INFRARED AND RAMAN SPECTRA INSTRUMENTATION OF INORGANIC COMPOUNDS H. E. Hallam, M.Sc., Ph.D., F.R.I.C. Department of Chemistry, University College of Swansea, Singleton Park, Swansea. Infrared and Raman spectroscopy are the two techniques commonly used for the study of molecular vibrations. During the last two decades i.r. spectro- scopy has achieved paramount importance as a structural diagnostic tool, particularly in the field of organic chemistry. For two reasons, its application to inorganic compounds has developed more slowly. First, all organic com- pounds give i.r. spectra but some inorganic materials (monatomic anions and cations) do not ; secondly, many metal-ligand (M-L) frequencies occur below 400 cm-l, a region which has only recently become examinable by commercial spectrometers.As a result of instrumental developments there has been a surge of interest in the applications of i.r. spectroscopy to prob- lems in inorganic chemistry and the technique now plays an important r61e in undergraduate inorganic practical courses alongside that already estab- lished in organic chemistry. Ideally an i.r. study should be carried out in conjunction with a study of the Raman spectrum since this gives complementary information. The Raman effect in fact provided much of the early experimental data and is of particular value to inorganic chemistry because of its facility for studying aqueous solutions. The very recent introduction of commercial laser Raman spectrometers has also caused a resurgence of interest in the Raman spectra of inorganic compounds,l although the expense of this instrumentation will preclude it from becoming routine in teaching laboratories for some time.This review is limited to a discussion of the elementary principles and features of the i.r. and Raman spectra of inorganic molecules and ions as a guide to qualitative and structural analysis. No attempt is made to discuss the detailed theory nor to describe fully experimental procedures and instru- mentation. The general text edited by Davies2 adequately covers these aspects of both i.r. and Raman spectroscopy and also includes a chapter by Ebsworth on applications to inorganic chemistry. Sampling techniques and i.r. instrumentation are fully described in an excellent new manual3 edited by Miller.An outline of instrumentation and practical techniques is given here to provide a background to the discussion of vibrational spectra. A spectrometer is an. instrument which measures the amount of radiation transmitted by a substance as a function of the wavelength (A), frequency (Y = c/A), or wavenumber (ij = v / c = 1/A) of the radiation. Hallam 39 Infrared A schematic diagram of an i.r. spectrometer is shown jn Fig. 1. Radiation, from an electrically-heated filament which emits frequencies over a wide spectral range, is reflected through the sample and reference cells by mirrors. The reference beam is attenuated by a comb which can be moved into and out of the optical path until it is equal in intensity to the sample beam.The two beams are alternately sent to the thermocouple detector by a rotating chopper mirror. This allows the transmitted intensity at each frequency to be compared with the intensity in the absence of sample. Before reaching the detector the alternating beam is dispersed into its component frequencies by means of an alkali halide prism or a diffraction grating. The spectrum is scanned by rotating the prism or grating so as to alter the frequency being received by the detector. When the sample is not absorbing, the detector receives a constant signal; if the sample absorbs at a particular frequency the sample transmission is lowered. The thermocouple thus receives an alternating signal which is amplified and fed to a servo motor which drives the attenuating comb into the reference beam until the two beams are balanced.The comb is linked to a pen which records the attenuation, i.e. sample transmission (T = transmitted intensity/incident intensity I/IoO) on a chart moving in synchronism with the rotation of the prism/grating. The resultant plot of Ill0 versus v is the i.r. spectrum of the sample. There are several routine i.r. spectrometers available commercially which cover the range 4000 to 400 cm-1 and one which extends to 250 cni-l. More sophisticated (and more expensive) instrumentation is required to reach the very far i.r., i.e. down to tens of cm-1. Raman When an intense beam of light of frequency v is incident upon a transparent medium a small amount of the radiation energy is scattered, even if the medium is rigorously freed from dust particles. The scattered energy consists almost entirely of radiation of the