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
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 and . 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.
Functional Groups and "Fingerprint" Regions
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 and 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 stretch||2,950 – 2,850||Medium or strong|
|Alkenyl Bond Vibrations|
|Alkenyl stretch||3,100 – 3,010||Medium|
|Alkenyl stretch||1,680 – 1,620||Variable||Stretches between 3,010 and 3,100 cm–1 are generally confirmation of unsaturation.|
|Alkynyl Bond Vibrations|
|Alkynyl stretch||2,260 – 2,100||Variable|
|Aromatic Bond Vibrations|
|Aromatic bend||860 – 680||Strong|
|Aromatic bend||1,700 – 1,500||Multiple, medium|
|Alcohol Bond Vibrations|
|Alcohol stretch (H-bonded)||3,600 – 3,200||Strong, broad|
|Alcohol stretch (free)||3,700 – 3,500||Strong, sharp|
|Alcohol stretch||1,150 – 1,050||Strong|
|Carboxylic Acid Bond Vibrations|
|Carboxylic acid stretch||3,600 – 2,500||Broad, variable|
|Carboxylic acid stretch||1,780 – 1,710||Strong|
|Amine Bond Vibrations|
|Amine stretch||3,500 – 3,300||Medium||Primary amines produce two stretch absorptions, secondary amines produce only one, and tertiary amines produce none.|
|Nitrile Bond Vibrations|
|Nitrile stretch||2,260 – 2,220||Medium|
|Nitro 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 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 stretch||2,830 – 2,695||Medium|
|Ketone Bond Vibrations|
|Ketone stretch||1,750 – 1,680||Strong|
|Ester Bond Vibrations|
|Ester stretch||1,750 – 1,735||Strong|
|Amide Bond Vibrations|
|Amide stretch||3,700 – 3,200||Medium||Primary amides produce two stretch absorptions, secondary amides produce only one, and tertiary amides produce none.|
|Amide stretch||1,690 – 1,630||Strong|
|Alkyl Halide Bond Vibrations|
|Alkyl Halide stretch||1,400 – 1,000||Strong|
|Alkyl Halide stretch||800 – 600||Strong|
|Alkyl Halide stretch||750 – 500||Strong|
|Alkyl Halide stretch||~500||Strong|
In general, carbon to hydrogen bonds () give stretches that appear above 2,800 cm–1. However, the hybridization of the carbon can affect absorbance, with appearing around 2,900 cm–1, around 3,100 cm–1, and around 3,300 cm–1. Carbonyl groups () 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 () and the stretches from a variety of bending vibrations (i.e., bending and 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.