
%0 Journal Article
%A Doddrell, DM 
%A Kleinschmidt, KM 
%T Nuclear spin-lattice relaxation in a multi-spin system. A spin 1/2 particle dipolar coupled to a spin 1 and spin 3/2 particle. <sup>13</sup>C spin relaxation in urea
%J Australian Journal of Chemistry
%D 1980/12/01
%V 33
%N 12
%P 2571-2583
%K 
%R https://doi.org/10.1071/CH9802571
%U https://www.publish.csiro.au/paper/CH9802571 
%X Density matrix theory is used to show that for S = 1/2, 1, 3/2 the decay of the I-magnetization (I = 4) in a two-spin IS dipolar coupled system is governed by the differential equation                  d<Iz>/dt= -k1{J(&#969;I-&#969;S)+3J(&#969;I)+6J(&#969;I+&#969;S)}{<Iz>-I0}                         -k2{6J(&#969;I+&#969;S)-J(&#969;I-&#969;S)}{<SZ>-S0} where k1 and k2 are positive constants dependent on S and the dipolar interaction constants. It follows that if <S2> = 0, as arises on S- irradiation, or <SZ> = S0, as in paramagnetic transition-metal complexes, the decay of the I-magnetization is exponential. The theory is used to analyse 14N,2H dipolar-induced 13C spin relaxation in deuterated urea. It is shown that at low temperatures dipolar interactions dominate the relaxation whereas at high temperatures spin rotation effects become important. The relaxation times are very long; at 300 T1 is 114s. 