incident frequency (Rayleigh scattering) but also contains frequencies ( V - Av) and ( V + Av) where the Raman shift A v corresponds to a molecular vibrational frequency.If the frequency of the incident beam is within the visible range the scattered radiation can be examined by means of a visible spectrometer. To excite Raman spectra it is necessary to use an extremely intense source of mono- chromatic radiation; the mercury arcs developed for this purpose are now being replaced by 1asers.l Figure 2 shows schematically the essential com- ponents of a recording Raman spectrometer.SAMPLING TECHNIQUES The vibrational spectrum of a molecule arises from its internal vibrations and we can best interpret it when the molecule is in its simplest and most repro- ducible surroundings. Ideally this is in the gas phase where, at low pressures, we can consider the molecule to be isolated and thus free from the external influence of neighbouring molecules. Unfortunately few molecules have 40 R. I. C. Review2 L 0 -.- f I V u' Hallam q- I Flat contact Sample tube I A$-- Hemispherical lens Fig. 2. Schematic diagram of a Raman spectrometer. sufficient vapour pressure at room temperature to provide enough absorbing molecules for a gas-phase spectrum to be obtained, and thermal decomposi- tion often occurs if higher temperatures are employed to increase the vapour- phase population.The closest possible approach to the ideal state usually is to study the substance in dilute solution in an inert solvent, As far as possible highly polar solvents should be avoided because of the resulting strong solute-solvent interactions. However, since the majority of inorganic compounds are highly polar ionic molecules they are generally only sufficiently soluble in polar solvents-in particular, water. Aqueous solution spectra, however, are still preferable to spectra of the solid state, in which environment the molecule/ion is greatly perturbed by strong inter- actions with neighbouring molecules/ions in the crystal lattice. The difficulty with the solution technique is that the solvent has its own vibrational spectrum and, in spectral regions where the solvent is strongly absorbing, the solute cannot be examined.Water, being a simple triatomic molecule, has a limited number of vibrational frequencies and might be expected to be a suitable i.r. solvent. Unfortunately the very large changes in dipole moment accompanying the vibrations of the water molecule lead to extremely intense absorption in the i.r. The situation is worsened by features arising from the presence of higher molecular aggregates, (H20)n, due to the strong hydrogen-bonding properties of water. It i s thus impossible to obtain good i.r. spectra of aqueous solutions over a wide frequency range although the use of D2O does extend the available range.The polarizability of the water molecule, however, changes little during its vibrations. This leads to a very weak Raman scattering and makes water an excellent solvent for Raman spectroscopy. 1.r. cells A simple gas cell consists of a 10 cm length of wide bore glass tubing with i.r.- transparent windows fitted to the ends. Window materials commonly used are rock-salt (transparent to 600 cm-l), KBr (to 350 cm-l), CsI (to Dispersing prism * o r grating 42 jd Recorder F Amplifier Laser source 7 1 A ~ / / ’ Y P h oto m u I t ~ p I I er detector +- R.I.C. Reviews 200 cm-1)-these are relatively expensive and water-soluble. CaFz (to 1000 cm-I), AgCl (to 450 cm-1) and Irtran-4 (500 cm-l) are water-insoluble but also expensive.Polythene (Rigidex) is a cheap and useful window material below 400 cm-l. Solution cells consist of two windows separated by a spacing washer of suitable thickness-usually 0.1 to 10 mm. Pure liquids can usually be ex- amined simply as a capillary film between two windows. Raman cells These are much simpler than i.r. cells and usually consist of glass tubes with a flat glass end-window. Greater care however must be taken in the Raman technique to filter solutions to remove all dust particles, since these act as scattering centres. With both techniques the examination of powdered solids presents some difficulty due to reflection from the numerous crystal faces. However, using ‘front illumination’ it appears1 that excellent Raman spectra can now be obtained directly from a powder in a sample tube.Two techniques have been developed which permit satisfactory i.r. spectra of solids to be obtained : the oil-mull technique and the pressed-disc technique. Both involve (a) finely grinding the sample so that the average particle size is less than the wave- length of radiation being used (i.e. less than ca 3 p) so that