Correlation diagram orbital

Connecting the energy-ordered orbitals of reactants to those ofproducts according to symmetry elements that are preserved throughout the reaction produces an orbital correlation diagram. 

Orbital-correlation diagram for the reaction C2H2 -h C-----> C3H2... 

Figure 1.1. Orbital correlation diagram illustrating the distinction between normal electron demand (leftside) and inverse electron demand (right side) Diels-Alder reactions.

The cyclobutene-butadiene interconversion can serve as an example of the reasoning employed in construction of an orbital correlation diagram. For this reaction, the four n orbitals of butadiene are converted smoothly into the two n and two a orbitals of the ground state of cyclobutene. The analysis is done as shown in Fig. 11.3. The n orbitals of butadiene are ip2, 3, and ij/. For cyclobutene, the four orbitals are a, iz, a, and n. Each of the orbitals is classified with respect to the symmetiy elements that are maintained in the course of the transformation. The relevant symmetry features depend on the structure of the reacting system. The most common elements of symmetiy to be considered are planes of symmetiy and rotation axes. An orbital is classified as symmetric (5) if it is unchanged by reflection in a plane of symmetiy or by rotation about an axis of symmetiy. If the orbital changes sign (phase) at each lobe as a result of the symmetry operation, it is called antisymmetric (A). Proper MOs must be either symmetric or antisymmetric. If an orbital is not sufficiently symmetric to be either S or A, it must be adapted by eombination with other orbitals to meet this requirement. 

We have now considered three viewpoints from which thermal electrocyclic processes can be analyzed symmetry characteristics of the frontier orbitals, orbital correlation diagrams, and transition-state aromaticity. All arrive at the same conclusions about stereochemistiy of electrocyclic reactions. Reactions involving 4n + 2 electrons will be disrotatory and involve a Hiickel-type transition state, whereas those involving 4n electrons will be conrotatory and the orbital array will be of the Mobius type. These general principles serve to explain and correlate many specific experimental observations made both before and after the orbital symmetry mles were formulated. We will discuss a few representative examples in the following paragraphs. 

An orbital correlation diagram can be constructed by examining the symmetry of the reactant and product orbitals with respect to this plane. The orbitals are classified by symmetry with respect to this plane in Fig. 11.9. For the reactants ethylene and butadiene, the classifications are the same as for the consideration of electrocyclic reactions on p. 610. An additional feature must be taken into account in the case of cyclohexene. The cyclohexene orbitals tr, t72. < i> and are called symmetry-adapted orbitals. We might be inclined to think of the a and a orbitals as localized between specific pairs of carbon... 

Fig. 11.10. Orbital correlation diagram for ethylene, butadiene, and cyclohexene orbitals.

When the orbitals have been classified with respect to symmetry, they can be arranged according to energy and the correlation lines can be drawn as in Fig. 11.10. From the orbital correlation diagram, it can be concluded that the thermal concerted cycloadditon reaction between butadiene and ethylene is allowed. All bonding levels of the reactants correlate with product ground-state orbitals. Extension of orbital correlation analysis to cycloaddition reactions involving other numbers of n electrons leads to the conclusion that the suprafacial-suprafacial addition is allowed for systems with 4n + 2 n electrons but forbidden for systems with 4n 7t electrons. 

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.

Figure 15.12 Orbital correlation diagram for cyclobutane formation...

Figure 15.21 Orbital correlation diagram for the disrotatoric ring closure of butadiene...

Fig. 6. Orbital correlation diagram for the photoelectron spectra of 1,2-dithietes (the orbital energies given are the negative ionization energies, -/ j in eV).

We will conclude this section on theory with such a case. In Section 8.3 it was shown that the influence of substituents on the rate of dediazoniation of arenediazonium ions can be treated by dual substituent parameter (DSP) methods, and that kinetic evidence is consistent with a side-on addition of N2. We will now discuss these experimental conclusion with the help of schematic orbital correlation diagrams for the diazonium ion, the aryl cation, and the side-on ion-molecule pair (Fig. 8-5, from Zollinger, 1990). We use the same orbital classification as Vincent and Radom (1978) (C2v symmetry). 

More detailed insight into the interaction of NO with the Cu M5 site can be inferred from the simplified Kohn-Sham orbital correlation diagram shown in Figure 2.11. 

Figure 1. Walsh type orbital correlation diagram (EH MO calculations [6]) for transforming linear H3P-Pt-PH3 (D3h) to a bent (C2V) geometry. Only the valence MOs and the relevant metal contributions to the MO wave functions are shown. 

The techniques which can be used for the construction of orbital correlation diagrams are well known 2,5,8,20) an(j neeq not be reviewed... 

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)... 

FIGURE 9. Orbital correlation diagram for l-azabicyclo[4.4.4]tetradec-5-ene (32), 1 -azabicyclo 4.4.4 -tetradecane (34) and bicyclo[4.4.4]tetradec-l-ene (35)... 

The plane indicated serves to construct the orbital correlation diagram using the methylene ground state or o -configuration, the lowest singlet being Ai or (see Fig. 8). 

Fig. 8. Orbital correlation diagram for the addition of methylene to ethylene through transition state a (Pig. 7) 109)...

This rule can be understood from the orbital correlation diagram shown in Scheme 7.22, where the symbols S and A denote symmetric and antisymmetric orbitals, respectively (Bellville and Bauld 1982, Bauld et al. 1983). 

An orbital correlation diagram has been presented for linear and bent M—N—O systems in five-co-ordinate tetragonal molecules, and the diagram can be extended to six-co-ordinate complexes with only minor modifications. From this study, it is concluded that the antibonding nature of the level to be filled under symmetry, and not its composition, lead to distortion and bending of the M—N—O unit, and that v(NO) is not a good guide to the linear... 

Fig. 9. Orbital correlation diagram for Dth B4H48-, BsHs , and 0>, BeHe ". (MO, molecular orbitals.)...

SYMMETRY PROPERTIES OF MOLECULAR ORBITALS CORRELATION DIAGRAMS... 

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