Localized electron model limitations

The local response model of the van der Waals interaction implies a corresponding model of the polarization potential, hence of the long-range limit of the bound-free correlation potential for an electron outside a polarizable charge distribution [12]. The implications of this observation are discussed in the present paper. 

There are many studies of the transfer of electrons from enzymes to substrates, across biological membranes, to (or from) electrodes from (or to) substrates, between adsorbed molecular dyes and semiconductor particles, within synthetic films and nano-scale arrays, within molecular wires , and so on. Only a few, general comments will be offered on these topics here. The basic physics of molecular electron transfer does not change with the scale of the system, as long as identifiable molecular moieties are present with at least partly localized electronic configurations. The nature of the properties observed, the experimental probes available, and the level of theoretical treatment that is useful may be very different. Different approaches, different limiting models are used for extended arrays (or lattices) of very strongly coupled moieties. 

So, honest disagreements exist among chemists as to the best Lewis structures for molecules that, at least at first glance, appear to exceed the octet rule. This uncertainty shows the limitations of the Lewis model with its localized electron pairs. Note, however, that even with its limitations, it is still very useful because of its simplicity. The ability to obtain a reasonable bonding picture with a back-of-the-envelope model has led to the enduring influence of the Lewis model. ... 

The extent to which individual electron pairs are localized in distinct spatial regions has been carefully analyzed by Bader and Stephens (1975) using the minimum fluctuation criterion. These authors arrive at the conclusion that the model of spatially localized pairs is appropriate for LiH, BeH2, BH3, and BH-r, it is borderline for CH4, but in NHj, OH2, FH, Ne, N2, and F2, the motions of the valence electrons are so strongly inter-correlated, the localized pair model ceases to afford a suitable description. Moreover, their results provide no physical basis for the view that there are two separately localized pairs of nonbonded electrons in H20. This clearly shows the limit of the Lewis electron pair concept which otherwise has practically disappeared in Linnett s theory. 

The resonance concept is one way of overcoming some of the limitations of the localized electron pair model, but such cases are treated more naturally by molecular orbital theory, which is not limited to bonds involving two atoms (see Topics C6 and C7). 

In another paper [1] in this volume we have discussed some basic notions of the spectroscopy of charge transfer (CT) crystals and molecular conductors, mainly based on ver> simple dimeric models. The practical applicability of these models is limit to dimerized systems with localized electron states. This situation most often occurs for half-filled systems with intermediate to strong electron correlations. 

Specific Electric Conductivity The specific electric conductivities a of LGS, LGN and LGT were measured using a four-point technique at temperatures between 795 and 1000 K on crystal slices cut perpendicular to the Z-axes. The temperature dependence of the conductivities in dielectric materials with a limited defect density are caused by localized electronic states, and thus the data yield a straight line in a plot of In c versus 1/T, the slope of which is a measure of the activation energy AE of the charge carriers in a Mott-type transition model. These activation energies were found to be 1.1 (LGS), 1.0 (LGN) and 0.9 (LGT) eV, respectively. The rather strong increase in electric conductivity (LGS 10 Q cm at 530°C, 5 X lO- Q- cm- at 730 °C LGT lO- fi- cm" at 600 °C, 8 x 10-"Q- cm- at 730 °C) appears to limit the high-temperature applications of these materials. 

Occam s razor does not claim that a simpler explanation is correct, merely that it is more likely to be true. Both the MO and VB models (when pudred to its mathematical limits) give identical descriptions of bonds, but simple VB theory also needs to be modified with the concept of hybridization before it can predict molecular shapes correctly. Organic chemists draw VB-like structures and use curly arrows to show the movements of relatively localized electron pairs from one place to another. 

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