Monday, 3 December 2007

Automatic Processing of 13C NMR spectra

The days in which chemists had a lot of time to spend in the processing and evaluation of their NMR spectra has probably gone. Synthetic or medicinal chemists should use their precious time working on their lab benches whilst NMR spectroscopists are usually devoted to get the most from the NMR instruments (optimizing or designing new pulse sequences) and resolving the most challenging cases in structure elucidation/verification. Thus, in my opinion, any method permitting to automate processing steps would be very useful in accelerating spectral analysis in the framework of NMR structure determination.

Here I would like to introduce a very simple processing scheme which can greatly simplify the automatic processing of 13C NMR spectra. For the time being I will simply outline the operations required but I will leave for a future blog entry an explanation on how the method actually works under the hood.

The method starts by first multiplying the FID by a Cosine Bell function (the squared version would also work) combined with a 45ยบ linear ramp function:

Next, we will apply the Fourier Transform followed by the magnitude calculation of the resulting frequency domain spectrum. That’s it! The resulting spectrum will exhibit the same resolution as a standard phase corrected spectrum despite of being in magnitude mode!. As I wrote, I will explain why this is so in this blog shortly

So, the advantages of this method are that neither phase nor baseline correction are required. It is true that, in the last years, automatic algorithms for phase and baseline corrections have become very reliable, but they are not bullet proof and sometimes they require manual tuning in order to get optimal results. For example, in the figure below you can see (top) a 13C spectrum with a severe baseline roll caused by the corruption of the first points of the FID. Whilst backward linear prediction or efficient baseline correction algorithms (see this) could be used to resolve this problem, the method I presented here will yield a very good spectrum with no user intervention at all (bottom).

That is all for now. I will soon answer some questions such as why these window functions are used and why phase and baseline correction are not needed but in the meantime, should you need any clarification, just drop me an email or leave a comment here.


Manuel P Pacheco said...

Well, I have tried the method described in a number of 13C spectra, and must admit that while it works fine for some, in some instances it results in a significant loss of peaks in the spectra. Mostly when the first order phase correction is far out of its optimal value.

Carlos Cobas said...

Hi Carlos,

In principle the method should not be sensitive to any phase distortions. The only problem I could see would be a very low SNR, but other than that I don't know ... Could you please send me one data set?