Characterization of Inorganic Compounds1,2,3
Author: J. M. McCormick
Last Update: August 24, 2011
The methods of characterization for inorganic compounds, by which we mean compounds containing a metal, are not dissimilar to those you learned in Organic Chemistry lab. The nature of the compounds preclude some methods, but it also opens up others. The following is a partial list of different physical methods, what they tell you, and their applicability to characterizing inorganic compounds. Methods of characterization in blue are those primarily used by inorganic chemists and are usually required for publishing.4
Inorganic compounds are often ionic, and so have very high melting points. While some inorganic compounds are solids with accessible melting points, and some are liquids with reasonable boiling points, there are not the exhaustive tabulations of melting/boiling point data for inorganic compounds that exist for organics. In general, melting and boiling points are not useful in identifying an inorganic compound, but they can be used to assess its purity, if they are accessible.
Inorganic compounds, in contrast to many organic compounds, are very colorful. Unfortunately, color alone is not reliable indicator of a compound’s identity, but it is useful when one is following a separation on a column.
You can get information about the arrangement of the particles in the solid from the shape of a well-formed crystal, or by observing the visual changes in the crystal when it is rotated under a polarizing microscope. Crystal shape is used as a means of mineral identification, but mineralogists have the advantage over chemists in that their crystals are the result of very long, slow crystallization processes that often result in large, well-formed crystals.
Elemental analysis is one of the most useful methods available to characterize a compound. You can do some elemental analyses yourself using standard procedures (gravimetric, colorimetric, AA), but it is often more convenient to pay someone else to do it. For a C, H, N analysis a professional laboratory requires ~10mg of sample and ~$50, but it is definitely worth it.
In inorganic chemistry this is most often used to determine the molar mass of compounds. When mass spectroscopy data are combined with an elemental analysis, the chemical formula of the substance can be determined. Analysis of a compound’s fragmentation pattern can be used to gain structural information. This is not usually done because of the complex fragmentation patterns of inorganic compounds, and because other methods are available for structure determination.
Most often used to separate a product from a complex reaction mixture. If a known sample of the compound is available, it can be identified in a reaction mixture by spiking the analyte with the known. Most chromatography in inorganic chemistry is on solutions (e. g., column chromatography and HPLC, both normal and reverse-phase), because the high boiling point of many inorganic compounds precludes GC analysis.
Other Physical Properties
In the old days taste and smell were used to characterize compounds. However, these are no longer recommended because of the potential toxicity of new compounds.
A useful physical property that can be used to distinguish between ionic and non-ionic compounds is “crunchiness.” When crushed with a spatula most ionic compounds will feel crunchy, while non-ionic compounds will not. This should not be tried with azide, perchlorate or fulminate salts because these tend to be explosive.
UV-Vis Absorption Spectroscopy
The number, energies and intensities of a transition metal compound’s absorption bands in the UV-Vis and near IR can be used to determine the general type of atom bound to a metal and the geometry about the metal. There are some complicating factors, which we will discuss in lecture, but there is enough published about the spectroscopy of transition metal compounds to help you sort things out.
Circular Dichroism (CD) Spectroscopy
CD spectroscopy measures the degree to which a sample rotates circularly polarized light. CD is limited to enantiomerically pure, optically-active compounds; an equal mixture of enantiomers will give no CD spectrum (enantiomeric pairs having CD spectra that are mirror images of each other and so would cancel out). CD is similar to absorption spectroscopy, but because of different selection rules for the electronic transitions, peak intensities and widths may be different between CD and absorption (the energy of a peak must still, however, be the same). This is a very useful technique for studying metal-containing proteins and enzymes because the chirality of the peptide induces a CD signal from any metal bound to the protein.
IR Absorption Spectroscopy
Can be used just like in organic chemistry to fingerprint a compound. In simple compounds the number, energy and intensity of the IR transitions are directly related to the geometry of compound and to which atoms are bound to which other atoms. Unfortunately, some metal-ligand vibrations, and many of the vibrations for the heavier elements occur outside the frequency window of most commercial instruments. For complex compounds involving large organic moieties, the IR becomes more difficult to interpret. IR is useful to determine the presence of complex counter ions like PF6–, ClO4–, BF4–, because they have distinctive absorptions in the IR. Click here to view the Vibrational Frequencies page.
