Twisted form

The overall picture of the many results which have been obtained with hetero-substituted cyclohexane rings is a very consistent one. Cyclohexane itself in its lowest energy conformation adopts the so-called chair conformation, as depicted in Figure 3 by the two outer formulae (a, b). These are contained in energy wells ca. 42 kJ moP deep. Another conformation, of low abundance in cyclohexane at normal temperatures, but which is important in some substituted derivatives, is the twist form (c, d). This is ca. 22 kJ moP less stable than the chair forms, and it lies on the lowest-energy pathway between them. 

The twist form is also called the flexible form, because without angle strain a left-handed twist (c) can be converted into a right-handed one (d), by way of a third form ca. 7 kJ mol higher in energy (and actually not strictly an intermediate, since it occupies a maximum on the energy profile) which is the boat form (e). 

One interesting exception to the generalization that chair forms predominate in this series is shown by the tetrathian (44), which prefers the twist form in the solid phase, and also in CS2 solution at 0 C both chair and twist forms coexist in solution, with a free energy difference of ca. 3.5 kJ between them (67JA5978, 68JA2450). 

The Z,Z,Z,Z,Z-isomer is required by geometry to have bond angles of 144° to maintain planarity and would therefore be enormously destabilized by distortion of the normal trigonal bond angle. The most stable structure is a twisted form of the , Z,Z,Z,Z-isomer. MO (MP2/DZd) calculations suggest an aromatic stabilization of almost ISkcal for a conformation of the , Z,Z,Z,Z-isomer in which the irmer hydrogens are twisted out of the plane by about 20°, but other calculations point to a polyene structure. ... 

Trioxane 210 has been used as a model system by Gu and coworkers to study the antimalarial drug artemisinin 211 (Scheme 137) [97CPL234, 99JST103]. It is the boat/twist form rather than the chair conformer of 210 that describes the subunit in 211. Moreover, geometric parameters and vibrational frequencies can only reliably be computed at the DFT level and by post-Hartree-Fock methods. B3-LYP/6-31G calculations on the conformers of 3,3,6,6-tetramethyl-1,2,4,5-tetroxane show that the chair conformer is stabilized with respect to the twisted conformer by about -2.8 kcal/mol [00JST85]. No corresponding boat conformer was found. 

Fig. 3.64 Some snapshots of the evolution of a twisted form of the 9-neighbor voting rule M5 (see equation 3.79) the initial density p = 0.50.

For cyclohexane, there are two extreme conformations in which all the angles are tetrahedral. These are called the boat and the chair conformations and in each the ring is said to be puckered. The chair conformation is a rigid structure, but the boat form is flexible and can easily pass over to a somewhat more stable form known as the twist conformation. The twist form is 1.5 kcal mol (6.3 kJ mol )... 

As the triplet energy of the sensitizer becomes less than that necessary to excite either form of the butadiene (Et < 50 kcal/mole), it is proposed that energy is transferred via nonvertical excitation to lower energy twist forms of the diene triplets.(11> The product distribution again reflects the ground state population of s-cis and s-trans forms ... 

Fig. 18. Top transition coordinates (with symmetry species) of conformational transition states of cyclohexane (top and side views). Hydrogen displacements are omitted. The displacement amplitudes given are towards the C2v-symmetric boat form, and towards >2-symmetric twist forms (from left), respectively. Inversion of these displacements leads to the chair and an equivalent T>2-form, respectively. Displacements of obscured atoms are given as open arrows, obscured displacements as an additional top. See Fig. 17 for perspective conformational drawings. Bottom pseudorotational normal coordinates (with symmetry species) of the Cs- and C2-symmetric transition states. The phases of the displacement amplitudes are chosen such that a mutual interconversion of both forms results. The two conformations are viewed down the CC-bonds around which the ring torsion angles - 7.3 and - 13.1° are calculated (Fig. 17). The displacement components perpendicular to the drawing plane are comparatively small. - See text for further details.

An excess of the twisted form (B) was prepared from the photostable 2,2 -bis(trifluoromethyl)-129c by laser flash photolysis, and its decay to the folded form (A) was followed photometrically. From the rate constants at four temperatures, AH (B- A) = 16.1 0.7 kcal/mol was obtained. For this compound, AH° (A B) = 4.2 kcal/mol had been found, and the sum of these two values agrees well with AG (E- Z) = 21.5 0.3 kcal/mol obtained by band-shape analysis, which probably corresponds to a A// value of 20 to 21 kcal/mol, assuming AS to be -2 to -5 e.u. 

At 25°C, the cyclohexane molecules mainly have the chair form. The equilibrium concentration of the isomeric twist form is —10 " mol dm On ionization, the solvent cation-radicals in the chair form are predominant. Electron transfer between the chair form of the cyclohexane cation-radicals and the chair-shaped surrounding cyclohexane (neutral) molecules is very fast, since it requires minimum reorganization energy. However, the chair-form cation-radical sometimes approaches a minor part of the neutral molecules in the twisted form. Because the twisted cyclohexane has lower IP, the twist-shaped molecules scavenge the cation-radicals in the chair form. 

As noted, the twisted conformer, which has a lower IP, rapidly scavenges the chair form of the cation-radical. Being endothermic, the backward transfer is relatively slow, and equilibrium is reached in 20-30 min. Thus, the electron transfer can be described as a series of periods of very fast hole migration between the chair forms and intermittent migration with the participation of the twist forms. 

STEREOCHEMICAL TERMINOLOGY, lUPAC RECOMMENDATIONS TWIDDLING PERSISTENCE Twist form,... 

Conformations taken up by seven-membered rings are termed twist (7), chair (C), twist-boat (TB), and boat (B) conformers, akin to the related conformers for six-mem-bered rings (Fig. 11). Most often a twist form is the lowest-energy conformation in a... 

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