Combining 15N-CP/MAS NMR and quantum chemistry to reveal the occurrence of proton transfer: The case of the aminobenzothiazole/carboxylic acid adduct

The binding of the pesticide metabolite aminobenzothiazole (ABT) to a model humic acid was studied. It is shown that, by comparing quantum chemically calculated and experimental 15N NMR chemical shifts, proton transfer occurs in complexes of ABT with aminobenzoic acid (AB) and thiophene carboxylic acid (TC). For extended structures, i.e. the crystalline form of AB–ABT and TC–ABT, only in the latter proton transfer is observed, in agreement with previously reported analysis based on IR spectroscopy. Thus it is concluded that in the bimolecular complexes as model systems for structures found in humic substance, here specifically AB–ABT, proton transfer may be different as compared to the crystalline state. Moreover, it was shown that combining experimental NMR work and quantum chemical calculations can reveal the presence of proton transfer.