Nitrogen Quadrupole Hyperfine Structure in the Rotational Spectrum of 2-,3-,and 4-Fluoro-Benzonitrile.A Comparative High Resolution Microwave Fourier Transform Study

A microwave Fourier transform study of the rotational spectrum of 3-fluoro-benzonitrile was carried out to study the N quadrupole coupling and to give improved rotational constants and centrifugal distortion parameters. In order to fully exploit the high resolution inherent to the experimental technique, frequencies, linewidths, intensities and phases were directly fitted to the observed transient emission signals. The quadrupole coupling constants are discussed in comparison to those of the related molecules 2-fluoro-benzonitrile, 4-fluoro-benzonitrile, and benzonitrile itself. For this comparison a sufficient number of hfs-patterns of the latter molecules was remeasured to derive coupling constants of comparable reliability. The four molecules may be grouped into two pairs. In benzonitrile and in 3-fluoro-benzonitrile the CN-bond shows a smaller deviation from cylindrical symmetry than in 2-and 4-fluoro-benzonitrile.