THE ELECTRONIC SPECTRUM OF THE BUTADIENE CHROMO PHORE: ABSORPTION AND CIRCULAR DICHROISM OF $\alpha$- AND $\beta$-PHELLANDRENE

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1976

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Ohio State University

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The absorption and circular dichroism spectra of (-)-$\alpha$-phellandrene (I) and (-)-$\beta$T-phellandrene(II) have been measured in the region 300-135 am. Solution spectra were measured to 160 nm. The spectra were interpreted in terms of the cis- and trans- geometries of the parent chromophore 1,3 butadiene. The overlapping valence and Rydberg states were unambiguously separated by comparison of the vapor phase and solution phase spectra. For the trans isomer, a broad intense valence band near 170 nm has been characterized as magnetic-dipole allowed and electric-dipole forbidden in the parent molecule. This band is interpreted as containing the $2^{1}A_{g}, 3^{1}A_{g}$ quasi-degenerate complex. A magnetic-dipole forbidden band on the high energy wing of the N $\rightarrow$ V transition is assigned as $\sigma(7a_{g}) \rightarrow \Pi^{*}$ (possibly containing some Rydberg character). For the cis isomer, both $B_{2}$ valence states being electric as well as magnetic-dipole allowed have been identified. Evidence for an electric-dipole allowed and magnetic-dipole forbidden $A_{1}$ state around 155 nm is presented. In addition the $\sigma(7a_{l}) \rightarrow \Pi^{*}$ transition has been assigned.

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Author Institution: Department of Chemistry, University of Southern California

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