reflections and refractions at the particle faces are minimized, and (b) dispersing the particles in a medium (liquid or solid) of approximately the same refractive index so that scattering by reflection and refraction is further reduced and Rayleigh scattering is also reduced. Oil mull technique An oil mull is made by grinding the solid sample with a mulling agent such as liquid paraffin (Nujol) and placing a few drops of the resulting paste squeezed between two plates as for a liquid film.The method has the dis- advantage that the ‘solute’ spectrum is overlaid by the absorptions due to the mulling oil. In the case of Nujol (a mixture of c20-c30 alkanes) these are at 2900, 1460, 1375, and 720 (w) cm-1 and result from stretching and bending modes of the C-H groups. If the solid being mulled has absorptions at or near these frequencies a second mull has to be prepared using a per- fluorocarbon oil (Fluorolube ; Kelex). Pressed-disc technique This consists of grinding a few milligrams of the solid with about 0.5 g of a dry powdered alkali halide, placing the mixture in a special die and com- pressing it with a hydraulic press to a small disc ca 1 mm thick under a pressure of several tons.The method is commonly known as the KBr disc technique since it was developed for organic compounds studied by spectro- meters going down to 650 or 400 cm-l. For inorganic compounds where it is usually necessary to go to lower frequencies it is necessary to use CsI (Special grade CsI for this purpose is now available from BDH). The technique has the advantage that the halide (if pure) does not have specific absorption bands. Its main disadvantage is moisture pick-up during preparation. It is Hnllam 43 probably the method to be recommended for inorganic solids although thought must always be given to the possibilities of halogen exchange and the effect of the alkali-halide matrix environment on the ‘solute’ frequencies.VIBRATIONAL SPECTROSCOPY AND MOLECULAR STRUCTURE The combination of i.r. and Ranian spectroscopy can be used to determine unequivocally the structure of small molecules of high symmetry; the term molecule in this sense includes a radical or an ion. The spatial arrangement of atoms determines the molecular symmetry which in turn determines whether or not a particular vibrational frequency will appear in the i.r. and/or Raman spectrum. It is not the purpose here to discuss symmetry theory (refs 4 and 5 provide good accounts); suffice it to say that the combina- tion of all symmetry elements (rotations and reflections) determines the point group to which the molecule belongs.This in turn determines the number of vibrational modes to be expected, and their i.r. and Raman activity, for a particular molecular structure. Thus, observation of the number of bands appearing in the i.r. and/or Raman spectrum of a molecule may decide the structure or may eliminate certain possible structures. For complex polyatomic molecules it is generally impossible to apply group theory and symmetry principles. Nevertheless, vibrational frequencies can yield valuable structural information on a semi-empirical basis using either i.r. or Raman spectroscopy since in general all the frequencies will be i.r.- and R(aman)-active. A molecule consists of n atoms of varying masses linked together by (n - 1) chemical bonds of varying strengths.The atoms are not held rigid but can move together in straight-line translation and rotate and vibrate periodically about a mean position. There are two types of fundamental vibrations : stretching, in which the distance between two atoms increases or decreases but the atoms remain on the same bond axis, and bending or deformation, in which the position of the atom changes with respect to the original bond axis. To describe these complex motions three coordinates must be specified for each atom, i.e. 3n coordinates or degrees of freedom. Of these, three describe the translation as a rigid molecular unit whilst another three describe the rotation of a non-linear molecule (two for a linear molecule). Thus each molecule has 3n - 6 (3n - 5 for a linear molecule) internal degrees of freedom which, when executed, cause a distortion of the molecule. These distortions are the normal modes of vibration; such a mode is one in which the centre of gravity of the molecule does not move and in which all of the atoms move with the same frequency and in phase.Each mode is independent of the others and a molecule may execute each of these normal modes of vibration simultaneously. They may, however, not always be different for, as a consequence of geometrical symmetry, two or more vibrational frequencies may coincide-these are said to be degenerate. Of all vibrations, only those which involve a change of dipole moment are i.r.