Recent Publication:

Can We Identify Salt-cocrystal Continuum State Using XPS? No

X-ray Photoelectron Spectroscopy (XPS) is used to understand the nature of acid-base crystalline solids, to know whether the product is a salt (proton transfer, O-···H-N+) or cocrystal (neutral adduct, O-H···N). The present study was carried out to explore if intermediate states of proton transfer from COOH to Nitrogen (the proton resides in-between hydrogen bonded to O and N, O···H···N) can be differentiated from salt and cocrystal using N 1s XPS spectroscopy. The intermediate states of proton transfer arise when the pKa difference between the acid and the conjugate base is between -1 and 4, -1 <DpKa< 4, a situation common with COOH and pyridine functional groups present in drug molecules and pharmaceutically acceptable coformers. Complexes of pyridine N bases with aromatic COOH molecules in nine salts/cocrystals were cocrystallized and their N 1s core binding energy in XPS spectra were measured. The proton state was analyzed by single crystal X-ray diffraction. Three new complexes were crystallized and analyzed by XPS spectra (without knowing their X-ray structures), to assess the predictive ability of XPS spectra in differentiating salt-cocrystal intermediate states against the extremities. The XPS results were subsequently confirmed by single crystal X-ray data. 1:1 and 1:2 complexes of 3,5-dinitrobenzoic acid and isonicotinamide exist as salt and salt-cocrystal continuum, respectively, as shown by the N 1s core binding energy. The proton states of the crystalline solids by XPS are in good agreement with the corresponding crystal structures. Other complexes, such as those of 3,5-dinitrobenzoic acid with 1,2-di(4-pyridyl)ethylene exhibit salt-cocrystal continuum, maleic acid with 1,2-di(4-pyridyl)ethylene and acridine are salts, 2-hydroxybenzoic acid and acridine is a salt, and complex of 3,5-dinitrobenzoic acid and 3-hydroxypyridine is a salt, while fumaric acid with 1,2-di(4-pyridyl)ethylene and acridine are cocrystals. Furthermore, three new acid-base complexes of 3,5-dinitrobenzoic acid with phenazine, 4-hydroxypyridine and 4-cyanopyridine were studied initially by XPS and then confirmed by X-ray diffraction. In summary, since the N 1s binding energies cluster in a narrow range as cocrystal (398.9-399.6 eV) and salt (400.9-401.9 eV), it is possible to differentiate between neutral cocrystal and ionic salt, but the salt-cocrystal continuum values in XPS spectra are not clustered in a distinct intermediate range of energy to be of predictive value.

Authors:  S. Tothadi, T. R. Shaikh, 
  • R. Dandela, 
  • C. P. Vinod
  • , and 
  • A. K. Nangia
  •  Publication date : Aug 4th, 2020

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