C–H▪▪▪S Hydrogen Bonds in Ampicillin and Amoxicillin Crystals Investigated by Sulfur K-Edge X-Ray Absorption Near-Edge Structure Spectroscopy and Single-Crystal X-Ray Structure Analysis

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Sulfur K-edge X-ray absorption near-edge structure (XANES) spectroscopy was evaluated for its ability to detect non-conventional C–H▪▪▪S hydrogen bonds in crystals of the sulfur-containing penam antibiotics ampicillin and amoxicillin. The XANES spectra of the nearly isomorphous crystals of ampicillin trihydrate and amoxicillin trihydrate were very similar, whereas that of ampicillin anhydrate displayed unique features. Single-crystal X-ray structure analyses revealed that the C–H▪▪▪S hydrogen bond geometries and the chemical types of the hydrogen donors differed between the isomorphous trihydrate crystals and ampicillin anhydrate crystal. These observations demonstrate that the shapes of the sulfur K-edge XANES spectra are dependent on the nature of the C–H▪▪▪S hydrogen bonds. Sulfur K-edge XANES spectroscopy shows promise for use in the detection and analysis of non-covalent interactions, including hydrogen bonds to sulfur atoms, within active pharmaceutical ingredients.