Nuclear Magnetic Resonance Standards for Aqueous Solutions

Abstract

FOR the nuclear magnetic resonance investigation of interaction in solution, careful consideration must be given to the choice of standard against which changes in chemical shift are to be measured. An external standard is in many ways ideal, but a correction has then to be applied to the observed chemical shift, due to differences in bulk magnetic susceptibilities. The true chemical shift δ is given by the equation: where + Δν is the observed upfield shift, in p.p.m., of a line with respect to the reference, and χ_ref and χ_sol the volume magnetic susceptibilities of the reference and solution being investigated. The value of g is determined by the shape of the interface of contact between the reference and the solution. Since it is somewhat tedious to measure susceptibilities², many investigators have either ignored any corrections (thereby invalidating the work³) or have used internal standards, for which no susceptibility corrections have to be applied.