Energy barriers, to planarity

Figure 6.41 Potential energy curves and vibrational energy levels for an inversion vibration when the barrier to planarity is (a) infinite, (b) moderately low and (c) zero...

If the spin-spin information was being transmitted by the normal through-bond mechanism the upfield three proton signal would be expected to occur as a doublet because these protons are the only ones which can assume the required planar zig-zag conformation 77>78h Preliminary results, using the change in chemical shift method 79>, indicates that the energy barrier to rotation is of the order of 20 k.cal.mole O. As expected the silicon compound (39) shows a nine proton doublet... 

The energy barrier to this conformational change is about 22 kJ mol . There is no reason why any one particular carbon should be out of the plane, and at room temperature there is rapid interconversion of all possible variants. Again, a planar form would feature as the energy maximum in tlie interconversions. The conformation with four carbons in plane and one out of plane is termed an envelope conformation. This terminology comes from the similarity to an envelope with the flap open. 

Although planar structures for 111 and 112 were not attained, it is still likely that these novel diazabiaryls can serve as chiral ligands with C2 symmetry. In support of this likelihood, the resolution of 112 was accomplished recently on swollen, microcrystalline triacetylcellulose by Jan Sandstrom. The free energy barrier to ring inversion of 112 was found to be about 101 kJ/mol, through a thermal racemization process using chiral 112. ... 

TABLE 3. Free-energy barriers to rotation through the planar state (kcalmol 1), C1—C2 bond lengths (pm) and twist angles (deg) for compounds 8-10... 

However, investigation of square planar (olefin) Pt(II)complexes has shown that the energy barrier to the interconversion of the rotamers can be low67) therefore, it is possible (Fig. 14, paths a + b and a + b ) that for each diastereomeric Tt-complex only one rotamer is formed in the interaction between catalyst and olefin and that the rotameric equilibrium is reached very rapidly. 

Thiabenzene 256 and its benzologues 1-thianaphthalene 257, 2-thianaphthalene 258 and 9-thiaanthracene 259 are also potentially antiaromatic 8ji electron systems provided they are planar. However, they adopt a boat conformation in which the S atom lies above the plane of the C atoms, thereby creating a 671 electron homoaromatic system with ylidic character. Calculations show that the energy barrier to inversion at S increases in the order 259 < 257 < 256 < 258 and the calculated dipole moments indicate the greatest ylide character is found in 259. The relative stability is in the order 259 > 257 > 258 > 256 (Table 19) <2006HAC376>. 

Since, in the excited state, the fluorine atoms may be above or below the plane of the benzene ring the potential function for vn is W-shaped, like that in Figure 6.41(b). Fitting the observed vibrational energy levels to the potential function in Equation (6.93) gives the height of the barrier to planarity as 78 cm 1. 

Here, v represents the surface tension of a smooth film, whereas r, and r2 are the principal radii of the formed hole. Suh (1983) has shown that for the case of a tellurium film, energy barrier to restore a planar surface is small and pit formation ensues for a melted textured surface. 

Such an interpretation is also supported by the 13C-NMR data. An ortho, para electronic shift of this type has recently been proposed to explain the optical spectral differences between free and metal-bound phenoxides in the transferrins (67). In enterobactin, however, this tautomerism is of fundamental conformational importance as it lowers the energy barrier to rotate the amide bond to —47° from the planar trans configuration (Fig. 6), as is required in order to form a mononuclear octahedral complex. Upon reorientation, the amide proton becomes sterically more shielded from interaction with the solvent and this is reflected in its temperature coefficient which (in absolute value) drops from 4.7 X 10-3 ppm/deg in enterobactin to 1.3 X 10-3 ppm/deg in the Ga3+ chelate. Conversely, the driving force to form the mononuclear complex will favor the electronic delocalization implied in the above tautomerism. 

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