Locating conical intersections

B. A Quantitative Example The Photochemistry of 1,4-Cyclohexadiene (CHDN) VII. Comparison with Other Methods for Locating Conical Intersections... 

We can now proceed to discuss the phase-change rule and its use to locate conical intersections. 

VII. COMPARISON WITH OTHER METHODS FOR LOCATING CONICAL INTERSECTIONS... 

In Chapter VIII, Haas and Zilberg propose to follow the phase of the total electronic wave function as a function of the nuclear coordinates with the aim of locating conical intersections. For this purpose, they present the theoretical basis for this approach and apply it for conical intersections connecting the two lowest singlet states (Si and So). The analysis starts with the Pauli principle and is assisted by the permutational symmetry of the electronic wave function. In particular, this approach allows the selection of two coordinates along which the conical intersections are to be found. 

In order to be able to characterize the PES of excited states, locate conical intersections, and derive mechanisms for photophysics and photochemistry, efficient electronic structure methods for excited states are required. In the following section we give a brief overview of the current state of methodological developments in electronic structure methods applicable to excited states. 

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... 

VII. Comparison with Other Methods for Locating Conical Intersections... 

Transient absorption experiments have shown that all of the major DNA and RNA nucleosides have fluorescence lifetimes of less than one picosecond [2—4], and that covalently modified bases [5], and even individual tautomers [6], differ dramatically in their excited-state dynamics. Femtosecond fluorescence up-conversion studies have also shown that the lowest singlet excited states of monomeric bases, nucleosides, and nucleotides decay by ultrafast internal conversion [7-9]. As discussed elsewhere [2], solvent effects on the fluorescence lifetimes are quite modest, and no evidence has been found to date to support excited-state proton transfer as a decay mechanism. These observations have focused attention on the possibility of internal conversion via one or more conical intersections. Recently, computational studies have succeeded in locating conical intersections on the excited state potential energy surfaces of several isolated nucleobases [10-12]. 

Recent reports detail a computational analysis of the photodimerization of 1,3-butadiene that located conical intersections for concerted [4+4] and [2+2] cycloaddition paths [19,20] and have correlated these results with the products observed experimentally [21]. 

Locating Conical Intersections of Born-Oppenheimer Potential... 

Below we will describe two algorithms for locating conical intersections. The first searches in a particular branching space, while the later searches in larger spaces. 

Dick B, Haas Y, Zilberg S. Locating conical intersections relevant to photochemical reactions. Chem Phys. 2008 347 65-77. 

This review summarises and discusses the advances of computational photochemistry in 2012 and 2013 in both methodology and applications fields. The methodological developments of models and tools used to study and simulate non-adiabatic processes are highlighted. These developments can be summarised as assessment studies, new methods to locate conical intersections, tools for representation, interpretation and visualisation, new computational approaches and studies introducing simpler models to rationalise the quantum dynamics near and in the conical intersection. The applied works on the topics of photodissociation, photostability, photoisomerisations, proton/charge transfer, chemiluminescence and bioluminescence are summarised, and some illustrative examples of studies are analysed in more detail, particularly with reference to photostability and chemi/ bioluminescence. In addition, theoretical studies analysing solvent effects are also considered. We finish this review with conclusions and an outlook on the future. 

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