MICROWAVE SPECTRA OF DEUTERIUM ISOTOPOLOGUES OF {\em cis}-HEXATRIENE

Abstract

Several deuterium isototopologues of {\em cis}-hexatriene have been synthesized in sub-millimole amounts. They include a mixture of the 1,1-d$_{2}$, {\em cis}-1-d$_{1}$, and {\em trans}-1-d$_{1}$ species and a sample of the 2-d$_{1}$ species. Microwave spectra for {\em b}-type transitions were observed in the 12-21 GHz range on the jet-beam, mini-cavity FT instrument at Kent State University. {\em cis}-Hexatriene has a small dipole moment of approximately 0.05 D. For the 1,1-d$_{2}$ species, 11 lines were found, and for each of the two 1-d$_{1}$ species, 9 lines were observed. For the 2-d$_{1}$ species, 10 lines were measured. Ground state rotational constants were fitted to the transitions using quartic centrifugal distortion constants computed with the B3LYP/cc-pVTZ model for each isotopologue. In prior work, rotational constants for the $^{13}$C species of {\em cis}-hexatriene were observed in natural abundance. Thus, we are close to having sufficient experimental information for determining a semiexperimental equilibrium structure for {\em cis}-hexatriene. The principal goal is to assess the effect of pi-electron delocalization on the C$_{6}$ backbone in comparison with butadiene.