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Explain The Procedure To Use An Infrared Spectrum To Determine The Presence Of Functional Groups

Referencing Styles : APA | Pages : 1

Infrared spectroscopy, also referred to as vibrational spectroscopy or IR spectroscopy encompasses the contact between matter and infrared radiation. It comprises of an assortment of methods that are typically founded on the principles of absorption spectroscopy. Showing consistency with different spectroscopic techniques, infrared spectrum is generally used for the identification and assessment of chemical elements. Samples that are observed thorough IR spectrum can be liquid, solid, or gas. The technique or method of IR spectroscopy is directed through the use of an apparatus that is commonly referred to as an infrared spectrometer or spectrophotometer, with the aim of developing an IR spectrum. Visualisation of the IR spectrum is generally accomplished in graphical format that depends on absorbance of infrared light or the transmittance of the light on vertical axis vs. the wavelength frequency on horizontal axis. Characteristic units used for measuring frequency in the IR spectra are namely, reciprocal centimeters, also referred to as wave numbers that are denoted by the sign cm−1. IR wavelength units are usually denoted in micrometers (previously referred to as "microns"), which in turn is represented by the symbol μm. There exists a reciprocal association between wavelength frequencies with wave numbers. A laboratory tool that uses this method is a Fourier transform infrared (FTIR) spectrometer.

While conducting two-dimensional IR spectrum, the infrared segment of the electromagnetic spectrum gets typically separated into three different regions, namely, the near-, mid- and far- infrared ranges that are so-called owing to their association to the visible spectrum. The near-IR higher-energy region, roughly 14000–4000 cm−1 (0.7–2.5 μm wavelength) has the capability to excite harmonic or overtone molecular vibrations. In contrast, the mid-infrared, roughly 4000–400 cm−1 (2.5–25 μm) is typically used for studying the essential vibrations and related rotational-vibrational assembly. This is in contrast to the far-infrared, roughly 400–10 cm−1 (25–1000 μm) that lies next to the microwave section, and has reduced energy, thus most often being used for the study of rotational spectroscopy.

IR spectroscopy is based on the fact that all kinds of chemical molecules are able to absorb frequencies, which are representative of their arrangement. These absorptions generally happen at resonant frequencies, which represents the frequency of absorbed radiation that equals the vibrational frequency. These energies are also influenced by the atom mass, form of the molecular potential energy surfaces, and the allied vibronic coupling.

With the aim of making a vibrational mode IR active" that is present in a sample, it needs to be related to the modifications in the dipole moment. There is no need of creating a permanent dipole is since the rule necessitates only an alteration in dipole moment. Furthermore, a chemical molecule can vibrate in numerous ways, and every one of those processes are referred to as a vibrational mode. Molecules that contain N number of atoms are referred to as linear molecules having nearly 3N-5 degrees of vibrational modes, in contrast to nonlinear molecules that comprise of 3N – 6 degrees of vibrational modes, which are also known as vibrational degrees of freedom. Thus, it can be stated that one of the most prevalent application of IR spectroscopy is to identify different organic compounds. Some diatomic molecules only comprise of one vibrational band and one bond, and if that particular molecule is found to be symmetrical, the band cannot be detected in the IR spectrum. However, it can be selected in Raman Spectrum.

Presence of a covalent bond between two different atoms is generally considered as two different objects that have mass m1 and m2, respectively, and are connected by means of a spring. Hence, it is anticipated that the bond would stretch and compress with a particular vibrational frequency that is generally represented by an equation where k refers to the force constant of the spring, µ denotes the reduced mass, and c refers to the speed of light.


