Structural dynamic fluxionality

Fig. 1.59. Dynamic structural fluxionality of Aus upon deposition, adsorption of reactants (O2, CO) and oxidation reaction. Note that the most stable geometric structure for Aug in the gas phase is planar. On an F center of the MgO(lOO) surface, Aug shows two energetically very close lying isomers, a quasi-planar structure (not shown here, as this isomer reveals unfavorable energetics in the reaction path of the oxidation reaction) and the bicapped octahedron shown in this figure as structure a and b. Upon adsorption of oxygen, the bicapped octahedral structure is transformed into a bicapped bipyramid (structures c, d, and e). Interstingly, when freezing the structure of the bicapped octahedron no adsorption of O2 is possible. Thus, the reaction takes place only when the bicapped bipyramidal structure is populated. Upon adsorption of CO only minor structural changes are observed...

Molecular Motions and Dynamic Structures. Molecular motions are of quite general occurrence in the solid state for molecules of high symmetry (22,23). If the motion does not introduce disorder into the crystal lattice (as, for example, the in-plane reorientation of benzene which occurs by 60° jumps between equivalent sites) it is not detected by diffraction measurements which will find a seemingly static lattice. Such molecular motions may be detected by wide-line proton NMR spectroscopy and quantified by relaxation-time measurements which yield activation barriers for the reorientation process. In addition, in some cases, the molecular reorientation may be coupled with a chemical exchange process as, for example, in the case of many fluxional organometallic molecules. ... 

Homoleptic complexes have been obtained also with tetrakis(l-pyrazolyl)borates, e.g., [ B(pz)4 2Cd] and [ B(3-Mepz)4 2Cd] (both P2 jc, Z = 2) in both compounds structure analyses the ligands have been shown to coordinate trihapto, i.e., with one pz ring free. In both cases Cd has a distorted octahedral environment, with averaged structural data very similar to those for the tris(l-pyrazolylhydridoborate complexes.201 Variable-temperature 3H NMR studies of these and of mixed complexes with tris- and bis(l-pyrazolylhydridoborates indicate fluxional behavior (coalescence temperatures and barriers for the dynamic processes are given). 

Secondly, for some crystalline systems, the structure obtained by diffraction techniques may be incomplete. For example, in some cases the diffraction data may not reveal dynamic aspects of the solid-state structure (as in the case of fluxional organo-metallics) and in others it may not be possible to distinguish clearly between different atoms (as for example 27A1 and 29Si in zeolites) and a combination of the NMR and x-ray data will yield a more complete and meaningful description of the structure. 

The synthesis, X-ray structure, and dynamic NMR properties of the arachno 10-vertex cluster [9,9-(PMe2Ph)2-9,6,8-PtS2B7H7] has been reported the compound was fluxional via a vertex flip of the Pt(PMe2Ph)2 moiety.84... 

The CH cation 1, protonated methane, is the parent of hypercoordinated carbocations containing a five coordinated carbon atom. It is elusive in solution and has not been observed by NMR spectroscopy but gas-phase infrared investigations have shown its fluxional structure which has been proven by ab initio molecular dynamic simulation.18... 

Numerous studies concerning the structure and dynamics of the CH5+ have been reported (5-7, 16, 18, 67-74). Figure 2 shows the Bom-Oppenheimer molecular dynamics (BOMD) calculations for CHS+, calculated at B3LYP/6-31G level. The structures (a) and (b) correspond to snapshots from the dynamics. Figure 2 (c) shows the superimposed structures from the dynamics, indicating the high fluxionality of CH5+. 

Dynamic intramolecular rearrangements are observed for a variety of diene-metal complexes at, or near, ambient temperature. This stereochemical non-rigidity may be detected by variable temperature NMR experiments40 in which the signals observed for a static structure coalesce into time averaged signals for the fluxional process. For purposes of this section, processes with activation energies > ca 25 kcal mol 1 or which are irreversible will be considered to be isomerization phenomena and will be discussed in Section IV. 

V. G, J. Am. Chem. Soc., 1995, 117, 12863-12864). Dynamic 13C NMR evidence indicates that the complex is fluxional. A structure was postulated, based on semi-empirical calculations, in which the NO+ is attached to a single n bond. Use principles of orbital interaction theory to discuss the structure and bonding of such a complex (see also Figure 4.8 and question 4, Chapter 11). 

The observant reader will undoubtedly have noted that no evidence of any stereochemical nonrigidity (on the NMR time scale) has been obtained for any of the mononuclear complexes derived from OFCOT. However, the dinuclear complex 88 does exhibit fluxional behavior (151). If the solid-state structure of 88 were rigidly maintained in solution, the 19F-NMR spectrum should show eight resonances due to symmetry inequivalent fluorines. However, only four broad resonances are present in the 19F NMR spectrum at room temperature, indicative of a dynamic process that interconverts the two enantiomorphs of 88. Two mechanisms have been proposed for such interconversions in COT analogues a glide and a twitch (Scheme 3). The... 

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