Planar carbon electronic states

Although sp3 hybridization is the most common electronic state of carbon, it s not the only possibility. Look at ethylene, C2H4, for example. It was recognized more than 100 years ago that ethylene carbons can be tetravalent only if they share four electrons and are linked by a double bond. Furthermore, ethylene is planar (flat) and has bond angles of approximately 120° rather than 109.5°. 

In the transition state there is a p orbital at the carbon atom in the middle that shares one pair of electrons between the old and the new bonds. Both these pictures suggest that the transition stale for an S]sj2 reaction has a more or less planar carbon atom at the centre with the nucleophile and the leaving group arranged at 180° to each other. 

Since the electronic state of the molecule in this case is similar to carbon, sp ), the SO3 molecule will contain three a and one tt bonds, and the molecule will be planar, the angle between the a bonds being 120°. The same electronic structure of sulphur will occur in the sulphate ion in which the main structure will be... 

It has also been previously shown73 that the fluctuation pattern of electron density in the ELF basins provides a consistent description of pseudopericyclic and pericyclic bonding in concerted processes such as thermal chelotropic decarbonilation reactions.74 Experimental support for planar pseudopericyclic transition states in chelotropic decar-bonilations has been recently reported.75 ELF picture of bonding reveals that for the eight transition states analysed (see Scheme 3), the departing CO can be visualized in terms of a carbon monoxide structure with a Tone pair region on the carbon atom. 

Fig. 5.13 Schematics of the C2v geometrical structure in the CH4 cation with the electronic state. Sites marked as a" are coplanar with the carbon p-orbital with an unpaired electron and sites e lie in the nodal plane of the g-orbital. H atoms prefer the co-planar a site and D atoms favor the nodal plane site e [13, 54]...

An important feature is that the electronic states dominated by orbitals in the boron plane couple strongly to specific phonon modes, making pair formations favorable. This explains the high transition temperature. The analysis of the authors [28] suggests comparable or higher transition temperatures may result in layered materials based on boron, carbon, and nitrogen with partially filled planar orbitals. 

For a correct description of the latter, the effects of electron correlation must be exactly accounted for, as was noted earlier. This is necessary in the first place because of crossing of the orbitals when reaction moves along the disrotatory route giving rise to the 1,4-biradical structure of the transition state XXIIa [37]. Although a planar carbon skeleton is retained in XXIIa, its structure clearly indicates an asynchronous character of rotations of the methylene links on the MERP ... 

In the case of a planar linear polyene the molecular orbitals may be classified according to their symmetry under reflection through the molecular plane, a orbitals are symmetric under reflection, n orbitals are antisymmetric. In discussing the low energy excited singlet states of linear polyenes it is appropriate to limit attention to many electron states built from orbitals with n symmetry (the 7C-electron approximation). In the simplest treatments these 7C-electron orbitds may be written as linear combinations of 2p-7c atomic orbitals, one per sp2 hybridized carbon atom. In the case of an all-trans planar polyene which has 2/m symmetry, these 7C-electron orbitals will transform as Ay or Bg. Since the number of electrons is even, the many electron states will have either Ag or Bu symmetry. 

A considerable number of experiments have shown that symmetrical PMDs in the ground state have an aH-trans configuration and are nearly planar with practically equalized carbon—carbon bonds and slightly alternating valence angles within the polymethine chain (1,3,5,22,23). This is caused by some significant features of the PMD electron stmcture. 

One important structural feature on which to focus is whether the nitrogen atom lies in the same plane as the three carbon atoms. Electron diffraction experiments have found the ground state to be slightly non-planar. You can determine the planarity of the structures you compute by examining the sum of the three C-N-C angles (for a planar molecule, the sum will be 360°) and by looking at the values of the C2-N-C4-O and C3-N-C4 Hg dihedral angles (in a planar structure, both will be 0°). 

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