Infrared (IR) spectroscopy is a fundamentally important spectroscopic analysis technique available to scientists today. It is a powerful technique that helps in specific functional group identification in a cost-effective and non-destructive manner. This article in our spectroscopic series is specifically dedicated to discussing the basics, the working principle, and the importance of IR spectroscopy in the analytical chemistry world.
What is IR spectroscopy
IR spectroscopy is a type of molecular spectroscopy. It is based on the interaction of chemical compounds with the infrared region (2.5 to 16 µm) of the electromagnetic spectrum. The electrons present in the molecules undergo vibrational transitions on absorbing low-energy infrared radiations. IR spectroscopy manipulates these vibrational energy changes to detect the functional groups present in the molecule.
A vibrational energy change originates from the bending and/or stretching of the atoms bonded to one another in the molecule. The molecules must possess a non-zero net dipole moment in order to undergo these vibrational transitions.
What are IR radiations
Infrared (IR) radiations lie between visible and microwave regions of the electromagnetic spectrum. The wavelength of mid-range IR radiations ideally ranges from 2.5 to 16 µm. Radiations in the range of 0.8 to 2.5 µm are known as near-infrared radiations while those in the range of 16 to 103 are called far infrared. Wavelength is inversely proportional to the frequency or energy of a radiation.
The more commonly used unit in IR spectroscopy is wavenumber (cm-1) which is the reciprocal of wavelength. The higher the vibrational frequency of a functional group, the greater the wavenumber (cm-1) at which it absorbs the IR radiations. Thus, different functional groups present in a molecule can be identified by their characteristic vibrational frequencies.
Vibrational transitions in IR spectroscopy
A molecule can undergo different types of vibrational transitions. Vibrational energy levels lie in between the electronic energy levels of a molecule. Thus, an electron requires comparatively less energy to change its fundamental vibration state in IR spectroscopy as opposed to that required for electronic excitation in UV-Vis spectroscopy.
There are two main modes of fundamental vibrations, namely:
- Stretching vibrations
It refers to the rhythmical movement of two bonded atoms along their bond axis. The bond length increases or decreases by stretching vibrations of atoms.
- Bending or deformation vibrations
It refers to the movement of atoms such that the bond length stays constant while the bond angle changes. Bending vibrations can be sub-categorized into rocking, scissoring, twisting, and wagging.
However, we should keep in mind that only those vibrations are infrared active which leads to a change in the net dipole moment of the molecule. Therefore, symmetrical H2, N2, and CO2 molecules cannot be precisely identified via IR spectroscopy. Contrarily, asymmetric molecules containing carbonyl (C=O) and/or hydroxyl (O-H) functional groups can undergo infrared active vibrations and can be effectively examined using IR spectroscopy.
Important terminology in IR spectroscopy
- First overtone: Vibrational transition from the zeroth vibrational level (v0) to the first vibrational energy level (v1).
- Second overtone: Vibrational transition from the zeroth vibrational energy level (v0) to the second vibrational level (v2).
- Combination band (v1 + v2): A combination band appears on the IR spectrum when the electrons present in a molecule undergo two fundamental vibrational transitions at the same time.
- Difference band (v2-v1): A difference band appears on the IR spectrum when the molecular electrons are already present at a higher vibrational energy state (v1) and are further excited to another vibration level (v2). The energy supplied must be equal to the difference in the energies of the two vibrational levels i.e., v2-v1.
6 components of an IR spectrophotometer
An IR spectrophotometer consists of the following main components:
1. Radiation source
- Nernst filament lamp or Globar is used as the infrared radiation source.
- The Nernst filament is composed of a mixture of zirconium, thorium, and yttrium oxides. It emits IR radiations when electrically heated up to 1000-1800°C.
2. Beam splitter
- The IR beam is split into two beams of equal intensity by passing it through the beam splitter.
- One beam acts as a reference beam while the other passes through the sample cell.
3. Sample cell
- Extremely pure samples are required for IR spectroscopy.
- A solid sample is usually grounded and pressed into a pellet using an IR transparent potassium bromide (KBr) disk. It can also be prepared as a solution in an IR transparent solvent such as CCl4.
- The sample is then placed in the sample cell/ cuvette of path length 10 cm.
- Commercially available sample cells can also hold liquid and/or gaseous samples for IR spectroscopic analysis.
4. Optical chopper
- The optical chopper is a mirror that rotates at about 10 cycles/s so that the reference and the sample beams reach the monochromator alternately.
5. Monochromator
- The monochromator is a prism or a diffraction grating that selects a specific wavelength to reach the detector at a time.
6. Detector
- A thermocouple is often used as a detector in IR spectroscopy.
- The detector records the infrared absorption by calculating the difference between incident and transmitted IR intensities.
- Absorbed IR energy is converted into an electrical signal and plotted as a graph of absorbance versus wavenumber. This graph is called the IR spectrum.
Interpreting an IR spectrum
An IR spectrum consists of two main regions.
- Fundamental group region
- Fingerprint region
The fundamental group region records IR absorption in the range 4000-1600 cm-1. The peaks obtained in the fundamental group region are characteristic of the stretching vibrations of functional groups in a molecule.
