Conformation, molecular twist-boat

According to molecular mechanics (MM) calculations, the minimum energy conformation of the enolate is a twist-boat (because the chair leads to an axial orientation of the f-butyl group). The enolate is convex in shape with the second ring shielding the bottom face of the enolate, so alkylation occurs from the top. 

Figure 2. History of the ring torsion angle C1-C2-C3-C4 calculated from a molecular dynamics simulation of the motions of an a-D-glucopyranose molecule in vacuum which began in the C conformation and which subsequently underwent a transition to a twist-boat conformation. (Reproduced from Ref. 9. Copyright 1986 American Chemical Society.)...

Concerning 1,4-dioxane 16, its inversion has been studied by using ab initio molecular orbital theory at the HF/6-31G and BLYP/6-31G levels. The chair conformation is the lowest in energy, followed by the two twist-boats. The transition state connecting the chair and the twist-boats is a half-chair structure, in which four atoms in the ring are planar <1997PCA3382>. 

Standard ab initio molecular orbital calculations of 1,3,5-trithiane confirm the chair conformation to be the most stable conformer, next to the twist-boat conformation, being 3 kcal/mol higher in energy than the chair... 

A number of 1,3-dithianyl-substituted 1,4-disilacyclohexanes were synthesized (98TL3197) and one molecular structure (176) was studied by X-ray diffraction (cf. Scheme 57). The two 1,3-dithianyl rings adopt strain-free chair conformations, whereas the conformation of the 1,4-disilacyclo-hexane ring was a twist-boat. The twist form was attributed to steric repulsion between the exo-cyclic C-S bonds the interaction between the silicon and the two sulfur atoms was also considered. 

Nine energy-minimum structures of 1,5-dithiacyclooctane 810 (DTCO) were obtained by the ab initio molecular orbital method (6-31G ) the most stable was a twist-boat-chair structure (TBC) while the BC conformer, in which... 

The conformational properties of bridged biphenylenes, l,2,4,5-tetrahydrobiphenyleno[l,8-r/initio molecular orbital and density functional theory (DFT) methods. Studies on the Hartree-Fock (HF)/6-31G level of theory revealed that for 5, a plane symmetrical boat conformation was of the lowest energy. The twist, twist-boat, and chair conformations are less stable by 2.41, 5.02, and 2.62 kcal mol-1, respectively. Contrary, the twist conformation was found to be the most stable form for 4 <2003JMT(637)115>. 

Mechanistically, the requisite betaine intermediates [from dioxaphospholanes N and O] adopt either the chair ( C -> 4) or twist-boat ( 85, 82) conformations so that the C-O" and -O-+PPh3 groups can assume the requisite pre-transition state andperiplanar arrangement for suitable displacement of Ph3PO. From the results of molecular modeling studies (/.e., MacroModel II) on both the chair e.g., IC4) and twist-boat betaine e.g., 85, 82) conformers, the small energy differences between them suggest that the twist-boat ( 85 and 82) betaine conformers may also be... 

Conformations and barriers to inversion in A7V -dialkylperhydro-l,4-diazepine-2,3-dione and its mono- and di-thio-analogues have been studied by n.m.r. and by molecular-mechanics calculations. The results indicate a twist-boat conformation and also that the dominant part of the inversion barrier is steric rather than electrostatic in origin. The influence of temperature and substituents on the conformation of the 5-phenyl-l,5-benzodiazepin-... 

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