Raman spectroscopy is complementary to IR absorption spectroscopy. Both probe vibrations within a compound, but they have different selection rules. By considering what peaks are present, or absent, in the two spectra of a compound, one can determine geometry; at least in simple cases. Raman is also useful because it can, depending on instrument design, scan to very low frequencies (~100 cm-1), and thus observe transitions too low for IR absorption. A variant technique, resonance Raman, can be used to assign vibrations and identify ligands.
Nuclear Magnetic Resonance (NMR)
The workhorse of chemistry. While paramagnetic NMR is not impossible, NMR is usually performed on diamagnetic compounds. The NMR spectra of inorganic compounds are often more complicated than organics because other nuclei also have nuclear magnetic moments. So in addition to the familiar 1H (I = 1/2) and 13C (I = 1/2), there are ~90 other elements that have at least one NMR-active nucleus. Although less widespread than the standard solution NMR, solid state NMR and even single-crystal NMR have been used on materials that simply do not dissolve in any solvent.
The number of unpaired electrons in a transition metal compound is a very useful physical property to know. From a knowledge of the magnetism one can determine what metal is present, its oxidation state and even a rough idea of the metal’s structure. Magnetic measurements are often made with a Gouy balance, an Evans balance (a modified version of the Gouy balance, which we have in lab), or a SQUID (super-quantum interference device) magnetometer. A compound’s magnetic moment can also be determined by NMR using the Evans method. Its one drawback is that it is a bulk technique, and when used on mixtures the measured magnetic moment will be the weighted average of the moments of all of the paramagnetic species present.
Electron Paramagnetic Resonance (EPR or ESR)
While NMR is usually only for diamagnetic compounds, EPR is for paramagnetic compounds with an odd number of unpaired electrons (EPR can be done when there are an even number of unpaired electrons, but it is a much harder experiment). In this experiment a sample is irradiated with microwave radiation and the field is swept until resonance occurs. The field at which resonance occurs depends on the number of unpaired electrons, the geometry about the metal center and the metal’s ligands. In many ways EPR and NMR the same, and there is even a technique that combines them (ENDOR, electron-nuclear double resonance, spectroscopy).
Magnetic Circular Dichroism (MCD)
MCD is a hybrid technique based on the fact that all matter will rotate circularly polarized light in the presence of a magnetic field. It is most useful for paramagnetic compounds (either with an even or odd number of unpaired electrons). Because it is both a magnetic and a spectroscopic method, MCD can be used to measure a compound’s magnetic properties and electronic transitions. MCD is most powerful when used in conjunction with another method. It can also be used on a mixture, if transitions arising from different species can be identified. The main limitations of MCD is that it is usually performed at low temperature (< 77 K) and the samples must be strain-free glasses. The cost of the instrumentation and the cryogens have limited MCD to only a few groups worldwide.
For most inorganic chemists, electrochemistry means cyclic voltammetry (CV). CV has been called the “electrochemical equivalent of spectroscopy” because it can be used to determine 1) the oxidation state, 2) E0 of each redox process that the compound can undergo and 3) the kinetics of the redox process. CV is a standard characterization method in most inorganic laboratories.
Single-crystal X-ray diffraction is the most powerful X-ray technique for inorganic chemists. From precise measurement of the intensity and angles at which an X-ray beam diffracts off a crystal, the arrangement of the atoms can be reconstructed. Obviously, as a direct probe of structure crystallography is an invaluable characterization method for all types of compounds. Some inorganic compounds (e. g., rocks, minerals) can’t be obtained as single crystals. In these cases X-ray powder diffraction can be used to obtain the dimensions of the unit cell for use in identification (there is a large, indexed catalog of lattice constants for many minerals).
There are a large number of techniques for characterizing inorganic compounds. Some are limited to one or two elements (e. g., Mssbauer spectroscopy), while others require large, expensive instruments (e. g., EXAFS). These are not that important in every day work, but you should be aware of them and what their strengths and weaknesses are.
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- Drago, R. S. Physical Methods in Chemistry W. B. Saunders: Philadelphia, 1977.
- Ebsworth, E. A. V.; Rankin, D. W. H. and Cradock, S. Structural Methods in Inorganic Chemistry, 2nd Ed. Blackwell Scientific Publications: Boston, 1991.