The 1(01)-0(00) and 2(02)-1(01) rotational transitions of (HCCl)-Cl-35 and
(HCCl)-Cl-37 in the (H) over tilde (1)A(') ground vibronic state have been
observed with a Fourier transform millimeter-wave spectrometer. The HCCl mo
lecule is produced by discharging a gaseous sample of CH2Cl2 diluted in Ar
with a pulsed discharge nozzle. The effective rotational constant (B+C)/2,
the centrifugal distortion constant Delta (J), the nuclear quadrupole inter
action constants, and the nuclear-spin rotation interaction constant are de
termined for each isotopic species. The nuclear-spin rotation interaction i
s found to make a significant contribution to the hyperfine structure of th
is molecule, which originates from the relatively low-lying electronic exci
ted state. The nuclear quadrupole interaction tensor is highly asymmetric,
indicating a significant pi character of the C-Cl bond. This can be interpr
eted in terms of the backdonation of pi electrons from the chlorine atom to
the carbon atom. (C) 2001 American Institute of Physics.