Background:
Summary of Methods Used
Simple Example:
Theory
The Vibrations
Research Topics:
Fatty Acid Methyl Esters (FAMEs)
School of Chemistry
The Queen's University of Belfast
Non-Planar Porphyrins
For TtBP we know that 252 vibrational modes are expected, but in the experimental spectrum (repeated here) – as you can see - there are very much fewer than 252 modes observed, and this is because the experimental spectrum is a resonance Raman spectrum so that only totally symmetric vibrations are observed.
We can show that only 63 modes are of the appropriate symmetry – this was why we had to obtain a calculation with D2 symmetry and of these 63, only 41 are observed within our spectral range.
These are two representations of the calculated data.
In the top (pink) trace we found that some of the calculated vibrational bands were predicted to have very low intensity – in fact they’re so weak that you can’t see them at all on trace (a). But – since resonance Raman conditions had been used in the experiment, the calculated intensities were meaningless anyway so all we did to produce the green spectrum was to arbitrarily rescale all the weak bands making it possible to see all of the bands which are expected to appear in this region of the spectrum.
Comparison with the real spectrum shows that there is a good match in both the number and positions of bands. The vast majority of the remaining 41 bands are to be found in the spectra of porphyrins generally, so that left us with only the six or seven unusually strong low-frequency bands that are not seen in the spectra of planar porphyrins. So the question is…
