Electron interactions, multiple

According to the electron distribution of triafiilvenes, systems containing electron-deficient multiple bonds like TCNE, MAA, and ADD interact preferentially with the semicyclic double bond of triafulvenes. 

Since the parameters used in molecular mechanics contain all of the electronic interaction information to cause a molecule to behave in the way that it does, proper parameters are important for accurate results. MM3(2000), with the included calculation for induced dipole interactions, should model more accurately the polarization of bonds in molecules. Since the polarization of a molecular bond does not abruptly stop at the end of the bond, induced polarization models the pull of electrons throughout the molecule. This changes the calculation of the molecular dipole moment, by including more polarization within the molecule and allowing the effects of polarization to take place in multiple bonds. This should increase the accuracy with which MM3(2000) can reproduce the structures and energies of large molecules where polarization plays a role in structural conformation. 

Most of the (W,W)-carbenes are predicted to be linear and this substitution pattern results in a polarized two-electron three-center n system. Here also, the C—W bonds have some multiple bond character these (W,W)-carbenes are best described by the superposition of two ylidic structures featuring a positive charge at the carbene carbon atom. The most studied carbenes of this type are the transient dicarbomethoxycarbenes and the masked diborylcarbenes. Since no carbenes of the latter type have yet been isolated, they are not included in this chapter. Lastly, the quasilinear (D,W)-carbenes combine both types of electronic interaction. The D substituent lone pair interacts with the py orbital, while the W substituent vacant orbital interacts with the px orbital. These two interactions result in a polarized allene-type system with DC and CW multiple bonds. Good examples of this type of carbene are given by the transient halogenocarboethoxycarbenes and by the stable (phosphino)(silyl)- and (phosphino)(phosphonio)carbenes (see below). 

Due the nature of the substituents, all the stable singlet carbenes exihibit some carbon-heteroatom multiple-bond character and for some time their carbene nature has been a subject of controversy. One has to keep in mind that apart from dialkyl-carbenes, all the transient singlet carbenes present similar electronic interactions. As early as 1956, Skell and Garner drew the transient dibromocarbene in its ylide form based on the overlap of the vacant p-orbital of carbon with the filled p orbitals of the bromine atoms (Scheme 8.31). 

In order to appreciate some of the variety of problems that can be aided with EC-EPR, it is necessary to understand the basic theory of EPR and the instrumentation required to perform an EPR experiment. It is also necessary to understand how an unpaired electron interacts with the atomic nuclei in a free radical, because this interaction produces the multiple lines characteristic of an EPR spectrum. The magnitude of the spacings of these lines can be related to the unpaired electron distribution in a free radical, so it is also helpful to have... 

One of the major reasons for interest in this area is the ease with which the new hybrid materials properties can be varied by changing the metal, metal oxidation state, metal matrix, and polymer. Multiple metal sites are readily available. This allows the metal-containing system to have a high degree of tunability. This is due to the often strong electronic interaction between the metal... 

Aberg, T. Shake theory of multiple excitation processes. In Photoionization and other probes of many-electron interactions. Wuilleumier, F. (ed.), pp. 49-59. NATO Advanced Study Institute Series. New York Plenum Press 1976... 

Electron pairs involved in the n component of multiple (i.e double or triple) bonds behave somewhat differently from those in single (a) bonds (Section 7.4.1). Not only are there differences in geometry, hybridization, and s character (Section 9.3), but 7i-bond electrons interact with (delocalize into) neighboring a molecular orbitals in the molecule (hyperconjugation). Because of this delocalization, 71-bond electrons can communicate nuclear spin information over distances further than three bonds. Several examples of this type of long-range coupling are listed in Table 9.5. 

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