Fermi-resonances, matrix isolation

When pairs of lines of different intensity occur in a region where only a single one is expected on the basis of normal mode calculations (cf. below), these need not indicate a site splitting, but are often indicative of Fermi-resonances, that is, a (near) coincidence of an IR active fundamental with a combination band that borrows intensity from the former. Such phenomena can often only be detected by virtue of the high resolution that is available in matrix isolation IR spectroscopy. 

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