Matrix isolation is an experimental technique used in
chemistry and
physics
Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which r ...
. It generally involves a material being trapped within an unreactive matrix. A ''host'' matrix is a continuous
solid phase
In the physical sciences, a phase is a region of space (a thermodynamic system), throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, index of refraction, magnetizati ...
in which ''guest'' particles (atoms, molecules, ions, etc.) are embedded. The guest is said to be ''isolated'' within the ''host'' matrix. Initially the term matrix-isolation was used to describe the placing of a
chemical species
A chemical species is a chemical substance or ensemble composed of chemically identical molecular entities that can explore the same set of molecular energy levels on a characteristic or delineated time scale. These energy levels determine the wa ...
in any unreactive material, often
polymers
A polymer (; Greek '' poly-'', "many" + ''-mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic an ...
or
resins
In polymer chemistry and materials science, resin is a solid or highly viscous substance of plant or synthetic origin that is typically convertible into polymers. Resins are usually mixtures of organic compounds. This article focuses on natu ...
, but more recently has referred specifically to
gas
Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma).
A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...
es in
low-temperature solids. A typical matrix isolation experiment involves a guest sample being diluted in the gas phase with the host material, usually a
noble gas
The noble gases (historically also the inert gases; sometimes referred to as aerogens) make up a class of chemical elements with similar properties; under standard conditions, they are all odorless, colorless, monatomic gases with very low ch ...
or
nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
. This mixture is then deposited on a window that is cooled to below the melting point of the host gas. The sample may then be studied using various
spectroscopic
Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wa ...
procedures.
Experimental setup
![Apparatus for Matrix Isolation](https://upload.wikimedia.org/wikipedia/commons/4/46/Apparatus_for_Matrix_Isolation.svg)
The transparent window, on to which the sample is deposited, is usually cooled using a compressed
helium
Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
or similar refrigerant. Experiments must be performed under a high vacuum to prevent contaminants from unwanted gases freezing to the cold window. Lower temperatures are preferred, due to the improved rigidity and "glassiness" of the matrix material. Noble gases such as
argon
Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as ...
are used not just because of their unreactivity but also because of their broad optical
transparency in the solid state. Mono-atomic gases have relatively simple
face-centered cubic (fcc) crystal structure
In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric patterns ...
, which can make interpretations of the site occupancy and
crystal-field splitting of the guest easier. In some cases a
reactive
Reactive may refer to:
*Generally, capable of having a reaction (disambiguation)
*An adjective abbreviation denoting a bowling ball coverstock made of reactive resin
*Reactivity (chemistry)
*Reactive mind
*Reactive programming
See also
*Reactanc ...
material, for example,
methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Ea ...
,
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
or
ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous wa ...
, may be used as the host material so that the reaction of the host with the guest species may be studied.
Using the matrix isolation technique, short-lived, highly-reactive species such as
radical ions and reaction intermediates may be observed and identified by
spectroscopic
Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wa ...
means. For example, the solid noble gas
krypton
Krypton (from grc, κρυπτός, translit=kryptos 'the hidden one') is a chemical element with the symbol Kr and atomic number 36. It is a colorless, odorless, tasteless noble gas that occurs in trace amounts in the atmosphere and is often ...
can be used to form an inert matrix within which a reactive ''F
3−'' ion can sit in chemical isolation. The reactive species can either be generated outside (before deposition) the apparatus and then be condensed, inside the matrix (after deposition) by irradiating or heating a precursor, or by bringing together two reactants on the growing matrix surface. For the deposition of two species it can be crucial to control the contact time and temperature. In ''twin jet'' deposition the two species have a much shorter contact time (and lower temperature) than in ''merged jet''. With ''concentric jet'' the contact time is adjustable.
