ANALYSIS OF ROTATIONAL STRUCTURE IN THE HIGH-RESOLUTION INFRARED SPECTRA OF THE {\em TRANS}-HEXATRIENE-1,1-{\em D}$_{2}$ AND -{\em CIS}-1-{\em D}$_{1}$ SPECIES

Abstract

Hexatriene-1,1-{\em d}$_{2}$ with some admixture of the {\em cis}-1-{\em d}$_{1}$ and {\em trans}-1-{\em d}$_{1}$ species was synthesized by reaction of 2,4-pentadienal and (methyl-{\em d}$_{3}$)-triphenylphosphonium iodide (Wittig reagent). The {\em trans} isomer was isolated by preparative gas chromatography, and the high-resolution (0.0015 cm$^{-1}$) infrared spectrum was recorded on a Bruker IFS 125HR instrument. The rotational structure in two C-type bands for the 1,1-{\em d}$_{2}$ species was analyzed. For this species the bands at 902.043 and 721.864 cm$^{-1}$ yielded composite ground state rotational constants of {\em A}$_{0}$ = 0.801882(1), {\em B}$_{0}$ = 0.041850(2), and {\em C}$_{0}$ = 0.039804(1) cm$^{-1}$. For the {\em cis}-1-{\em d}$_{1}$ species the C-type band at 803.018 cm$^{-1}$ gave {\em A}$_{0}$ = 0.809384(2), {\em B}$_{0}$ = 0.043530(3), and {\em C}$_{0}$ = 0.041321(2) cm$^{-1}$. By iodine-catalyzed isomerization, we have obtained some of the much less favored {\em cis} isomer and hope to obtain microwave spectra for its three deuterium-substituted species. The rotational constants reported here contribute to data needed for determining a semi-experimental structure for {\em trans}-hexatriene, which should show that the structural consequences of {\em pi}-electron delocalization increase with the chain length of polyenes.