Infrared Spectroscopy

    References: Carey, pp 518-523 and Pavia et al., pp A14-A31

    For starters, consider the possible motions available to a molecule containing n atoms. Since each atom is capable of moving in three dimensions [components along the orthogonal or mutually perpendicular x-, y-, and z- coordinate axes, as defined by us for our own convenience], there are a total of 3n possible ways for the molecule to move. 
    Of this total, three modes are translational -- the molecule as a whole moves in the x-, y- or z- direction without change in any of its bond angles or distances. The available translational kinetic energy states form a continuum for all practical purposes. 
    An additional three modes are rotational -- the molecule as a whole can rotate around its axes parallel to the x-, y-, or z- directions we have defined without change in any of its bond angles or distances. Rotational kinetic energy is quantized; the spacing of the allowable rotational kinetic energy levels is such that transitions between them can be produced by absorption of electromagnetic radiation in the microwave portion of the electromagnetic spectrum. 
    This leaves a total of 3n-6 vibrational modes of motion, in which bond lengths and/or angles may change. [For completely linear molecules, there are 3n-5 possible vibrational modes, since one of the rotational modes is unavailable.] Vibrational kinetic energy is also quantized, and the spacing of the allowable vibrational kinetic energy levels is such that transitions between them can be produced by absorption of electromagnetic radiation in the infrared or IR portion of the electromagnetic spectrum. 
    If one models a covalent bond as a spring connecting two point masses, the fundamental vibrational frequency of the resultant harmonic oscillator will depend on the masses of the atomic nuclei and the "force constant" of the spring [strength of the bond]. As a result, each type of covalent bond has its own characteristic frequency ranges of infrared light absorbed during a transition between levels. There are additional frequencies absorbed when transitions involving more than two atoms are considered. The main value of IR spectroscopy is that it permits the identification of functional groups present in a molecule. See the "IR Primary Clues"  for more on this.
    Because the number of allowable vibrational modes will be much larger than the number of functional groups present, there will be additional IR absorptions unique to each molecule. Thus, by comparing an IR spectrum to that of a known sample -- if it is available -- we may even be able to identify a specific compound.

    Rather than reinvent the wheel and try to do what others have already done better, let me now direct you to the following sites [new window for each, close when done], where you will find some very useful introductions to infrared spectroscopy:

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© Ronald W. Rinehart, 2002-2007