Infrared Spectroscopy A Versatile Tool in Practical Chemistry Courses Volker ~ i s k a m p , 'Wolfgang Fichtner, Volker Kramb, Alexander Nintschew, and J e n s Stefan Schneider Fachhochschule Darmstadt, Deparfment of Chemical Engineering, Hochschulstrasse 2, 64289 Darmstadt, Germany With the objective of reducing the amount of waste i n our practical chemistry courses ( 1 )we were able to downscale the preparation of inorganic compounds significantly (in some cases to microscale synthesis) and introduce IR spectroscopy a s a convenient tool for gaining structural information about the samples. The IR studies described in this paper have been included successfully i n our Introductory Practical Chemistry Course and make students familiar with IR spectroscopy a t a n early stage in their curriculum. By combining the preparation of samples with their spectroscopic analysis, the course program has been improved a s regards both methods and content. The students are shown how to prepare samples of basic inorganic compounds for IR measurement, practice the fabrication of KBr discs and learn how to operate the IR grating spectrometer (Perkin Elmer 841).The fundamentals of vibrational spectroscopy a r e explained (2) a n d t h e recorded spectra interpreted. Special attention is given to the correlation of the observed spectroscopic data with the bond lengths and bond strengths in selected compounds. Cyanide, Cyanate, Thiocyanate, and Cyano Complexes One important goal of introductory practical chemistry courses i s to teach young scientists and engineers how to deal with hazardous substances i n a safe and responsible way. Cyanide and some of its related compounds serve a s examples for potentially dangerous compounds which have to be handled carefully. Aqueous solutions containing cyanide are detoxified conveniently either by oxidation with HzOz to form cyanate (3) (eq 1) or by reaction with sodium thiosulfate to form thiocyanate ( 4 ) (eq 2). HzO, + NaCN 3 NaOCN + HzO
(1)
NazSz03+ NaCN -t NaSCN + NazS03
(2)
The cyano compounds treated in the course include the potassium hexacyanoferrates a s well-known examples of octahedral complexes and the related Prussian blue as a conveniently prepared blue pigment (eq 3).
'Author to whom correspondence should be addressed.
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Figure 1. Resonance structures of the cyanide, cyanate, and thiocyanate ion. Structure and bonding of the cyanide anion can be illustrated by the two limiting structures in Figure 1, where the negative charge can be located a t the electronegative nitrogen atom (hard base) or a t the carbon atom (soft base). The C-N bond can be described as being somewhere between a double and a triple bond. The C-N stretching vibration in KCN requires a n energy of 2080 em-'. The oxidation products of the cyanide ion, i.e. the cyanate and thiocyanate ions show C-N vibrations a t 2168 cm-' and 2049 cm-', respectively. This indicates that (compared to the cyanide ion) the C-N bond is stronger in cyanate and weaker in thiocyanate, as a large wavenumber corresponds w i t h a h i g h e r vibrational energy. T h e mesomeric equilibrium in Figure 1 shows that the dominating resonance structure is the one with the negative charge a t the oxygen atom. The C-N bond in OCN- therefore tends to have more triple than double bonding character and is strengthened compared to CN-. The most important resonance structure of SCN-, on the other hand, is the one with the negative charge a t the nitrogen atom. The IR spectroscopic study of the hexacyanoferrate complexes, with the cyano ligands bonded via the carbon atom to the Fe(I1) and FeUII) center, reveals further information. According to the acid-base theory by Pearson (51,the interaction between the soft base center of the CN- and the soft cation Fez+is stronger than that between CN- and the hard cation Fe3+. A strong C-Fe bond leads to a weaker C-N bond, and a s expected, the C-N stretching vibration in the yellow potassium hexacyanoferrate(I1) requires less
-
-
M
-I
M
N@
-12m-
1 2 '
/
01 // N ;
01 -
Nltro Complex
Nirito Complex
Figure 2. Isomers of the coordinated nitrite Ion: nitro and nitrito cornplexes.
Oxalic Acid
Oxalato Ligand
mean N-0 bond length in K~[CO(NOZ)~I (111pm (10)) is significantly shorter than in NaNOz (125 pm (11)). Dithionate and Thiosulfate Potassium dithionate (K2S206),a compound with a S-S single bond, is produced in our laboratory course by oxidizing potassium disulfite with manganese dioxide (eq 5). (5) &SzO, + MnO, + HzO+ Mn(OH), + K2S206 The S-S stretching gives a signal at 1242 cn-'. Sodium thiosulfate (NazSz03)also is prepared in the practical course from sodium sulfite and sulfur. The characteristic structural feature of this multi-facetted compound (12) is the partly doubly bonded S=S entity that is analogous to an S = 0 bond in the sulfate ion. The S-S stretchingvibration is found at higher energy (1635 cm-')).Again these findings correlate well with crystallographic data as these differences are reflected in the S-S bond lengths (KZSzO6= 216 pm (13) Oxalate Dianion and NazSz03= 197 pm (14)).