In contrast to that, the fingerprint region lies at the right end of an IR spectrum. It represents IR absorptions below 1600 cm-1. The peaks in this region mainly appear due to the bending vibration of atoms present in a molecule. Vibrational overtones and combination bands also appear in the fingerprint region.
The fingerprint region is more important for the structural identification of a molecule through IR spectroscopy. Two different chemical compounds can have the same functional group moieties such as an O-H group, a C=C, or a C=O group, etc., therefore their fundamental group regions may display similar peaks. But the peaks obtained in their fingerprint regions will vary for sure. In this way, IR spectroscopy helps in structural elucidation and compound identification.
The peaks obtained in the fundamental group and the fingerprint regions of an unknown compound’s IR spectrum should exactly superimpose with that obtained for a pure compound from the IR library.
The importance of Hooke’s law in IR spectroscopy
Hooke’s law empirically relates the stretching frequency of a bond or molecule with different factors as shown below.
where ѵ is the stretching frequency or wavenumber measured in cm-1, f is the force constant that describes the strength of a bond while mA and mB represent the mass of bonded atoms A and B.
From Hooke’s law, we can infer that the greater the strength of a bond, the higher will be its stretching frequency that is required to change its vibrational state of motion. Functional groups containing double and triple bonded atoms lie at higher wavenumbers i.e., at the left end of an IR spectrum. On the other hand, single bonded groups are situated at the far-right end of the IR spectrum.
The hydroxyl (OH) group lies at the far left of an IR spectrum because it possesses a high force constant thus a higher stretching frequency. Conversely, the stretching frequency decreases as the electronegativity of the atom bonded to H increases. As a result, an amine (N-H) group absorbs at a lower stretching frequency as compared to the O-H group.
The fundamental stretching frequencies of some commonly known functional groups are as shown in the table below.
| Functional group | Stretching frequency (cm-1) |
| Alkane (-CH3) | 2970-2880 |
| Alkene (-CH=CH-) | 3095-3010 |
| Aryl (C6H5-) | 3040-3010 |
| Ketone (-COC-) | 1700-1600 |
| Aldehyde (-CHO-) | 1740-1700 |
| Carboxylic Acid (-COOH) | 1725-1680 |
| Alcohol (-CH2-OH) | 3560-3500 |
| Esters (-COO-R) | 1750-1735 |
| Amides (-CONH-) | 1650-1550 |
| Free OH group | 3650-3600 |
| H-bonded OH group | 3500-3450 |
| Amine (-NH-) | 3400-3310 |
| Nitro (-NO2) | 1560-1535 |
| Halogen (-X) | 1100-500 |
A specific range of frequencies is given for each functional group because the absorption also depends on the chemical environment of the group. For instance, an alkane (C-H) functional group present as CH3 in a molecule absorbs at a slightly different frequency as opposed to the C-H present as -CH2– or -CH=C- etc.
An H-bonded OH group absorbs IR radiations at a lower frequency as compared to the free OH group. This is because intramolecular H-bonding lengthens the O-H bond length thus decreasing the force constant (f) a bit. The stretching frequency of O-D (D stands for deuterium, an isotope of hydrogen) is lower than O-H because D has higher mass (m) and stretching frequency is inversely proportional to m.
A broad rounded peak is obtained in the IR spectrum for a free OH group while the peak for H-bonded OH is comparatively less broad such as that obtained for alcohol molecules.
Practice further some very useful tips and tricks for interpreting IR spectra here.
What is IR spectroscopy used for
- IR spectroscopy is used in the pharmaceutical sector for identifying unknown drugs and enantiomers.
- IR spectroscopy is important in research and development for the characterization of newly synthesized chemical compounds and polymers.
- IR spectroscopy helps in food quality control by detecting trace adulterants in a food product that leads to food spoilage.
- The progress of a chemical reaction can be studied mid-way via IR spectroscopic analysis.
- IR spectroscopy is important because it facilitates the analysis of chemical compounds in their inherent physical states (solid, liquid, or gas).
Read other applications of spectroscopy in our article what are the uses of analytical spectroscopic techniques in chemistry.
IR spectroscopy holds a special significance in qualitative analysis. However, a possible limitation of IR spectroscopy is that it usually does not support quantitative analysis of chemical samples as it doesn’t obey the Beer Lamberts’ law. The instrument and working principle discussed in this article primarily belong to the traditional dispersive IR spectroscopic technique. An advanced version of IR spectroscopy that overcomes many of the challenges faced by the traditional technique is Fourier transform infrared (FTIR) spectroscopy.
You may also like to read about one of the most technically advanced spectroscopic techniques so far i.e., NMR spectroscopy.
References
1. Capuano, E. and S. M. van Ruth (2016). Infrared Spectroscopy: Applications. Encyclopedia of Food and Health. B. Caballero, P. M. Finglas and F. Toldrá. Oxford, Academic Press: 424-431.
2. Chemicals, A. R. (2017). “Infrared spectroscopy.” Analytical procedures and general instructions, ACS.
3. Smith, B. C. (2016). “The infrared spectroscopy of alkenes ” Spectroscopy 31(11): 28-34.
4. Stuart, B. (2005). Infrared Spectroscopy. Kirk‐Othmer Encyclopedia of Chemical Technology, Wiley.