Infrared spectroscopy is the study of how infrared light interacts with molecules. This can be assessed in three different ways: absorption, emission, and reflection. This technique is most commonly employed in organic and inorganic chemistry. Chemists use it to identify functional groups in molecules. IR Spectroscopy measures the vibrations of atoms, which The IR transmitter continuously emits IR light, while the IR receiver constantly detects reflected light. If the light is reflected back by an object in front of it, the IR receiver detects this. This is how the object is detected by the IR sensor. The usual IR absorption range for covalent bonding is 600 - 4000 cm-1. The graph depicts the regions of the spectrum where the following bond types typically absorb. For example, a sharp band at 2200-2400 cm-1 could suggest the presence of a C-N or C-C triple bond.
Although hydrocarbon molecules only include C-H and C-C bonds, the infrared spectra produced by C-H stretching and C-H bending provide a wealth of information. In alkanes, which have extremely few bands, each band in the spectrum can be assigned.
C–H stretch from 3000–2850 cm-1
C–H bend or scissoring from 1470-1450 cm-1
C–H rock, methyl from 1370-1350 cm-1
C–H rock, methyl, seen only in long chain alkanes, from 725-720 cm-1
In alkenes compounds, each band in the spectrum can be assigned:
C=C stretch from 1680-1640 cm-1
=C–H stretch from 3100-3000 cm-1
=C–H bend from 1000-650 cm-1
In alkynes, each band in the spectrum can be assigned:
–C≡C– stretch from 2260-2100 cm-1
–C≡C–H: C–H stretch from 3330-3270 cm-1
–C≡C–H: C–H bend from 700-610 cm-1
In aromatic compounds, each band in the spectrum can be assigned:
C–H stretch from 3100-3000 cm-1
overtones, weak, from 2000-1665 cm-1
C–C stretch (in-ring) from 1600-1585 cm-1
C–C stretch (in-ring) from 1500-1400 cm-1
Note that this is at slightly higher frequency than is the –C–H stretch in alkanes. This is a very useful tool for interpreting IR spectra. Only alkenes and aromatics show a C–H stretch slightly higher than 3000 cm-1.
Functional Groups Containing the C-O Bond: Alcohols have IR absorptions associated with both the O-H and the C-O stretching vibrations.
O–H stretch, hydrogen bonded 3500-3200 cm-1
C–O stretch 1260-1050 cm-1 (s)
The carbonyl stretching vibration band C=O of saturated aliphatic ketones appears:
C=O stretch – aliphatic ketones 1715 cm-1
α, β –unsaturated ketones 1685-1666 cm-1
If a compound is suspected to be an aldehyde, a peak always appears around 2720 cm-1 which often appears as a shoulder-type peak just to the right of the alkyl C–H stretches.
H–C=O stretch 2830-2695 cm-1
C=O stretch: Aliphatic aldehydes 1740-1720 cm-1; α, β –unsaturated aldehydes 1710-1685 cm-1
The carbonyl stretch C=O of esters appears:
C=O stretch-aliphatic from 1750-1735 cm-1 ;α, β –unsaturated from 1730-1715 cm-1
C–O stretch from 1300-1000 cm-1
The carbonyl stretch C=O of a carboxylic acid appears as an intense band from 1760-1690 cm-1. The exact position of this broad band depends on whether the carboxylic acid is saturated or unsaturated, dimerized, or has internal hydrogen bonding.
O–H stretch from 3300-2500 cm-1
C=O stretch from 1760-1690 cm-1
C–O stretch from 1320-1210 cm-1
O–H bend from 1440-1395 and 950-910 cm-1
Organic Nitrogen Compounds
N–O asymmetric stretch from 1550-1475 cm-1
N–O symmetric stretch from 1360-1290 cm-1
Organic Compounds Containing Halogens: Alkyl halides are compounds that have a C–X bond, where X is a halogen: bromine, chlorine, fluorene, or iodine.
C–H wag (-CH2X) from 1300-1150 cm-1
- C–X stretches (general) from 850-515 cm-1
Fourier transform infrared (FTIR) spectroscopy is a technique for obtaining infrared spectra. Infrared light is guided through an interferometer before passing through the sample (or vice versa). A changing mirror within the apparatus alters the distribution of infrared light that passes through the interferometer. An "interferogram" is a directly recorded signal that shows light output as a function of mirror position. The Fourier transform is a data-processing technique that turns raw data into the expected result (the sample's spectrum): light output as a function of infrared wavelength (or, equivalently, wavenumber). As previously stated, the sample's spectrum is always compared to a standard.
The "dispersive" or "scanning monochromator" approach is an alternative way to obtain spectra. This method involves irradiating the sample consecutively with different single wavelengths. The dispersive approach is more prevalent in UV-Vis spectroscopy, but it is less useful in the infrared than the FTIR technique. One reason why FTIR is favored is known as "Fellgett's advantage" or the "multiplex advantage." Information at all frequencies is captured concurrently, which improves both speed and signal-to-noise ratios. Another is known as "Jacquinot's Throughput Advantage": A dispersive measurement must detect significantly lower light levels than an FTIR measurement. There are various benefits and drawbacks, but almost all modern infrared spectrometers are FTIR equipment.