Infrared Spectroscopy

Infrared (IR) Spectroscopy

Infrared (IR) spectroscopy measures the interaction between organic molecules and infrared light and is used to determine the functional groups present in an organic compound.

The identification, or determination, of organic structures is one of the most important tasks in organic chemistry. Generally, it is possible to perform this task by comparing the physical properties of a compound to a known standard (e.g., melting point, boiling point, etc.) or by using chemical tests to determine the presence of specific functional groups. Unfortunately, these methods are not useful for compounds that have not been synthesized or fully characterized before or that are very complex. Additionally, relatively large samples are needed, as these methods tend to be destructive to the sample. Most spectroscopic techniques (such as IR spectroscopy), on the other hand, tend to be nondestructive.

Spectroscopy is the study of how light interacts with matter. Spectroscopy techniques are used to determine the constituents of a sample of unknown chemical composition. In addition, the typical analysis only requires a few milligrams of material, and most spectroscopy techniques are nondestructive.

Absorption spectroscopy is the measurement of the amount of light absorbed by a compound as a function of the wavelength of light. Both infrared spectroscopy and ultraviolet spectroscopy are types of absorption spectroscopy. Infrared spectroscopy (IR spectroscopy) is a method that observes the vibrations of bonds and provides evidence of the functional groups present. In comparison, ultraviolet spectroscopy (UV spectroscopy) is a method that observes electronic transitions and provides information on the electronic bonding in the sample.

Covalent Bond Vibrations

When infrared (IR) light interacts with organic molecules, it is too weak to break covalent bonds but instead just makes the covalent bonds vibrate, which can translate to stretching or bending of the covalent bonds.

Infrared light (IR light)is the region of the electromagnetic spectrum that corresponds to frequencies from just below the visible light frequencies to just above the highest microwave and radar frequencies. Infrared light occurs between wavelengths of 8×105cm8\times10^{-5}\;{\text{cm}} and 1×101cm1\times10^{-1}\;{\rm{cm}}. Because energy and frequency are inversely proportional to wavelength, infrared light is lower in energy and has longer wavelengths than visible light. Because infrared light is lower in energy, it does not have the necessary amount of energy to cause electronic transitions. But it can cause covalent bonds to vibrate. Electronic transitions are when an electron moves from one energy level to another energy level.

Molecules are made of atoms connected by covalent bonds. Some models of covalent bonds depict the bonds as rigid and unchanging until broken. However, it would be more accurate to describe bonds as elastic like a spring. If the bond is stretched or compressed, a spring force will return the bond to equilibrium position and the atoms will vibrate. If the vibration results in a change of the molecule's dipole moment, the direction of the permanent dipole, then the molecule will absorb infrared energy. Molecules absorb infrared light only at specific frequencies and wavelengths, and their vibrational transitions correspond to distinct energies.

Molecules vibrate in several different ways. Two atoms will undergo a stretching/compression motion as if connected by a spring. Three atoms connected by covalent bonds can undergo a variety of stretching, bending, and twisting movements, such as symmetric or asymmetric stretching, rocking, and scissoring. Symmetric stretching is when bonds around the central atom are stretching in opposite directions. Asymmetric stretching is when bonds around the central atom are stretching in the same direction. Rocking is when atoms around the central atom are moving back and forth in the same direction. Scissoring is when atoms around the central atom are moving back and forth in opposite directions.

The infrared spectrum (IR spectrum) is a graph of the energy absorbed or transmitted by a molecule as a function of the frequency or wavelength of light. The frequency at which the vibrations occur in an IR spectrum is dependent on two factors: the stiffness of the bond and the masses of the atoms attached to the bond. Stronger bonds, such as carbon-hydrogen bonds, are generally stiffer, which means they need more energy to stretch or compress and will appear at higher frequencies in the IR spectrum. The vibrational speed of bonds of different strength can be compared when atoms of equal weight are attached to a carbon or central atom. Stronger bonds will vibrate faster than weaker bonds. Heavier atoms vibrate more slowly than lighter atoms. This means that frequency decreases as atomic weight increases.

