Pure rotational transitions of a new carbon-chain radical, CCCl, a chlorine derivative of CCH, have been observed for the first time by Fourier-transform microwave spectroscopy. The radical has been produced in a supersonic free jet by a pulsed discharge in CCl4 diluted to 0.3% with Ne. Transitions with spin splittings were observed for two isotopomers, (CCCl)-Cl-35 and (CCCl)-Cl-37, in the regions at 12.6 GHz for N=1-0 and 25.2 GHz for N=2-1, respectively. The radical shows a spectral pattern for a molecule with (2)Sigma symmetry as is the case for CCH. Hyperfine splittings due to the Cl nucleus were also clearly resolved. The molecular constants have been precisely determined for the two isotopomers. Ab initio calculations at the MRCI level with the cc-pVTZ basis set have revealed that the first excited electronic state corresponding to the (2)Pi state at linear geometry is very close to the ground electronic state, and the two states are more strongly interacting with each other than the case of CCH. Based on the results of the ab initio calculations and the determined hyperfine constants, it was found that a conical intersection exists due to a strong vibronic coupling in the vicinity of the ground state, and as a result the CCCl radical has a bent structure in the ground state. (C) 2003 American Institute of Physics.