Pressure shifts of acetonitrile ground state parameters

The rotational spectrum of methylcyanide (acetonitrile) in the ground vibrational state was measured in the spectral region from 50 to 275 GHz using the new Prague millimeterwave spectrometer that was built in close cooperation with the Gisbert Winnewisser Cologne group. Resolved or partly resolved quadrupole hyperfine structures of the rotational transitions with lower quantum numbers as well as with low differences of quantum numbers J–K were observed. All rotational transitions were systematically measured at different sample pressures (at least in interval 1–15 μbar) to determine a pressure dependence of the corresponding rotational transition frequencies. Experimental accuracies of well developed lines using both the Doppler limited as well as sub-Doppler saturation layouts were better than 5 kHz. Such accuracies made possible to observe a reproducible linear pressure dependencies of all transition frequencies and to study the pressure dependence of rotational, centrifugal distortion and hyperfine parameters. The results obtained in this study indicate a molecular shrinking with the increasing pressure.