Pyramidal carbon

Lewars, Modeling Marvels, DOI 10.1007/978-1-4020-6973-4, Springer Science+Business Media B.V. 2008 

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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). 

Although the pyramidal carbon is the most unusual feature of this molecule, it is of... 

The energetically unfavorable situation with pyramidal carbon atoms in double bonds was early realized by Bredt. His famous rule12,13, which states that a carbon atom in a C=C bond cannot be a bridgehead in a bicyclic system, is based on studies of camphenes and pinenes and is thus intended to be valid only for five- and six-membered rings. 

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...

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. 

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. ... 

Bowlane (43) and some of its homologues 44-49 have been the first molecules possessing a pyramidal carbon atom in the ground state to be proposed as plausible synthetic targets on the basis of MM calculations (35). Work is in progress in this laboratory on other hypothetical molecules that should have pyramidal carbon atoms in the ground state. 

Apart from the iconic pyramidal carbon, the stracture shows only one clearly somewhat unusual geometric feature, namely exceptionally long base-to-apex CC bond lengths of 1.672 A (Fig. 2.4). CC single bonds are typically about 1.54 A and a CC bond over 1.60 A is exceptionally long [14, 15]. The record seems to be 1.72 A for the sp -sp bond of a tetraphenc> clobutene fused to a phenanthrene ring [16],... 

Other Molecules with Possibly Pyramidal Carbon... 

Besides pyramidane, mat r other molecules with pyramidalized carbon have been examined computationally. Much of the earlier computational work on carbon with unusual configurations was carried out by the groups of Dodziuk and of Radom. The former group explored various unusual configurations at carbon (planar pyramidal, and carbon tetracoordinate with a CCC angle close to linear), while the latter focused on planar carbon, as discussed in Chapter 1, and on pyramidal carbon. 

Bowlane was (Chapter 1) the starting point in the search for a molecule with tmly planar carbon. Drawn conventionally (9), it appears to possess a pyramidal carbon, but ab initio calculations indicate that the carbon in question actually disposes two of its bonds nearly linearly, with the other two pointing downward ... 

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 ... 

In a review of pyramidal carbon species [18], Rasmussen and Radom approach these from the viewpoint of their potentially planar carbon molecirles [35,36] from which one cap has been removed (Fig. 2.11). The resulting molecides are called hemialkaplanes and hemispiroalkaplanes. The former have a neopentane moiety capped by a cycloalkane, the latter a spiropentane moiety capped by a cycloalkarre. Molecules representing decapped alkaplanes do not have truly pyrarrridal carbon, as pointed out above for bowlane. However, decapping spiroalkaplanes can lead to genuine pyramidal-carbon stractures in particular, the iconic pyranridane. 

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