which is the molecule with the least substitutional effect. The data for the other substituted pyridines (4-chloropyridine, 4-cyanopyridine, and perfluoropyridine) will be given in forthcoming publications together with a discussion of the quadrupole cou pling constants.We used our MWFT spectrometers in the range from 5.3 to 26.4 GHz described elsewhere [5][6][7][8].Dry pyridine, 99.5%, was purchased from Merck, Darmstadt, and used without further treatment.[4-D]Pyridine was in our stock for more than 14 years. It was purchased from Merck, Sharp and Dohme, Canada.The spectra of both of the pyridines were re corded at temperatures of -2 5°C to -4 5°C and pressures below 0.2 Pa (1.5m Torr). The measure ments are given in Tables 1 a and A first order hfs analysis [10] proved to be suffi cient. The rotational constants were taken from [3]. They are given with the determined coupling con stants / gg, g = a, b, c in Table 2
We reinvestigated the quadrupole coupling of benzonitrile by an improved technique of microwave Fourier transform (MWFT) spectroscopy and evaluation method. The error of the coupling constants was reduced by one order of magnitude.
The complicated nuclear quadrupole hyperfine structure and methyl torsional fine structure in the rotational spectra of N,N-dimethylformamide and N-nitrosodimethylamine have been studied using microwave Fourier transform spectroscopy. It has been found that both molecules are rather similar in terms of their parameters of methyl group internal rotation as well as in terms of their amino nitrogen quadrupole coupling constants.
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