Confonner equilibrium

Fig. lA Theoretical quantitative data on the confonnational equilibrium of butane (1) and methoxyethane (II). Abscissae MeC-CMe or MeC-OMe torsion angles ordinates (a) 10 molar fractions by angle degree (b) kcal mol (from Jorgensen et al. 1981 Jorgensen and Ibrahim 1981) (reproduced with kind permission from the American Chemical Society). 

Van der Waals complexes can be observed spectroscopically by a variety of different teclmiques, including microwave, infrared and ultraviolet/visible spectroscopy. Their existence is perhaps the simplest and most direct demonstration that there are attractive forces between stable molecules. Indeed the spectroscopic properties of Van der Waals complexes provide one of the most detailed sources of infonnation available on intennolecular forces, especially in the region around the potential minimum. The measured rotational constants of Van der Waals complexes provide infonnation on intennolecular distances and orientations, and the frequencies of bending and stretching vibrations provide infonnation on how easily the complex can be distorted from its equilibrium confonnation. In favourable cases, the whole of the potential well can be mapped out from spectroscopic data. 

When two confonnations of a molecule are in equilibrium with each other, the one with the lower free energy predominates. Why is equatorial methylcyclohexane more stable than axial methylcyclohexane ... 

Other substituted cyclohexanes are similar- to methylcyclohexane. Two chair confonnations exist in rapid equilibrium, and the one in which the substituent is equatorial is more stable. The relative fflnounts of the two confor-rnations depend on the effective size of the substituent. The size of a substituent, in the context of cyclohexane confor-rnations, is related to the degree of branching at the atom connected to the ring. A single... 

Since the energy barriers between the conformers at room temperature are small compared to the product kT, the respective molecules exist as an equilibrium mixture of different confonners. 

A ground state heterogeneity due to the presenee of an equilibrium between different eonformers. The relative orientations Trp-heme are rigid and different from one eonformer to another each conformer presents a speeifie fluoreseenee lifetime. The three exponential fluoreseenee decay from the hemoglobin rules out the presenee of the two static confonnations T and R supposed to be in equilibrium. 

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