(26) This is called here the kinetic term since it depends on the kinetic rate constants k AB and k BA , and cannot be expressed as a population-weighted average. Unlike the R 0 S and R leak S terms, the kinetic term R kin S has no counterpart in the absence of chemical exchange. The term R kin S expresses the direct influence of the chemical exchange process on the singlet relaxation, through the stochastic modulation of the chemical shift differences by the chemical process.