Bond Vibrations

Bonds can vibrate in a variety of ways. Two atoms will undergo a stretching/compression motion as if connected by a spring. Three atoms connected by covalent bonds can undergo a variety of movements: symmetric stretching, asymmetric stretching, rocking, or scissoring (bending).

Functional Groups and "Fingerprint" Regions

Infrared (IR) spectra tables show the common absorbances of different functional groups. Additionally, the shape and intensity of the stretch can give information about the functional group.

In general, the most useful information regarding the identity of a molecule will be what functional groups are present, and this can be found in the functional group region. A functional group is a group of atoms with specific physical, chemical, and reactivity properties. Additionally, some functional groups can be considered combinations of different bond types. An ester is a common example of this; the CO2R contains both C=O {\rm{C{=}O}} and CO{\rm{C{-}O}} bonds, and both bond types are functional groups that are seen in the functional group region. Because molecules absorb infrared light only at specific frequencies and wavelengths and their vibrational transitions correspond to distinct energies, functional groups have characteristic absorptions.

In infrared spectroscopy, wave numbers (cm–1) are used to measure the location of stretches. Wave numbers (cm–1) are the inverse of wavelength (cm), which means the wave number is proportional to energy/frequency, while wavelength is inversely proportional to energy/frequency. The infrared region of the electromagnetic spectrum is 4,000–400 cm–1, and on an IR spectrum this region can be further subdivided into the functional group section (4,000–1,500 cm–1) and the fingerprint section (<{<} 1,500 cm–1).

Please note that the IR spectrum shows the stretch, but the stretch is often (and incorrectly) referred to as peaks. This has become common usage, so it is important to understand that the word peaks (while incorrect) is often used. The vibrational bending and stretching of various types of bond groups and the frequencies at which these vibrations occur can be found in IR spectra data tables or tables of characteristic absorptions for functional groups. Additionally, these tables will often indicate what type of stretch each functional group will give. For example, an alcohol (OH) will give a very intense rounded stretch between 3,600 and 3,200 cm–1, while a secondary amine (NH) gives one sharp stretch and a primary amine (NH2) gives two sharp stretches between 3,500 and 3,300 cm–1.

