Twist geometry

In this geometry, only twist elastic energy is involved, and the elastic energy is given by 

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The complex 65 was synthesized by reaction of the imidazolinium salt with the precursor ruthenium complex 67 (catalytically inactive) in the presence of silver carbonate (Scheme 42). The complex being air-stable and stable on silicagel was isolated in 52% yield after chromatography. The diastereomeric and enantiomeric purity of 65 was determined by HPLC analysis and found to be above 98% (de and ee). The molecular structure was determined by X-ray analysis and showed the unusual twist geometry of this complex. 

Molecular orbital calculations on ethylene indicate that the lowest energy excited singlet and triplet states have a twisted geometry.(2) This geometry helps minimize electron-electron repulsion. Figure 9.1 gives the calculated... 

This is likely a [ 2, + 2J cycloaddition from the first excited singlet state, and suggests that this state either does not have the twisted geometry noted for... 

In order to understand the origin of the breakdown of the SR methods away from equilibrium, consider the torsional potential in ethylene (Figure 2). While at its equilibrium geometry ethylene is a well-behaved closed-shell molecule whose ground and n-valence excited states can be described accurately by SR models (except for the doubly excited Z-state), it becomes a diradical at the barrier, when the Jt-bond is completely broken (13). Thus, at the twisted geometry all of ethylene s Jt-valence states (N, T, V, and Z) are two-configurational, except for the high-spin components of the triplet. 

The radical cation of [2,2,2]bicyclooct-2-ene and its 2,3-dimethyl derivative possess two interconverting twisted geometries with lowered symmetry. This structure is attributed to a pseudo-Jahn-Teller effect caused through vibronic coupling25. 

Acyclic a,jS-unsaturated ketones do not provide much photochemistry, except for facile geometric isomerization about the double bond. Although some also isomerize to /3,y-unsaturated ketones, others, such as mesityl oxide, apparently undergo no net reaction upon absorption of irradiation. Their low reactivity might result from a rapid relaxation to an appreciably more twisted geometry in their triplet states than can be obtained in the cyclic enones. Below are some examples of typical photochemical behavior.426,427... 

The production of buta-1,3-dienes (37) by reaction of 1,2-diarylcyclopropenes with dihalocarbenes is thought to involve electrophilic attack of the carbene to give a dipolar intermediate (38).51 The addition of carbene to CO and H2C=0 has been studied by MNDO calculations.52 Photolysis of diethyl diazomalonate in a CO matrix at low temperature gave rise to ketenes by immediate trapping of the postulated carbene (39).53 The major products of reaction between dichlorocarbene and cyclone were CO and the gem-dichloro species (40).54 The predominance of this pathway over formation of the dichlorooxirane or the cyclopropane is attributed to the aromatic nature of the carbonyl ylide and its twist geometry. 

It is interesting to consider also the triplet states. We have shown that T, lies close to Sj for TICT states of a simple dative bond but locally excited triplets certainly lie well above locally excited singlets. Therefore, a very strong donor-acceptor combination in the case of the complex dative bond will be needed in order to obtain minimum in T, for twisted geometries. 

Meyer, W. P. Martin, J. C. Substituent effects of alkoxy and amino groups directly bonded to cationic carbon in the perpendicularly twisted geometry. 2-Oxa- and 2-aza-l-adamantyl tosy-lates./. Am. Chem. Soc. 1976, 98, 1231-1241. 

The first examples of 3,6-(perfluoroalkyl)-l,2-dithiins have been obtained through the oxidative deprotection and cyclisation of (Z,Z)-l,4-6 (t-butylthio)-l,4-6w(perfluoroalkyl)-1,3-butadienes. They adopt a similar twist geometry to the parent dithiin and during electrochemical oxidation appear to form a planar radical cation (Scheme 29) <03JOC8110>. 

Figure 7.9 The gyroid membrane identified in Ascobolus. (a)-(b) Projections along the [331] and the [311] direction, generated at a distance of 3/4,3/4,1/4, and 3/4,1/4,1/4, respectively, (c) Corresponding computer generated projections. Note the part of unfolded continuous membrane shown in (b), with a remaining twisted geometry. Figures (a) and (b) are reproduced from [62], with permission.

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