Inexpensive Low-Frequency IR Windows Using Polyethylene Bags Miguel Angel Us& Universidad de Zaragoza, 50009 Zaragoza, Espatia
The use of polyethylene IR windows in the 6 W 2 0 0 cm-' ranee described recentlv in this Journal bv V. C. "~has been ~-~ D&! and their advantages over t i e traditional CsI piates or KBr discs have been ~horouehlvdiscussed. As an alternative to the commercial po1yeth;le~ewindows, the use of less expensive Tupperware seals or coffee can lids has been proposed, but (as seen in Figures 1 and 4 of the reference cited), these cell components are only moderately transparent in the low-frequency region (lowering the baseline down to -60% transmittance), because of their thickness. Polyethylene sheets have been used in our laboratory as a support for Nujol mulls for more than 15 years, not only by undergraduate students but also in research, and no incompatible material has been found so far. The source of these sheets are simple transparent polyethylene bags, (as used in households), which sell a t about $8.50 for a package containing 500 units having a size of 30 X 22 cm each. A small rectangle of the bag (-- 2 X 3 cm) is cut out and the Nujol mull is sandwiched between the two lavers of polvmer. As theeost (about 0.017centsperspectrumj is solow, thesheets mav be dis~osedof after acquisition of the data. since thk Nujol mull technique is used, spectra can be recorded over the entire range 4000-200 cm-', taking into account that in the 2975-2815 (CH2 stretching of polyethylene) (2) and 1475-1450 cm-' (CH2 bending)2 regions the ~
mineral oil also absorbs strongly (less than 50% transmittance). If thin sheets are used, the only other strong absorptions of polyethylene occur a t 730 and 720 cm-' (CH2rocking),2and they can even be used as a reference. The baseline outside these regions remains a t higher than 80% transmittance. The spectra of cis and trans-IPt(~Phs)zC121(Figs. 1 and 2, respectively) in the 600-200 em-' region demonstrate the usefulness of far infrared spectroscopy. The geometry of these compounds can be assigned from the number of bands in the 550-500 cm-' region (triphenylphospbine internal modesP or the 350-290 cm-' region (Pt-CI ~tretcbing).~ The transparency of the polyethylene sheets does not greatly depend on the exact thickness of the film, and thus the inconvenience of handling very thin supports (e.g., wrapping film) can be avoided. On the other hand, thenonrigidity of these sheets mevents their uses as a vacuum seal so that for an evacuable cryostat, such as the one described by Dew,' the thick windows are to be preferred. C. J. Chem. Educ. 1987,64,975. Ronmann, K. J. C M . Phys. 1955,23,1355-1356. Mastin, S. H. horg. Chem. 1974, 13,1003-1005. Adams. D. M.; Chatt. J.; Gerrat J.; Westland. A. D. J. Chem. Soc.
'Dew, V.
1984,734-739.
600 Figure 2. IR spectrum of translFt(~~h.)&l,I
412
Journal of Chemical Education
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