Enhanced effect of magnetic field gradients using multiple quantum NMR spectroscopy applied to self-diffusion coefficient measurement

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NMR spectroscopy can be used to measure molecular self-diffusion coefficients using a modified spin-echo pulse sequence known as the longitudinal eddy current delay (LED) method which incorporates pulsed magnetic field gradients. The effect of such gradients on multiple quantum coherences is investigated and shown to be of benefit for the measurement of diffusion coefficients using modest gradient strengths. The multiple quantum coherences involved in the spin operator 8I x I y I y S y of a weakly coupled SI3 spin system (13CH3 at natural abundance, in alanine in this case) were studied. The multiple quantum coherences included the quadruple-quantum (QQ) order of (3I + S), double-quantum (DQ) order of (3I - S) and (3I + S), and zero-quantum (ZQ) order of (I - S) with effective magnetogyric ratios of (3γH + γC), (3γH - γC), (γH + γC) and (γH - γC) respectively. The results show that the use of higher level quantum coherences for diffusion coefficient measurement can have the practical effect of significantly enhancing the gradient strength and this is important for slowly diffusing species.