Phosphorus pentafluoride
Encyclopedia
Phosphorus pentafluoride, P
F5
, is a phosphorus halide
. It's a colourless gas at room temperature and pressure.
analysis gives similar values: P−Fax = 158 pm and P−Feq = 153 pm.
Fluorine-19 NMR
spectroscopy, 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. 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 their timescales are significantly shorter than for NMR spectroscopy.
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
F5
Fluorine
Fluorine is the chemical element with atomic number 9, represented by the symbol F. It is the lightest element of the halogen column of the periodic table and has a single stable isotope, fluorine-19. At standard pressure and temperature, fluorine is a pale yellow gas composed of diatomic...
, is a phosphorus halide
Phosphorus halides
There are three series of binary phosphorus halides, containing phosphorus in the oxidation states +5, +3 and +2. All twelve compounds have been described, in varying degrees of detail, although serious doubts have been cast on the existence of PI5....
. It's a colourless gas at room temperature and pressure.
Structure
Single-crystal X-ray studies indicate PF5 molecule has two distinct P−F bonds (axial and equatorial): P−Fax = 158.0 pm and P−Feq = 152.2 pm. Gas-phase electron diffractionElectron diffraction
Electron diffraction refers to the wave nature of electrons. However, from a technical or practical point of view, it may be regarded as a technique used to study matter by firing electrons at a sample and observing the resulting interference pattern...
analysis gives similar values: P−Fax = 158 pm and P−Feq = 153 pm.
Fluorine-19 NMR
Fluorine-19 NMR
Fluorine-19 nuclear magnetic resonance is an analytical technique. 19F has a spin of 1/2, and a relative abundance of 100 % and a high magnetogyric ratio, making measurements very fast . Integrals are reliable due to the lack of a nuclear Overhauser effect...
spectroscopy, 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
Pseudorotation
The IUPAC defines pseudorotation as "a conformational change resulting in a structure that appears to have been produced by rotation of the entire initial molecule and is superimposable on the initial one, unless different positions are distinguished by substitution or isotopic labeling...
via the Berry mechanism
Berry mechanism
The Berry mechanism, or Berry pseudorotation mechanism, is a type of vibration causing molecules of certain geometries to isomerize by exchanging the two axial ligands for two of the equatorial ones. It is the most widely accepted mechanism for pseudorotation...
, 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. The explanation was first described by R. Stephen Berry
R. Stephen Berry
R. Stephen Berry is a U.S. professor of physical chemistry.He is the James Franck Distinguished Service Professor Emeritus at The University of Chicago and Special Advisor to the Director for National Security, at Argonne National Laboratory...
, after whom the Berry mechanism is named. Berry pseudorotation influences the 19F NMR spectrum of PF5 since NMR spectroscopy operates on a millisecond
Millisecond
A millisecond is a thousandth of a second.10 milliseconds are called a centisecond....
timescale. Electron diffraction and X-ray crystallography do not detect this effect as their timescales are significantly shorter than for NMR spectroscopy.