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fluxional molecule
Fluxional molecules are molecules that undergo dynamics such that some or all of their atoms interchange between symmetry-equivalent positions. Because virtually all molecules are fluxional in some respects, e.g. bond rotations in most organic compounds, the term fluxional depends on the context and the method used to assess the dynamics. Often, a molecule is considered fluxional if its spectroscopic signature exhibits line-broadening (beyond that dictated by the Heisenberg Uncertainty Principle) due to chemical exchange. In some cases, where the rates are slow, fluxionality is not detected spectroscopically, but by isotopic labeling.
==Carbonium ion==
The prototypical fluxional molecule is the carbonium ion, which is protonated methane, CH5+. In this unusual species, whose IR spectrum was recently experimentally observed and more recently understood, the barriers to proton exchange are lower than the zero point energy. Thus, even at absolute zero there is no rigid molecular structure, the H atoms are always in motion. More precisely, the spatial distribution of protons in CH5+ is many times broader than its parent molecule CH4, methane.〔For an animation of the dynamics of CH5+, see http://www.theochem.ruhr-uni-bochum.de/research/marx/topic4b.en.html〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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