Fluxional Processes in Boranes and Carboranes

Quasistructural molecules

“…For the particular case of fluxional molecules, it is worth discussing how small or large the barriers need to be in order for the molecule to show fluxional behavior. McKee 87 provides molecular examples for the following types of "fluxionality": (a) valence (e.g., the Cope rearrangement of bullvalene 88 ), (b) proton transfer (e.g., malonaldehyde 89 ), (c) ring whizzer (e.g., transition metal complexes with cyclic ligands 90 ), (d) pseudorotation (e.g., PF 5 91), and (e) nonclassical bonding (e.g., boranes and carboranes 92 ). McKee 87 also points out that core-shell clusters (e.g., gold nanoparticles 93 ) and transitionmetal complexes (e.g., mixed-valence dinuclear ruthenium complexes 94 ) often exhibit fluxional behavior.…”

Three-Dimensional Aromaticity in Polyhedral Boranes and Related Molecules

“…An important property of some of the deltahedral boranes is the stereochemical nonrigidity or fluxionality of some of the polyhedra. 71 Such fluxionality can be recognized experimentally by nuclear magnetic resonance (NMR) spectra (e.g., 1 H or 11 B) with fewer distinct resonances than would be predicted by the symmetry of the deltahedron. In addition, fluxional deltahedra can exhibit a temperature-dependent NMR spectrum with all of the resonances predicted from the symmetry of the deltahedron being observed at low temperatures but coalescence of some of the sets of resonances into single resonances being observed at higher temperatures.…”

Ab Initio Study of Rearrangements on the (CH)2(BR)2, RH, and NH2 Potential Energy Surfaces

“…The leastmotion reaction path for interconverting 1 and 2 is a diamondsquare-diamond (DSD) step. 30 However, the DSD pathway is blocked by a HOMO-LUMO crossing similar in nature to that found in the rearrangement of (CH) 2 (BH) 3 and (CH) 2 (BH) 4 . 31,32 The reaction path interconverting 1 and 2 maintains a C 2 axis.…”