The group to which I belonged, the Han Vos group, studied the photophysics of Ru and Os polypyridyl species. A useful tool to probe the excited state is Raman combined with deuteration of specific ligands. When you do a vibrational frequency calculation, you specify what isotopes to use when calculating the frequencies. In fact, you can instantly repeat the calculation for other isotopes without redoing the whole analysis. In this way you can calculate the frequencies for the deuterated and undeuterated forms.

This gives you a set of peaks but you don't exactly know which correspond to which. For example, in the diagram below for [Ru(bpy)

_{3}]

^{2+}, if you just consider the

^{1}H and

^{2}H spectra, it's fairly obvious which corresponds to v

_{5}but you're relying on guesswork for the peaks on the left. However, it's useful to know which peaks correspond to which; one reason for this is to develop forcefields for IR calculation (I forget the details).

Anyhoo, the trick I thought of is to use isotopes with fractional masses intermediate between

^{1}H and

^{2}H. Then it's fairly easy to trace the shift in the peaks using the diagram above. Another way to figure out the correspondence would be to look at the wiggle vectors for the frequencies and match up the ones with corresponding wiggles. I used this to verify my results.

And it turned out that according to my awesome diagram which was never published, the peaks had been misassigned in the literature. v

_{9}/v'

_{9}and v

_{10}/v'

_{10}were not corresponding; instead it was v

_{9}/v'

_{10}and v

_{10}/v'

_{9}. Take that literature!!

For more info, check out Chapter 5 of my thesis which I've just found someone has scanned in and deposited in DCU's Institutional Repository. Nice.

## 2 comments:

The trick of changing the mass of an individual atom is indeed a useful one. Sometimes for example, one might want to "purify" a mode (get rid of its mixing with other modes) to study just the bond (for example a "pure" carbonyl stretch). In which instance, starting from the force field, one can reduce the mass of other atoms to something very tiny indeed. This eliminates the mode mixing (also in some corners known as the Duschinksky effect).

Another imaginative abuse of isotopes!

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