Tuesday, September 9, 2008
Double Quantum Filtered COSY
COSY spectra are very useful in structure elucidation as they provide correlations between coupled spins. Often, NMR spectra have large singlet signals from uncoupled protons (such as t-butyl methyls, methoxy protons, excess water or a solvent signal) which provide no information in the COSY spectrum and perhaps even get in the way of looking for smaller coupled spins. In such cases one can use a double quantum filtered COSY sequence rather than a standard COSY 90 or COSY 45 sequence. Double quantum filtered COSY spectra filter out uncoupled singlets. A comparison of a standard COSY 90 and a double quantum filtered COSY sequence for ethyl acetate is shown below. One can see that the singlet is present in the COSY 90 spectrum but absent in the double quantum filtered COSY spectrum.