Microwave Spectra of cis-1,3,5- Hexatriene and Its 13 C Isotopomers; An r s Substitution Structure for the Carbon Backbone Richard D. Suenram, Brooks H. Pate, Alberto Lessari, Justin L. Neill, and Steven Shipman Department of Chemistry, University of Virginia, Charlottesville, VA Norman C. Craig, Robin A. Holmes, and Matthew C. Leyden Department of Chemistry and Biochemistry, Oberlin College, Oberlin, OH

GOAL: determine the semi-experimental equilibrium (r e ) structure of cis-1,3,5-hexatriene. This structure will test the expectation that p- electron delocalization increases with chain length in conjugated polyenes. Thus, the lengthening of the “C=C” bonds and the shortening of the “C–C” bonds should be greater in hexatriene than in butadiene. RESULT: Although cis-hexatriene has a small dipole moment, the microwave spectra of the normal species and the 13 C isotopomers in natural abundance have been recorded, and a provisional r s /r e structure for the C 6 backbone has been determined. The overall goal of this investigation is determining the semi-experimental equilibrium (re) structure of cis-1,3,5-hexatriene. This structure will test the expectation that -electron delocalization increases with chain length in conjugated polyenes. Thus, the lengthening of the “C=C” bonds and the shortening of the “C–C” bonds should be greater in hexatriene than in butadiene. Although cis-hexatriene has a small dipole moment, the microwave spectra of the normal species and the 13C isotopomers in natural abundance have been recorded, and a provisional rs substitution structure for the C6 backbone has been fitted.The overall goal of this investigation is determining the semi-experimental equilibrium (re) structure of cis-1,3,5-hexatriene. This structure will test the expectation that -electron delocalization increases with chain length in conjugated polyenes. Thus, the lengthening of the “C=C” bonds and the shortening of the “C–C” bonds should be greater in hexatriene than in butadiene. Although cis-hexatriene has a small dipole moment, the microwave spectra of the normal species and the 13C isotopomers in natural abundance have been recorded, and a provisional rs substitution structure for the C6 backbone has been fitted.

Preliminaries

Synthesis

Initial scans were recorded on the new broadband ( GHz) chirped-pulse Fourier transform microwave spectrometer with jet-beam-cooled sampling in the Brooks Pate Laboratory. The MW excitation pulse is a linear sweep, chirped-pulse of 1  s. The FID is acquired for 20  s. At a rep rate of 3.5 Hz, 10,000 scans were accumulated in 45 min. Subsequent spectroscopy, including all the work on the 13 C species, was done with a mini Balle-Flygare- type FT cavity instrument with jet-beam-cooled sample introduction. Instrumentation

25 MW Lines for Normal Species

Rotational Constants Inertial defect of uÅ 2 implies a planar structure

r s /r e Structure Vibration-rotation constants (alphas) were computed with the B3LYP/cc-pVTZ (UFG) model. Equilibrium rotational constants were determined from alpha sums and ground state rotational constants. Inertial defects were reduced to ~0.04 uÅ 2 for the equilibrium rotational constants. Geometric parameters for the C 6 backbone came from an r s treatment.

r s /r e Structure for the C 6 Backbone

Conclusions MW spectra have been observed for cis-1,3,5- hexatriene (0.05 D dipole) and its three 13 C isotopomers in natural abundance. The ground state inertial defect demonstrates that hexatriene is planar. An r s /r e structure for the C 6 backbone shows evidence of greater pi-electron delocalization than in butadiene. Deuterium isotopomers must be synthesized for use in determining a complete semi-experimental r e structure. Work toward the semi-experimental r e structure of the trans isomer is also underway.

Acknowledgments National Science Foundation NSF MRI and NSF CRIF:ID CHE The Dreyfus Foundation Oberlin College Department of Chemistry and Biochemistry