Double bond distortions torsional

In an oversimplified picture, nonradiative decay in U and C is controlled by a torsional motion about the C(5)C(6) double bond, while in the canonical G tautomer out-of-plane deformations of the six-membered ring are chiefly responsible for internal conversion. In the case of G, the canonical, biologically relevant, 9H-keto form indeed exhibits photophysical properties which are distinctly different from other tautomers. Its excited state lifetime, for example, is the shortest of all tautomers. This is a consequence of its pronounced out-of-plane distortions absent in other tautomers. 

Shea, K. J., Kim, J. S. Influence of strain on chemical reactivity. Relative reactivity of torsionally distorted double bonds in MCPBA epoxidations. J. Am. Chem. Soc. 1992, 114, 3044-3051. 

Structural features of olefins with distorted double bonds have been discussed within the deformation space defined by the eight bond angle deformations. The out-of-plane bond angle distortions are of particular interest because they are involved in addition reactions of the double bond. The symmetrical Blg-type deformation is related to concerted anti-additions, whereas the J3lu-type distortion (cf. Table 1) is appropriate for concerted syn-addition and those reactions that involve three-center intermediates and the formation of transition metal complexes. Twist or torsion is due to the Alu-type oop distortion and may be related to addition reactions, which in principle would lead—in the extreme case of a 90° twist angle—to an eclipsed rather than a staggered arrangement. 

In 1972, Mock considered double-bond reactivity and its relationship to torsional strain, by which he understood the strain imposed on a double bond in medium-ring fra 5-cycloalkenes or by steric compression of large cis substituents [28]. He argued that the loss of 7t overlap due to a torsion about the double bond can be partially compensated by rehybridization in these two situations, leading, respectively, to syn and anti pyramidalization of the double bond consequently, such bonds will favor different modes of addition (cis and trans). The proposition was supported by examples of X-ray structures of strained olefins, STO-3G energy calculations for the twisted and pyramidalized ethylene geometries, and by analysis of the out-of-plane vibrational frequencies of ethylene. Mock concluded that small ground-state distortions may produce sizable effects in the transition states. 

The effect of introducing /j -hybridized atoms into acyclic molecules was discussed in Section 2.2.1, and it was noted that torsional barriers in 1-alkenes and aldehydes are somewhat smaller than in alkanes. Similar effects are seen when sp centers are incorporated into six-membered rings. Whereas the energy barrier for ring inversion in cyclohexane is 10.3 kcal/mol, it is reduced to 7.7 kcal/mol in methylenecy-clohexane ° and to 4.9 kcal/mol in cyclohexanone. The conformation of cyclohexene is described as a half-chair. Structural parameters determined on the basis of electron diffraction and microwave spectroscopy reveal that the double bond can be accommodated into the ring without serious distortion. The C(l)—C(2) bond length is 1.335 A, and the C(l)-C(2)-C(3) bond angle is 123°. The substituents at C(3) and C(6) are tilted from the usual axial and equatorial directions and are referred to as pseudoaxial and pseudoequatorial. 

The molecular structure of Sc2-naph (Fig. 11) was reminiscent of previously reported yttrium naphthalene complexes (Fryzuk et al., 2000). The naphthalene is distorted from planarity with C2/C3 and C2A/C3A bending in opposite directions from the plane composed of the other six carbon atoms (ca. 20° torsion angle). The C-C bonds within the naphthalene are best described as two isolated double bonds (C2=C3 and C2A=C3A), with short distances averaging 1.37 A, and a 6C, 871-electron system for the six coplanar center carbon atoms. Each scandium ion binds to Cl through C4 (or CIA through C4A) with similar distances averaging 2.51 A. These features are also reminiscent of those of the lithium naphthalene dianion [Li(TMEDA)]2(p-ri ri -CioH8) (TMEDA = tetramethylethylenediamine) (Melero et al., 2009). 

Out-of-Plane Torsion. When three bond orbitals are confined to be planar, as are the bonds around the ethylenic double-bonded carbons, or the bonds around the C and N which form the planar peptide unit, there is a force which resists the distortion of the planar structure. The out-of-plane torsion can be envisaged as the angle x between the planes through the points (1,2,3) and (1,2,4) in the structure 1 = 2<, which intersect along the axis 1 = 2. 

---