title: Salt or co-crystal? Determination of protonation state by X-ray photoelectron spectroscopy (XPS).
abstract: Combined (15)N ssNMR and X-ray photoelectron spectroscopy (XPS) investigations for theophylline, a theophylline co-crystal, and a theophyllinium salt demonstrate that XPS allows direct observation of the degree of proton transfer, and thus identification of whether a salt or a co-crystal has been formed. The presence of a strongly binding-energy-shifted N 1s XPS peak with protonation indicates a salt (C==NH(+)), while this peak is unmistakably absent in the co-crystal. XPS should be considered as an alternative and complementary technique to single crystal X-ray diffraction and solid-state nuclear magnetic resonance spectroscopy (ssNMR).
authors.affiliation: School of Chemical Engineering and Analytical Science, The University of Manchester, Sackville Street, Manchester M60 1QD, UK. joanna.stevens@postgrad.manchester.ac.uk
authors.forename: Joanna S
authors.initials: JS
authors.lastname: Stevens
authors.forename: Stephen J
authors.initials: SJ
authors.lastname: Byard
authors.forename: Sven L M
authors.initials: SL
authors.lastname: Schroeder
mesh_terms: D003460:Crystallization; D056951:Photoelectron Spectroscopy / Q000379:methods*; D011522:Protons; D012492:Salts / Q000737:chemistry*; D013806:Theophylline / Q000737:chemistry*
chemical_list: D011522:Protons; D012492:Salts; D013806:Theophylline