- active and are capable of absorbing all or part of any i.r.radiation of the corresponding frequency incident upon the molecule. Vibrations which involve a change of polarizability are R-active. R. I. C. Reviews 44 In simple molecules the activity of a vibration may be determined by inspection of the normal mode. In a polyatomic molecule having a centre of symmetry (a), the vibrations symmetric with respect to the cs ( g vibra- tions) are R-active/i.r.-inactive, but the vibrations antisymmetric with respect to the cs (u vibrations) are i.r.-active/R-inactive. This is called the mutual exclusion rule. It should be noted however that this rule may not hold in polyatomic molecules having several symmetry elements besides a cs. Inorganic compounds will generally have their i.r. spectra measured in the solid state (as an oil mull or in an alkali halide pressed disc).It is therefore important to bear in mind site symmetry-the local symmetry of the crystal- line environment around the cs of the molecule in the unit cell. The site symmetry is usually lower than the molecular symmetry in the free state, so that the selection rule for the gaseous state is relaxed in the crystalline state. Hence bands forbidden in the gaseous state may appear weakly, and the degenerate vibrations may split in the crystalline state. For example, calcite and aragonite, two different crystalline forms of calcium carbonate, exhibit different spectra although their chemical compositions are the same. In addition to the internal molecular vibrations, solid state spectra will be further complicated by the presence of lattice vibrations; these are due to the translation and torsional motions of the molecule as a whole.Vibra- tional frequencies may also be split due to coupling between neighbouring molecules. Miller and Wilkins6 made the first systematic survey of the i.r. spectra of inorganic substances. Their reference collection covers 159 compounds and lists characteristic frequencies of 33 polyatomic ions in the range 5000- 650 cm-I. It has recently been extended7 down to 300 cm-1. Most polyatomic ions exhibit' characteristic frequencies which are thus useful for qualitative analysis; a selection of these is shown in Figs. 3-5. A bibliography of inorganic i.r. spectra has been compiled by Lawsong but unfortunately it is badly classified and incomplete.An excellent text by Nakamoto5 covers simple ions in detail and also several classes of coordina- tion compounds. A comprehensive text by Adamsg has just appeared which gives an authoritative account of metal-ligand frequencies. A good introduc- tion to Raman spectroscopy in inorganic chemistry is given by Woodwardlo and a recent bibliographic review has been compiled by R. N. and M. K. Jones.ll From this brief introduction it is apparent that knowledge of all the vibrational frequencies, together with their i.r. and Raman activity, makes it possible to decide between alternative proposed structures having different symmetries. The most important of these structures are outlined in the following sections.DIATOMIC MOLECULES Diatomic molecules have only one vibrational mode-along the chemical bond; its frequency, v, in cm-1 is given by Haflarn 45 where f is the force constant of the bond, p the reduced mass, and c the velocity of light. Mode - * p = MzMl//(Mz + M d X2 Molecules (Point group Dooh) In a homonuclear molecule the dipole moment of the molecule is unchanged during the vibration, therefore the vibration is not i.r.-active. The polariza- bility change during the vibration, however, causes it to be R-active. X Y Molecules (C, v> In a heteronuclear diatomic molecule both the dipole moment and the polarizability change during the vibration and the vibration is thus both i.r.- and R-active. Diatomic vibrational frequencies are well-known and can be found in reference 5; Table 1 lists some of these.From the standpoint of inorganic chemistry the most important are the diatomic ions, e.g. the [O-HI- ion is characterized by a strong sharp band at 3700-3500cm-l whereas the [C=N]- gives a medium sharp absorption at 2250-2050cm-l. The nitro- sonium ion (NO)+ in nitric acid absorbs at 2220cm-1. LINEAR TRIATOMIC MOLECULES The (3n - 5) = 4 normal modes of vibration are shown below: .-c-b v Description VI Symmetric stretching VQ - (The + and - signs indicate displacement out of the plane of the molecule.) X3 Molecules (D mh) The symmetrical stretching mode VI clearly involves no dipole moment change and is thus i.r.-inactive but is R-active. vz and v3 are i.r.