Tuesday, January 29, 2008
Decoupling is the application of radiation at one frequency while observing other frequencies. In the heteronuclear case, the decoupling frequency and the observation frequency differ by many tens of MHz and the decoupling radiation can be applied while the points of the FID are being sampled. In the homonuclear case however, the decoupling frequency lies within the spectral width for observation and the decoupling radiation can only be applied during the time between sampling points in the FID (the dwell time). If the radiation is applied while sampling points then the receiver would be overwhelmed by the decoupler, thus obliterating the spectrum. Homonuclear decoupling is an available option on most NMR spectrometers and used to be used very commonly as a means to assign proton spectra. It has largely been replaced by the 2D COSY experiment. The scheme for homonuclear decoupling is depicted in the figure below. The blue spectrum is a standard 1H spectrum of 3-heptanone. the red spectrum is of the same compound with the protons of carbon 1 irradiated. In the case of the standard spectrum, there is overlap between the protons of carbons 2 and 4. In the spectrum where homonuclear decoupling has been applied, one can clearly see that the protons from carbon 2 are a singlet whereas those from carbon 4 are a triplet.