ANALYSIS OF THE ROTATIONAL STRUCTURE IN THE HIGH-RESOLUTION INFRARED SPECTRUM OF {\em TRANS}-HEXATRIENE-1-$^{13}${\em C}$_{1}$

Author Institution: Department of Chemistry and Biochemistry, Oberlin College, Oberlin, OH 44074; Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA 99352Hexatriene-1-$^{13}${\em C}$_{1}$ was synthesized by reaction of 2,4-pentadienal and (methyl-$^{13}${\em C})-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 was analyzed. For this species the bands at 1010.7 and 893.740 cm$^{-1}$ yielded composite ground state rotational constants of {\em A}$_{0}$ = 0.872820(1), {\em B}$_{0}$ = 0.0435868(4), and {\em C}$_{0}$ = 0.0415314(2) cm$^{-1}$. The ground state rotational constants for the 1-$^{13}$C species were also predicted with Gaussian 03 software and the B3LYP/cc-pVTZ model. After scaling by the ratio of the observed and predicted ground state rotational constants for the normal species, the predicted ground state rotational constants for the 1-$^{13}$C species agreed within 0.005 \% with the observed values. Similar good agreement between observed and calculated values (0.016 \%) was found for the three $^{13}$C species of the {\em cis} isomer. We conclude that ground state rotational constants for single heavy atom substitution can be calculated with adequate accuracy for use in determining semi-experimental equilibrium structures of small molecules. It will be unnecessary to synthesize the other two $^{13}$C species of {\em trans}-hexatriene