Oxalic Acid, Oxalate, and Oxalate Complexes Fiaure 3. Structure of oxalic acid, oxalate dianion, and the flve-memberedchelate ring in the Our last example, oxalic acid, also plays oklato complexes. a significant role in inorganic analytical chemistry and in coordination chemistry energy (2043 cn-') than in the red relative (2116 cn-'1. (15).In the coursework oxalate complexes of copper and These results ought to initiate a discussion on the stability chromium are produced by straightforward complex forand toxicity of the hexacyanofemate ions. Students learn mation (eqs 6 and 7). that the yellow salt is a stable 18-electron complex that CuS04+ 2 KzCzO, + 2 HzO . does not give off cyanide. In contrast, the less stable red 17-electron complex hydroyzes easily in alkaline solution and releases cyanide. Students have now obtained IR spectroscopic data that provide proof of the significant differences in the stability of these two complexes. Another group of students who have been asked to record the IR spectrum of Prussian Both oxalato complexes feature five-membered chelate blue will find that the C-N stretching in this compound rings as shown in Figure 3. requires an energy of 2080 cn-', which is exactly between The infrared data of these complexes are compared with the two potassium hexacyanoferrates. those of oxalic acid and of potassium oxalate. The stretchThese spectroscopic data can be interpreted as evidence ing of the C-O(2) double bond in oxalic acid (119 pm (16)) that the oxidation state of the iron center in Prussian Blue requires a n energy of 1688 cm-'. The same signal in averages 2.5 and help to explain the rapid electron transK2C20r(with four equivalent O-atoms) requires much less fer from Fe(I1) to Fe(II1) via the bridging cyanide ligand energy (1591 ern-')) which is plausible as the C-0 bond or(6-8). der in this salt is 1.5, and all C-0 bonds are significantly longer (123 pm (17)).The carbouyl stretching in the chroNitrite and Nitro Cobaltate mium complex requires an energy of 1681 cn-' which is In the nitrite anion the bond order of the N-0 bond almost identical with the value found in free oxalic acid. In equals 1.5. Of the two signals observed in the N-0 stretchthe copper complex, the same vibration requires less ening region, the anti-symmetric N-0 vibration of KNOz reergy (1668 cn-')). quires an energy of 1381 cm-'; whereas, the symmetric viAgain a n explanation is offered by the acid-base bration requires 1267 em-' (9). When complex formation theory by Pearson ((5) Cf. discussion above on M-C occurs, the nitrite ion can be coordinated to the metal via bonds i n t h e p o t a s s i u m h e x a c y a n o f e r r a t e s ) . A a n oxygen as well as a nitrogen atom leading either to a stronger bond i s formed between oxygen a n d t h e nitrito or a nitro complex (see Fig. 2). h a r d Cr(II1) center t h a n between oxygen and t h e Potassium hexanitrocohaltate(III) is used in our introsofter Cu(I1) center. The strong Cr-O(1) bond leads ductory course as an example of a nitro complex (eq. 4) to a pronounced C-O(2) double bond (see Fig. 3). The weaker Cu-O(1) bond induces a weaker C-O(2) bond in the copper oxalato complex. As a result of the complex formation, the electron denSummary sity is removed from the nitrogen atom, and, therefore, IR spectroscopy is a versatile method for the structural the N-0 bonds are strengthened in comparison to the analysis of chemical compounds. A comparatively easy-touncoordinated nitrite ion. Hence, the N-0 stretching viunderstand theory, the facile preparation of samples and brations in the coordinated N O 2 ligand require more easy spectrometer operating make IR spectroscopy a suitenergy (anti-symmetric: 1421 em-'); symmetric: 1334 cmable method for freshman courses. Our work demonstrates (9)). In the discussion of these IR spectroscopic obserthat this topic can contribute to a better understanding of vations a comparison can be made with data from X-ray structure and bonding in courses covering Inorganic and crystallography. The findings correspond well as the Volume 72 Number 10 October 1995
