Spectral Identification of Short-Lived Molecules

Energy minima have all real frequencies, while molecules with one or more imaginary frequencies are not minima. 

Short-lived molecules may often be identified by their infrared spectra measured at extremely low temperatures. In most cases, the experimental spectrum will be incomplete, although a few characteristic lines or bands are often sufficient to decide among alternative structures. 

Assign the vibrational frequencies in the experimental spectrum using the data from the structure you selected. 

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