Knowledge Bank

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 $21Ag,31Ag$ quasi-degenerate complex. A magnetic-dipole forbidden band on the high energy wing of the N $\to$ V transition is assigned as $\sigma (7ag)\to \Pi \ast$ (possibly containing some Rydberg character). For the cis isomer, both $B2$ valence states being electric as well as magnetic-dipole allowed have been identified. Evidence for an electric-dipole allowed and magnetic-dipole forbidden $A1$ state around 155 nm is presented. In addition the $\sigma (7al)\to \Pi \ast$ transition has been assigned.