Analysis of the rotational structure in the high-resolution infrared spectra of trans-hexatriene-1,1-d2 and -cis-1-d1

Mixtures of trans-hexatriene-1,1-d2, -cis-1-d1, and -trans-1-d1 have been synthesized. Anharmonic frequencies and harmonic intensities were predicted with the B3LYP/cc-pVTZ model for the out-of-plane (a″) modes of the three isotopologues. Assignments are proposed for most of the a″ vibrational modes above 500 cm−1. Ground state (GS) rotational constants have been determined for the 1,1-d2 and cis-1-d1 species from the analysis of rotational structure of C-type bands in the high-resolution (0.0015 cm−1) infrared spectra in a mixture of the three isotopologues. The GS constants for the 1,1-d2 species are A0 = 0.8018850(6), B0 = 0.0418540(6), and C0 = 0.0397997(4) cm−1. The GS constants for the cis-1-d1 species are A0 = 0.809388(1), B0 = 0.043532(2), and C0 = 0.041320(1) cm−1. Small inertial defects confirm planarity for both species. These ground state rotational constants are intended for use in determining a semiexperimental equilibrium structure and evaluating the influence of chain length on π-electron delocalization in polyenes.