Chemistry of hydride ion and proton in anion-encaging crystals: Mayenite and apatite
September 4-8, 2016
Chemical shifts, d, of ‘H-’ ion in ionic crystals evaluated by 1H nuclear magnetic resonance (NMR) frequently exhibit values typically attributed to ‘H+’ To resolve this problem, the link between geometrical structure and chemical shift of H- ions have been systematically studied. Ca- and Sr-mayenite with chemical formula of [Ca24Al28O64]4+·2X2-and [Sr24Al28O64]4+·2X2- were employed as model materials. Their cubic lattice have specific site constructed by ‘cage’, which is partially occupied by monovalent or divalent guest anions including (X2- =) 2F-, 2Cl-, 2OH-, O2-, O22-, 2O2-, 2O-, or 2H- (Fig. 1). By severe reducing chemical treatment, electron (e-) is also incorporated in the cage with a similar manner with the guest anions, and contributes to an electronic conductivity in mayenite. For 1H-NMR study, it is convenient that both H+ (as OH- ion) and H- ion can be placed in same environments with slightly different sizes.
Experimentally collected chemical shift data were found to change systematically with the distance, dM-H, between hydrogen and coordinating electropositive cation, and the distance, dO-H…O, between OH- ion and hydrogen-bonded oxygen. These dependency were reproduced by ab initio approach. It was also found that the electron density near the hydrogen nucleus in an OH- ion (formally H+ state) exceeds that in an H– ion. This behavior is the opposite to that expected from formal valences, and can explain the mystery of NMR chemical shifts. The relationship of d (ppm) = 0.070 dM-H (pm) - 11.5 (Eq. 1) was established by the present study, and is useful for resolving H– species that are masked by various states of H+ ions, like the example of apatite described below.
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Katsuro Hayashi, "Chemistry of hydride ion and proton in anion-encaging crystals: Mayenite and apatite" in "Nonstoichiometric Compounds VI", ECI Symposium Series, (2016). https://dc.engconfintl.org/nonstoichiometric_vi/23