The total scattering pattern of these solutions (Figures ) is a complex sum of structural correlations. When the neutron diffraction patterns of two isotopically different but chemically identical solutions (assumed to be structurally isomorphic) are subtracted from each other, all of the correlations cancel out except those of the substituted nuclei. This is the core of the neutron diffraction technique with isotopic substitution (NDIS). For instance, subtracting the neutron scattering patterns of two D 2 O solutions, one containing D TMA D Ac and the other H TMA D Ac yields the so-called the first order difference ∆S X D Ac H TMA (Q) that can be expressed as a function of only 6 structural correlations (Equation 1). Similarly, subtracting the signals from solutions containing H Ac rather than D Ac yields the first order difference ∆S X H Ac Hnon (Q) (see Equation ). These two first order differences (See Figures ) report on the correlations between non-exchangeable H on TMA and every other atom (X) in the system in units of mbarns (mb). Using standard literature methods, the prefactors in the equations were computed from the atomic concentration and neutron scattering lengths of the various elements in the system. The differences between Equations 1 and 2 are highlighted in bold. ∆S