- Look at a large number of spectra with similar functional groups and see the commonalities. If you want to be an organic chemist, you will wind up doing this as a matter of course; the online databases at NIST and SDBS are certainly a big help in doing this.
- High level MO calculations can predict the set of "normal vibrations" in a molecule, and actually predict the IR and Raman spectra. I have done this a lot in these pages. However, this is merely a prediction; it doesn't always work, and we almost never get a sense of the appearance of the spectrum (broad or sharp peaks) from the calculation.
- Since the vibrational behavior of a bond depends on the masses of the things on the ends, changing the mass at one end (e.g., substituting a heavy isotope for one atom) takes advantage of the mathematical form of Hooke's law:
We can use this to predict how a change of isotope, say from 1H to
We see one of the major things that happens is that the C-H vibration at 3323 cm-1 moves to 2593 cm-1; the ratio of frequencies is 1.28. This is lower than expected, but the carbon is attached to a bunch of heavier stuff--this perturbs the calculation.
There are other changes, of course. The CC stretch at 2120 cm-1 is affected; it should be at 2047 cm-1; there is actually a weak peak just below 2000 cm-1. You'll note other changes in the fingerprint region, likely due to the C-C-H bending vibrations changing.