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THE ORIGIN OF THE IR SPECTRA-STRUCTURE CORRELATIONS CHART
Norman B. Colthup
71 Strawberry Hill Avenue, Unit 704
Stamford, Connecticut 06902
Infrared spectroscopy measurements were made as early as 1906 by Dr. William W. Coblentz at the U.S. Bureau of Standards. In the late 1930's the spectroscopists at the American Cyanamid Research Laboratories in Stamford Connecticut, built what has been considered to be one of the very first infrared spectrometers to be used for industrial applications in the chemical industry. This was a large tabletop single beam instrument with a boxlike cover whose joints were sealed with wax; hence the name "the wax instrument". The infrared beam went consecutively through two prisms that were made from large natural NaCl crystals of Russian rock salt that came from the salt mines of Siberia. After this, the beam went to a variable angle Littrow mirror and then back through the two prisms, and on to the exit slit and to the thermocouple.
The thermocouple detector output of this instrument was amplified by a sensitive double galvanometer system mounted on a large iron block suspended from the ceiling by elastic cables. The block had vanes below it that were immersed in oil for damping. Even so, on windy days the building swayed enough to cause problems. The thermocouple signal went to the first galvanometer that caused a small rotation of the galvanometer mirror. A constant light beam was reflected from this mirror and was focused on the edge of a photocell. The maximum thermocouple signal resulted in deflection of only a few millimeters of this beam onto the photocell. The signal from the photocell that was much stronger than the signal from the thermocouple went to the second galvanometer. A light beam reflected from its mirror, went to two side-by-side photocells attached to a recorder pen. If this beam was centered so that both photocells were activated equally, there was no recorder pen motion, but if the beam was more on one photocell than the other, a motor re-centered the photocells on the beam. This beam chaser recorder system was designed at the Stamford Labs and worked well for years. Later, a commercial DC amplifier became available. This was the instrument I first worked with in the middle 1940's.
During these days, R Bowling Barnes was director of the physics department and Van Zandt Williams was assistant director. Urner Lidell was followed by Robert C. Gore as group leader of the infrared group. All of these men had done early research in infrared spectroscopy. In 1944, these men published a book entitled Infrared Spectroscopy, Industrial Applications and Bibliography", Reinhold Publishing Co., which contained 363 IR spectra run mostly between 2000 and 1000 cm-1 along with basic IR theory and applications. Qualitative analysis and group frequencies known then were discussed in about nine pages and a statement was made that here, a great deal of work had to be done.
As a total beginner, I was interested in learning IR spectral interpretation using group frequencies as presented in that book. However, these were mostly in text form spread over a number of pages, which I found somewhat hard to remember. For my own use, I assembled these into a primitive chart shown in the figure, using data only from that one book. This was no big deal, but it proved to be a convenient way to present this kind of data. At a given frequency, you could see all the groups that absorbed at that frequency, and for a given group, you could see all the frequencies where that group absorbed. To my knowledge, this was the first IR spectra-structure correlation chart. It was never published, but it was reproduced in one of the first Perkin-Elmer instruction manuals for their first IR spectrometers in the middle '40s.
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