Spectroscopy
Within the host matrix, the
rotation and
translation
Translation is the communication of the meaning of a source-language text by means of an equivalent target-language text. The English language draws a terminological distinction (which does not exist in every language) between ''transla ...
of the guest particle is usually inhibited. Therefore, the matrix isolation technique may be used to simulate a spectrum of a species in the
gas phase
In the physical sciences, a phase is a region of space (a thermodynamic system), throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, index of refraction, magnetiza ...
without rotational and translational interference. The low temperatures also help to produce simpler spectra, since only the lower electronic and vibrational
quantum states
In quantum physics, a quantum state is a mathematical entity that provides a probability distribution for the outcomes of each possible measurement on a system. Knowledge of the quantum state together with the rules for the system's evolution i ...
are populated.
Especially
infrared (IR) spectroscopy, which is used to investigate
molecular vibration, benefits from the matrix isolation technique. For example, in the gas-phase IR spectrum of
fluoroethane some spectral regions are very difficult to interpret, as vibrational quantum states heavily overlap with multiple
rotational-vibrational quantum states. When fluoroethane is isolated in
argon
Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as ...
or
neon matrices at low temperatures, the rotation of the fluoroethane molecule is inhibited. Because rotational-vibrational quantum states are quenched in the matrix isolation IR spectrum of fluoroethane, all vibrational quantum states can be identified. This is especially useful for the validation of simulated infrared spectra that can be obtained from
computational chemistry.
History
Matrix isolation has its origins in the first half of the 20th century with the experiments by photo-chemists and physicists freezing samples in liquefied gases. The earliest isolation experiments involved the freezing of species in transparent, low temperature organic
glass
Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling ( quenching ...
es, such as EPA (ether/isopentane/ethanol 5:5:2). The modern matrix isolation technique was developed extensively during the 1950s, in particular by
George C. Pimentel. He initially used higher-boiling inert gases like
xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
and
nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
as the host material, and is often said to be the "father of matrix isolation".
Laser vaporization in matrix isolation spectroscopy was first brought about in 1969 by Schaeffer and Pearson using a
yttrium aluminum garnet
Yttrium aluminium garnet (YAG, Y3 Al5 O12) is a synthetic crystalline material of the garnet group. It is a cubic yttrium aluminium oxide phase, with other examples being YAlO3 (YAP) in a hexagonal or an orthorhombic, perovskite-like form, and ...
(YAG) laser to vaporize carbon which reacted with hydrogen to produce acetylene. They also showed that laser-vaporized
boron would react with HCl to create BCl. In the 1970s, Koerner von Gustorf's lab used the technique to produce free metal atoms which were then deposited with organic substrates for use in
organometallic chemistry. Spectroscopic studies were done on reactive intermediates in around the early 1980s by Bell Labs. They used
laser-induced fluorescence
Laser-induced fluorescence (LIF) or laser-stimulated fluorescence (LSF) is a spectroscopic method in which an atom or molecule is excited to a higher energy level by the absorption of laser light followed by spontaneous emission of light. It was f ...
to characterize multiple molecules like SnBi and SiC. Smalley's group employed the use of this method with
time-of-flight mass spectrometry
Time-of-flight mass spectrometry (TOFMS) is a method of mass spectrometry in which an ion's mass-to-charge ratio is determined by a time of flight measurement. Ions are accelerated by an electric field of known strength. This acceleration res ...
by analyzing Al clusters. With the work of chemists like these,
laser-vaporization in matrix isolation spectroscopy rose in popularity due to its ability to generate transients involving metals, alloys and semi-conductor molecules and clusters.
See also
*
Host–guest chemistry In supramolecular chemistry, host–guest chemistry describes complexes that are composed of two or more molecules or ions that are held together in unique structural relationships by forces other than those of full covalent bonds. Host–guest ch ...
*
Inert gas
An inert gas is a gas that does not readily undergo chemical reactions with other chemical substances and therefore does not readily form chemical compounds. The noble gases often do not react with many substances and were historically referred to ...
*
Van der Waals interactions
*
Radicals
References
Further reading
*
*
*Ball, David W.,
Zakya H. Kafafi, et al., ''A Bibliography of Matrix Isolation Spectroscopy, 1954-1985'', Rice University Press, Houston, 1988
{{Reaction mechanisms
Spectroscopy
Physical chemistry
Reaction mechanisms