In the Laboratory
Sampling Techniques for Organic Solids in IR Spectroscopy: Thin Solid Films as the Method of Choice in Teaching Laboratories Patty L. Feist Department of Chemistry and Biochemistry, University of Colorado, Boulder, Boulder, CO 80309;
[email protected] The influx of modern FTIR spectrometers to undergraduate organic chemistry teaching laboratories allows for rapid acquisition of IR spectra by large numbers of students. In fact, we have found that the rate-limiting step in student acquisition of IR spectra is the preparation of the samples, especially samples of solid organic compounds. Over a period of ten years, we have instructed our students in several different sampling techniques. One method stands out as the best method for students: thin solid films. This method is listed in current organic laboratory technique and spectroscopy text books (1–4) but is somewhat under-emphasized, and thus some instructors might not realize the advantages of thin solid films for IR spectroscopy in teaching laboratories. Sampling techniques for transmission IR spectroscopy of solid organic compounds include Nujol mulls, potassium bromide (KBr) pellets, and neat (thin-film) samples. Samples can also be run by diffuse reflectance IR spectroscopy, a method that requires very little sample preparation. A Nujol mull is prepared by grinding the organic solid with Nujol (mineral oil) using a mortar and pestle for several minutes and placing the mull between two salt plates. This method is easy and fast, and it does not require special equipment. However, the Nujol absorbs in the IR region (2920– 2850, 1460, and 1380 cm᎑1), possibly obscuring absorption bands from the organic compound and complicating the interpretation of the spectrum. A KBr pellet is a mixture of the organic compound with KBr, which is pressed into a transparent pellet or “window”. Unlike Nujol, KBr is transparent in the IR region. We have tried various methods of pressing KBr pellets, such as minipresses (nut-and-bolt KBr dies), a mechanical hand-held press, and blotter paper with a mechanical press (5), in an effort to find a method that we could teach to a lot of students in a short time. Preparation of a good pellet by either the Nujol mull or the KBR pellet method requires a significant amount of time, experience, practice, and skill. Another disadvantage is that each pellet-pressing method requires specialized equipment ranging in cost from $100 to many hundreds of dollars. We found that student misuse of equipment required not only the initial equipment investment but also replacement purchase costs. Thin solid films of organic compounds are prepared by placing a solution containing the compound on a salt plate and allowing the solvent to evaporate, depositing a thin layer of the solid compound on the salt plate. (Details are presented in the Method section.) The method is fast and easy, and does not require specialized equipment. Since the samples are prepared “neat” (undiluted), the spectra are free from interfering absorbance bands.
Another method for obtaining IR spectra of organic solids is diffuse reflectance spectroscopy. The method itself is quite fast and easy: the solid sample is simply ground with KBr (or not) and placed in a special cup in the instrument. A disadvantage of the method is that reflectance spectroscopy in general is not as sensitive as transmission spectroscopy (6 ). Also, the method requires a special accessory for the FTIR instrument, which costs several thousand dollars. Method Dissolve a small amount of solid organic sample (5–10 mg) in a few drops of a volatile solvent. Suitable solvents include methylene chloride, diethyl ether, and pentane. Place a drop of this solution on a salt plate (NaCl, 20 mm × 5 mm). Allow the solvent to evaporate; you should observe a thin solid film on the plate. If you do not, add another drop of the solution and allow it to evaporate. Place the plate in the V-shaped sample holder inside the FTIR and record the IR spectrum. If the peaks are of too low an intensity, remove the salt plate, add more of the solution, evaporate, and rerun the FTIR. If the peaks are of too high an intensity, remove the plate and clean it. Dilute the solution of solid compound with solvent and repeat the above process until a suitable spectrum is obtained. (Photos of this preparation are online at http://orgchem.colorado.edu/procedures/IR/ IRsolids.html.) Discussion The spectrum of 1-(hydroxymethyl)naphthalene run as a thin solid film is shown in Figure 1. Compare this spectrum with the spectrum of the same compound run as a KBr pellet, Figure 2. The thin solid film sample shows a greater percent transmittance through the sample at all wavenumbers and less baseline slope. The minor differences in the spectra may arise from differences in intermolecular interactions due to different concentrations of the compound in the two sampling techniques. Our method of preparing a thin solid film is essentially the same as that described in four laboratory technique or spectroscopy textbooks (1–4). (We found that about 20 other texts did not mention the technique at all.) Silverstein (1) states that deposited films are useful only when the material is deposited as microcrystals or as a glassy film. Pavia (2) does not note any reservations about the method of “dry films”, but relegates this sampling technique to the very end of the solid sample preparation section. Eaton (3) states that the method works if a solid forms a translucent film on the plate. Kemp (4 ) states that the method of “solid films” works well
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In the Laboratory
Figure 1. Uncorrected spectrum of 1-(hydroxymethyl)naphthalene run as a thin solid film. (Nicolet Avatar 320.)
Figure 2. Uncorrected spectrum of 1-(hydroxymethyl)naphthalene run as a KBr pellet. (Nicolet Avatar 320.)
for polymers and waxy or fatty materials, but in many cases the film is opaque and thus does not allow a good spectrum. Despite the reservations of Silverstein, Eaton, and Kemp, we have found that even rather opaque films of solid samples can be scanned to give acceptable IR spectra. Moreover, the majority of organic compounds we have tried give good spectra. When the films are translucent, as in the spectra of 1-(hydroxymethyl)naphthalene (Fig. 1), the spectra are much better than those run as KBr pellets. Of the methods discussed, the thin solid film is the easiest and most economical and produces excellent spectra free from interfering bands. We encourage all laboratory instructors to try this method so that students can routinely run spectra of organic solids.
liner, who first showed a perfect IR spectrum of a thin solid film to these disbelieving eyes.
Acknowledgments I thank the many organic chemistry graduate students at CU Boulder who use this technique, especially Marty Ber-
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Literature Cited 1. Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric Identification of Organic Compounds, 5th ed.; Wiley: New York, 1991; p 100. 2. Pavia, D. L.; Lampman, G. M.; Kriz, G. S.; Engel, R. G. Introduction to Organic Laboratory Techniques, A Microscale Approach, 3rd ed.; Saunders: Fort Worth, TX, 1999; p 753. 3. Eaton, D. C. Laboratory Investigations in Organic Chemistry; McGraw-Hill: New York, 1989; p 643. 4. Kemp, W. Organic Spectroscopy, 3rd ed.; Freeman: New York, 1991; pp 52–53. 5. Gerry, H. E.; Craig, R. E. R. J. Chem. Educ. 1989, 66, 616. 6. Nicolet Instrument Corporation. Introduction to Sample Handling; Nicolet Instrument Corporation Home Page; http://www. nicolet.com/labsys/ (accessed Nov 2000).
Journal of Chemical Education • Vol. 78 No. 3 March 2001 • JChemEd.chem.wisc.edu
