Jt-electron distribution

The Jt-electron distributions for 3- and 4-phenyl-1,2-dithiolium cations, derived from an improved Huckel method, have been compared with bond length data and UV spectra. The phenyl substituents appear to have only a small influence on the n-electron structure of the 1,2-dithiolium ring, but the 7t-bond order of the linkage connecting the phenyl and dithiolium rings is more pronounced with the 3-phenyl isomer. ... 

Hall s formula (69) can be obtained from Eq. (74), because for AHs the topological matrix is of the form (31). In the case of NAHs A2 A-1 have non-zero diagonal elements Jt-electron distribution is thus not uniform and the dipole moments have values very different from zero. The rules governing the charge displacements in NAHs are discussed in Section VII. 

Ever since the work of Hiickel on the stability of Jt electron systems and the famous 4n + 2 rule for ring systems, energetic criteria have remained a basic measure for the characterization of aromaticity. In molecular orbital (MO) theory, a delocalization energy (DE) can be defined as the difference between the energy Eix of a model system with localized ji electron pair bonds and the energy dei of the real system with a delocalized jt electron distribution. 

Of course, we cannot expect the description to reflect all the details of the charge distribution in the butadiene molecule, but one may expect this approach to be able to reflect at least some rough features of the Jt electron distribution. If the results of more advanced calculations contradicted the rough particle-in-box results, then we should take a closer look at them and... 

The jt-electron density distribution in nitronates can be described by borderline resonance structures A-D (Scheme 3.84). 

In the preceding chapters we have seen how new bonds may be formed between nucleophilic reagents and various substrates that have electrophilic centres, the latter typically arising as a result of uneven electron distribution in the molecule. The nucleophile was considered to be the reactive species. In this chapter we shall consider reactions in which electrophilic reagents become bonded to substrates that are electron rich, especially those that contain multiple bonds, i.e. alkenes, alkynes, and aromatics. The jt electrons in these systems provide regions of high electron density, and electrophilic reactions feature as... 

Nitrogen is more electronegative than carbon, and this influences the electron distribution in the Jt-electron system in pyridine through inductive effects, such that nitrogen is electron rich. In addition, the... 

It was discovered, however, that the spherical aromaticity of the icosahedral fullerenes C20, Cjq and CgQ depends on the filling of the Jt-sheUs with electrons [107]. As pointed out in Section 14.3.1 no distortion of the cage structure is expected in these fullerenes if their shells are fully filled. Closed-shell situations are realized if the fullerene contains 2(N -1-1) Jt electrons. This is closely related to the stable noble-gas configuration of atoms or atomic ions [108]. In this case the electron distribution is spherical and all angular momenta are symmetrically distributed. Correlation of the aromatic character determined by the magnetic properties is shown in Table 14.3. 

The only electrons that might be useful in the kind of attraction we have discussed so far are the lone pair electrons on bromine. But we know from many experiments that electrons flow out of the alkene towards the bromine atom in this reaction—the reverse of what we should expect from electron distribution. The attraction between these molecules is not electrostatic. In fact, we know that reaction occurs because the bromine molecule has an empty orbital available to accept electrons. This is not a localized atomic orbital like that in the BF3 molecule. It is the antibonding orbital belonging to the Br-Br G bond the c orbital. There is therefore in this case an attractive interaction between a full orbital (the Jt bond) and an empty orbital (the o orbital of the Br-Br bond). The molecules are attracted to each other because this one interaction is between an empty and a full orbital and leads to bonding, unlike all the other repulsive interactions between filled orbitals. We shall develop this less obvious attraction as the chapter proceeds. 

Examples of 1,3-dipoles include diazoalkanes, nitrones, carbonyl ylides and fulminic acid. Organic chemists typically describe 1,3-dipolar cycloaddition reactions [15] in terms of four out-of-plane 71 electrons from the dipole and two from the dipolarophile. Consequently, most of the interest in the electronic structure of 1,3-dipoles has been concentrated on the distribution of the four Jt electrons over the three heavy atom centres. Of course, a characteristic feature of this class of molecules is that it presents awkward problems for classical valence theories a conventional fashion of representing such systems invokes resonance between a number of zwitterionic and diradical structures [16-19]. Much has been written on the amount of diradical character, with widely differing estimates of the relative weights of the different bonding schemes. 

The electron distribution also shows evidence of aromatic character. The jT-electron population of the carbene carbon is 0.362 (with the 4-3IG basis) indicating moderate delocaUzation from the carbon-carbon double bond. This electron transfer also contributes to a substantial dipole moment (3.35 D with 6-3IG ) with the carbene center at the negative end. 

As a conclusion, the simple analytical form of the potential energy surface allows to calculate the minimal energy path, step by step from HS to the LS energy minimum. It is obvious that along the path the contributions of the different modes will change. At HS only JT active modes contribute. After the first step the symmetry is lowered and the other modes as mentioned will mix in. This allows getting very detailed picture on the interaction between the deformation of the electron distribution and the displacements of the nuclei. 

Case of V > A vibronic ( i + P) (g) e -problem. Providing t > 0 (ground state E) and strong transfer, the lower sheet of the adiabatic surface represents the so-called Mexican hat characteristic of the E e-JT problem [46,47]. In this case the electronic distribution is dynamically averaged, and the system behaves as fully delocalized. The decrease of the electron transfer and/or the increase of the vibronic coupling results in the appearance of three minima (pseudo-JTE),... 

A) A general tendency to acquire aromatic character or to avoid antiaromaticity is exhibited by polycyclic anions. This tendency is fulfilled by sustaining modes of electron delocalization and charge distributions which would result in aromatic character or reduced antiaromatic contributions. A (4n + 2)jt-electron path of conjugation offers the largest contribution to the character of the system. Applying... 

---