Microwave spectrum and molecular structure of the cyclic C3H radical

Abstract

The rotational spectral lines of the cyclic C3H radical in the 2B2 ground electronic state are observed by microwave spectroscopy. The radical is produced in an absorption cell by discharging a mixture of C2H2, CO, and He. The spectral lines of two 13C isotopic species are also observed. The radical is a planar molecule with C2v symmetry, and the rs structure is determined as follows: rs(C–H)=1.0760 Å, rs(C–C)=1.3771 Å, and rs(C–CH)=1.3739 Å. From the observed inertial defect, the vibrational frequency of the C–C antisymmetric vibration is estimated to be about 500 cm−1, which is fairly low for the frequency of the C–C stretching mode. The low vibrational frequency arises from the vibronic interaction between the ground electronic state and the low-lying 2A1 electronic state. On the basis of the hyperfine coupling constants obtained for the 13C species, the unpaired electron is found to be almost evenly distributed among the three carbon atoms. Such delocalization of the unpaired electron is consistent with the observed geometrical structure.