Laboratory microwave spectroscopy of the cyanomethyl radical, CH2CN

The microwave spectrum of the cyanomethyl radical (CH2CN) in its ground electronic state ((X) over tilde B-2(1)) has been observed for the lowest three rotational transitions (N = 1-0 to 3-2) using a Fourier-transform microwave spectrometer in combination with a pulsed-discharge nozzle. The observed frequencies may thus be used as precise rest frequencies of the radical with a frequency accuracy of several kilohertz. A total of 92 hyperfine components of the five rotational transitions for both ortho (K-a = 0) and para (K-a = 1) species have been measured, leading to refinement of the hyperfine coupling constants for the nitrogen and hydrogen nuclei of the molecule. One of the transition frequencies for the para species coincides within our experimental accuracy to U39927, observed in the spectral line survey toward TMC-1. This study has confirmed that the cyanomethyl radical has a permanent dipole of approximately 3.5-4 D, which differs from the value reported previously.