Double-quantum filtered 1H MAS NMR spectra.

It is shown that straightforward double-quantum filtered (1)H MAS NMR experiments yield spectral lineshapes that permit to estimate the minimum number of (1)H spins in a cluster. The approach may offer an alternative to multiple-quantum experiments for the characterisation of (1)H spin clusters of moderate size. The duration of the double-quantum excitation period has to be chosen suitably, it is necessary to find a practical compromise between optimum double-quantum filtration efficiency and optimum information content of the spectral lineshapes. Some (1)H MAS NMR experiments on partially deuterated maleic acid are reported as well as numerical simulations.