ENHANCED EFFECT OF MAGNETIC-FIELD GRADIENTS USING MULTIPLE-QUANTUM NMR-SPECTROSCOPY APPLIED TO SELF-DIFFUSION COEFFICIENT MEASUREMENT

NMR spectroscopy can be used to measure molecular self-diffusion coeff icients using a modified spin-echo pulse sequence known as the longitu dinal eddy current delay (LED) method which incorporates pulsed magnet ic field gradients. The effect of such gradients on multiple quantum c oherences is investigated and shown to be of benefit for the measureme nt of diffusion coefficients using modest gradient strengths. The mult iple quantum coherences involved in the spin operator 8I(x)I(y)I(y)S(y ) of a weakly coupled SI3 spin system ((CH3)-C-13 at natural abundance , in alanine in this case) were studied. The multiple quantum coherenc es included the quadruple-quantum (QQ) order of (3I + S), double-quant um (DQ) order of (31 - S) and (31 + S), and zero-quantum (ZQ) order of (I - S) with effective magnetogyric ratios of (3 gamma(H) + gamma(C)) , (3 gamma(H) - gamma(C)), (gamma(H) + gamma(C)) and (gamma(H) - gamma (C)) respectively. The results show that the use of higher level quant um coherences for diffusion coefficient measurement can have the pract ical effect of significantly enhancing the gradient strength and this is important for slowly diffusing species.