Excited state orientational

These reactions are believed to proceed through a complex of the alkene with a singlet excited state of the aromatic compound (an exciplex). The alkene and aromatic ring are presumed to be oriented in such a manner that the alkene n system reacts with p orbitals on 1,3-carbons of the aromatic. The structure of the excited-state species has been probed in more detail using CAS-SCF ab initio calculations. ... 

A consequence of the orientation of the 11-carbonyl function towards the C-19 methyl group which is retained in the excited state is the exclusive functionalization at C-19. Ring cleavage products of the Norrish II type are not observed but the reaction is rather sensitive to conformational changes in the substrate. In a series of experiments conducted under comparable conditions (24 hr irradiation) the yield of cyclobutanols drops... 

From the standard orientation, we see that the plane of the molecule is the XY plane. Both orbitals are composed of only p components, indicating that they are Jt orbitals. Thus, this excited state corresponds to the Jt— Jt transition. 

Figure 1. Schematic of the radial cuts of the ground- and excited-state potential energy surfaces at the linear and T-shaped orientations. Transitions of the ground-state, T-shaped complexes access the lowest lying, bound intermolecular level in the excited-state potential also with a rigid T-shaped geometry. Transitions of the linear conformer were previously believed to access the purely repulsive region of the excited-state potential and would thus give rise to a continuum signal. The results reviewed here indicate that transitions of the linear conformer can access bound excited-state levels with intermolecular vibrational excitation.

Fig. 4.13 Combined magnetic hyperfine interaction for Fe with strong electric quadrupole interaction. Top left, electric quadrupole splitting of the ground (g) and excited state (e). Top right first-order perturbation by magnetic dipole interaction arising from a weak field along the main component > 0 of the EFG fq = 0). Bottom the resultant Mossbauer spectrum is shown for a single-crystal type measurement with B fixed perpendicular to the y-rays and B oriented along...

First, we note that the cr-cr nature of the lowest excited state of long-chain polysilanes predicted by the semiempirical calculations described in detail below implies a transition moment direction lying approximately along the chain direction rather than perpendicular to it. Evidence for such an orientation of the transition moment has been obtained in measurements on solid polysilane samples (19,36). This... 

Assuming that the enone component of the oriented 77-complex is in its n -> 77 excited state and the olefin is situated above the a,/9 double bond of the enone, one can explain the mode of addition by the calculated charge distribution of the n - 77 excited state ... 

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