What is the relation between the chemical interaction of materials and peak intensity changes of FTIR spectrum? how can i use the peak intensity changes for interpretation? is it true to use peak intensity changes for interpretation?
The different vibrations of the different functional groups in the molecule give rise to bands of differing intensity. This is because the derivative of μ with respect to x is different for each of these vibrations. For example, the most intense band in the spectrum of octane is at 2971, 2863 cm-1 and is due to stretching of the C-H bond. One of the weaker bands in the spectrum of octane is at 726cm-1, and it is due to long-chain methyl rock of the carbon-carbon bonds in octane. The change in dipole moment with respect to distance for the C-H stretching is greater than that for the C-C rock vibration, which is why the C-H stretching band is the more intense than C-C rock vibration.
Another factor that determines the peak intensity in infrared spectra is the concentration of molecules in the sample. The equation that relates concentration to absorbance is Beer's law,
The absorptivity is the proportionality constant between concentration and absorbance. The absorptivity is an absolute measure of infrared absorbance intensity for a specific molecule at a specific wavenumber. For a pure sample, concentration is at its maximum, and the peak intensities are true representations of the values of the derivative of µ with respect to x for different vibrations. However, in a mixture, two peaks may have different intensities because there are molecules present in different concentration.
The different vibrations of the different functional groups in the molecule give rise to bands of differing intensity. This is because ∂μ∂x∂μ∂x is different for each of these vibrations. For example, the most intense band in the spectrum of octane shown in Figure 3 is at 2971, 2863 cm-1 and is due to stretching of the C-H bond. One of the weaker bands in the spectrum of octane is at 726cm-1, and it is due to long-chain methyl rock of the carbon-carbon bonds in octane. The change in dipole moment with respect to distance for the C-H stretching is greater than that for the C-C rock vibration, which is why the C-H stretching band is the more intense than C-C rock vibration.
The absorptivity is the proportionality constant between concentration and absorbance, and is dependent on (¶µ/¶x)2. The absorptivity is an absolute measure of infrared absorbance intensity for a specific molecule at a specific wavenumber. For pure sample, concentration is at its maximum, and the peak intensities are true representations of the values of ¶µ/¶x for different vibrations. However, in a mixture, two peaks may have different intensities because there are molecules present in different concentration.
Peak position is more important than peak intensity in interpretation of structural changes.
The peak intensity in the FT- IR spectrum is related to the change in dipole moment that occurs during the vibration. Consequently, vibrations that produce a large change in dipole (e.g. C=O stretch) result in a more intense absorption than those that result in a relatively modest change in dipole (e.g. C=C). Vibrations that do not result in a change in dipole moment (e.g., a symmetrical alkyne C triple bond C stretch) will show little or no absorption for this vibration.
Also,Peak intensity in the FT- IR spectroscopy is useful in quantitative analysis by apply Beer's Law
"Peak position is more important than peak intensity in interpretation of structural changes."
This is correct insofar as most infrared spectroscopists never learnt to quantify intensities properly.
Otherwise, interpreting intensities was seen as the next big step in the 50ies of the last century. Here is one reason why:
Article The Intensities of Carbonyl Bands in the Infrared Spectra of Steroids1
A peak intensity is more important to interpret the FTIR spectra. Because the intensity of the peaks depends on molecular interactions, etc., may possible in that compound...
I fully and heartily agree! Actually there are means to properly evaluate peaks nowadays which go far beyond simple band fitting:
Article Quantitative Evaluation of Infrared Absorbance Spectra – Lor...
Together with the insight, that oscillator strength is equivalent to integral absorbance, this allows to go also for concentration determination:
Article Beer's Law‐Why Integrated Absorbance Depends Linearly on Concentration
First, as in any spectroscopy the peak intensity depends on the concentration of substance - if we are talking about the solution spectra. In any IR handbook it can be also read that the intensity of the band is proportional to the square of the transition moment, which means the change in the dipole moment as a result of of the vibration causing the band.
Anna Dołęga In this respect I would not trust any IR book which I do not have written myself... ;-) Seriously, in general it is the integrated absorbance or the (squared) oscillator strength, which is proportional to the concentration in general, not only for solutions, except if local field effects (see e.g. Lorentz-Lorenz formula) play a role:
Article Beyond Beer's Law: Spectral Mixing Rules
Article Beyond Beer's Law: Revisiting the Lorentz‐Lorenz Equation
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