Solvent dependent configurations

Figure 3. Solvent dependence of the measured decay time of dichlorobis(5fi-dimethyl-1,10-phenanthroline )iridiurYh-(III) chloride at 77°K. Solvent A, 1 1 glycerol-water B, 4 1 glycerol-water C, 4 1 methanol-water D, glycerol E, 2 1 glycerol-ethanol F, 1 1 glycerol-ethanol G, ethanol and H, 4 1 ethanol-methanol, Insert proposed solvent-dependent configuration interaction mechanism.

G.R. Mitchell The first thing is that there are very, very strong solvent dependent configurations that can be generated, which people have studied in gels. Obviously, when you dry out the films you get quite close to a gelling point. Have you done any scattering smdies on the films ... 

In solution the compounds exhibit solvent-dependent colours in dilute solution in non-polar solvents, planar monomers are present but in more concentrated solutions oligomerization occurs. In the solid state a dimeric structure has been identified (X-ray, Figure 2.30) with R = Ph there is a staggered configuration (Rh-Rh 3.193 A) but with other isocyanides (R = 4-FC6H4) the configuration is eclipsed. 

The cavitands are essentially synthesized from their resorc[4]arene precursors which are readily obtained by resorcinol condensation with aldehydes. The main feature comes from the different configurations that are expected for this tetrameric species and the relative thermodynamical stability of each isomer, which has been widely investigated by several authors. In addition, the conformational mobility of the resorc[4]arene molecules will depend on substitution at the upper and lower rims [28, 36, 40, 41]. The first attempt to synthesize a phosphorus bridged cavitand was to treat resorc[4]arene la (1, R=CH3) with phenylphosphonic dichloride or phenylphosphonothioic dichloride. Only inseparable isomer mixtures were obtained and isolation of the desired cavitands was not possible [42]. The first isolated phosphorylated resorcinol-based cavitand was described in 1992 by Markovsky et al., who prepared compound D from la and four equivalents of o-phenylenechlorophos-phate in the presence of triethylamine [43, 44]. For this compound, a tautomeric temperature and solvent dependent equilibrium exists between the spirophosphorane structure and the cyclic phosphate form (Scheme 4). 

In the experiments of Weinstein and Firestone [26], it was shown that the spectrum of the electron in mixed solvents depends on the mixture of the two components, for example, an alcohol and an ether. If complete relaxation were possible, one would expect that the solvent molecules would interchange and the result would be the lowest energy configuration. This does not appear to occur. 

The orientation of association of the benzene molecule about the pyrrole NH group is open to some conjecture. It was assumed from IR studies118 that the orientation was orthogonal (18). Such a model is also compatible with solvent-dependence studies of the molar Kerr constant.155 The model does not, however, adequately account for the solvent effect of benzene upon NMR chemical shifts of the a- and /3-protons of pyrrole (see Section III, B, 3, a). The NMR observations are best explained in terms of a nonplanar configuration (19) resulting not from NH-7T bonding but, instead, from a 7r-dipole interaction.151 In an attempt to confirm the orthogonal model, LeFevre et a/.155... 

Whereas amines 80A and 80B are stable in solution, acetamides 81A and 81B show a solvent-dependent equilibration between atropoisomers. These equilibrations can be measured by CD since the conversion of (aft)-to (aS)-configurated isomers in this series of compounds is accompanied by a reversal of the sign of the Cotton effects at 260 nm. Also, protons 5-H of the (aft) and (a5) conformers could easily be distinguished in the H-NMR spectra. 

N-Substituted 3-aminocrotonic esters (67) tend to adopt a planar or near-planar structure. As a consequence of their push-pull nature these molecules show an increased facility for Z-67-is-67 isomerization around the C=C double bond as well as restricted rotation around the C—N and C—COOR2 single bonds. The IR and XH-NMR spectra of simple 3-(alkylamino)crotonic esters have shown that these substances exist either in the liquid state or in solution as equilibrium mixtures of the Z and E configurations, respectively. The position of the equilibrium is solvent-dependent, and the energy difference between the isomers varies from ca 7.3 kJ mol-1 in non-polar solvents to ca 0.8 kJ mol"1 in dimethyl sulphoxide, the intramolecularly bonded Z-form 68 or 69 being the most stable111-113. 

The lithiation of /ra .s-A-alkyl-2,3-diphenylaziridines such as 51 was reported to be completely a-regioselective while the stereochemical course of the lithiation-trapping sequence was found to be solvent dependent <07OL1263>. Retention of configuration was observed in hexane, ether, or toluene, while coordinating solvents such as THF or toluene/crown ether produced inversion. Related isomerization and dimerization reactions of a-lithiated terminal aziridines were also reported <07JOC10009>. 

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