Vibrational Spectroscopy [12 marks] 2. (a) Describe the similarities and differe
ID: 908329 • Letter: V
Question
Vibrational Spectroscopy [12 marks] 2. (a) Describe the similarities and differences between Dispersive IR and Fourier Transform JR Spectrometers including the advantages and disadvantages of each. (b) Describe the origin of Rayleigh, Stokes and Anti-Stokes bands in a Raman spectrum using words and diagrams. (c) Describe how vibrational spectroscopy can be used to determine the structure and environment of biological molecules including, whether it is free, chemically bound, solvated or coordinated to a metal ion.Explanation / Answer
Similarities between dispersive ir and raman spectroscopy
Both techniques are based on the interaction of electromagnetic waves with the lattice waves or phonons.
Both interact with phonons
Both techniques are however not unable to measure dispersions of phonons
The chemical structure of a molecule is the dominating effect that determines vibrational frequencies via the strengths of the vibrating bonds and the masses of the vibrating atoms. In spite of that, the chemical structure of a protein cannot be deduced from the infrared spectrum because of many overlapping bands. Changes in chemical structure however can be detected and an important example is a change of protonation state of side chains which is often essential for protein function. Here, infrared spectroscopy seems to be the method of choice since the protonation state of most side chains is reflected in the spectrum
The electron density of a given bond in a molecule is influenced by neighbouring groups within the molecule via inductive and mesomeric effects. The latter is the reason for amide groups absorbing at lower wavenumbers than carbonyl groups of carboxylic acids
Redox reactions are the basis of the energy delivering processes in living organisms. They affect the electron density distribution of a given molecule and thus will alter its vibrational spectrum.
As pointed out by Deng and Callender vibrational spectroscopy is exceptionally sensitive to changes in bond strength since a change of 0.02% can be easily detected. As bond strength and bond length are directly related bond distortions smaller than 1 pm in the course of a catalytic reaction can be monitored with astonishing accuracy.
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