Structural dynamical fluxionality

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... 

Structural dynamical fluxionality Interconversion between different structural isomers at finite temperature. 

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. 

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. ... 

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... 

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). 

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