-active/R- inactive (centro-symmetric molecule : mutual exclusion rule). Azide ion [N=N=N]- is a good example: 1360 cm-l ( v I ) , 650 cm-1 ( VZ), 2040 crn-l(v3).XY2 Molecules (Dooh) The three modes again have the same activity as the corresponding modes of X3; common examples are carbon dioxide and carbon disulphide (Table 2). 46 Symbol S I Asymmetric stretching 1 vas R.I.C. Reviews V Table I : Vibrational frequencies (cm-1) of diatomic n Ilecules and ions Molecule Ion Li+[ 0 HI- Na+ [ 0 HI- N a+ [0 D]- K+[C N] - Na+[CN]- [NO]+ 4161 2994 3962 2886 - 0 2883 * 9 2230 233 I 892 317 665 V 3678 3637 268 I 2080 2080 2220 Table 2: Vibrational frequencies (cm-1) of linear XS and XY2 molecules and ions Molecu I e or ion (1343)" 658 497 I344 667 397 213 645 (667) 2349 I533 555 204 I 2360 coz cs2 XeF2 K+[N3]- [NOzl+ 1400 * Average of a Ferrni resonance doublet.The presence and structure of the nitronium ion [NOz]+ in mixtures of nitric acid and sulphuric acid has been demonstrated from their Raman spectra. X YZ Molecules (Cmv) The three modes are both i.r.- and R-active; hydrogen cyanide is a typical such molecule; this and several others are given in Table 3. The symbols in brackets give an approximate description of the vibrational frequency, v2 7 I 2 (6C-H) 569 (6C-D) 524 589 637 628) Table 3: Vibrational frequencies (cm-1) of linear XYZ molecules and ions Molecule or ion Vl HCN DCN cos 33 I I (vC-H) 2630 ( vC-D) 859 I286 N2O K+[NCO]- 2165 2052 K+ [ N CS]- 487 470) I v3 2097 ( VC 3 N) I925 ( VC = N) 2064 2224 I207 739 HaIlam 4 47 e.g.v (C-H) represents a stretching vibration which is largely localized in the C-H bond and v(C==N) one which is essentially concerned with the stretching of the triple bond. Table 4: Vibrational frequencies (cm-1) of bent triatomic molecules and ions Molecule or ion V l v2 I I10 I043 3756 2788 (2627) 969 1618 1261 3657 267 I 2615 688 1318 I328 3213 I151 3263 I362 739 705 I595 I178 I183 320 750 828 I540 518 I242 332 3626 I800 605 Note: (a) that stretching (v) frequencies are higher than bending (6) frequencies; (b) that the anti- symmetric stretching (vss) frequency is usually higher than the symmetric (vs) one (exceptions above are 0 3 and [NOzI-). BENT TRIATOMIC MOLECULES The (3n - 6) = 3 normal modes of vibration are shown below: The vibrations are both i.r.- and R-active whether the molecule is symmetri- cal, X3 or XY2 (C2,) or asymmetrical, X X Y or XYZ (Cs).Table 4 lists some typical examples. 48 R.I.C. Reviews PLANAR TETRA-ATOMIC MOLECULES (3n - 6) = 6 normal modes of vibration are to be expected: E) XY3 Molecules (D3h) Of the four vibrations above ~1 (the ‘breathing’ frequency) clearly involves no change in dipole moment and is thus i.r.-inactive but is R-active; vz on the other hand will be i.r.-active but R-inactive; v3 and v4 are both i.r.- and R-active.Table 5 gives some examples. Table 5: Vibrational frequencies (cm-1) of planar XY3 molecules and ions Molecule or ion ~~ BC13 480 652 47 I I069 so3 244 532 995 I330 La3+ [B03]3- ;A:] N a+ [ N 0 3 1 - Ca2+[C03]2- 83 I 879 939 I068 I087 692 706 1275 I405 I460 (cal cite) X Y2Z (C2 v ) and X YZ W ( Cs) molecules Successive replacement of the Y atoms by different atoms lowers the sym- metry to CzV and then to Cs with a consequent change in selection rules. For both cases degeneracy is removed and all six vibrations are i.r.- and R-active (see Table 6). Hallam . 49 v2 v1 Molecule I- COBrCl I828 PYRAMIDAL TETRA-ATOMIC MOLECULES The six normal modes of vibration are shown below: 3336 3338) * 2327 PH3 PC13 XY3 Molecules (GV) All four vibrations are both i.r.- and R-active; common examples a reshown in Table 7.NH3 Table 7: Vibrational frequencies (cm-I) of pyramidal X Y 3 molecules and ions u1 Molecule or ion v3 v2 v4 3414 242 I 494 975 826 96 I 737 K+[C103]- [IO3]- [S0312- [Se03]2- * Bands split due t o inversion doubling. 507 930 779 1010 807 Chlorine trifluoride is an interesting example of an XY3 molecule in that it is found to have six i.r.- and R-active fundamentals. This means 50 v5 v4 v3 --- 932 * 968) 1628 1121 189 486 330 496 374 260 620 390 633 43 2 - V6 R. I. C. Reviews that the molecule is neither symmetrical planar nor pyramidal.Diffraction studies have shown that the molecule is T-shaped with one Cl-F bond longer than the other two; from symmetry considerations it should therefore be formulated as an XYZZ molecule belonging to the CzV point group (see Table 8). Molecule v3 v5 v4 v2 v1 V6 SOFz SOCl2 FClF2 530 344 752 806 492 364 I333 1251 528 XYzZ (CZ,) and XYZW (Cs) molecules Substitution of one Y atom lowers the symmetry to CzV and substitution of two Y atoms by different atoms lowers it further to Cs. In both cases the degeneracy is removed yielding six vibrational frequencies, all of which are i.r.