PNNL

Abstract

What is the true oxidation state of formally ‘H–’ ions incorporated in an oxide host? This question frequently arises in connection with chemical shifts of 1H nuclear magnetic resonance (NMR) spectroscopy, which exhibit values typically attributed to H+. To resolve this controversy, we systematically investigate the link between geometrical structure and chemical shift of H- ions in an oxide host, mayenite, with a combination of experimental and ab initio approaches. We demonstrate that the electron density near the hydrogen nucleus in an OH- ion (formally H+ state) exceeds that in an H– ion. This behaviour is the opposite to that expected from formal valences. We deduce a relationship between the chemical shift of H– and the distance from the H– ion to the coordinating electropositive cation. This relationship is pivotal for resolving H– species that are masked by various states of H+ ions.