Evaluated Infrared Reference Spectra

S.E. Stein

This collection of infrared spectra was originally edited and published by the Coblentz Society.

A detailed discussion of the Coblentz IR spectral collection, acquired primarily on prism and grating instruments, has been given in the Deskbook of Infrared Spectra[1, PDF with text]. A brief discussion of issues is presented below concerning the comparison of spectra in this collection with spectra acquired on modern FTIR instruments.

  1. Resolution: For prism-based spectrometers, the spectral resolution of a given prism material depends directly on the rate of change of its refractive index versus wavelength, which in turn depends on wavelength. Materials used for prism manufacture tend to show a general decline in resolution with decreasing wavelength over the conventional IR region. This noticeably broadens IR spectral features in the 3,000 cm-1 region. See, for example, tables of refractive index for crystals and glasses in the American Institute of Physics Handbook[2]. Changes in resolution over a spectrum for grating instruments are less noticeable, partly because they occur repetitively over each of several grating changes needed to acquire a complete spectrum. In both grating and prism instruments, resolutions in the low frequency (fingerprint) region are usually not very far from the “natural” resolution of the sample. However, very sharp features can be noticeably broadened in this region.
  2. Pen Widths and Irregularities: In the digitization process spurious variations can occur due to varying clarity of spectral lines and overlapping grid lines. If a minor feature is of interest, it is may be necessary to visually examine the original image to determine whether it is real or spurious.
  3. Absorbance Scale: Spectra were selected for inclusion in the Coblentz collection based on a consensus of evaluators. In some cases, in order to clearly show weak features, spectra were recorded at levels where the maximum absorbance was high. Under these conditions relative absorbances of major bands become less uncertain, since accuracy depends on the distance between the band maximum and the maximum absorbance limit, which can become very small. This must be kept in mind when comparing these spectra to those acquired under higher levels of transmission.
  4. Sample Conditions: Many of the spectra in the Coblentz collection were painstakingly measured in solution in “split solvent” cells. It is important to keep in mind that the spectrum of a compound in solution can depend on its concentration in solution and features may be significantly different as a pure liquid or solid, and can differ drastically from gas phase spectra. Solvents used in split solvent measurements were selected to minimize solvent absorptions over the complete IR spectra range.
  5. Instrument Class: Spectra in the Coblentz collection were obtained primarily on dispersive instruments (prism, grating and combinations) and were digitized long after their measurement. For reasons described above, care is needed when comparison spectra to those acquired on FTIR instruments. Differences in resolution, sample state, impurities and dynamic range need to be kept in mind when examining these spectra.
  6. Time of Spectral Acquisition/Evaluation: For spectra where a date was not provided on the spectrum, an estimate was made based on the approximate sequence number in the collection.

References

  1. Smith, A.L., The Coblentz Society Desk Book of Infrared Spectra in Carver, C.D., editor, The Coblentz Society Desk Book of Infrared Spectra, Second Edition, The Coblentz Society:Kirkwood, MO, 1982, pp 1-24. A PDF file of this article is available (reproduced with permission of the Coblentz Society).
  2. D. E. Gray, ed., American Institute of Physics Handbook, Third Edition, McGraw Hill:New York, 1972.

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