IR Absorptions for Functional Groups

Functional Group) Characteristic Absorption(s) (cm–1) Intensity Interpretation of Spectra
Alkyl Bond Vibrations
Alkyl CH{\rm{C{-}H}} stretch 2,950 – 2,850 Medium or strong
Alkenyl Bond Vibrations
Alkenyl CH{\rm{C{-}H}} stretch 3,100 – 3,010 Medium
Alkenyl C=C{\rm{C{=}C}} stretch 1,680 – 1,620 Variable Stretches between 3,010 and 3,100 cm–1 are generally confirmation of unsaturation.
Alkynyl Bond Vibrations
Alkynyl CH{\rm{C{-}H}} stretch ~3,300 Strong
Alkynyl CC{\rm{C{\equiv}C}} stretch 2,260 – 2,100 Variable
Aromatic Bond Vibrations
Aromatic CH{\rm{C{-}H}} stretch ~3,030 Variable
Aromatic CH{\rm{C{-}H}} bend 860 – 680 Strong
Aromatic C=C{\rm{C{=}C}} bend 1,700 – 1,500 Multiple, medium
Alcohol Bond Vibrations
Alcohol OH{\rm{O{-}H}} stretch (H-bonded) 3,600 – 3,200 Strong, broad
Alcohol OH{\rm{O{-}H}} stretch (free) 3,700 – 3,500 Strong, sharp
Alcohol CO{\rm{C{-}O}} stretch 1,150 – 1,050 Strong
Carboxylic Acid Bond Vibrations
Carboxylic acid OH{\rm{O{-}H}} stretch 3,600 – 2,500 Broad, variable
Carboxylic acid C=O{\rm{C{=}O}} stretch 1,780 – 1,710 Strong
Amine Bond Vibrations
Amine NH{\rm{N{-}H}} stretch 3,500 – 3,300 Medium Primary amines produce two NH {\rm {N{-} H}} stretch absorptions, secondary amines produce only one, and tertiary amines produce none.
Nitrile Bond Vibrations
Nitrile CN{\rm{C{\equiv}N}} stretch 2,260 – 2,220 Medium
Nitro NO{\rm{N{-}O}} stretch 1,600 – 1,500 and 1,400 – 1,300 Strong Stretches must appear in both regions for a nitro to be present.
Aldehyde Bond Vibrations
Aldehyde C=O{\rm{C{=}O}} stretch 1,740 – 1,690 Strong This stretching absorption is one of the strongest and can be useful for determining both the number and type of carbonyls present.
Aldehyde CH{\rm{C{-}H}} stretch 2,830 – 2,695 Medium
Ketone Bond Vibrations
Ketone C=O{\rm{C{=}O}} stretch 1,750 – 1,680 Strong
Ester Bond Vibrations
Ester C=O{\rm{C{=}O}} stretch 1,750 – 1,735 Strong
Amide Bond Vibrations
Amide NH{\rm{N{-}H}} stretch 3,700 – 3,200 Medium Primary amides produce two NH{\rm{N{-}H}} stretch absorptions, secondary amides produce only one, and tertiary amides produce none.
Amide C=O{\rm{C{=}O}} stretch 1,690 – 1,630 Strong
Alkyl Halide Bond Vibrations
Alkyl Halide CF{\rm{C{-}F}} stretch 1,400 – 1,000 Strong
Alkyl Halide CCl{\rm{C{-}Cl}} stretch 800 – 600 Strong
Alkyl Halide CBr{\rm{C{-}Br}} stretch 750 – 500 Strong
Alkyl Halide CI{\rm{C{-}I}} stretch ~500 Strong

This table of IR characteristic absorptions for functional groups gives the bending and stretching of various types of bond groups and the frequencies at which these occur. Additionally, it indicates what type of stretch each functional group is likely to give. In transmittance IR spectra, strong stretches will extend to near to the bottom of the spectra, while medium stretches will only go around halfway, and variables can be medium or strong. Sharp stretches will come to a point, while broad stretches are more rounded.

In general, carbon to hydrogen bonds (CH{\rm{C{-}H}}) give stretches that appear above 2,800 cm–1. However, the hybridization of the carbon can affect absorbance, with C(sp3)H{\rm{C(}}{sp}^3{\rm{){-}H}} appearing around 2,900 cm–1, C(sp2)H{\rm{C(}}{sp}^2{\rm{){-}H}} around 3,100 cm–1, and C(sp)H{\rm{C(}}{sp}{\rm{){-}H}} around 3,300 cm–1. Carbonyl groups (C=O{\rm{C{=}O}}) will produce very strong stretches that appear between 1,780 and 1,630 cm–1. The different types of carbonyls also produce stretches at specific absorptions; for example, aldehydes give a stretch at 1,740–1,690 cm–1, while amides give a stretch at 1,690–1,630 cm–1. Providing there is no overlap between stretches, it is possible to use this information to determine both the number of carbonyls and the type present in a molecule.

Most of the spectrum is considered the functional group region (4,000–1,500 cm–1), and most functional groups can be found in this region (alkanes, alkenes, alkynes, carbonyls, amines, alcohols, nitriles, etc.). The section below 1,500 cm–1 is called the fingerprint section. This small section contains the stretches from alkyl halide bonds (CX{\rm{C{-}X}}) and the stretches from a variety of bending vibrations (i.e., CH{\rm{C{-}H}} bending and CC{\rm{C{-}C}} bending). Identifying individual bonds in this region is much more difficult than identifying individual bonds in the functional group region. This is because there tend to be a lot of stretches and a lot of overlap between stretches. It is more useful to compare the fingerprint region of a sample to that of a known compound to confirm identity.