Taking into consideration the equation 1, frequency is considered proportional to the spring strength, and also displays an inverse relationship with the mass of the aforementioned objects. Therefore, N-H, C-H, and O-H bonds are expected to have increased stretching frequencies when compared to C-O and C-C bonds, owing to the fact that hydrogen has been identified as a light atom. Triple and double bonds are also considered as sturdier springs, therefore a C-O double bond demonstrates an increased stretching frequency, in comparison to a C-O single bond. Furthermore, it is also known that infrared light acts in the form of an electromagnetic radiation, and comprises of wavelengths that range from 700 nm to 1 mm, which in turn demonstrates consistency with relative bond strengths. Thus, when infrared light is absorbed by a molecule having a frequency that matches usual vibrational frequency present in a covalent bond, energy emitted from the radiation brings about an upsurge in bond vibration amplitude. Thus, in a carbonyl group (C-O double bond), more time is spent by the electrons around the atom of oxygen, in comparison to the carbon atom owing to the greater electronegativity of oxygen, compared to carbon. Henceforth, there occurs a net dipole moment that leads to the development of a partial negative charge on the oxygen atom, and a subsequent partial positive charge on the carbon atom. In contrast, there is no net dipole moment in a symmetrical alkyne taking into consideration the fact that two discrete dipole moments present on either side nullify each other. Thus, it can be stated that a carbonyl group stretch demonstrates presence of an intense band in IR spectrum, in comparison to a symmetrical internal alkyne that depicts a small band for C-C triple bond stretching. In order to determine the presence of functional groups in an unknown compound in IR spectrum, the following steps must be followed:

  1. The IR spectrometer must be turned on and it must be allowed to warm up.
  2. The unknown sample must be obtained from the instructor, followed by recording the appearance and letter of the obtained sample. Gaseous sample need a sample cell that contains a long path length, with the aim of compensating for the diluteness. The path length of the unknown sample is dependent on the sample concentration. Liquid samples are also sandwiched between two salt plates (sodium chloride) that are transparent to the beam of IT light and prevent the introduction of any line in the spectrum. Solid sample preparation would encompass sample crushing with mineral oil, or grinding the sample with purified salt. The sample will initially be dissolved in a non-hygroscopic solvent, following which a solution drop will be deposited on the surface of the NaCl cell. This will then be evaporated, followed by an analysis of the film developed. Care will also be taken to ensure that the thickness of the film is not much, else it will prevent passage of light. In addition, while conducting photoacoustic spectroscopy the need for including a sample treatment is minimum. The solid or liquid unknown sample will be placed inside a sample cup that will eventually be inserted inside the photoacoustic cell, followed by its sealing for measurement.
  3. The background spectrum must be collected in an appropriate manner.
  4. With the use of a metal spatula, a small amount of the unknown sample must be placed under the probe.
  5. The probe must be twisted until it gets locked into an appropriate place.
  6. The IR spectrum of the unidentified sample must be recorded. The beam of IR light will be initially split into two halves and will also be passed through the sample and the reference, respectively. The beam will then pass through a splitter, followed by a detector.
  7. This step must be necessary for obtaining a high quality spectrum.
  8. The absorption frequencies that are suggestive of the functional groups that are present must be recorded.
  9. The probe must be cleaned with acetone.
  10. The record must be printed out.
  11. The spectrometer must be turned off.
  12. The obtained spectrum must be analysed.

Thus, the aforementioned steps will facilitate measuring the frequencies of functional groups that are present in an unknown sample, characteristic to their corresponding structures. It is imperative to record the spectrum of both the unknown sample and a reference that will help in controlling several variables that might create an impact on the spectrum. The simplest reference that can be used is to replace the unknown sample by air. However, on finding that the unknown sample is a dilute solute that gets dissolved in water, measuring pure water in similar beaker will act as a reference measurement that would nullify all other instrumental properties.  Furthermore, one common method that will be used for comparing a reference encompasses measuring the reference, followed by replacement of the reference with the unknown sample, and eventually measuring it. This method is not seamlessly dependable; if the infrared lamp has greater brightness at the time of reference measurement, followed by reduced brightness during sample measurement, it is anticipated that the measurement will be misleading.


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