Special Topic 2.5 Conical intersections

V 2 is the electronic coupling term between the zero-order states at the crossing point, h is Planck s constant, v (not id) is the nuclear velocity along the reaction coordinate as the molecule approaches the crossing point and si s21 is the difference between the slopes of the two intersecting PESs at this point. Even in systems for which an accurate calculation of p 12 is possible nowadays, the Landau Zener model provides a useful first estimate. 

The zero-order diabatic PESs that cross at the origin of the reaction coordinate are shown as dotted lines and the interaction Vi2, chosen at 0.05 eV, leads to an avoided 

Diabatic reactions are rarely initiated from triplet states, because the interaction terms V12 between states of different multiplicity, which are due to spin orbit coupling (Section 4.8) are small. Even if surface crossings with the singlet ground-state PES do occur in certain regions of the (3N — 6)-dimensional space of the internal nuclear coordinates and the molecule does explore such a region in its lifetime, it is unlikely to find the minute escape hatch to the ground state PES. 

Class The reaction proceeds by formation of reactive intermediates such as a radical pair, a carbene, a biradical or a zwitterion, which have low-lying excited states and are therefore situated near a minimum on the excited state PES. Many photoreactions belong to this class (see Section 5.4). 

IR-absorbing dyes with high quantum yields of fluorescence are hard to come by. Explain. [Low rate constants of fluorescence (Equation 2.11), fast competing IC (Equation 2.22)]. 

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Class Diabatic reactionss proceed directly from the excited state to the ground state of the photoproduct via geometries, at which the two surfaces cross (conical intersections, Special Topic 2.5) or nearly cross (avoided crossings, funnels). In such reactions, no intermediates other than the excited state reached by absorption can usually be detected, because the molecule arrives on or near to a cusp of the ground-state surface and immediately proceeds to a stable product. 

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