Such vibrations are said to be infrared active. In order for a vibrational mode to absorb infrared light, it must result in a periodic change in the dipole moment of the molecule. Some bonds absorb infrared light more strongly than others, and some bonds do not absorb at all. The power of infrared spectroscopy arises from the observation that different functional groups have different characteristic absorption frequencies. It turns out that it is the infrared region of the electromagnetic spectrum which contains frequencies corresponding to the vibrational frequencies of organic bonds. The difference in energy between the two vibrational states is equal to the energy associated with the wavelength of radiation that was absorbed. If a molecule is exposed to electromagnetic radiation that matches the frequency of one of its vibrational modes, it will in most cases absorb energy from the radiation and jump to a higher vibrational energy state – what this means is that the amplitude of the vibration will increase, but the vibrational frequency will remain the same. The energy of molecular vibration is quantized rather than continuous, meaning that a molecule can only stretch and bend at certain ‘allowed’ frequencies. These complex vibrations can be broken down mathematically into individual vibrational modes.

At room temperature, organic molecules are always in motion, as their bonds stretch, bend, and twist. Note that IR light has relatively longer wavelengths (is lower in energy) than UV or visible light.Ĭovalent bonds in organic molecules are not rigid sticks – rather, they behave more like springs. Therefore, shining infrared light on a sample of a compound can help diagnose the types of functional groups and bonds are in the molecule. When an organic molecule absorbs infrared light as energy, that causes the chemical bonds to stretch (or bend or rotate depending on how much energy is put in). In this case, the reduced mass of the C-H system, 12*1/(12+1) = 0.9, is less than that of the C-C system, 12*12/(12+12) = 6, so the higher mass C-C system has a relatively lower stretching frequency. These two systems have the same reduced mass, each with two carbon atoms, but the stronger C=C double bond has a larger force constant, therefore the stretching frequency of C=C is faster/higher. To extend this to a chemical bond, consider the simplified two-body examples below. From this relationship, we can further conclude that (1) a system with a larger reduced mass will have a lower stretching frequency (the spring will stretch and contract slower if it has heavier masses on each end), and (2) a system with a larger force constant will have a higher stretching frequency (a tighter-wound spring will contract faster after being stretched). Therefore, we can model the stretching or oscillations of this bond with the equation derived from physics, ν = 1/(2π)*√(k/μ), where ν = the frequency of the oscillation (how fast the bond is stretching and contracting), k = the force constant (strength of the bond), and μ = the reduced mass of the system, m 1m 2/(m 1+m 2). An analogy often used for a chemical bond between two atoms is a spring with a mass at each end. 26-38.By interpreting the frequencies of infrared light absorbed by a molecule, one can identify bond-stretching motions and therefore gain structural information such as what type of functional groups and chemical bonds are present.Ībsorption of infrared radiation by an organic compound causes bonds within that molecule to be stretched, and this phenomenon can be used to gain information about the molecular structure. (2005), 'Determining Functional Groups of Commercially Available Ink-Jet Printing Reactive Dyes Using Infrared Spectroscopy', Research Journal of Textile and Apparel, Vol. Functional group and chemical structure.Experimental results revealed that the reactive dyes under determination contained the same functional groups and chemical compounds as the reference reactive dyes. Although other instrumental analytical methods can help to determine the actual chemical composition of reactive dyes, the IR technique alone can still provide important structural information about the commercially available reactive dyes.

An infrared (IR) spectroscopic technique was used to determine the possible functional groups and chemical compounds present in commercially available ink-jet printing reactive dyes of four primary colours, i.e.

Ir spectrum functional groups