Confonner stability

A drop of a dilute solution (1%) of an amphiphile in a solvent is typically placed on tlie water surface. The solvent evaporates, leaving behind a monolayer of molecules, which can be described as a two-dimensional gas, due to tlie large separation between tlie molecules (figure C2.4.3). The movable barrier pushes tlie molecules at tlie surface closer together, while pressure and area per molecule are recorded. The pressure-area isotlienn yields infonnation about tlie stability of monolayers at tlie water surface, a possible reorientation of tlie molecules in tlie two-dimensional system, phase transitions and changes in tlie confonnation. Wliile being pushed togetlier, tlie layer at... 

Thermal stahility. Yor applications of LB films, temperature stability is an important parameter. Different teclmiques have been employed to study tliis property for mono- and multilayers of arachidate LB films. In general, an increase in temperature is connected witli a confonnational disorder in tire films and above 390 K tire order present in tire films seems to vanish completely [45, 46 and 45] However, a comprehensive picture for order-disorder transitions in mono- and multilayer systems cannot be given. Nevertlieless, some general properties are found in all systems [47]. Gauche confonnations mostly reside at tire ends of tire chains at room temperature, but are also present inside tire... 

Thermal stability. The tliennal stability of SAMs is, similarly to LB films, an important parameter for potential applications. It was found tliat SA films containing alkyl chains show some stability before an increase in tire number of gauche confonnations occurs, resulting in melting and irreversible changes in tire film. The disordering of tire... 

Stabilization of the syn conformer in the gas phase is explained rather intuitively in terms of the extra stabilization due to increased interactions between the H atom in the OH group and the O atom in C=0 group. As one can see in Figure 5, the extra stabilization in the anti confonner in aqueous solution arises from the solvation energy, especially at the carbonyl oxygen site. 

Fig. 1.29. Interactions between two hydrogen Is orbitals and carbon 2p orbitals stabilize the eclipsed confonnation of propene.

Disubstituted cyclopropanes exemplify one of the simplest cases involving stability differences between stereoisomers. A three-membered ring has no confonnational mobility, so the ring cannot therefore reduce the van der Waals strain between cis substituents on adjacent carbons without introducing other strain. The situation is different in disubstituted derivatives of cyclohexane. 

Section 3.13 Higher cycloalkanes have angles at carbon that are close to tetrahedral and are sufficiently flexible to adopt confonnations that reduce their torsional strain. They tend to be populated by several different confonna-tions of similar stability. 

Preferential Stabilization of Different Confonners of the Rare Gas-Dihalogen Complexes... 

Filled orbitals which cannot physically separate because they are part of the same molecule may have substantially larger overlap than the intennolecular case, and the destabilizing interaction, Asu, may be much larger than the stabilizing interaction, Ael. The net repulsion will lead to confonnational changes so as to minimize the repulsive interactions. If the interaction cannot be avoided by confonnational change, then as the result of the interaction, a pair of electrons is raised in energy. The system has a lower... 

The operation of the anomeric effect and the stabilization of carbocations are beautifully illustrated in a confonnational study of 2-oxanol (2-oxacyclohexanol) (Smith, B. J., J. Am. Chem. Soc., 1997, 119, 2699-2706). 2-Oxanol prefers the OH axial fonn by 12 kJ/mol and, upon protonation of the OH group, spontaneously loses water to fonn the oxonium ion. Use principles of orbital interaction theory to explain ... 

The overall yield of the reaction is 98 % as a >98 2 regioisomeric mixture. The major isomer is 88 12 endolexo-rmxtxae. It can be explained from consideration of the stability of plausible triplet diradicals and the ISC of the confonners in the formation of flie product. The e/idb-conformer shows favorable spin-orbit interactions for formation of a sigma bond in the product, whereas exo-conformer faces steric restriction for this orbital interaction. 

The confonnation of fluoroalkane derivatives is of considerable contemporary interest. The influence of fluorine on the conformation of vicinally substituted compounds is based on a stabilizing gauche interaction that relies upon hyperconjugation between a C-H bond and the low lying a orbital of the C-F bond. The energy of this interaction is estimated to be 0.8 kcal mol", modest but... 

The most remarkable feature of AdC/(7) is that it goes through a maximum, at a temperature 7 s where AdS(7, ) = 0 and the stability of the native confonnation, AjOX/), is therefore only enthalpic (see Figure 29). 

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