Biradicaloid geometries

Now in addition to all these Si and Ti minima which have counterparts in the ground state surface, minima (or funnels) in Si and Ti can also be expected at biradicaloid geometries. These are geometries at which the molecule, in the MO description, has two approximately... 

Many types of biradicaloid geometries can be envisaged for a given molecule.19 They can be derived from its ordinary geometry (minimum... 

On a somewhat higher level of qualitative MO argumentation, one can allow for the fact that the total energy of the molecule is not related only to the sum of energies of occupied orbitals, but also to certain electron repulsion terms. This leads to a better understanding of the nature of electronic states of molecules at biradicaloid geometries 19>U2> and, in particular, of the difference between Si and Ti hypersurfaces.19) We... 

V. Bonacic-Koutecky, K. Schoffel, and J. Michl, Theor. Chim. Acta, 72,459 (1987). Critically Heterosymmetric Biradicaloid Geometries of Protonated Schiff Bases—Possible Consequences for Photochemistry and Photobiology. 

In this chapter, we review the progress we have made to date, and describe the issues that need to be addressed by further studies. The chapter is divided into three parts. The first deals with the ultrafast internal conversion of photoexcited nucleobases, and the role the out-of-plane deformation (twisting of a double bond, leading to biradicaloid geometry) plays in the photoprocess. The second is concerned with the highly efficient ICT process in dialkylaminobenzonitriles and related EDA molecules, where in-plane bending of the triple bond (which yields ira state of... 

In both cases, asynchronous, or biradicaloid, geometries are favored by methods which neglect differential overlap (MINDO/2, MINDO/3, and CNDO/2), while syn-... 

Here, we consider the two simplest and most fundamental cases of biradicaloid geometries, reached from ordinary geometries by stretching a single bond or by twisting a double bond. These two cases together underlie much of organic photochemistry. 

The antiaromatic geometry found along the concerted path of ground-state-forbidden pericyclic reactions, which is topologically equivalent to an antiaromatic Hiickel [4n]annulene or MObius [An + 2]annulene, is a particularly interesting type of biradicaloid geometry. (Cf. Section 4.4.) Other biradicaloid geometries and combinations of those mentioned are equally possible. 

Figure 4.26. Examples of biradicaloid geometries expected to be favorable for spin-orbit coupling overlap leading to covalent interaction y between the localized nonbonding orbitals (full lines) and nonzero overlap after the action of the angular momentum operator /" on one of them (dotted lines) a) partial double-bond twist, b) disrotatory, and c) conrotutory ring closure, d) disproportionation, and e) fragmentation.

The global term pericyclic funnel will be used to refer to the funnel or funnels in the S surface that occur at the critically heterosymmetric biradicaloid geometries reached near the halfway point along the path of a thermally forbidden pericyclic reaction, and the minima in S, that are encountered along one-dimensional cuts along reaction paths that miss the conical intersections (in particular, those along high-symmetry paths, which pass... 

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