Infrared Spectroscopy and Near Infrared Spectroscopy

Infrared and near-infrared spectroscopy study the interaction of infrared and near-infrared light with matter, respectively. They are widely used to identify molecules or determine their structure or composition. This relies on the fact that different molecules produce different (N)IR signals due to their possible vibrational and rotational transitions.

Vibrational transitions occur in molecules when electrons are raised to a higher vibrational state or downgraded to a lower vibrational state. These vibrational states arise as a result of "vibrational modes," which involve stretching or bending of bonds. The amount of stretching (change in bond length) or bending (change in bond angle) is quantified, so transitions between different vibrational states can only occur through the absorption or emission of photons with specific energies. These energies—which lie in the infrared region of the EM spectrum—depend on the atoms involved in each bond, the number of bonds and their relative orientation. Therefore, different molecules have different IR signals, with specific signals corresponding to specific functional groups. These signals can be used to identify molecules.

The rotational transition is related to the quantized angular momentum of the molecule, which is generated by the rotation of the atomic nucleus around its center of mass in the gas phase. An increase in angular momentum corresponds to a transition to a higher spin state, while a decrease in angular momentum corresponds to a transition to a lower spin state. Again, the energies of these transitions correspond to photons in the infrared region of the electromagnetic spectrum.

Infrared radiation can be further divided into three spectral regions: near-infrared, mid-infrared, and far-infrared. Each of these areas is associated with a different type of transformation.

wave number

Infrared signals are usually discussed in terms of wavenumbers, not wavelengths, λ. The unit of wavenumber is cm-1 and can be calculated by:

Near Infrared Spectroscopy

Near-infrared (NIR) radiation is the region of energy and wavelength closer to the visible spectrum, corresponding to wavelengths of 0.7-2.5 μm (12,800-4,000 cm -1 ). In molecular spectroscopy, this region corresponds to overtones and combined vibrational transitions.

The transition of an electron between the vibrational ground state v 0 and the first excited vibrational state v 1 is called the ground state. This is indicated by the red arrow in the image below. Any transition from v 0 to a higher vibrational state is called an "overtone transition". For example, the transition v 0 → v 2 is called "first overtone", and v 0 → v 3 is called "second overtone". These are indicated by the blue and gold arrows in the image below, respectively.

Combinatorial vibrational transitions occur when two or more vibrational modes are excited by a single higher-energy photon.

The selection rules of quantum mechanics forbid overtones and combinatorial transitions. However, they can still happen, but very slowly. For this reason, the absorption of these transitions is very weak, and NIR spectroscopy is not a very sensitive technique.

Mid-infrared spectrum

The mid-infrared (MIR) region spans wavelengths from 2.5-50 μm (4,000-200 cm -1 ) and is associated with fundamental vibrational transitions (v 0 →v 1, red arrows in the upper panel) and rotational vibrational transitions.

Rotational transitions mostly occur between different rotational states in the same vibrational state. However, both types of conversions may occur simultaneously. These transitions are called rotational vibrational transitions or rotational vibrational transitions and are accompanied by the absorption or emission of MIR photons. The figure below shows an example of a possible vibration transition.

Far Infrared Spectrum

The far infrared (FIR) is the lowest energy region of the infrared spectrum, with wavelengths in the 50-1,000 μm (200-10 cm -1 ) range. In molecules, it participates in rotational and low-energy vibrational transitions.

The wavelengths, wavenumbers and associated molecular transitions for NIR, MIR and FIR are summarized in the table below.