Answered: Explain the theory of IR spectroscopy…

Biomedical Instrumentation Systems

1st Edition

ISBN:9781133478294

Author:Chatterjee

Publisher:Chatterjee

Chapter16: Fiber Optics And Lasers In Bioinstrumentation

Section: Chapter Questions

Problem 8P

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Question

Explain the theory of IR spectroscopy and its applications in biology.

Expert Solution

Step 1

firstly lets look at what infrared radiation is and what are the consequences of a molecule is irradiated with IR rays

IR rays fall below the UV-visible region in the electromagnetic spectrum with a wavelength of the range 700 nm to 25 mm
it extends from the red region of VIBGYOR [visible region] to the microwave region
in terms of amplitude, its waves are longer than the visible rays
chemical bonds connecting atoms behave like 'springs', ie, that are capable of being stretched and compressed when energy is applied. this constitutes the 'vibrational energy of the molecule'
these bonds are also flexible ie they can bend or be deformed
so, if a molecule has n atoms it will possess a total of 3n-6 fundamental vibrations of which, 2n-5 are bending vibrations and n-1 are stretching vibration
when an IR active molecule absorbs light rays, it will absorb a very specific wavelength that will be missing in the transmitted light, this creates an absorption band in the spectrum. such a spectrum is called IR spectrum. these lines/bands are quite distinct.
IR spectroscopy is also called functional group spectroscopy or fingerprint spectroscopy because each molecule [and its functional group] absorb a very distinct wavelength of light that will appear as a band on the spectrum. based on the location, wave number of the signal, one can ascertain the functional group in question.

Step 2

how do we ascertain a functional group?

its simple. the spectrum/spectralgraph has distinct regions:

functional group region whose range is $\geq 1500{\text{ cm}}^{-1}$
fingerprint region and the range is $<1500{\text{ cm}}^{-1}$
In this region there are many troughs which form an intricate pattern which can be used like a fingerprint to determine the compound
the table below is used to determine the functional group in the test molecules

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