- and R-active (Table 8). PLANAR PENTA-ATOMIC MOLECULES The (3n - 6) = 9 normal modes of vibration of a square-planar molecule are shown below: --- 390 284 434 748 455 326 v5 410 I94 703 v 4 si, s - / +Y- Hallam 51 XYq MoIecuIes (D4h) Of the above seven vibrations the three symmetrical modes, VI, vz and v4, involve no change of dipole moment and are i.r.-inactive but R-active.The v3, V6, and v7 modes are R-active but i.r.-inactive. The v5 fundamental vibrational frequency is inactive in both i.r. and Raman but can be observed in the hyper-Raman effect or as an overtone, It will be seen that no funda- mental vibration in the i.r. appears in the Raman and vice versa. This 'rule of mutual exclusion' indicates that the molecule has a cs and confirms that it is not tetrahedral. Some of the few molecules, and ions, which possess this structure are given in Table 9.v1 Molecule or ion - 543 v3 29 I v2 502 XeF4 R b+ [A u C 141- 347 3 24 144 TETRAHEDRAL PENTA-ATOMIC MOLECULES The nine normal modes are depicted below: 52 v7 v4 v5 --- V6 (221) 235 586 171 358 I23 (I:: R . I. C. Reviews XY4 Molecules ( T d ) All four vibrations are R-active but only ~3 and v4 are i.r.-active. Examples are shown in Table 10. Molecule or ion I v1 1 v2 I v3 Table 10: Vibrational frequencies (cm-1) of tetrahedral XY4 molecules and ions SiH4 C12F4 2180 908 459 cc14 SnC14 [NH4]+CI- [ PO41 3- Nai[S04]2- 3 68 3040 970 983 [Zn C1412- 28 I ~ ~~ ~~ * Fermi resonance doublet. 910 628 2183 1281 970 43 5 218 3 I 4 I06 I680 358 454 I30 82 X Y3Z (C3 ,,), X Y2Z2 (C2 v) X Y2Z W ( Cs) and X YZ W V ( Cs) molecules Successive replacement of the Y atoms lowers the symmetry which splits the degenerate vibrations and activates the i.r.-inactive vibrations to six in xY3z and nine in X Y2Z2, X Y2Z W and X YZ WV.A large number of these molecules exists and the vibrational spectra of many have been recorded; Tables 11 and 12 give a small selection. Molecule --- v1 486 408 I082 995 v2 1290 1035 786 669 or ion POC13 voc13 [FSO3]- [ S z 0 3 ] 2 - Table 12: Vibrational frequencies (cm-1) of tetrahedral XYZZZ and XY2Z'Jv molecules v2 v3 Molecule ------- S 0 2 F 2 553 497 848 608 SOzBrF v1 1269 1228 v5 v4 360 450 274 461 1502 1460 v4 -- 58 I 504 1287 I123 V6 I93 I29 409 335 v3 267 I65 566 446 v5 337 249 592 54 I v7 539 270 (:3* 131 I400 500 622 403 3 I45 I080 I106 277 Hallam V6 53 x Y5 MOLECULES Five (v3 to v7) are i.r.-active and six (v1, VZ, v5 to V8) R-active.Examples are listed in Table 13. P V7 Trigonal bipyramidal X Y5 (D3h) molecules The (3n - 6 ) = 12 normal vibrations are as follows: P b Mole- cule v2 v1 ---- PF5 PC15 SbC15 640 370 307 817 395 356 v3 945 441 V 5 V8 V6 4 P v4 576 30 I --- 534 28 I I00 I82 514 26 I I66 1026 58 I 399 74 Tetragonal pyramidal X Y5 (C4v) molecules Molecules possessing this structure are rare, examples being BrF5 and IF5.They should exhibit nine R-active vibrations of which only six should also be i.r.-active. OCTAHEDRAL xY6 (Oh) MOLECULES The (3n - 6) = 15 normal modes are illustrated below: 4 An octahedral molecule is centro-symmetric, thus the mutal exclusion rule applies, i.e. R-active vibrations (vl, v2 and ~ 5 ) are i.r.-inactive and the i.r.-active modes (v3 and v4) are R-inactive; V6 is hyper-Raman active only but can often be estimated from i.r.-active combination frequencies. Octahedral coordination is common in inorganic chemistry and many i.r. and Raman studies have been made of compounds which exhibit it; examples are listed in Table 14. POLYATOMIC MOLECULES There also exists a rapidly-expanding wealth of information on more complex compounds.These include ammine and amido, nitro and nitrito, aquo and hydroxo, cyano, carbonyl, acetylacetonates, and many others. Few studies on complex compounds of low symmetry yield complete structural informa- tion but approximate approaches based on the concept of localized group Hallam 55 v2 v1 v3 v4 Molecule or ion s Fs u Fs 775 668 31 I 344 644 532 3 29 320 [ S n C I 612- [ Pt GIG] 2- vibrations often yield useful information. The concept has proved extremely useful when dealing with metal-ligand vibrations particularly with complexes of heavy metal atoms. For example, they frequently provide clues as to the bonding site of a ligand. For illustration, in the case of the nitro group NO,, there are two possible sites of ligand attachment, at the oxygen or the nitrogen atom: 615 I89 I 86 825 825 940 626 294 330 M\o/N\o (11) 1330 1180 /O M-N \o Or (1) N-attachment (I) is unlikely to have such a pronounced effect on the NO, frequencies (vsN02 1300; vasN02 1260; 8NO2 825; see p.48) as would oxygen attachment (11). The