Nuclear quadrupole tensors for Cl-35 and Cl-37 in cis-1-chloro-2-fluoroethylene obtained by detection of perturbation-allowed Delta J=2 and Delta J=3 transitions

The nuclear quadrupole hyperfine structure in rotational transitions of cis-1-chloro-2-fluoroethylene was measured for both Cl-35 and Cl-37 containing isotopomers in the millimeter-wave region. Near degeneracies of the 9(3,7) and 10(2,9) levels of (ClHC)-Cl-35=CFH and of the 19(9,10/11) and 17(10,7/8) levels of (ClHC)-Cl-37=CFH cause strong perturbations of the hyperfine patterns and give rise to perturbation-allowed DeltaJ = 2 and DeltaJ = 3 transitions. Lamb-dip measurements of such perturbed hyperfine patterns and of forbidden DeltaJ = +2 and perturbation-enhanced DeltaJ = 0 transitions of (ClHC)-Cl-35=CFH provided an accurate determination of chi (ab), the off-diagonal element of the inertial nuclear quadrupole tensor. For (ClHC)-Cl-37=CFH, chi (ab) was determined for the first time thanks to the observation of perturbed hyperfine patterns and of forbidden DeltaJ = +3 and DeltaJ = -2 transitions. Additional measurements of hyperfine structures led to improved values for the diagonal elements of the chi tensor of both Cl-35 and Cl-37. Moreover, the complete inertial nuclear quadrupole tensor was evaluated from the field gradient at the chlorine nucleus computed by using the Multi-Configuration Self-Consistent Field (MCSCF) approach and employing basis sets of triple-zeta quality: very good agreement with the experiment was obtained. (C) 2000 Academic Press.