Microwave rotational spectrum and ab initio computations on 4-cyanopyridine: Molecular structure and hyperfine interactions

The cm-wave rotational spectrum of 4-cyanopyridine was studied on a pulsed supersonic jet-expansion Fourier-transform microwave spectrometer. The ground state rotational parameters for the parent and four ¹³C and two ¹⁵N isotopologues were determined with high accuracy in natural abundance including all quartic centrifugal distortion constants and nuclear quadrupole coupling parameters of the two nitrogen nuclei. With the study of both singly substituted ¹⁵N species, the nuclear origins of the hyperfine structures could be differentiated and the respective constants identified unambiguously. Different approaches have been used to determine the structure: empirical (r_s), ab initio and semi-experimental (r^se _e) methods. The semi-experimental approach improves the accuracy significantly, whereas the Kraitchman's method fails again.