following sets of frequencies have been observed.12 (1) 1400 1480 (11) and have been assigned to structures I and 11 respectively and serve to dis- tinguish readily between, for example, the isomeric nitropentammine and nitritopentammine cobalt (111) complex anions, [CO(NH3)5N02I2+ and [CO/NH3)50N0I2+. A more detailed application13 is in the structure of the carbonates. The structure of the free ion (D3h) and its spectrum has been discussed (p.49). When the group is covalently bonded to other groups as in organic carbonates and hydrogen carbonates, or is coordinated to a metal ion by a bond which has partial covalent character, the symmetry of the group will be changed from that of the free ion. As a result of the decreased symmetry additional bands will arise due to the removal of degeneracy. The ranges of these for the various possible structures are shown in the correlation diagram in Fig. 3. These readily allow one to distinguish between unidentate and bidentate carbonate ligands. Studies of polynuclear metal carbonyls have shown that terminal (C=O) groups consistently absorb at 2100-2000 cm-1 whereas bridging (C= 0) groups which are of lower bond order, absorb at lower frequencies, 1900- 1800 cm-1.Similar but less comprehensive studies have been made on cyano complexes where it appears that a bridging cyano group absorbs at a higher frequency than does a terminal cyano group. 56 V 6 v5 I- 524 202 (363) (144) I 58 I62 R. I. C. Reviews \ +--=- \ . / / / HaIlam i a, 0 -0 - 0 -0 9 \ \ \ \ \ h iI----- - 0” v 57 I I I I I T I I I 1 I I I so;- HSO, so; - s,o,z - s,o;- Se0;- SCN- c to, cio; Po:- cr0:- Mn0:- w0:- BrO, 10, vo; Cr202,- 400 1200 1400 1600 200 600 1000 800 cm-’ d Fig. 4. Characteristic i.r. frequency ranges of some polyatomic inorganic ions. CORRELATION TABLES As with organic compounds, measurements of i.r.(and Raman) spectra of large numbers of compounds of similar structural types allow correlation tables of characteristic group frequencies to be drawn up. It should be stressed however that these are of far less value than their counterparts in organic chemistry. This is to be expected since organic molecules have a very limited range of atomic masses and force constants compared with inorganic molecules. Furthermore, most metal-ligand vibrations are at lower frequencies than C-X frequencies and therefore the possibilities of vibrational interaction are greater. The i.r. correlation table illustrated (Fig. 4) will serve as a preliminary guide towards spectral interpretation and qualitative identification; references 58 R.I . C. Reviews 5-7 and 9 should be used for any serious qualitative analysis. Great care must, however, be exercised in this empirical approach because frequency ranges are affected by many factors such as physical state, coupling of vibrations, and the nature of the cation or anion. Most of the data in Fig. 4 refer to the solid state which is the physical state in which inorganic com- pounds are usually examined. AQUEOUS SOLUTIONS Water is the commonest solvent used in inorganic chemistry and it is there- fore essential that vibrational spectroscopy should contribute to our know- ledge of the structure and reactions of inorganic species in aqueous solution. Because of the very weak scattering by liquid water, aqueous solutions are ideally studied by the Raman technique.With the introduction of the new laser sources these studies now embrace1 highly coloured ions. Recent developments in sampling techniques however have revived interest in the i.r. of aqueous solutions. Multi-reflection ATR (attenuated total reflection) units allow the i.r. beam to be reflected many times from the interface of a solution in contact with a crystal of relatively higher refractive index such as silver chloride or germanium. This circumvents the practical difficulties of filling and cleaning the very short path length transmission cells necessary for aqueous solutions and ATR cells provide quite satisfactory i.r. spectra of aqueous solutions over a reasonable spectral range.It appears, however, that the ATR technique possesses no inherent advantage over the transmission technique for aqueous solutions and very recently it has been demonstrated14 that careful use of conventional transmission methods using water-insoluble window materials (IRTRAN-2) can yield acceptable spectra. Goulden and Manning14 have studied a large number of inorganic compounds in aqueous solutions of varying pH over the range 950-1550 cm-l. In contrast to the corresponding solid state spectra they find the absorption bands of the dissolved ions appear over very limited frequency ranges and are in- dependent of the other