Solvated electron
A solvated electron is an electron that is in solution in a solvent, in particular that is not bound to an atom or molecule . The name is derived from solvation . The solution containing solvated electrons is also referred to as an electride solution .
The solution of alkali metals in liquid ammonia produces a typical blue coloration, which W. Weyl already mentioned in 1864 in his work on metal ammonium compounds . In this work, an as yet unknown species was suggested as the cause of the blue color. It was not until 1962 that Edwin J. Hart (1910–1995) and Jack W. Boag were able to identify this species as an electron that is released by ionizing an atom of the alkali metal in question and goes into solution.
generation
In polar solvents such as water or alcohols , but also in non-polar solvents such as alkanes , the solvated electron can be generated artificially by radiolysis or photolysis . Different generation mechanisms such as Charge Transfer to Solvent (CTTS), proton transfer or ionization are possible. The lifetime of the solvated electron in these solvents can be a few hundred nanoseconds . However, electron scavengers in the solvent significantly reduce the service life.
Spectroscopic properties
Probably the most frequently studied physical property of the solvated electron is its absorption spectrum . It is characterized by a broad, structureless band that extends over wide areas of the visible and infrared spectral range. Depending on the solvent, the maximum absorption is in the range from 700 to 1100 nm , which explains the blue coloration observed. The shape, width and position of the absorption band depend on the type of solvent, pressure and temperature.
Empirically, the absorption depending on the photon energy can be described as Lorentz-shaped for energies above that with maximum absorption and Gaussian-shaped below.
The exact cause for the creation of this curve shape is not yet well understood, as its simulation based on theoretical models delivers good qualitative results, but cannot quantitatively describe the measured values satisfactorily.
Theoretical models
Various models are discussed in the literature to explain the spectroscopic properties of the solvated electron:
- cavity model
- dielectric-continuum model
The cavity model is based on the assumption that the solvated electron is surrounded by a number of solvent molecules that form a solvation shell around the electron. Through the interaction with this shell ( cavity ), the electron sees a potential in which bound states exist equivalent to a quantum mechanical system. Optical transitions between these states lead to the observed absorption band. Due to the limited number of molecules in the first solvation shell, the potential can only be viewed as roughly spherical symmetry, which is why the bound states of the electron are often referred to as s- or p-like in the literature. Theory work also shows evidence of a split of the excited state into three sub-states (removal of the degeneracy ).
The latest " ab initio " calculations show clear indications that support this model.
Investigation methods
Most of the early experimental work on the solvated electron deals with the spectral properties of the equilibrated ground state under different conditions:
- different solvents
- Additives such as salts (ions) in different concentrations
- pressure
- temperature
With increasing knowledge about the static properties of the solvated electron, the need also increased to investigate the formation process, which takes place on the picosecond time scale. A common investigation method for this is ultra-short-term spectroscopy : the generation process is set in motion via photolysis using an ultra-short laser pulse. Then the development of the absorption band over time until the final formation of the solvated electron is examined. The question of the intermediate steps in which the solvated electron is generated is of interest here.
In addition, it is possible to stimulate the equilibrated ground state to a higher state and to observe the relaxation dynamics that follow.
particularities
The solvent water has always been of particular interest due to its high relevance for chemistry and biology. Probably for this reason, the independent term hydrated electron has become established for the solvated electron in water .
See also
Individual evidence
- ↑ W. Weyl: About Metallammonium Compounds . In: Annals of Physics . tape 197 , no. 4 , 1864, p. 601-612 , doi : 10.1002 / andp.18641970407 .
- ^ W. Weyl: About the formation of ammonium and some ammonium metals . In: Annals of Physics . tape 199 , no. 10 , 1864, p. 350-367 , doi : 10.1002 / andp.18641991008 ( digitale-sammlungen.de ).
- ^ Edwin J. Hart, JW Boag: Absorption Spectrum of the Hydrated Electron in Water and in Aqueous Solutions . In: Journal of the American Chemical Society . tape 84 , no. 21 , 1962, pp. 4090-4095 , doi : 10.1021 / ja00880a025 .