IR - triple bond versus double bond

[reising1]

I'm a little confused by something fairly simple.

A triple bond has an IR peak at 2100-2260 and a double bond at 1620-1680. These are measured in Hertz. I thought that double bonds had more vibration than triple bonds, so shouldn't the Hertz for double bind be higher?

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[FuzzyMemory]

There is a difference between the resonant frequency (wavenumber) of absorbance and the number of vibrational modes a molecule has. The peak values you mentioned pertains specifically to the C-C stretch wavenumbers in an IR graph. Of course, stretching is not the only mode of vibration these bonds have, so they will have other peaks, though typically not very diagnostic in an IR graph.

A triple bond is stronger than a double bond, and thus the wavenumber of absorbance is higher. Remember that the value of the peaks in an IR graph pertains to the natural frequency at which that particular mode of vibration absorbs IR waves.