Pyramidal carbon structure

Silicon s tetravalent pyramid crystalline structure, similar to tetravalent carbon, results in a great variety of compounds with many practical uses. Crystals of sihcon that have been contaminated with impurities (arsenic or boron) are used as semiconductors in the computer and electronics industries. Silicon semiconductors made possible the invention of transistors at the Bell Labs in 1947. Transistors use layers of crystals that regulate the flow of electric current. Over the past half-century, transistors have replaced the vacuum tubes in radios, TVs, and other electronic equipment that reduces both the devices size and the heat produced by the electronic devices. 

The comparably weak enhancement of the benzylic protons and the relatively strong C polarization for the benzylic carbons can be explained if the benzylic carbon adopts a pyramidal structure. Although radicals containing pyramidal carbon are relatively rare, the existing examples are derived from strained-ring... 

JA4155). However, when the 6-31G basis set is employed, the Cs structure with the pyramidalized carbon atom turns out to be a local minimum on the PES (90JA4155). 

The available, rather low level calculations (HF/STO-3G, HF/STO-3G and HF/3-21G, all at the HF/STO-3G optimized geometries) show that the pyramidal aromatic structure 63a is more stable than the two planar structures. In the analogous carbon cations, on the other hand, the planar cations 64b and 65b are considerably lower in energy than the... 

Five-Center Four-Electron Bonding Structures. The potential for the existence of 5-center 4-electron (5c-Ae) bonding structures 558 have recently been surveyed by Tantillo and Hoffmann980 [calculations at the B3LYP/6-31G(d) level], A cation with three anthracenyl units joined around the C -H-C -H--C core with two approximately trigonal pyramidal carbon atoms and one five-coordinate trigonal bipyramidal carbon was found to have 5c-Ae bonding. The anticipated existence of... 

Fig. 5.48 The hydrocarbon pyramidane, C5H4, evidently (pyramidane has not been synthesized) has a lone pair of electrons on its pyramidal carbon atom, like carbene (methylene), CH2. While the lone pair on CH2 is no surprise (draw the Lewis structure for the singlet), a cycloalkane with an unshared electron pair is remarkable...

Two transition structures with a retention (192, TSret) and an inversion (193, TSinv) configuration (Figure 2) were optimized for 1,3-silyl migration in allylsilane at HF/6-31G, MP2/6-31G and DFT/6-31G levels. The TSjnV 193 was found to be a distorted trigonal bipyramid (TBP) around the silicon with the two allylic carbons at the equatorial positions different from the TS illustrated by Kwart and Slutsky297,302, while 192 has a distorted square pyramid (SP) structure around silicon. Analysis of the orbital interaction in the transition states showed that the major stabilization of 193 was caused by the MO interaction as predicted by the Woodward-Hoffmann rules, while the major stabilization in 192 was ascribed to the subjacent orbital control. 192 was more stable than 193 at... 

Whereas the structural assignment by one-dimensional 13C NMR spectroscopy is unambiguous in the case of D2-Cj(,2,51 Achiba and co-workers were able to determine the carbon atom connectivity by 2D 13C NMR INADEQUATE (incredible natural abundance double-quantum transfer experiment) analysis performed on an isotopically enriched sample (20% 13C).58 In particular, they found that the observed chemical shifts correlate well with the curvature of the spheroid, the more strongly pyramidalized carbon atoms being shifted toward lower magnetic field. 

Theoretical calculations on a model system have predicted that the ground-state conformation of the anion is the pyramidal Q structure (Fig. 5) (62). It is noteworthy that the C, structure has unusually long C-Si bonds and a butadiene-like geometry of the carbon skeleton. The planar Qu form is the transition state for pyramidal inversion between equivalent Cj structures, the inversion barrier being 16.2 kcal/mol (6-31 G ). 

Bridgehead Double Bonds. Typical bridgehead olefins represented by 44 (cf. Table 4) have been extensively discussed by Bredt (3). Inspection of molecular models, which do not allow for rehybridization at the carbon atoms of the double bond, suggest that structure 68 should prefer extensive torsional deformations. Extended Hiickel calculations showed, however, that the structure of a bridgehead double bond is best described by the overlap of a spx hybridized orbital at the bridgehead with an orbital at the adjacent carbon atom of high p character. This type of n bond is favored over a structure with two strongly pyramidalized carbon atoms (79). 

FIGURE 4.15 Orbital hybridization models of bonding in methyl radical, (a) If the structure of the CH3 radical is planar, then carbon is sp -hybridized with an unpaired electron in a 2p orbital. (6) If CH3 is pyramidal, carbon issp -hybridized with an electron in an sp orbital. Model (a) is more consistent with experimental observations. 

As a starting point for the mechanistic discussion, it is useful to review the structural features of the excited states. The first singlet excited state of butadiene. Si, can be approximated as the i >2->--LUMO tt-tt transition. The lack of fluorescence from this excited state indicates that a very facile path exists for nonradiative energy transfer. The S2 state has doubly excited character and relaxes to a structure with ionic character that can rotate at the pyramidal carbon but not at the allyl fragment. The minimum energy of the Tj state corresponds to the allyl-methylene diradical with a nearly 90° twist and slight pyramidalization at the methylene carbon. In substituted systems, one or the other of the zwitterion structures (e.g., allyl cation versus allyl anion) may be favored. ... 

The C4v structure is a transition state leading to the relative-ininimum C2v species [32], Structures 6, 7 and 8, rather than housing a pyramidal carbon, also turn out to be of relevance to the question of how nearly two bonds at a tetracoordinate carbon can approach linearity (the relevant CCC angle of 7 is calculated to be 178°). Another molecule that, drawn intuitively, looks like it may have a pyramidal carbon, but on subjection to computation turns out to be otherwise, is 1,3-dehydroadamantane geometry optimization by molecular mechanics,semiempir-ical, or ab initio methods show the deceptive atom to be tetrahedral, albeit strongly distorted ... 

Proposed reaction sequence for generation of a structure containing a pyramidal carbon. 

Figure 13.8 Examination by SEM of the MWCNT-based structures (a) perpendicular aligned carbon nanotubes (b) the latter after a physicochemical treatment forming pyramid-like structures (c) network of cross-linked carbon nanotube walls forming cavities.

The cyclopropenyl radical 1 has been studied experimentally and theoretically. Experimentally, cyclopropenyl radical is determined by ESR to have Cs symmetry with a pyramidalized carbon. The structure of the cyclopropenyl radical has been calculated by Glukhovtsev et al. and by Merrill and Kass, and the C—H bond angle at the radical center... 

There have been many studies aimed at deducing the geometiy of radical sites by examining the stereochemistry of radical reactions. The most direct kind of study involves the generation of a radical at a carbon which is a stereogenic center. A planar or rapidly inverting radical would lead to racemization, whereas a rigid pyramidal structure should... 

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