INFRARED SPECTRA OF INORGANIC COMPOUNDS

Publisher Summary

This chapter discusses the experimental, theoretical, and empirical correlations between functional organic groups and the infrared spectrum. The application of infrared spectroscopy to the identification of inorganic compounds is less successful. In obtaining infrared spectra of inorganic solids, an experimental complication arises from possible chemical reaction between the inorganic compound and the infrared window material or support medium. The chapter presents many examples of spectra of inorganic compounds in the solid phase. The majority of these compounds are crystalline solids in which the crystallographic unit cell contains several polyatomic ions or molecules. Optical modes called lattice modes of vibration result from the motion of one polyatomic group relative to another within the unit cell. Lattice modes occur in the region 400–10 cm−1 and are characteristic of specific crystal geometry. They are used as fingerprints for an inorganic compound in much the same way as the internal modes of vibration of organic compounds are used in the region 4000–400 cm−1.

INTRODUCTION

The utility of infrared spectroscopy to the organic chemist is perhaps unsurpassed within the framework of most modern laboratories. Experimental, theoretical, and empirical correlations between functional organic groups and the infrared spectrum have been thoroughly studied and reported. The vast body of literature devoted to the results of these studies provides a rather solid base for use by the analytical spectroscopist. Through the efforts of several authors this accumulated literature has been summarized and reviewed in several excellent books (1–7).

The application of infrared spectroscopy to the identification of inorganic compounds has been somewhat less successful. Many simple inorganic compounds such as the borides, silicides, nitrides, and oxides, do not absorb radiation in the region between 4000 and 600 cm−1 which, for many years, was the extent of the infrared region covered by most commercial spectrometers. Only within the last 10 years have instruments become available which include the region below 600 cm−1, and it has been even more recent that instrumentation has been developed to cover the far-infrared region between 200 and 10 cm−1. These are the regions in which most inorganic compounds absorb infrared radiation.

The region 4000–600 cm−1 has proved to be very useful for the identification of polyatomic anions of the type CO32−, SO42−, NO3−, etc. When standard spectra are available, a compound such as KNO3 can easily be distinguished from NaNO3 or Ca(NO3)2, but in the absence of standard spectra, specific identification of a cation–anion pair is usually not possible by infrared spectroscopy. The differences between the spectra of KNO3 and Ca(NO3)2, for example, are largely due to two effects: (1) the extent to which the cation perturbs the internal vibrations of the anion and (2) changes in the crystal structure of the system. The latter is more pronounced in the far-infrared region than in the region 4000–600 cm−1. These effects are usually not predictable.

In obtaining infrared spectra of inorganic solids, an experimental complication arises from possible chemical reaction (cation exchange) between the inorganic compound and the infrared window material or support medium. The literature contains many examples of standard spectra of inorganic compounds in which this type of chemical reaction has obviously taken place. Care has been exercised in the preparation of samples here so as to avoid this difficulty.

In the present compendium, spectra of inorganic compounds in the solid phase are presented. The majority of these compounds are (powdered) crystalline solids in which the crystallographic unit cell may contain several polyatomic ions or molecules. The internal modes of vibration of the polyatomic group generally occur in the region 4000–400 cm−1; many of these have been extensively documented in the literature. Other optical modes called lattice modes of vibration result from the motion of one polyatomic group relative to another within the unit cell. Lattice modes generally occur in the region 400–10 cm−1 and are characteristic of a specific crystal geometry. They can be used as fingerprints for an inorganic compound in much the same way as the internal modes of vibration of organic compounds are used in the region 4000–400 cm−1. The purpose of this work is to present reference spectra and empirical spectra-structure correlations. We do not intend to cover the theoretical aspects of the solid state. For this the reader is referred to several excellent review articles and books (8–13).

EXPERIMENTAL

The mid-infrared spectra were scanned using a Beckman Model IR-9 and two Herscher-Dow prism grating spectrometers in the region 3800–400 cm−1 and a Perkin-Elmer Model 225 in the region 3800–200 cm−1. Far-infrared spectra were scanned on a Beckman Model IR-11 in the region 600–45 cm−1. Extensive descriptive material about the instrumentation is given in several books (14–16).

