Thursday, July 17, 2008

Multiplet Analyzer

Not all the spectra are second order. Actually, today's marketing insists to generalize that, in our new century, almost all the spectra are first order. Such a generalization makes sense if you process a spectrum (or less) per year, otherwise your destiny is to meet, sooner than you expect, something second-order. It's true that you are allowed to describe your spectrum as a sequence of generic multiplets, avoiding a more detailed analysis. This is tolerated but hasn't become the recommended practice yet. That said, there are certainly a lot of first-order multiplets in our spectra and extracting shifts and J it's an easy but tedious task. Can the computer help us? See for example the following spectrum of 1-pentyne in CDCl₃.

(The peak at 1.56δ is an impurity). There are, from left to right, a triplet of doublets, a triplet, a sextet and another triplet. The manual extraction of parameters is easy. Let's start from the sextet, because it's a curious rarity. It's enough to know the frequencies of the two external lines. The distance, divided by 5, gives the J. The sum, halved, gives the chemical shift. If you use a pocket calculator, there is the risk of a wrong transcription of the values from the monitor to the calculator and from the calculator back to the computer (for example into MS Word). The risk is low, but not zero. Besides this risk, the whole operation is time-consuming. Finally, it's not cool: you have to move cyclically from the NMR software to the pocket calculator to MS Word, etc...
Being it a simple operation, it can be performed automatically by the computer. It will not start directly from the data points, but from the same values used by an operator: the list of frequencies (peak-picking), the intensities at these frequencies, the list of integrals. These lists can also be generated automatically; in conclusion the whole process can be performed in automatic fashion. In practice, however, it's not convenient. Why? Suppose that the maker of the program claims that the automatic method works in the 99.9% of the cases. That's a generous claim, difficult to trust. For example, if the program doesn't recognize our impurity at 1.56δ as such (see picture above), it will fail to recognize the multiplet as a sextet. But, even if we believe the claim, how do we know if our spectrum belongs to the 99.9% or to the 0.1% of failures? We are forced to check the output with care. The time saved with the automatic processing will be lost for the check.
In my opinion, it's best to perform the integration and the peak-picking first (manually or automatically) and perform the check at this stage: the inspection is visual, not textual, therefore faster. In the case shown by the picture, this is the moment to remove the entry "1.56" from the list of frequencies. When all is ready, proceed with the computer-assisted extraction of the NMR parameters. A picture is more explanatory. Refer to your software or give a glimpse to this tutorial.
Although not a rule, Multiplet Analyzers ignore and remove the roof effect. Take for example our sextet. It can be recognized as such only if the intensity ratio is 1:5:10:10:5:1. The leftmost multiplet also has 6 lines, but the intensity ratio is 1:1:2:2:1:1, therefore it's recognized as a triplet of doublets. The roof effect prevents the recognition, therefore it's removed by averaging (symmetrization). For example, if the ratio of a triplet is 0.9:2:1.1, the computer calculates the average of the outer lines and the result is the theoretical 1:2:1. There's a notable exception. When there are two lines only, a single solution is possible, the doublet. When the lines are three, if we assume that all nuclei have spin = 1/2, the solution is still unique.
The roof effect can also exploited, in these cases at least. Like the old textbooks say, the chemical shift of a doublet doesn't correspond to the middle frequency, but to the center of mass. This is the single case I know when a multiplet analyzer performs a second-order analysis. I don't know if the center-of-mass rule is general and if all today's program observe it.
There's another, more obvious, rule to follow: any two coupling partners must show an identical splitting. In practice, the values extracted from two multiplets are rarely identical. Only the user can decide what to do. She can either:
  1. Substitute the original values with an average value.
  2. Remove what appears to be the less accurate value and put in its place the splitting shown by the partner.


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