Bond torsion

Figure 7-13. Cross-terms combining internal vibrational modes such as bond stretch, angle bend, and bond torsion within a molecule.

Torsional strain (Section 3.6) The strain in a molecule caused by electron repulsion between eclipsed bonds. Torsional strain is also called eclipsing strain. 

Intramolecular allylzinc aldehyde additions lead, in one synthetic step, to bicyelic a-methylene-y-lactones41. Interestingly, the (Z)-isomer gave only the r/5-fused bicyclic products, which arises from a. vyw-selectivc addition, while the ( >isomeric bromide furnished a cisjtrans mixture (yield 56%, cisjtrans 72 28)41 indicating that the double-bond torsion and the intramolecular carbonyl addition have similar rates. A similar sequence was the key step in a confertin synthesis28. 

P212121 Z = 4 D, = 1.413 R = 0.041 for 3,165 intensities. Thepyran-oside conformation is 4C1, with Q = 58 pm, 6 = 5°. The primary alcohol group is gauche-trans. The linkage-bond torsion-angles are O-5-C-l-0-1 -C-7 = — 87°, C-1-0-1-C-7-0-7 = — 67°. The two rings of the monoterpene moiety are inclined to each other, with C-C-C-C torsion-angles, about the common bond, of 149 and —97°. 

Z = 8 D, = 1.84 R = 0.059 for 2,587 intensities. In the asymmetrical unit, there are two molecules that have similar conformations. The glycosyl dispositions are anti (45.8°, 40.9°) in both molecules. The D-ribosyl conformation is 2Tj (157.6°, 32.1°) in one molecule, and 2T3 (165.1°, 31.9°) in the other. The exocyclic, C-4 -C-5 bond torsion-angles are gauche+ (51.9°, 53.9°) in both molecules, and the C-5 -0-5 bond torsion-angles are trans (172.5°, 176.6°). The two UDP molecules form a dimer coordinated by three K+ ions. There is no metal-ion or water bridge between the pyrophosphate chain and the uracil base of the same molecule. The three K+ ions are coordinated by oxygen atoms of... 

C13HuC1N307 2,2 -Anhydro-[5-chloro-l-(3,5-di-0-acetyl-/ -D-arabino-furanosyl)-6-oxocytosine] (ACAFCC)166 I4t Z — 8 D = 1.56 R = 0.066 for 818 intensities. The glycosyl disposition of the anhydronucleoside is constrained to the syn (— 68.8°) orientation. The conformation of the D-arabinofuranosyl group is a flattened 4E (232.6°, 18.0°), and the exocyclic, C-4 -C-5 bond torsion-angle is g auche+ (50.2°). The adjacent bases are connected by N-H O hydrogen-bonds between the N-4 atom of one molecule and the carbonyl oxygen atom (0-4) of another. The twofold-symmetry-related bases are stacked, with an interbase separation of 329 pm. 

The angle 6 is defined according to Scheme 2, the exact definitions of the angles in the last column, describing the double bond torsion, being given in Reference 38. Here we note only that 0 ° torsion means a planar olefin. The values are reported only for dimethyl-substituted double bonds. 

The synthesis and molecular structures of 6,7,13,14-dibenzo-l,5,8,12-tetraselena-cyclotetradecane and 1,5,9-triseIenacyclododecane have been reported and they have been shown to adopt conformations which maximize the number of possible gauche [C-Se-C-C] bond torsion angles a series of complexes has also been demonstrated . 

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