Bent conformation

Supplementary to this subject, a recently published ab-initio calculation of silicondicarbonyl 0 = C = Si = C = 0 should be mentioned [195]. With an extended basis set (DZP functions) a bent C2 structure with C—Si—C 80.0°, Si—C 1.871, C —O 1.126 A is calculated the bent conformation is energetically more stable by 76.7 kcal/mol than a linear Dxh structure [196], These results also allow a reinterpretation of the experimental data (IR spectroscopy) for silicondicarbonyl [197]. 

Figure 4.11 (a) The folded or bent conformation of cyclobutane, (b) The bent or envelop form of cyclopentane. In this structure the front carbon atom is bent upward. In actuality, the molecule is flexible and shifts conformations constantly... 

Cyclopentane assumes a slightly bent conformation => relieves some of the torsional strain. 

Figure 4.21 Possible Fe-CO bent conformation binding modes.

The minimum of the triplet state corresponds to a larger Fe-O-O angle (131°) than the S = 1 state (121°), as we also found for a small FeP(Im)(02) system [6b]. On the other hand, an S = 0 closed-shell state is well separated in energy in the linear conformation (22 kcal mol-1 relative to the ground triplet state), but becomes very close to the ground state in the bent conformation (1.4 kcal mob1). Its electronic configuration can be shown schematically as ... 

Because the 3D-structure of the receptor was not yet known, we wanted to derive a pharmacophore model, to prove this hypothesis. Most of the H2-antagonists have much conformational freedom. Evaluation of available crystal data and a systematic conformational search revealed mainly bent conformations,... 

Muscimol and 4-PIOL are assumed to bind to the same binding site, but a direct superimposition of the amino groups and the 3-isoxazolol rings, which make up the pharmacophore elements in these compounds, seems impossible. However, owing to the flexibility of the arginine, this side chain can adopt an extended as well as a bent conformation, which makes it feasible to accommodate muscimol and 4-PIOL in the same binding pocket as illustrated in Fig. 6A. 

Figure 7.11 (A) Coordination of O2 and CO to heme iron in Mb and Hb. (B) Possible Fe-CO bent conformation binding modes.

The hexafluoroantimonate salt of the terthiophene cation-radical in which terminal thiophene was completely surrounded by bicycle[2.2.2] octane units was studied by x-ray crystallography (Nishinaga et al. 2004). In the crystal lattice, the authors did not find only one isomer. They wrote about the dominant form, which was an x-trans-trans conformer. The central thiophene ring remains free, and only the terminal rings are fused with the bicycle[2.2.2] octane units—this cation-radical acquires a bent conformation. As B3LYP calculations show, the all-anti conformer is more stable than the all-syn counterpart by 5.35 kJ moY (Yamazaki et al. 2006). 

The trans-bent conformation is not the preferred conformation in those ylides where the carbanion bears a substituent with a lone pair of electrons. Schmidbaur and coworkers have made a study of phosphino-substituted ylides and related species23,33-35,90-93. In the case of mono(phosphino)methylides there are a number of possible conformations, shown in Figure 3, all with the lone pair of electrons at the phosphino group perpendi-... 

Phosphorus ylides have been succinctly described23 as an easily pyramidalized car-banion stabilized by an adjacent tetrahedral phosphonium centre. They usually adopt the trans-bent conformation (Figure 2) but the barrier to rotation about P=C is very low (4-5 kJ mol 1). 

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