Electron delocalization in molecules

Once more, free-electron models correctly predict many qualitative trends and demonstrate the appropriateness of the general concept of electron delocalization in molecules. Free electron models are strictly one-electron simulations. The energy levels that are used to predict the distribution of several delocalized electrons are likewise one-electron levels. Interelectronic effects are therefore completely ignored and modelling the behaviour of many-electron systems in the same crude potential field is ndt feasible. Whatever level of sophistication may be aimed for when performing more realistic calculations, the basic fact of delocalized electronic waves in molecular systems remains of central importance... 

The more effective electron delocalization in molecules with extended conjugation narrows the energy gap between the HOMO and the LUMO. Thus, visible radiation of lower frequency (longer wavelength) is absorbed in the HOMO- LUMO electron transition. 

Barfield, M., Grant, D. M. (1961). The Dependence of Geminal Proton Spin-Spin Coupling Constants on Electron Delocalization in Molecules. Journal of the American Chemical Society, 83,4726-4729. 

Clearly, therefore, VBSCF constitutes a handy tool for studies of the role of electronic delocalization, in molecules that possess more than one Lewis... 

The SHG/SFG technique is not restricted to interface spectroscopy of the delocalized electronic states of solids. It is also a powerful tool for spectroscopy of electronic transitions in molecules. Figure Bl.5.13 presents such an example for a monolayer of the R-enantiomer of the molecule 2,2 -dihydroxyl-l,l -binaphthyl, (R)-BN, at the air/water interface [ ]. The spectra reveal two-photon resonance features near wavelengths of 332 and 340 mu that are assigned to the two lowest exciton-split transitions in the naphtli-2-ol... 

The cyclic conjugation and orbital phase are both continuous in benzene. Electrons delocalize in a cyclic manner. The cyclic conjugation is discontinuous in borazine in Scheme 33). Electrons cannot delocalize in a cyclic manner, but only between the neighboring pairs of donors and acceptors. There arises a fundamental question how electrons delocalize in the isoelectronic molecules where C=C bonds are replaced with N-B bonds. 

Despite spectacular successes with the modelling of electron delocalization in solids and simple molecules, one-particle models can never describe more than qualitative trends in quantum systems. The dilemma is that many-particle problems are mathematically notoriously difficult to handle. When dealing with atoms and molecules approximation and simplifying assumptions are therefore inevitable. The immediate errors introduced in this way may appear to be insignificant, but because of the special structure of quantum theory the consequences are always more serious than anticipated. 

In the ground state, aminomethylenemalonates possess an essentially planar geometry, which maximizes the electron delocalization in the molecules. In the heteropolar transition state, the plane of the groups R3 and NR R2 and the plane of the two carbonyl groups occupy orthogonal positions. More details of the dynamic and static stereochemistry of push-pull ethylenes, as in compounds 1 and 2, are discussed in two excellent reviews (73TS295 83TS83). 

The existence of many ionic structures in MCVB wave functions has often been criticized by some workers as being unphysical. This has been the case particularly when a covalent bond between like atoms is being represented. Nevertheless, we have seen in Chapter 2 that ionic structures contribute to electron delocalization in the H2 molecule and would be expected to do likewise in all cases. Later in this chapter we will see that they can also be interpreted as contributions from ionic states of the constituent atoms. When the bond is between unlike atoms, it is to be expected that ionic stmctures in the wave function will also contribute to various electric moments, the dipole moment being the simplest. The amounts of these ionic structures in the wave functions will be determined by a sort of balancing act in the variation principle between the diagonal effects of the ionic state energies and the off-diagonal effect of the delocalization. 

The organic chemist made an important step in the understanding of chemical reactivity when he realized the importance of electronic stabilization caused by the delocalization of electron pairs (bonded and non-bonded) in organic molecules. Indeed, this concept led to the development of the resonance theory for conjugated molecules and has provided a rational for the understanding of chemical reactivity (1, 2, 3). The use of "curved arrows" developed 50 years ago is still a very convenient way to express either the electronic delocalization in resonance structures or the electronic "displacement" occurring in a particular reaction mechanism. This is shown by the following examples. 

As structures b through e in Figure 8-1 show, delocalization of the unshared electron pair occurs throughout the ring, making these electrons less available for reaction. As a result of this electron delocalization, the molecule becomes less basic. 

Numerous additional tests to determine the effect of the carbon chain on the functional group were conducted using a homologous series. The effect of unsaturation is pronounced only in cases where the double bond is conjugated with the aldehydic functional group. For example, trans-2-hexenal ( ) had a AT value of 1.50 as compared to n-hexanal with a value of 0.93. The ease of electron delocalization in trans-2-hexenal, as shown below, increases the polarity of the molecule, hence its higher AT value. 

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