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Friday, July 11, 2014

1H Decoupled 1H NMR Spectra

13C NMR spectra acquired with 1H decoupling are particularly simple to interpret as every symmetrically unique carbon atom gives rise to a peak in the NMR spectrum.  One is usually able to simply count the number of carbons in a molecule by counting the peaks in the 13C NMR spectrum.  1H NMR spectra, on the other hand, are complicated by homonuclear 1H - 1H coupling such that many 1H resonances are complex multiplets spread over a frequency range of some tens of Hz.  Furthermore, multiplets often overlap complicating the interpretation of the data.  Historically, this problem has been tackled by using higher and higher magnetic field strengths which disperse the chemical shifts over a wider frequency range without affecting the value of the coupling constants.  The effect is higher chemical shift resolution at higher fields.  In the limit of infinite field, the width of the 1H multiplets would be insignificant with respect to the chemical shift differences and one would obtain 1H NMR spectra containing essentially singlets.  Of course, we do not have access to infinite fields however, it would be very desirable to collect 1H decoupled 1H NMR spectra consisting of a singlet for each 1H resonance, much like the 13C signals in proton decoupled 13C NMR spectra.  It is not possible to collect proton decoupled 1H NMR spectra in the same way as it is to obtain proton decoupled 13C NMR spectra since one would have to both observe and decouple all of the protons at the same time.  There are however very clever techniques to obtain such pure shift 1H spectra.1,2  They are based on selective refocusing pulses applied simultaneously with weak field gradients and hard 180° pulses allowing all chemical shifts to be measured at the same time but from different slices of the column of sample in the NMR tube.  For each resonance, the coupling from all of the coupling partners is refocused simultaneously.  The data are collected in a conventional 2D matrix with an incremented evolution time.  An FID is constructed by concatenating a chunk from each of the individual 2D time domain signals.  The Fourier transform of the reconstructed FID is a 1H decoupled 1H NMR spectrum.  An example of this is shown in the figure below for a sample of menthol using a Bruker AVANCE II 300 MHz NMR spectrometer.3  The lower spectrum is the conventional 1H NMR spectrum.  One can see that it consists of broad complex multiplets some of which overlap with one another.  The upper spectrum is the pure shift spectrum.  It is greatly simplified compared to the conventional spectrum in that all of the multiplets are collapsed into singlets and each of the 14 types of protons of menthol can be identified.
Obtaining such spectra comes at the cost of much reduced sensitivity and much greater data collection times.  There is however, interest in improving this with modifications in the sequence and the way in which data are collected.4

1.  Zangger and Sterk. J. Mag. Reson. 124, 486 (1997).
2. Aguilar, Faulkner, Nilsson and Morris. Angew. Chem. Int. Ed. 49, 3901 (2010).
3. Bruker User Library .
4. Castanar, Nolis, Virgili and Parella. Chem. Eur. J. 19, 17283 (2013).