Nitrogen Hyperfine Structure in the Rotational Spectra of Perfluoropyridine and Other Pyridine Derivatives

Abstract

We resolved and analysed the nitrogen nuclear quadrupole hyperfine structure (hfs) in the rotational spectrum of perfluoropyridine C 5 F 5 N. The results are discussed and compared with those of previously measured pyridine compounds.The nitrogen nuclear quadrupole coupling in perfluoropyridine has been reinvestigated by means of microwave Fourier transform (MWFT) spectroscopy [ Perfluoropyridine was first investigated by Doraiswamy and Sharma [6]. They used a conventional Stark modulated microwave spectrometer and reported the rotational and quadrupole coupling constants. The latter indicated an almost cylindrically symmetric electric field gradient tensor at the nitrogen nucleus.The reinvestigation with the higher resolution of our MWFT spectrometer in the frequency range from 8.0 to 12.4 GHz [7] led to more reliable values for the rotational and quadrupole coupling constants and to a confirmation of the symmetry mentioned above.The substance, a colorless, volatile liquid, was purchased from Aldrich Chemie, Steinheim. The spectra were recorded at temperatures of approximately -40 °C and pressures below 1.5 mTorr (0.2 Pa). A sample of the measurements is given in Figure 1, showing the JK_K + = 13 9 4 -13 8 5 and 20 10 , io -209, n rotational transitions, both triply split by the nitrogen nuclear quadrupole coupling. A listing of the recorded transitions is given in Table 1.The hfs analysis was carried out as described in frequencies were evaluated by adding the hfs corrections to the frequencies of the strongest multiplet components.The lower J transition frequencies were then used to refine the rotational constants assuming rigidity of the molecule. We used our programs HT1NQ [8] and DH14KS [9] for the hfs-and^DH9 [10] for the rigid rotor analysis. The results are given in Table 2.The interpretation of the nitrogen nuclear coupling in substituted pyridines follows Gordy and Cook [11], Because the two orbitals which form the N-C bonds are equivalent, there are only three electronic parameters unknown: the ionic character of the a bonds i a , the hybridization parameter al, and the polarization i n of the 7r-bond. From the CNC angle, which is known to be 117° [12] for pyridine and assumed to be the same for the substituted species, a\ can be evaluated to a\ = 0.312 [13].