Fluxional (or non-rigid) 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. Where such movement does not occur, the molecule may be described as a semi-rigid molecule.[1][2][3][4] Longuet-Higgins introduced the use of permutation-inversion groups for the symmetry classification of the states of fluxional (or non-rigid) molecules.[5][6]
A well-studied fluxional ion is the methanium ion, which is protonated methane, CH+
5.[7][8][9] In this unusual species, whose IR spectrum was recently experimentally observed[10][8] and more recently understood,[11][12][13] 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 CH+
5 is many times broader than its parent molecule CH4, methane.[14][15]
https://en.wikipedia.org/wiki/Fluxional_molecule
Phosphorus pentafluoride, PF5, is a phosphorus halide. It is a colourless, toxic gas that fumes in air.[1][2]
Preparation[edit]
Phosphorus pentafluoride was first prepared in 1876 by the fluorination of phosphorus pentachloride using arsenic trifluoride, which remains a favored method:[1]
- 3 PCl5 + 5 AsF3 → 3 PF5 + 5 AsCl3
Structure[edit]
Single-crystal X-ray studies indicate that the PF5 has trigonal bipyramidal geometry. Thus it has two distinct types of P−F bonds (axial and equatorial): the length of an axial P−F bond is distinct from the equatorial P−F bond in the solid phase, but not the liquid or gas phases due to Pseudo Berry Rotation.
Fluorine-19 NMR spectroscopy, even at temperatures as low as −100 °C, fails to distinguish the axial from the equatorial fluorine environments. The apparent equivalency arises from the low barrier for pseudorotation via the Berry mechanism, by which the axial and equatorial fluorine atoms rapidly exchange positions. The apparent equivalency of the F centers in PF5 was first noted by Gutowsky.[3] The explanation was first described by R. Stephen Berry, after whom the Berry mechanism is named. Berry pseudorotation influences the 19F NMR spectrum of PF5 since NMR spectroscopy operates on a millisecond timescale. Electron diffraction and X-ray crystallography do not detect this effect as the solid state structures are, relative to a molecule in solution, static and can not undergo the necessary changes in atomic position.
Lewis acidity[edit]
Phosphorus pentafluoride is a Lewis acid. This property is relevant to its ready hydrolysis. A well studied adduct is PF5 with pyridine. With primary and secondary amines, the adducts convert readily to dimeric amido-bridged derivatives with the formula [PF4(NR2)]2. A variety of complexes are known with bidentate ligands.[4]
Hexafluorophosphoric acid (HPF6) is derived from phosphorus pentafluoride and hydrogen fluoride. Its conjugate base, hexafluorophosphate (PF6–), is a useful non-coordinating anion.
https://en.wikipedia.org/wiki/Phosphorus_pentafluoride
In nuclear and particle physics, the concept of a neutron cross section is used to express the likelihood of interaction between an incident neutron and a target nucleus. In conjunction with the neutron flux, it enables the calculation of the reaction rate, for example to derive the thermal power of a nuclear power plant. The standard unit for measuring the cross section is the barn, which is equal to 10−28 m2 or 10−24 cm2. The larger the neutron cross section, the more likely a neutron will react with the nucleus.
An isotope (or nuclide) can be classified according to its neutron cross section and how it reacts to an incident neutron. Nuclides that tend to absorb a neutron and either decay or keep the neutron in its nucleus are neutron absorbers and will have a capture cross section for that reaction. Isotopes that fission are fissionablefuels and have a corresponding fission cross section. The remaining isotopes will simply scatter the neutron, and have a scatter cross section. Some isotopes, like uranium-238, have nonzero cross sections of all three.
Isotopes which have a large scatter cross section and a low mass are good neutron moderators (see chart below). Nuclides which have a large absorption cross section are neutron poisons if they are neither fissile nor undergo decay. A poison that is purposely inserted into a nuclear reactor for controlling its reactivity in the long term and improve its shutdown margin is called a burnable poison.
https://en.wikipedia.org/wiki/Neutron_cross_section
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