Both have very different functional groups. Take another look at the cyclohexanol and cyclohexanone spectra. And, you guessed it, no two fingerprints are alike. The peaks are due to the entire molecule, its fingerprint, rather than being from independent functional groups. It can be fun.įor you Sherlock Holmes fans, the region from 1400 to 990 cm-1 (7.2 -11.1 fim) is known as the fingerprint region. Check to see if you can find the C-H stretch and the C-O stretch that are in all four spectra, using the correlation table. They identify regions of the IR spectrum where peaks for various functional groups show up.

That’s why frequency correlation diagrams, or IR tables, exist (Fig. What about all the other peaks? You can ascribe some sort of meaning to each of them, but it can be very difficult. 119) might be ketones because of the characteristic C-O stretch at 1700 cm-1 (5.9/zm) in each. 118) have the characteristic OH stretch of alo-chols, they might just be alcohols. You’ve just correlated or interpreted four IRs.īecause the first two (Fig.

It’s not there for the alcohols and it is there for the ketones. But there is a C=0, and where’s that? The peak about 1700 cm-1 (5.9/^m). Is there an OH in 2-butanone? Of course not. There’s no OH peak about 3400 cm-1 (2.9/^m), is there? Should there be? Of course not. Now, consider a couple of ketones, 2-butanone and cyclohexanone (Fig. See the peak (some might call it trough) at about 3400 cm-1(2.9 [im)l That’s due to the OH group, specifically the stretch in the O-H bond, the OH stretch. Here’s a fine example of a pair of alcohols if ever there was one. And because the spectrum is laid out on a piece of paper, these specific energies become specific places on the chart. Specific functional groups absorb specific energies. With your sample in the sample beam, the instrument scans the IR spectrum. This range is also expressed as wavelengths from 2.5 to 15 micrometers (jUm). The units are called wave numbers or reciprocal centimeters (that’s what cm-1 means). The IR spectrum is the name given to a band of frequencies between 4000 and 650 cm-1 beyond the red end of the visible spectrum. Email: the chromatographies, which physically separate materials, infrared (IR) spectroscopy is a method of determining what you have after you’ve separated it.

Infrared spectroscopy chart