ions present. Correlation charts (Fig. 5) constructed from such data permit unequivocal identification of most common inorganic ions.Care must be exercised in the choice of pH and high concentrations should be avoided. At higher concentrations ionic interactions may lead to the appearance of additional bands as the symmetry of the free ion is reduced; the technique, of course, allows such interactions to be studied. The impetus now given to i.r. and Raman spectroscopy of aqueous solu- tions will provide precise knowledge of ionic species in aqueous media. For example, alkaline solutions of zincates and aluminates are generally now described as existing as [M(OH)4In- ions although earlier texts write these as Mot- species. Raman spectral5 of zincates are consistent with the presence of tetrahedral [Zn(OH)4I2- species but recent i.r. and Raman studies16 of aluminates fail to identify the presence of the corresponding [Al(OH)4]- ion.In very alkaline solutions (PH > 12.5) the species is linear AlO,. Near pH 12.5 A10, is in equilibrium with a second form which completely takes over at pH values down to pH 8. The second form is probably an octa- hedrally coordinated polymeric anion although a square planar [Al(OH)4]- Hallam 59 Ion so;- HSO, so: - HSO s,o; - so, aq CI 0, ClO, P o i - HP0:- H,PO, COf - HCOS NH; Cr,O 5- Fig. 5. i.r. Frequency correlation chart for aqueous solution spectra of some polyatomic inorganic ions (adapted from Goulden and Manning14). is not entirely ruled out. Clearly however, in the dissolution of Al(OH)3 in the alkaline conditions described in qualitative analysis schemes, it is the A10; species which is present. BOND PROPERTIES Numerous correlations exist between vibrational frequencies and other bond properties which can provide simple and rapid estimates of such properties.For example, the symmetrical stretching frequencies for a large number of halides are known and their values change in the same order as the stability constants where these are known. Metal-oxygen stretching frequencies for a limited number of hydrates are known and these indicate that the order of stability of the hydrates is, e.g. Mn2+ < Cu2+ > Zn2+, i.e. in agreement with the normal Irving-Williams order for the stability of the complexes. SUMMARY Both i.r. and Raman spectroscopy give essentially the same information about molecular structure.For centro-symmetric molecules they are exactly complementary and for complete structural information it is desirable to 60 I I I I I I I I I I I I I I I I t I I I I I I I I I I I1 crn-’ --t R.I.C. Reviews have both i.r. and Raman spectra for the compound. Qualitative inorganic analysis can, with profit, incorporate either technique and should be encour- aged at the undergraduate level for the identification of ions, since the spectra also lead to the structure of the ion. Had such techniques been used more frequently in the past many over- simplified descriptions in inorganic chemistry might have been avoided. One obvious example is the description of the metaborate ion as BO,. A simple examination of the i.r. and/or Raman spectra would have eliminated the existence of the linear [0-B=O]- ion in most metaborate systems although a more detailed spectroscopic study would have been necessary to suggest the chain or cyclic trimeric units such as that below which actually exist in these salts. 0 I I 3- REFERENCES Elsevier, 1963. 1 I. R. Beattie, Chem. in Britain, 1967, 3, 347. 2 Manse1 Davies (ed.), Infrared Spectroscopy and Molecular Structure, Amsterdam : 1965. 3 R. G. J. Miller (ed.), Laboratory Methods in Infrared Spectroscopy, London: Heyden, 4 I. J. Worrall, Molecular Symmetry, R.I.C. Lecture Series, 1967, No. 2. 5 K. Nakamoto, Infrared Spectra of Inorganic and Coordination Compounds, New York : Wiley, 1963. 6 F. A. Miller and C. H. Wilkins, Analyt. Chem., 1952, 24, 1253. 7 F. A. Miller, G. L. Carson, F. F. Bentley and W. H. Jones, Spectrochim. Acta, 1960, 10 L. A. Woodward, Quart. Rev., 1956, 10, 185. 16, 135. 8 K. E. Lawson, Infrared Absorption of Inorganic Substances, New York: Reinhold, 1961. 9 D. M. Adams, Metal-Ligand and Related Vibrations, London: Edward Arnold, 1967. 1 1 R. N. Jones and Magda K. Jones, Analyt. Chem., 1966, 38, 393. 12 R. B. Penland, T. J. Lane and J. V. Quagliano, J. Am. chern. Soc., 1956, 78, 887. 13 B. M. Gatehouse, S. E. Livingstone and R. S . Nyholm, J. chem. SOC., 1958, 3137. 14 J. D. S. Goulden and D. J. Manning, Spectrochim. Acta, 1967, 23A, 2249. 15 E. R. Lippincott, J. A. Psellos and M. C. Tobin, J. chem. Phys., 1952, 20, 536. 16 L. A. Carreira, V. A. Maroni, J. W. Swaine and R. C. Plumb, J. chem. Phys., 1966, 45, 2216. 61 Hallam