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

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We present the experimental set up of a microwave Fourier-transform spectrometer for the entire K band from 18 to 26.4 GHz. It is hereby shown that this type of spectroscopy is achieved above 18 GHz. Advantages and disadvantages are discussed. The performance is illustrated by the recordings of rotational transitions of various molecules.

The microwave spectrum of benzonitrile, C 6 H 5 CN, was thoroughly investigated by several authors [1 -5]. The restructure, the dipole moment and centrifugal distortion constants have been determined. For the nitrogen-hfs only an estimate exists [4]. By use of Microwave Fourier transform Spectroscopy [6, 7] we were able to investigate the nitrogen-hfs. Benzonitrile 99°/o was purchased from Merck, Darmstadt, and used after vacuum distillation. The spectra were taken at -50 to -60 °C and pressures down to 0.3 mT. The assignment of the measured lines was checked by a centrifugal distortion analysis [8, 9] using the intensity weighted mean of the hfs-components. 47 lines of Table 1 and Table 2 of [4] were fitted with F = 5 -4 7-6 6-5 Fig. 1. Nitrogen-hfs of the K-dublett 642 -541, 643 -542 measured with MWFT spectroscopy. Transient emission recorded in steps of 50 ns for 1024 points. Measuring time 10 min. for a 10 MHz range. Zeros up to 2048 points were added prior to the Fourier transformation. Spectral points at 10 kHz distance. Frequencies calculated by three point interpolation Lorentz profile assumed. Pressure 0.1 mT, temperature -57 °C.

An investigation of the 14N quadrupole hyperfine structure in the rotational spectrum of n-propyl isocyanide is presented. It was possible to resolve the multiplets by using microwave Fourier transform spectroscopy.