The samples were prepared as mulls, using as mulling agents Fluorolube for the region between 3800–1333 cm−1 and Nujol for the region between 1333–400 and 600–45 cm−1, the technique hereinafter being referred to as a “split mull.” In the mulling technique, finely ground particles are suspended in the mulling agent and the slurry is supported between two infrared transmitting windows. Samples were not subjected to prolonged grinding, but were treated in a routine manner, the grinding time seldom exceeding 10 minutes. Mechanical grinding devices were not employed. BaF2 windows were used in the region 3800–1333 cm−1, AgCl in the region 1333–400 cm−1, and polyethylene in the region 600–45 cm−1. These window materials are inert to reaction with respect to most inorganic compounds. Standard window materials such as potassium bromide, sodium chloride, cesium bromide, and cesium iodide were found to be highly prone to ion exchange with a number of inorganic compounds, and for this reason their use was avoided.

To illustrate the extent of ion exchange effects, pure samples of Pb(NO3)2 (verified by X-ray diffraction) were prepared as split mulls on sodium chloride, potassium bromide, cesium iodide, barium fluoride, and silver chloride plates. Spectra A and D in Fig. 1 are of pure Pb(NO3)2 and NaNO3, respectively, scanned as split mulls between BaF2 (3800–1333 cm−1) and AgCl (1333–400 cm−1) plates. Spectrum B is a freshly prepared Pb(NO3)2 split mull between NaCl plates, and spectrum C is the Nujol portion of that mull 2 hours after preparation, having been in intimate contact with the NaCl plates. The out-of-plane NO3− deformation of NaNO3 which occurs at 838 cm−1 is clearly present in spectra B and C. The band intensity increases with contact time (spectra B to C), indicating the continuing formation of NaNO3 by ion exchange between Pb(NO3)2 and the NaCl plate. Similar reactions were observed between Pb(NO3)2 and KBr and Csl.

Fig. 1 Spectrum A: (Pb(NO3)2. Spectrum D: NaNO3. A and D are scanned as split mulls between BaF2 (3800-1333 cm−1) and AgCl (1333-400 cm−1) plates. Spectrum B: Pb(NO3)2 scanned as split mull between NaCl plates. Spectrum C: Nujol portion of B 2 hours after preparation, having been in contact with NaCl plates.

The potassium bromide pellet technique for preparing samples was strictly avoided. Anomalies in the infrared spectra of inorganic compounds prepared by this technique have been extensively studied. In addition to a possible cation exchange reaction with KBr, the material under investigation may also undergo changes in crystalline form as a result of the high mechanical pressures (10,000 psi) used in the pelleting process. Extreme caution should be exercised when applying the potassium bromide pellet technique to obtain infrared spectra of inorganic compounds.

ARRANGEMENT OF SPECTRA

The spectra are arranged to bring together compounds containing similar anions, in order to facilitate recognition of characteristic group frequencies. The arrangement is based on the position in the periodic table of the central atom in the anion. Where there is no central atom (e.g., CN−) the anions are arranged by lowest group; thus CN− falls under C.

In grouping the anions by their central atom, these have been arranged in order of, first, increasing group number, then increasing atomic number within a group: B, Al, C, Si, N, P, O, S, F, Cl, Br. In subarrangement under a given central atom, for example, N, the anions are given in order of increasing number of N atoms in the anion: N3−, N24−, N3−, etc. The polyatomic anions are arranged in orders of decreasing ratio of N atoms to other atoms in the anion, such as N2O22−, NO2, NO3. Under a specific anion, individual compounds appear in the order of increasing atomic number of the cation within a given group. For the nitrates (NO3−) the order is NH4NO3; (Group I) NaNO3, KNO3 ··· CsNO3; (Group II) Ca(NO3)2 ··· Ba(NO3)2;...