MO configuration

For formaldehyde give the lowest MO configuration resulting from a n -n promotion and deduce the resulting states. 

Table 9.1 Molecular orbital (MO) configurations and equilibrium bond lengths of N2...

The four electrons involved in bonding can therefore be assigned to the MO configuration... 

In this analysis, the activation barrier for both C1-C6 and C1-C5 cyclizations of enediyne radical-anions can be described as the avoided crossing between the out-of-plane and in-plane MOs (configurations). One-electron reduction populates the out-of-plane LUMO of the enediyne moiety. At the TS (the crossing), the electron is transferred between the orthogonal re-systems to the new (in-plane) LUMO. This effect leads to the accelerated cyclization of radical-anions of benzannelated enediynes, a large sensitivity of this reaction to re-conjugative effects of remote substituents and the fact that this selectivity is inverse compared to that of the Bergman cyclization. Similar electronic effects should apply to the other reductive cyclization reactions that were mentioned in the introduction. 

The D-ma mo-configured 2-deoxyfluoro derivative 99 was prepared by Wong and his group, who exploited an aldolase-catalyzed carbon-carbon bond formation reaction208 and a 3,4-dideoxy-3-fluoro derivative (100, Scheme 26) of compound 11 was provided by Resnati and coworkers.225... 

The structure of CH2 was discussed by a completely MCVB treatment in Chapter 15. Here we look at it from an ROHF point of view. The structure of CH2 was uncertain for a number of years, but it is now known that the ground state is triplet with a bent geometry in C2V symmetry. Conventions dictate that CH2 be oriented with the C2-and z-axes coincident and the molecule in the y-z plane. Consequently the ground state is 5i, and the MO configuration is... 

In the previous section we saw how a linear combination of VB configurations may be utilized in order to generate states. The same procedure may also be utilized with MO configurations. The theoretical equivalence of these separate routes will be illustrated in the next section. 

The starting point for the MO procedure is the donor (D)-acceptor (A) model originally proposed by Mulliken (Mulliken, 1952a,b Mulliken and Person, 1969). Let us consider the possible electronic MO configurations of a donor and acceptor pair. The donor molecule is represented by an electron... 

Thus we see that the MO configuration, a2, has a precise VB equivalent. What is more, any MO representation may be converted to its VB analogue, and vice versa, by simply describing the MO configuration in terms of the atomic or hybrid orbitals from which it is composed. It follows, therefore, that just as R- -X is a poor VB representation of the R—X bond because it does not take into account ionic contributions, a2 as represented by (41), is also seen to be an unsatisfactory MO wave-function since it places an excessive emphasis on ionic configurations where the two electrons lie in the same hybrid orbital, and hence repel each other. 

This section illustrates therefore that both ground and excited states of the R—X bond may be simply described by either VB or MO configurations. These configurations are mutually related and an improved wave-function may be obtained through configuration mixing. 

The same basic diagram provides a detailed description of the twisting of a -bond (Salem, 1973 Salem and Rowland, 1971). This is illustrated in Fig. 10. The -bond in ethylene may exist in a number of predetermined states, each of which may be approximated by the appropriate MO configuration. The possible states are S0, the ground state, ( 2) Tj, the first... 

The reaction profile is generated from a linear combination of VB or MO configurations. For simplicity VB configurations are preferred. 

The He I photoelectron spectrum of N2 is shown in Fig. 7.4. Photo-electron energy increases to the right, and therefore (M + )- (M) increases to the left. Of course, it is the latter quantity that is of interest. The ground-state MO configuration is... 

MO configuration will be 2o I<7 In lirJ, (or 2o lfc23u lb )- Because the formerly nonbonding 1wux orbital is greatly stabilized (3o,) on bending, the water molecule is bent rather than linear. 

The configurationally most labile optically active square-pyramidal derivative shown to date is compound 25, simultaneously the first example of an optically active square-pyramidal complex having five independent ligands (67) (see Scheme 17). Starting with 25a in toluene solution at 0°C, the first-order approach to equilibrium proceeds with a half-life of 27 minutes. The extreme lability of the Mo configuration in 25a can be used for the following asymmetric transformation. If an equilibrium mixture of 25a and 25b that contains both isomers in the ratio —1 1 is cooled to -20°C, one isomer (25a) starts to crystallize. Because at -20°C, equilibration is still fairly rapid, 25b is steadily transformed into 25a until almost all the material is converted into solid 25a (67). No trace of the corresponding trans isomer can be seen in the H-NMR spectrum on equilibration. 

It must be pointed out that the classification into MC, LC and CT transitions (or excited states) is somewhat arbitrary and loses its meaning whenever the states involved cannot be described with localized MO configurations. The chemical and physical properties of these orbitally different excited states have been examined in detail by several authors2 3>6> 8-10) and will not be further discussed here. 

Here the first two determinants are the determinantal form of the Heitler-London function (eq 1), and represent a purely covalent interaction between the atoms. The remaining determinants represent zwitterionic structures, H-H+ and H+H, and contribute 50% to the wave function. The same constitution holds for any interatomic distance. This weight of the ionic structures is clearly too much at equilibrium distance, and becomes absurd at infinite separation where the ionic component is expected to drop to zero. Qualitatively, this can be corrected by including a second configuration where both electrons occupy the antibonding orbital, Gu, i.e. the doubly excited configuration. The more elaborate wave function T ci is shown in eq. 4, where C and C2 are coefficients of the two MO configurations ... 

Fig. 3. VB mechanisms for electron conduction in lithium with (a) 2pa AOs and (b)

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