Unexpected change in NOE with increasing temperature: Crosstalk between chemical exchange and cross relaxation in a NiN2S2 complex

Abstract

Typically, with increasing sample temperature the rotational correlation time of the molecule decreases, leading to a decrease in the relative nuclear Overhauser effect (NOE) enhancement. In characterizing the (O-ethyl-L-cysteinato)-Ni(II) in the solution state, the measured NOE enhancement increases with increasing sample temperature. This unexpected change is explained via crosstalk between nuclear spin cross relaxation and an additional chemical exchange phenomenon between the amino protons of the molecule. The case study using a relaxation matrix approach, with both cross relaxation and chemical exchange, could be essential to understanding a metal complex with a bidentate amino acid derived ligands.