Planar carbon

jt Nars,ModelingMarvels, yO 10.1007/978-l-4020-6973 l, Springer Science+Business Media B.V. 2008 

---

The chiral information of stereogenic centers in the allyl moiety of the precursor is destroyed on deprotonation. While an i/3-bound ion pair with a planar carbon frame is a chiral compound, usually rapid racemization takes place by intra- or intermolecular migration of the cation from one face to the opposite one. The sole exceptions known at present are secondary 2-alkenyl carbamates with X = dialkylaminocarbonyloxy21, in which the cation is tied by the chelating ligand, see Section 1.3.3.3.1.2. 

Trigonal planar geometry Is common In carbon compounds that have double bonds. For example, ethylene has two trigonal planar carbon atoms. Each carbon atom has three ligands two hydrogen atoms and one CH2 group. 

In agreement with the Hiickel rule those annulenes and dehydroannulenes which contain (4 n + 2) 77 electrons and a reasonably planar carbon skeleton appear to be aromatic. Aromaticity in annulenes is usually equated with positive resonance energy and the absence of bond alternation. The most direct method of measuring bond alternation is by single crystal X-ray diffraction. Unfortunately this method has been applied in only a few cases. 

The principle of building these planar carbon networks can also be extended to the third dimension in a manner similar to the construction of ful-lerenes from planar graphite sheets Thus, fullere-... 

Larger changes in bond lengths, as expected, are observed for more localized carbocations. Most of the structures available are for stabilized systems, such as protonated carbonyl compounds [e.g. the protonated cyclopropyl ketones referred to on page 110 (Childs et al., 1990), and dioxacarbocations (Paulsen and Dammeyer, 1973, 1976 Paulsen and Schuttpelz, 1979 Childs et al., 1986, 1991). It is normal to see one of the atoms of the counterion (in most cases MXJ or MX ) packing in the position expected for addition to the activated C=OH(R)+ system, apparently just within the sum of the van der Waals radii for the neutral centres (Childs et al., 1986). This can happen without significant pyramida-lization, however (Childs et al., 1991), and on both sides of the planar carbon centre it tells us little new about reactivity. 

The central event in an epimerization reaction is the removal of a proton from the chiral a-carbon of an amino acid residue. This event generates an enolate with a trigonal planar carbon atom. Replacement of the proton on the face opposite from which it was abstracted results in the inversion of the configuration of the a-carbon. In theory, this event can occur at any stage of peptide synthesis. In practice, however, epimerization is observed almost exclusively during the amide-bond-formation step. This discussion will be confined to that type of epimerization. 

However, you will find other representations of planar carbons with rather grossly distorted bond angles. For example, methanoic acid is planar with nearly 120° bond angles, but often is drawn with H—C—O angles of 90° and 180° ... 

Because the bisulfate anions occupy spaces between the planes of the graphite layers, they will separate the layers more than they are separated in pure graphite. The conduction between layers will be reduced. The conduction within the layers, however is enhanced because the partial oxidation produces some empty spaces in the delocalized molecular orbitals, which extend over the graphite sheet. The presence of these empty spaces makes it easier for the electrons to migrate through the solid along the planar carbon networks. 

Racemization of brinzolamide to the S isomer occurs under heat and light (pH independent) conditions (Fig. 113). This can occur via a radical mechanism from radical formation at the chiral center to form a resonance stabilized planar radical with hydrogen atom addition occurring on both sides of the planar carbon centered-radical to racemize the stereocenter (163). 

Lewars E (2008) Modeling marvels. Computational anticipation of novel molecules. Springer, New York, chapter 1, Planar Carbon, Introduction... 

A more fundamental reason is that predictions for molecules very different from those outside the training set should be less reliable than those for molecules similar to the ones used for parameterization. Therefore for investigating exotic species like, say, planar carbon or nitrogen pentafluoride AMI and PM3 are considered unreliable, and even noncorrelated ab initio calculations would be considered well short of definitive nowadays [1]. 

It has been reported that 1 //-bcnz / indene (1), l//-benz[e]indene (2), l//-benz[/g] acenaphthylene (3), and 1 //-cyclopenta[/]-phcnanthrene (4), possessing planar carbon frameworks with a single secondary C(.sy 3)H2 centre, exhibit modest acidity in the gas-phase and in DMSO.1 The origin of their amplified acidity compared with cyclopentane, cyclopentene, and cyclopentadiene is the more pronounced anionic resonance, which distributes the negative charge over the whole planar carbon skeleton via mobile n-electrons. 

SV detection was also achieved to detect catecholamines on a PDMS-quartz chip. Pyrolyzed photoresist films (PPF) were used as planar carbon electrodes. Since the photoresist (AZ4330) must adhere on a substrate for pyrolysis at 1000°C, a quartz plate that could withstand high temperature was selected. The LOD of dopamine decreased from 160 nM to 100 nM when the PPF was treated by piranha solution (HjSOyHjOj). It was because such a treatment increased the surface oxygen/carbon ratio, and hence the oxidation kinetics of dopamine wave improved [765]. 

In a small ring, the pyramidal anion formed by charge separation is much slower to invert because ring strain inhibits formation of a trigonal, planar carbon, so even if charge separation takes place in a polar solvent, the Li+ is likely to return to the same face from which it departed. 

---