Fluxional

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 volatile, air-sensitive Hquid species (CH3)2AlB3Hg and (CH3)2GaB3Hg are prepared by the direct reaction of the corresponding main group metal hahde and salts of the [B3Hg] ion, in the absence of solvent (178). The reaction of (CH3)2AlB3Hg and A1(BH 3 results in the species (BH 2AlB3Hg. These small metallaboranes are fluxional in solution and have limited thermal stability at room temperature. 

In another fluxional process involving ruthenium instead of rhodium, it has been shown that the rate-controlling step is the complex dissociation and that the ligand exchanges between the two annular nitrogen atoms by an intermolecular process. 

In solution, the phenyl derivative 12.29 (R = Ph) is fluxional. The mechanism of the fluxional process has been shown by an N NMR investigation of a partially N-labelled sample, Le., PhCN2 N3S3 ( N = 99% N), to involve a series of 1,3-nitrogen shifts (Section 4.8.4). Thermolysis or photolysis of 12.29 generates the corresponding 1,2,3,5-dithiadiazolyl radicals [RCNaSa]". ... 

The open face comprises a fluxional system involving the two atoms and the endo-Hi atom of the BH2 group)... 

AI(BH4)3 was the first fluxional compound to be recognized as such (1955) and its thermal decomposition led to a new compound which was the first to be discovered and structurally characterized by means of nmr ... 

This binuclear complex is also fluxional and has the structure shown in Fig. 7.4a. A1(BH4)3 reacts readily with NMe3 to give a 1 1 adduct in which Al adopts the unusual pentagonal... 

XeFs 49.5 Distorted octahedral (fluxional) Pentagonal bipyramidal or capped oct edral (1)... 

In addition to acting as an rf ligand, CgHg can coordinate in other modes, " some of which are illustrated in Fig. 19.33. Many of these complexes show fluxional behaviour in solution (p. 935) and the distinction between the various types of bonding is not as clear-cut as implied by the limiting structures in Fig. 19.33. 

Fe(CO)s is a highly toxic substance discovered in 1891, the only previously known metal carbonyl being Ni(CO)4. Like its thermally unstable Ru and Os analogues, its structure is trigonal bipyramidal (Fig. 25.10a) but its C nmr spectrum indicates that all 5 carbon atoms are equivalent and this is explained by the molecules fluxional behaviour (p. 914). 

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