Many compounds of potential interest for chemical forensic analysis are organic acids or bases. Examples include alkaloids and other biologically active natural products, and many synthetic compounds, particularly some pesticides and pharmaceuticals. Such compounds are commonly prepared as the salt form of the neutral molecule, because organic salts often have favorable properties such as greater stability and higher solubility in water compared to the free acid or base form. The particular counterion present in organic salts is a characteristic feature that can distinguish otherwise identical samples from each other, absent any active ion exchange processes. While ion-chromatography is often used to identify counterions in organic salts, circumstances may limit the utility of this method for some samples in particular instances. We have shown recently that solution state NMR spectroscopy offers an alternative way to accomplish counterion identification. Typically, in organic solvents, but not in water, the 1H and 13C chemical shifts of different salts of a given compound can exhibit characteristic variations depending on the type of counterion present. The magnitude and direction of each perturbed 1H and 13C chemical shift in an organic salt of interest together may constitute a signature distinguishing it from other salts of the same compound. This phenomenon was recently demonstrated in the alkaloid strychnine, and is now being extended to other compounds. The chemical basis for counterion-dependent NMR chemical shift perturbations in salt forms of organic acids and bases is currently being investigated.

Counterion-dependent NMR chemical shift perturbations in organic acid and base salts

246th National Meeting (2013)

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