The C-H stretch also occurs in the region around 3000 cm -1 but this is usually mostly obscured by the broad O-H absorption.We now need to match up the wavenumbers of bands in the spectrum with the wavenumbers given in Table A.2. In order to identify the bonds present in the molecule we first of all look at the region above 1500 cm -1. For example, using Table A2 we could identify the presence of C=O in a molecule but would not be able to distinguish between an aldehydes and a ketone.Ĭonsider the IR spectrum of butanoic acid shown. We can use an IR spectrum to identify the bonds present in a molecule but cannot always distinguish between functional groups. Table A.2 The precise wavenumber for a vibration for a particular functional group depends on the atoms adjacent to a particular bond thus C=O in a ketone will have a slightly different stretching frequency to C=O in an ester. The wavenumbers at which some bonds vibrate are shown in Table A.2.Īldehyde, ketone, carboxylic acids, ester To a good approximation the various bonds in a molecule can be considered to vibrate independently of each other. We are usually interested in identifying the bonds/functional groups in an organic molecule. They will both show very similar bands in the region above 1500 cm -1 because they have the same functional group but they will have different fingerprint regions. the infra red spectra of butanone and propanone can be distinguished using the fingerprint region.

Comparison of the spectrum in the fingerprint region with spectra in databases of IR spectra can be used to identify the molecule. The region below 1500 cm -1 is called the “fingerprint region” and is characteristic of the molecule as a whole. Thus, in the region above 1500 cm -1 in the IR spectrum of propanone there are two bands corresponding to the C-H stretch and the C=O stretch. The infra red spectrum can be used to determine the bonds present in a molecule. The troughs (usually called “bands”) thus represent wavenumbers at which radiation is absorbed. IR spectra are always looked at with the baseline (representing 100% transmittance, i.e. The printout of the spectrum then shows which frequencies (wavenumbers) are absorbed. In an IR spectrometer IR radiation in the range 400-4000 cm -1 is passed through a sample.

The major use of IR spectroscopy is in determining the structures of organic compounds. Structure Determination using IR Spectroscopy

Ir spectra