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coordination chemistry. Resonance equilibria can be semiquantitatively estimated on the basis of the straightforward model that the stretching of a strong interatomic bond requires more energy than that of a weaker bond. Students learn t o correlate the IR data from their samples with data from X-ray crystallography in the simple manner: large wave number + high vibrational energy + strong bond -+ short bond length. This interplay of synthesis and analysis improves the didactic value of any practical laboratory course and can lead to a better integral understanding of chemical concepts. Experimental Procedure
The compounds studied were obtained by standard procedures (18).Most of the preparations could be downscaled and carried out with less than 1g starting material. The coordination compounds Prussian blue, copper(II)oxalate, and chromium(1II)oxalate provide excellent examples for microscale preparations that were carried out as follows. K [ F ~ " F ~ " ' ( C N ) ~ IAn : aqueous solution of &[F~(CN)BI. 3H20(20mg in 2 mL H20)is added to a solution of 20 mg FeC13 . 6 H20in 1mL H20 and a drop of hydrochloric acid (37%)at 363 K. The dark hlue precipitate is isolated hy centrifugation and dried at 353 K. Kz[Cu(C20&(Hz0)z1: A hot aqueous solution of CuS04 (100 mg in 2 mL H20) is added to a solution of 310 mg K2C204 . 2 Hz0 in 5 mL H20 at 363 K. The mixture is stirred briefly and allowed to cool in a refrigerator (277 K) overnight. The light hlue crystals are filtered off, washed with a small amount of cold water and dried over silica. K3[Cr(CzOa)31:A solution of 200 mg K2Cs04. 2 Hz0 and 460 mg H2C204.2 H20 in 5 mL of H20 is added to a solu-
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tion of 200 mg K2Cr207in 4 mL H20 at 363 K. The mixture is stirred briefly and cooled in an icebath. The black crystals of &[Cr(C204)31are filtered off, washed with a small amount of cold water and dried over silica. Preparation of the KBr discs: Approximately 200 mg of dried KBr (analytical grade) and 5 mg of sample are thoroughly crushed and mixed in an agate mortar. In order to obtain a transparent disc the powder is pressed (50-60 bar) under vacuum for 5 min. If the IR absorption of the disc is too high or too low another disc of different concentration has to be made. It is one objective of this course that the students practice their experimental skills and gain a feeling for the right sample concentration. Literature Cited 1. Schneider J. S.: Wiskamp, V J. Chon Educ.. 1994.71.587. 2. Ranwell. C. N. FundnmrntolsofMolecuiorSpdrosopy, 3rd ed.: McGraw-Hill Ltd.: Landon, 1983;Chapter 3. 3. Helmling. 0.;Barenschee. E. R.;Oiehl. S. Chem. lad. 1990.3.24. 4. Forth. W.; Hensehler, D.: Rummel. W Pharmokoiopleund hrikologie,2nd ed.: Bibliopaphischea Institut:Mannheim. 1977. 5. pearson, R. G. Hard and sort Acids and Bases; Dowden Hutchinson Ross: Stmudsburg,1073. 6. Holleman,A. E;Wiberg. E.lahrhueh&rAnogonischhh Chemie,91sGlOOthed.; de Gryter:Berlin. 1985. 7. Nelson, S. M. I" ComprehensivecooniinationChmislry. lstod.;Wilkinson. G.:GilI d R. D.:McCleverty,J. A ; Eds;Pe~~amon Press: Oxford. 1987:Val. 4, pp l. 8. Buser, H. J.:Schwanenbseh.O.:Petter,W.: Ludi,A. Inorg Chem. 1977.16.2704, 9. Cotton, F. A ; Wilkinson, G. Advanced Inorganic Chemislry,4th ed.; John Wiley & Sons:New York. 1980;pp 173-174. 10. GmelinsHandbuchdrrAnorganiiihhhChemie,8thed.;VCHPublishers:Weinheim. 1061.Val. 58. Part AiCobalti,p 776. n. carpenter.G. 6.ACL. CWI. 1955.8,852. 12. Dhawale. S. W. J Chem Educ. 1993. 70. 12. 13. Msninez. S.; Garda-Blanco, S.:Riuoir LActo Crysf. 1956.9,145. 14. Taylor, P G.:Beevem, C. A A c f o Clyst. 1952,5,341.15. Scott. K L.; Wieghardt,K; Sykes.k G. 1nor.gChem 1973.12.655. 16. Cox, E. G.:Oouglll, M. W.; Jeffrey G.A. J. Chem. Soe. 1952.4854. 17. Jeffrey,G.A.:Perry. G. S. J.Am Chem Soc. 1954,76.5283. 18. Brauer, 0. Handhaoh ofFnpomtiueInorghnir Clzrmidry, 2nd ed.; Academic Press Inc:New York. 1963.Vols. 11.
