MICROWAVE SPECTRA OF {\em cis}-1,3,5-HEXATRIENE AND ITS $^{13}$C ISOTOPOMERS; AN $r_s$ SUBSTITUTION STRUCTURE FOR THE CARBON BACKBONE

Abstract

Author Institution: Department of Chemistry, University of Virginia, Charlottesville, VA 22904; Department of Chemistry and Biochemistry, Oberlin College, Oberlin, OH 44074Based on the presumed effects of $\pi$-electron delocalization, the adjustments in ``C=C'' and ``C-C'' bond lengths should increase with increasing length of polyene molecules. Thus, the adjustments in these bond lengths relative to localized bond lengths should increase from butadiene to hexatriene. Despite its weak dipole moment (0.05 D estimate from B3LYP/cc-pVTZ model), microwave spectra have been recorded for {\em cis}-1,3,5-hexatriene on the FT pulsed-jet broadband and cavity spectrometers at the University of Virginia. The small inertial defect ($\Delta$ = 0.16700 u\AA$^2$) shows that this molecule is planar. Spectra have also been recorded for the three $^{13}$C isotopomers in natural abundance on the cavity instrument. An $r_s$ substitution structure of the carbon backbone fitted to ground state rotational constants gives 1.343 \AA\ for the C$_1$=C$_2$ bond, 1.457 \AA\ for the C$_2$-C$_3$ bond, and 1.335 \AA\ for the C$_3$=C$_4$ bond in partial agreement with expectations. After synthesis of deuterium isotopomers of hexatriene and investigation of their microwave spectra, we intend to obtain a semi-experimental equilibrium ($r_e$) structure for direct comparison with theoretical predictions and full assessment of the structural effects of $\pi$-electron delocalization