Fluctional

FluR84 Flurry, R. L. Isomer counting for fluctional molecules. J. Chem. Educ. 61 (1984) 663-665. 

Si-P bonds are formed in the reaction with di- and tri-phosphinomethanides (Eqs.(l), (2)), and the resulting ylides 4 and 5 are fluctional in solution. In the case of monophosphinomethanide 3, both Si-C and Si-P bond formation is observed. The ylide 6 is rigid in solution on the NMR time scale, but it rearranges to the phosphinomethane derivative 7 within several days at 20 °C. 

A model with a pentacoordinate transition state II may account for the observed fluctionality and for the reluctance of 4 and 5 to rearrange to the respective phosphinomethane. 

For 10, the differences of axial and equatorial Si-P bond lengths are attributable to a "normal" cis-influence of the methyl groups, whereas for 9, the difference Si-Pax/Si-Peq is drastic. In fact, in the solid state, 9 rather appears as "a frozen transition state" between four- and hexacoordination (omitting pentacoordination). In solution, 9 is fluctional. 

Of course, as with any spectroscopic technique, FTIR should not be used in isolation but always in conjunction with other techniques. FTIR is most usefully paired with circular dichroism in both the UV (i.e., ECD) and, if possible, also in the IR (i.e., VCD))64 70 73 110-112 Coupling IR to NMR studies can usefully aid interpretation of spectra from fluctional structures and can lead to some site-specific insight)113-1151 Even the generality of X-ray diffraction results (especially for linear oligopeptides whose conformation can be strongly influenced by intermolecular interactions in the crystal) can be tested by comparison of solid-state and solution-phase IR data with solution-phase CD data.1"6 "71... 

Buhl, M. 2000. Density-fluctional study of vanadate-glycylserine isomer. J. Inorg. Biochem. 80 137-139. 

An examination of the structure of lanthanide(III) ion complexes in solution using nuclear magnetic resonance is made in considerable detail. The importance of hydration in both the inner and outer spheres is stressed. The structural data are then used in an attempt to understand the stability constants and the rates of exchange of bound ligands. Fluctional properties are also analysed. Finally, the importance of the structures, the stability constants and the kinetic properties are related to the effects of Ln(III) ions, and through them Ca(II) ions, in biological systems. 

Fig. 7. Rates, k, of fluctional motion of dipicolinate in [Lu(III)(dipicolinate)2(H20)3] at low temperature. Note the sudden change in log k with 1/r2...

The stretched porphycene 4.142 ([26]-porphyrin-(6.0.6.0)) was also recently reported by Vogel, et al. This macrocycle was prepared in 50% yield via a McMurry-type dimerization of dialdehyde 4.140, followed by a spontaneous dehydrogenation of the presumed non-aromatic intermediate (Scheme 4.3.11). The free-base form of [26]-porphyrin-(6.0.6.0) (4.142a) proved to be rather fluctional in a variety of solvents over the temperature range of 25 to -50 °C. This hindered... 

Early NMR spectroscopic studies provided support for the conclusion that hexaphyrin is a fluctional molecule that is able to adopt a nearly planar conformation in solution. However, according to molecular models, planarity can only be achieved when two opposing sets of exopyrrolic double bonds of the macrocycle... 

Prof Debra Searles Fluctional relations, free energy calculations and irreversibility... 

The crystal and molecular structure of (S)-7-phenyldinaph[2,l-h T,2 -d]arsole (82), which was obtained by spontaneous resolution of the racemate from hot methanol, reveals appreciable bending of the distorted naphthyl residues away from each other (Scheme 4) . The molecule is fluctional in solution on the NMR time scale, however, with similar barriers between the conformational isomers (atropisomers) for the 7-phenyl [AG 59 1 kJ mol" (259 K)] and the 7-methyl [AG 65 1 kJmol" (287 K)] compounds. The analogous phospholes are also unsuitable for resolution because of similarly low barriers to inversion of the atropisomers - Both arsenic ligands, when coordinated to iron(II) in complexes of the type [( -C5H5) l,2-C6H4(PMePh)2 FeL]PFg,... 

When cyclobutadiene is liberated as a transient species by oxidative decomposition of cyclobutadieneiron tricarbonyl (lead tetraacetate in pyridine) in the presence of cyclopentadicnone diethyl ketal the adduct I is obtained. Irradiation of 1 in acetone gives 9,9-diethoxyhomocubane, which on hydrolysis gives homocubanone. II. The corresponding alcohol, homocubanol. is of interest because the derived cation is a fluctional molecule.2... 

The X-ray structure of the latter complex (Fig. 5) shows that it contains a [HRh(/A-H)2RhH] core. Both complexes may be considered classical rhodium hydride species, although they are of a less common variety in that the ancillary amine ligands are purely room temperature, the complex of 4 is fluctional whereas that of 3 is rigid. 

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