Nonlinear chemical shifts and a new formula for the averaged shift in systems with two-position chemical exchange

Relative chemical shifts are known to be invariant to the value of the resonance frequency. Experience, however, has shown this frequency invariance with respect to the chemical shift to be violated in some molecules, to the effect that the absolute shift of the NMR line is nonlinearly dependent on the strength of the external magnetic field. In an earlier paper the authors carried out a theoretical investigation of the field dependence of the characteristics of the NMR spectra of systems with two-position exchange and showed that the nonlinear effect is attributable to the occurrence of asymmetrical chemical exchange between magnetically inequivalent positions. In this paper they further develop this theory by incorporating hyperfine splitting and temperature dependence into the calculation