Fluctional molecules

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

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... 

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 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,... 

Many examples of tbis4j e oCrearrangement are known, one of the more interesting being that o ullvalene, ch is converted into itself with a first-order rate constant of 3440 sec dl 25"C." At 100 C, the NMR spectrum of bullvalene exhibits only one peak at 5.78 r, indicative of the fluctional nature of the molecule. The free energy of activation for the rearrangement has been determined as 12.8 kcal/mol, a very low value indeed. ... 

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