Conformational interconversion solvent

Compound Substituents Solvent Conformational interconversion (AG /kcal mol-1) ring interconversion N-inversion (eae to aee)... 

The gas-phase thermodynamic (van t Hoff) parameters AH°, A5°, and AG029g for the syn-anti conformer interconversion of these gaseous alkyl nitrites are presented in Table 2. For MeONO this data compared well with those obtained by direct integration of slow exchange spectra. [27,22] The agreement of these gas-phase NMR thermodynamic parameters with microwave and theoretical data reinforce the validity of this technique applied to the syn-anti equilibria. Comparison of these gas-phase data with solution values yields information on the effects solvents have on the conformer equilibria, which in the particular case of these alkyl nitrites reflect a balance of steric and dielectric effects. 

Comparison of the barrier heights in the gas phase with those found in the liquid phase can elucidate the role of solvent internal pressure on conformational interconversion when dielectrical interactions are minimal. The magnitude of these interactions can be estimated by considering an activation volume, AV, for the process, defined as,... 

Figure 4 Schematic representation of a retention cycie pathway for the conformational interconversion of a globular protein or polypeptide, Pn.m> fi" solution in the mobile phase and in two unfolded states, and Pus> which occur in the presence of a liquid-solid interface involving immobilized nonpolar ligands of an RPC or HIC sorbent in the presence of an aquo-organic solvent, a kosmotropic or a chaotropic mobile phase system. If the globular protein or polypeptide undergoes a two-stage interconversion in the mobile phase and at the surface, the corresponding distribution process between the two chromatographic phases will involve the unfolded intermediates, Pum and PJjs- Also shown are the corresponding rate constants, k, for these interconversions. The subscripts refer to the native and unfolded states, N and U, respectively, and the mobile and stationary phase, M and S, respectively.

These kinetic parameters are very little affected by solvent composition over the range 0 to 35% dioxan/ Activation parameters AH and have been reported for conformational interconversion of the chelate ring in the [Fe(CN)4(l,3-propanediamine)] anion AH is 7.5kJmol higher here than for the analogous 1,2-ethanediamine anion/ ... 

These results allowed the proposal, at the beginning of the 1980s, of a different molecular model for cholesteric induction 65,66 This model is sketched in Figure 7.15 in the case when both nematic host and chiral guest have a biaryl structure. Nematic molecules exist in chiral enantiomorphic conformations of opposite helicity in fast interconversion. The chiral dopant has a well-defined helicity (M in Figure 7.15) and stabilizes the homochiral conformation of the solvent In this way, the M chirality is transferred from the dopant to the near molecule of the solvent and from this to the next near one and so on. This... 

Dynamic NMR shows the presence of two conformations in 1,3,5,7-tetroxocane a crown and a boat-chair. The equilibrium constant is strongly solvent and temperature dependent, with the highly symmetrical crown, which has a high dipole moment, having a lower entropy than the boat chair by about 6 J K-1 mol-1 (72JA1390,1389). Hie interconversion barrier is 50 kJ mol-1, and no ring inversion nor pseudorotation process has been detected in the boat-chair. 

From many studies it is known that the interconversion of conformations with the substituent in the equatorial and the axial positions occurs about 100,000 times per second, which corresponds to a transition-state energy (activation energy) of about 11 kcal mole-1 above the ground-state energy. The rate decreases as the temperature is lowered. If one cools chlorocyclohexane to its melting point (—44°), the substance crystallizes to give the pure equatorial isomer. The crystals then can be cooled to —150° and dissolved at this temperature in a suitable solvent. At —150° it would take about 130 days for half of the equatorial form to be converted to the axial form. However, when the solution is warmed to —60° the equatorial conformation is converted to the equilibrium mixture in a few tenths of a second. 

These steric interactions become more pronounced when we consider the introduction of an additional chelate ring in those complexes containing three didentate en ligands. The A XkX form of an [M(en)3]n+ cation is estimated to be 7-8 kJ mol-1 more stable than the A 888 diastereomer. This becomes particularly important when we consider kinetically inert complex cations, such as [Co(en)3]3+, where there is a significant barrier to the interconversion of the diastereomers. In practice, the conformation of the chelate rings in [Co(en)3]3+ salts depends upon the nature of both the anions and any additional solvent molecules in the lattice which can form hydrogen bonds with the en ligands. We will return to this topic in Chapter 7, where we discuss some reactions of [Co(en)3]3+ salts in which an extraordinary steric control is exerted. 

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