Question

Expt. 11/11x: Identification of an Unknown Carbonyl Compound by Infrared Spectroscopy

1. Briefly explain how an infrared spectrometer works.

Answer 1

Spectroscopy is based upon the idea of measuring the energy needed to produce a change from one energy level to another.The energy possessed by chemical particles is quantised; there can only be a number of definite energy values, rather than a whole range of energy values.Infrared spectroscopy takes advantage of the vibrational energy changes that occur in molecules when they are exposed to infrared radiation (10¹⁴ – 10¹³ Hz).Frequency and Wavelength are related to each other:
C = v
(C = Speed of Light)

The Wavenumber is the reciprocal of the wavelength (1/), and thus is a direct measure of the frequency.

(Infrared spectroscopy (IR spectroscopy or Vibrational Spectroscopy) involves the interaction of infrared radiation with matter( based on absorption spectroscopy).

Infrared radiation from a heat source is split into two beams. One of the beams goes through the sample, and the other goes through a reference chamber, ensuring that absorptions from water and carbon dioxide in the air are cancelled out.The beams are then sent along the same path, in separate alternating pulses, achieved using a beam chopper (rotating disc with segment cut out of it).

The beams are then analysed by passing them through a sample of sodium chloride or potassium chloride (both invisible to IR radiation) or through a diffraction grating. Only light of one particular frequency is allowed to focus onto the detector at one time. The spectrum is achieved by scanning the frequencies and recording the associated light intensities.When the sample is not absorbing, there will be no difference between the alternating pulses reaching the detector, so no signal is recorded.When a vibration is being excited, the sample beam intensity will be reduced and a signal generated. The record of an IR spectrum seems to be upside down (the baseline is at the top); however it is transmittance that is being recorded and this is at a maximum when no light is being absorbed.

For the interpretation, please take a solved example and try to understand carefully.

Thank you and good luck.