Fluxional molecule

Fluxional Molecules.— The two main areas of interest in current studies of fluxional molecules are the determination of the mechanism, for example whether 1,2- or 1,3-shifts are involved, and the determination of activation parameters. Some values of the latter are given in Table 1. 

The first example of a fluxional benzene-metal compound has been reported, the l,2,3,4-/e/ru/r / robenzenerhodium(i) compound (50). The 

An X-ray structural study of / -methylbenzylcyclopentadienylmolyb-denumdicarbonyl indicates that the benzyl ligand is 1,2,7-trihapto (51). Thus variable-temperature n.m.r. spectra of this and related compounds [(51) to (53)] can be interpreted in terms of fluxional behaviour, whose mechanism involves a a-benzyl intermediate and rotation about the C1-C7 bond. In contrast to these benzyl compounds, analogous 7r-2-thenyl (54) and 7T-3-thenyl (55) compounds show no evidence for fluxional behaviour in their n.m.r. spectra over a wide range of temperature.  

Fluxional group Benzyl Benzyl Benzyl Benzyl Benzyl 

Examples of fluxional behaviour may also be found in organometallic compounds containing more than one metal atom in the molecule, for example (C5H5)(CO)2Mo(C7H,)Fe(CO)3 and the cluster compound Ru3(C8H8)(CO)4, whose limited solubility unfortunately precludes satisfactory determination of kinetic parameters.  

—Fluxional Molecules. The newly prepared compounds bis-(6-phenyl-and bis-(6-t-butyl-l,3,5-trimethylcyclohexadienyl)iron(n) (14) exhibit a sort of fluxional behaviour in which a rocking or twisting motion of the cyclohexadienyl rings interconverts a- and w-bonds to the iron. The barriers to this motion are very similar for the phenyl and t-butyl com- 

There is wide-line n.m.r. evidence for fluxional behaviour even in the solid state, as, for example, for the cyclo-octatetraene complex (18) and the di-iron species Fe2(C8H8)(CO)6.  

We now turn to some fluxional molecules these feature dynamics in which atoms interchange between symmetry-equivalent positions, without bond breaking necessarily occurring. 

The CH5+ cation is isoelectronic and isostructural with the neutral molecule BH5. Both have equilibrium point group Cs. Comparisons between various relevant compounds yielded prediction of some NMR parameters of the methonium ion.102 

As a second example, we can cite a relatively simple molecule PF5, which was first measured by 19F NMR in the early days, in the gas phase.40 There was no 

Similar effects may occur in molecule C1F3, which has an equilibrium T-shaped structure (G = C2V)- Both gas and liquid-phase 19F NMR measurements have been reported (yielding an AB2-type spectrum),104 and some theoretical analyses exist.10 106 

If the two (or more) alternative configurations of a stereochemicaUy non-rigid molecule are chemically equivalent and have identical energies as, for example, in (3.36), the molecule is said to be fluxional. 

StereochemicaUy non-rigid molecules such as PHj, which undergo inversion, or PFj, which undergo pseudorotation, can be considered as simple examples of fluxional molecules. 

A second general example is provided by CO ligands, in complexes containing this ligand bonded in a variety of ways. Crystal structure measurements have revealed that there is an almost continuous range of 

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Figure A3.12.5. A model reaction coordinate potential energy curve for a fluxional molecule. (Adapted from [30].)...

In the above discussion it was assumed that the barriers are low for transitions between the different confonnations of the fluxional molecule, as depicted in figure A3.12.5 and therefore the transitions occur on a timescale much shorter than the RRKM lifetime. This is the rapid IVR assumption of RRKM theory discussed in section A3.12.2. Accordingly, an initial microcanonical ensemble over all the confonnations decays exponentially. However, for some fluxional molecules, transitions between the different confonnations may be slower than the RRKM rate, giving rise to bottlenecks in the unimolecular dissociation [4, ]. The ensuing lifetime distribution, equation (A3.12.7), will be non-exponential, as is the case for intrinsic non-RRKM dynamics, for an mitial microcanonical ensemble of molecular states. 

The iminophosphoranes (181) with imide bromides gave the amidino-phosphonium bromides (183) which, it appears from their temperature-variable n.m.r. spectra, are interesting fluxional molecules. The free energy of activation for the interconversion of (183a) and (183b) (R R = Pr R3 = Ph) is 17.2 0.9kcalmol-i. 

If one end of a chelate ring on an octahedral complex is detached from the metal, the five-coordinate transition state can be considered as a fluxional molecule in which there is some interchange of positions. When the chelate ring reforms, it may be with a different orientation that could lead to racemization. If the chelate ring is not symmetrical (such as 1,2-diaminopropane rather than ethyl-enediamine), isomerization may also result. For reactions carried out in solvents that coordinate well, a solvent molecule may attach to the metal where one end of the chelating agent vacated. Reactions of this type are similar to those in which dissociation and substitution occur. 

Equilibration of fluxional molecules must not be confused with resonance. In each electrocyclic reaction, the nuclei alter their positions as bond lengths and angles change. Interconversion of fluxional molecules also must not be... 

The fully cyclopropylated ylid 4 is a fluxional molecule exhibiting rapid or-proton exchange (15). 

The reaction of methyl tropyl ether-iron tricarbonyl with acid yields a tropylium complex, in which the ring formally contains an uncoordinated double bond (188), but is actually an example of a fluxional molecule. 

Tris(dimethylphenylphosphine)pentahydridorhenium(V) is a white, crystalline solid, mp 97-98°. It may be remelted several times without apparent decomposition and is stable for several months at 20°. It is soluble in cold benzene but much less soluble in cold ethanol or light petroleum ether. The rhenium-hydrogen stretching vibrations, v(Re—H), are (in Nujol mull) 1949 (w, sh), 1931 (m), 1905 (m), and 1852 (m) (in benzene solution) 1969 (sh), 1898, 1855, 1817 (sh) cm-1. In the nmr spectrum (benzene at 34°) the hydridic hydrogens are apparently equivalent (fluxional molecule)... 

There are two basic types of chemical processes reversible (leading to, and maintaining, an equilibrium mixture) and irreversible (proceeding in one direction, i.e., to completion). NMR methods can be used to study both types of processes. A common example of an irreversible process is a chemical reaction with a free energy more negative than ca. 20 kJ/mol. Reversible processes include interconversions of conformations by rotations around single bonds, interconversions of fluxional molecules and valence isomers, and proton transfers, to name a few. 

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