State Correlation Diagram approach

In Chapter 15, the state correlation diagram approach of the previous chapter is applied to a brief discussion of photochemistry in the manner of Dauben, Salem, and Turro [13]. A more comprehensive approach to this subject may be found in the text by Michl and Bonacic-Koutecky [14], Turro [15], or Gilbert and Baggott [16]. 

State Correlation Diagram (SCO) approach 27-8 statistical copolymerization see copolymerization statistical copolymers sea also copolymers definition 333... 

The photochemical disrotatory closure of butadiene to cyclobutene has been described with a state-correlation diagram, like that shown in Figure 21.4. It is based on the familiar orbital-correlation diagram of Woodward and Hoffmann," from which the intended correlations indicated by the dashed lines can readily be deduced. The solid lines indicate that there is an avoided crossing, which is put in as a result of the quantum mechanical noncrossing rule. It says that two states of the same total symmetry cannot cross. Instead, as they approach each other in energy, they will mix and separate, as the solid lines indicate. 

An alternative manner of building up the reaction profile for an SN2 reaction is based on the state correlation diagram (Shaik and Pross, 1982b Shaik, 1983, 1984). This approach is of theoretical interest since it provides further insight into the factors which govern barrier formation. 

Figure 62. Partial electronic state correlation diagram for N+-H2 system. At left, N+ is assumed to approach H2 along perpendicular bisector of bond. At right, collinear approach is assumed. Crossings that are avoided in the more general conformations of Cs symmetry indicated by dotted lines.480...

Reference [63]). In these two approaches only the designation of the orbitals and states is different the outcome, i.e., the state correlation diagram, is the same. 

The State Correlation Diagram (SCD) approach introduced by Shaik and Pross appears similar in some respects. However, the LUMO, HOMO and the first two excited stales are considered, (refer Figure 1.5) Thus, if we consider the interaction of the radical with the olefin in its ground (singlet) state (R -i-C=C ) and excited (triplet) state (R -i- C=C ) and two charge transfer... 

The novelty in the present approach is the absence of potential energy surfaces. It is sufficient to identify the stationary states involved in a given mechanism. An example of this is given in our paper [22]. Only state correlation diagrams are retained. The chemical interconversion is seen from a perspective of the stationary scattering approach. The asymptotic states also include the transition structures. Since we are interested in molecular mechanisms, these intermediates states are depicted to emphasize the chemical change. They actually define path histories. 

This is a very powerful rule, and it is especially useful when there are several components to a pericyclic reaction. With several components it is often difficult to identify the appropriate HOMOs and LUMOs for an FMO analysis, and difficult to quickly write an orbital or state correlation diagram. In such cases, aromatic transition state theory, or the generalized orbital symmetry rule, are the easiest approaches for analyzing the reaction. It is your decision as to which works best for you. 

State correlation diagram for a [2+2] cycloaddition. There is a substantial barrier on the ground state energy surface, but the first excited state surface approaches the ground state surface, and a funnel forms that allows the excited state to exit to the ground state, facilitating the reaction. 

Contents Molecular Orbitals. - Chemical Reactivity Theory. - Interaction of Two Reacting Species. - Principles Governing the Reaction Pathway. - General Orientation Rule. - Reactivity Indices. - Various Examples. -Singlet-Triplet Selectivity. - Pseudoexcitation. -Three-species Interaction. - Orbital Catalysis. -Thermolytic Generation of Excited States. - Reaction Coordinate Formalism. - Correlation Diagram Approach. 

Fig. 11. Electronic state correlation diagram for the low-lying states of NH2 At the right, a collinear approach of to H2 is assumed. At the left, approaches H2 along the perpendicular bisector of the hydrogen molecule [14].

A more complete analysis of interacting molecules would examine all of the involved MOs in a similar wty. A correlation diagram would be constructed to determine which reactant orbital is transformed into wfiich product orbital. Reactions which permit smooth transformation of the reactant orbitals to product orbitals without intervention of high-energy transition states or intermediates can be identified in this way. If no such transformation is possible, a much higher activation energy is likely since the absence of a smooth transformation implies that bonds must be broken before they can be reformed. This treatment is more complete than the frontier orbital treatment because it focuses attention not only on the reactants but also on the products. We will describe this method of analysis in more detail in Chapter 11. The qualitative approach that has been described here is a useful and simple wty to apply MO theory to reactivity problems, and we will employ it in subsequent chapters to problems in reactivity that are best described in MO terms. I... 

Fig. 13.3. Orbital correlation diagram for one ground-state ethene and one excited-state ethene. The symmetry designations apply, respectively, to the horizontal and vertical planes for two ethene molecules approaching one another in parallel planes.

The most widely used qualitative model for the explanation of the shapes of molecules is the Valence Shell Electron Pair Repulsion (VSEPR) model of Gillespie and Nyholm (25). The orbital correlation diagrams of Walsh (26) are also used for simple systems for which the qualitative form of the MOs may be deduced from symmetry considerations. Attempts have been made to prove that these two approaches are equivalent (27). But this is impossible since Walsh s Rules refer explicitly to (and only have meaning within) the MO model while the VSEPR method does not refer to (is not confined by) any explicitly-stated model of molecular electronic structure. Thus, any proof that the two approaches are equivalent can only prove, at best, that the two are equivalent at the MO level i.e. that Walsh s Rules are contained in the VSEPR model. Of course, the transformation to localised orbitals of an MO determinant provides a convenient picture of VSEPR rules but the VSEPR method itself depends not on the independent-particle model but on the possibility of separating the total electronic structure of a molecule into more or less autonomous electron pairs which interact as separate entities (28). The localised MO description is merely the simplest such separation the general case is our Eq. (6)... 

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