Conical intersections decay

Figure 14 Illustration of the general procedure used to locate the initial relaxation direction (IRD) toward the possible decay products, (a) General photochemical relaxation path leading (via conical intersection decay) to three different final structures, (b) Potential energy surface for a model elliptic conical intersection plotted in the branching plane, (c) Corresponding energy profile (as a function of the angle a) along a circular cross section centered on the conical intersection point and with radius d.

Figure 2.18 Energy profiles along the ESRP describing the excited-state relaxation from the FC point to the conical intersection decay point of cis-CgHgNHg (data from Garavelli et al. ). (Small square) the (lB -like) branch of the cis trans photoisomerization...

The first study was made on the benzene molecule [79], The S ISi photochemistry of benzene involves a conical intersection, as the fluorescence vanishes if the molecule is excited with an excess of 3000 crn of energy over the excitation energy, indicating that a pathway is opened with efficient nonradiative decay to the ground state. After irradiation, most of the molecules return to benzene. A low yield of benzvalene, which can lead further to fulvene, is, however, also obtained. 

A photochemical reaction coordinate has two branches an excited state branch and a ground state branch that is reached after decay at a conical intersection. Thus a conical intersection between ground and excited states of a molecule is a precursor to ground state reactivity, and conforms to the above definition of a reactive intermediate. The main focus of our article will be to develop this idea. In Figure 9.1b, we show the energy profile for a photochemical reaction with a conical intersection... 

Conical intersections are involved in other types of chemistry in addition to photochemistry. Photochemical reactions are nonadiabatic because they involve at least two potential energy surfaces, and decay from the excited state to the ground state takes place as shown, for example, in Figure 9.2a. However, there are also other types of nonadiabatic chemistry, which start on the ground state, followed by an ex-cnrsion npward onto the excited state (Fig. 9.2b). Electron transfer problems belong to this class of nonadiabatic chemistry, and we have documented conical intersection... 

Radiationless decay takes place in the coordinates X X2 as one passes through the conical intersection diabatically (the VB structure does not change, see, for example. Fig. 9.4). 

However, the initial excitation is to the enol S2 7jr-7jr state. It is clear from Figure 9.25 that there is an extended conical intersection seam between the ln-7nl6n-8n excited states and the ground state. Thus, the system can decay... 

Computational Photochemistry, Conical intersections, Radiationless decay, Ab initio... 

Figure 11-5. Energy level diagram of minima and conical intersections involved in the radiationless decay in uracil. Energies and structures taken from Ref. [147, 224]...

The first study, by Ismail et al. [153], used the CASSCF method with a 6-31G basis set and an active space of 14 electrons in 10 orbitals to locate conical intersections and pathways connecting them to the Franck Condon region. Two such conical intersections were identified in that work, the ci2 and ci3, as defined above. In that work the barrier leading to ci2 was calculated to be 10 kcal/mol, too high to make this conical intersection relevant. But the barrier leading to ci3 was found to be much smaller, 3.6 kcal/mol, and it was concluded that ci3 is involved in the dominant decay path. Reaching this intersection requires first a conical intersection between the nn state, which is vertically the Si state, and the non state, which is vertically the S2 state. Merchan and Serrano-Andres followed up this study [140] using a method... 

A question that becomes obvious at this point is what happens to the molecules that have similar structures to the natural bases but have different photophysical properties, i.e. they fluoresce. These molecules have similar main structure to the bases, similar ring systems and double bonds, and so, according to the previous discussion, similar conical intersections should be expected. If that is true, and conical intersections facilitate efficient radiationless decay, why do these molecules fluoresce instead of decaying nonadiabatically That is a question that has occupied a number of scientists and some answers and insights are given in the following section. 

Matsika S (2004) Radiationless decay of excited states of uracil through conical intersections. J Phys Chem A 108 7584... 

Coe JD, Martinez TJ (2005) Competitive decay at two- and three-state conical intersections in excited-state intramolecular proton transfer. J Am Chem Soc 127 4560... 

Yamazaki S, Kato S (2007) Solvent effect on conical intersections in excited-state 9H-adenine radiationless decay mechanism in polar solvent. J Am Chem Soc 129 2901—2909... 

Fig. 26. Schematic representation of the decay mechanisms of the hydroxymethyl radical. The 3s Rydberg state and the ground state have a conical intersection leading to the ground state H2CO + H products. (From Hoffman et al.170)...

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