Conformational interconversion pathways

Conformational preferences have been studied for many other C Hs-X-CgHs fragments (X = CH2, CR2, C = 0, NH, NR, O, S, SO2) [42], as well as for nitrobenzenes with C(sp ) or N(sp ) substituents in ortho-position [43], If the terminal phenyl groups in the above examples are replaced by triptycyl-units, molecular analogs of bevel gear systems are obtained [44]. The conformational interconversion pathway mapped by bis(9-triptycyl)methane and related molecules follows almost ex-... 

The pioneering applications of molecular mechanics to coordination compounds were isomeric and conformational analyses [1,2]. Applications involving the computation of conformer equilibria discussed in this chapter are studies of solution structure refinements [217,255, 256], racemate separations [293-295], and the evaluation of conformer interconversion pathways [237, 238, 296]. The importance of conformer equilibria in the areas of electron-transfer rates and redox potentials is discussed in Chapter 11, and many examples discussed in the other chapters of Part II indicate how important the prediction of conformational equilibria is in various fields of coordination chemistry. 

In a series of papers entitled Structural Systematics, Orpen and co-workers used the GSD to examine a number of geometric aspects of /7-block metal complexes including torsional flexibility and conformer interconversion pathways for phosphine and diphosphine ligands, carbonyl ligand migration between metal centers (see also Figure 9,... 

Fischer S, P D J Groothenuis, L C Groenen, W P van Hoorn, F C J M van Geggel, D N Reinhoudt and M Karplus 1995. Pathways for Conformational Interconversion of CaUx[4]arenes. Journal of the American Chemical Society 117 1611-1620. 

Malloy, T. B., Jr., Conformational Barriers and Interconversion Pathways in some Small-Ring Molecules, 11, 97. 

Fig. 7 Correlations between the two OC-O torsional angles x and 2 for acyclic C-O-C-O-C systems [6]. The ellipsoidal curve marks the suggested pathway for conformational interconversions. Reprinted from Cosse-Barbi and Dubois (1986), copyright 1986, with kind permission from Pergamon Press Ltd, Headington Hill Hall, Oxford 0X3 OBW, UK.

For bicyclic and polycychc systems, fewer conformations and interconversion pathways need to be considered than is the usual case for monocyclic systems. For example, placing a CHj bridge on a... 

Energy profiles in Figure 14 also reveal that planar furanose forms are often of lower energy than puckered conformers. For example, relative conformational energies determined for 7 with the 3-21G basis set indicate that the planar conformer is more stable than the Eq conformer in 8, the planar conformer is calculated to be more stable than. These observations suggest that the conformational dynamics of some furanose rings may not be completely described by pseudorotation in these cases,conformer interconversion may occur by both inversion and pseudorotational pathways, with the latter being the more preferred route. 

Barriers, Conformational, and Interconversion Pathways in Some Small... 

When several sp3-hydridized substituents are attached to a planar framework, their mutual interactions lead to conformational states and interconversion pathways that complicate the simple pictures given in... 

Conformational interconversion is a process that cannot be observed experimentally. However, the energy profile associated with interconversion can be mapped using molecular mechanics. Then, representative structures along the minimum energy pathway can be predicted and visualized. For example, the mechanism of conversion from the X to the <5 conformer of [Co(en)(NH3)4]3+ has been calculated in this way11801. 

Derivative 178 containing a dibenzo-34-crown-10 ring interlocked with macrocycles incorporating two 4,4 -dipyr-idyl moieties tethered by different aryl spacers acts as bistable [2]catenane <20060L2119>. Variable-temperature (VT) NMR studies were used to determine the activation energy required for the conformational interconversions and to demonstrate that, by appropriate incorporation of bulky groups on one or both of the aryl linkers, it was possible to block one or both of the two circumrotation pathways. 

Hendrickson has investigated symmetrical interconversion pathways between cyclooctane conformations. In some cases at least, these paths are probably not as low-energy as unsymmetrical paths, which unfortunately are more difficult to calculate. In the crown family, which includes the chair-chair and twist-chair-chair forms, the situation is quite clear. The twist-chair-chair is of lowest energy and conversion to the chair-chair form involves only the energy difference between the two forms. Thus the chair-chair is a transition state for the degenerate interconversion of twist-chair-... 

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