Conical intersections and intersystem crossings

Light absorption leading to electronic excitation is commonly accompanied by radiative reemission as the electron decays back to the ground potential energy surface. However, in certain cases (dependent on excited-state radiative lifetime and potential features to be described below) the system returns to the ground electronic surface without optical emission, a so-called radiationless transition. Such non-radiative transition processes are important features of reaction pathways on both ground and excited surfaces. 

However, given the distinctive geometry (I/O-l 1.2) of the CI(So/Si) species, we are free to use more informative wavefunction methods to describe bonding propensities of the nascent species. In particular, for the ground-state So species of principal interest we can conveniently revert to single-configuration methods as employed in previous chapters. Because DPT methods often exhibit spurious numerical behavior 

The NRT bond orders of Figs. 11.16c point to significant oxetene (cyclic enol ether) character of the CI-Sq species, with pronounced diradical character and high polarity of the strained C(2)0(8) long-bond. The a-spin NRT description is dominated by the cyclic enol-like oxetene pattern CIi, with weaker admixture of the corresponding keto-like pattern Cl2 , namely, 

The jS-spin NRT description differs dramatically at all four skeletal centers, with leading structures 

As shown in Fig. 11.16b, all four skeletal centers exhibit large spin density values (INSDI 0.5), indicative of extreme (teriOTadical) spin-polarized 

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