Infrared and Raman Vibrational Spectra of Methane

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This Demonstration shows how vibrational transitions in a methane molecule ) are associated with changes in its bond dipole moments. An IR-active vibrational transition occurs only when the vectorial sum of the individual C-H bond dipoles undergoes a change. If there is no such change in the dipole moment, the vibration is IR inactive, but still possibly active in the Raman spectrum. Each type of vibration is classified according to the symmetry group of the molecule [1].

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The popup menu selects the type of vibration, and the trigger initiates the vibrational motion. The vibration of each bond is shown in the graphic. When the sum is different from zero, the change in molecular dipole moment is represented by a red arrow. The IR spectrum is listed as the last option; the vibrational peaks have been highlighted to distinguish them from rotational transitions at frequencies just slightly higher and lower than the vibrational peaks.

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Contributed by: D. Meliga and S. Z. Lavagnino (December 2016)
Additional contributions by: G. Follo and A. Ratti
Open content licensed under CC BY-NC-SA


Snapshots


Details

A nonlinear molecule with atoms has vibrational degrees of freedom. For methane (), , and there are nine vibrational modes.

Taking account of degeneracy, just four types of vibrations can take place. Degeneracy occurs when different vibrations have the same frequency. The first type of vibration is nondegenerate, while the third type has a twofold degeneracy, and both the second and fourth types have threefold degeneracies [2].

Snapshot 1: symmetric stretching leading to an IR-inactive vibration

Snapshot 2: asymmetric stretching leading to an IR-active vibration

Snapshot 3: IR rotational-vibrational spectrum

References

[1] C. E. Housecroft and A. G. Sharpe, Inorganic Chemistry, 2nd ed., New York: Pearson/Prentice Hall, 2005.

[2] D. A. McQuarrie and J. D. Simon, Physical Chemistry: A Molecular Approach, Sausalito, CA: University Science Books, 1997.



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