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Hydrogen Atoms in Boehmite: A Single Crystal X-Ray Diffraction and Molecular Orbital Study

Published online by Cambridge University Press:  01 July 2024

Roderick J. Hill
Roderick J. Hill*
Affiliation:
CSIRO Division of Mineral Chemistry, P.O. Box 124, Port Melbourne, Victoria, Australia, 3207
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Abstract

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The crystal structure of natural boehmite, AlOOH, from Tveidalen, Langesundsfjorden, Norway, with a = 3.693(1), b = 12.221(2) and c = 2.865(1) Å, was refined from single crystal X-ray diffraction data (MoKα maximum 2θ = 120°) to an Rw value of 0.032 in space groups Amam and A21am. In both cases the proton was found to occupy positions of the type (x, 0.038, 0), where x = 0.104 and 0.396, about 0.75 Å from the donor oxygen atom. This arrangement corresponds to the presence of chains of asymmetric hydrogen bonds with random polarity parallel to the a axis, and indicates that the effective space group is Amam. No significant electron density was observed either at the center (0, 0, 0) or at the position (1/4, y, 0), corresponding to symmetric and bifurcated hydrogen bonds, respectively, between the layers of octahedra.

Calculations of the total energy of a representative portion of the boehmite structure as a function of hydroxyl orientation, using semiempirical molecular orbitals of the Hückel type, are in agreement with the observed electron density distribution and suggest a preference for (1) sp3, rather than sp2, hybridization of the donor oxygen atom orbitals, and (2) asymmetric, rather than symmetric hydrogen bonds. Disorder of the hydrogen atom positions within individual hydrogen bond chains is shown to be much less likely than disorder between adjacent fully coherent chains.

Резюме

Резюме

Кристаллическая структура натурального бемита, АlOOН, Твейдалена, Лангезундс- фьерден, Норвегия, с а = 3,693(1), b = 12,221(2), и с = 2,865(1) Å была усовершенствована на основе данных по рентгеновской однокристальной дифракции (МоKα; максимум 2θ = 120°) до величины Rw = 0,032 в пространственных группах Aman и А21am. В обоих случаях было обнаружено, что протон занимает положения типа (х, 0,038, 0), где х = 0,104 и 0,396, около 0,75 Å от донатора кислорода, соответственно присутствию цепных асимметрических водородных связей со случайной полярностью параллельной оси а и указывая на то, что эффективной пространственной группой является Aman. Значительная плотность электронов не наблюдалась ни в центре (0, 0, 0), ни в положении (1/4, у, 0), соответствующем симметрическим и раздвоённым водородным связям, соответственно, между слоями октаэдра.

Расчёты полной энергии характерной части структуры бемита, как функции ориентации гидроокиси, при использовании полуэмпирических молекулярных орбит типа Глюкла находятся в согласии с наблюдаемым распределением плотности электронов, указывая преимущественно на: (1) скорее sp3, чем sp2, гибридизацию орбит донатора кислорода, и (2) скорее асимметрические, чем симметрические водородные связ. Показано, что неупорядоченность положений атомов водорода внутри индивидуальных цепей водородных связей, вероятно, при комнатной температуре, намного меньше, чем неупорядоченность между соседними вполне когерентными цепями. [Е.С.]

Resümee

Resümee

Die Kristallstruktur von natürlichem Boehmit, AlOOH, von Tveidalen, Langesundsfjord, Norwegen, mit a = 3,693(1), b = 12,221(2), und c = 2,865(1) Å, wurde anhand von Einzelkristalldiffraktometerdaten (MoKα, Maximum 2θ = 120°) auf einen Rw-Wert von 0,032 in den Raumgruppen Amam und A21am verfeinert. In beiden Fällen besetzt das Proton die Position (x, 0,038,0), wobei x = 0,104 und 0,396 ist und etwa 0,75 Å vom Donor-Sauerstoffatom entfernt ist. Dies stimmt damit überein, daß Ketten von assymmetrischen Wasserstoffbrücken vorhanden sind, deren ungefähre Polarität parallel zur a-Achse liegt, was daraufhindeutet, daß die tatsächliche Raumgruppe Amam ist. Es wurde keine bemerkenswerte Elektronendichte beobachtet, weder im Zentrum (0, 0, 0) noch in der Position (1/4, y, 0), entsprechend symmetrischen bzw. sich gabelnden Wasserstoffbrücken zwischen den Oktaederschichten.

Berechnungen der Gesamtenergie eines repräsentativen Teils der Boehmitstruktur als eine Funktion der Hydroxyl-Orientierung mittels halbempirischer MO-Rechnungen (Hückel-Methode) stimmen mit der beobachteten Verteilung der Elektronendichte überein. Dies deutet darauf hin, daß (1) ehereine sp3—als eine sp2—Hybridisierung des Donor-Sauerstofforbitals erfolgt, und (2) eher assymmetrische als symmetrische Wasserstoffbrücken vorhanden sind. Es zeigt sich, daß eine Unordnung von Wasserstoffatom-Positionen innerhalb bestimmter Ketten von Wasserstoffbrücken bei Raumtemperatur weniger wahrscheinlich ist als eine Unordnung zwischen benachbarten koherenten Ketten. [U.W.]

Résumé

Résumé

La structure cristalline d'une boéhmite naturelle AlOOH de Tveidalen, Langesundsfjorden, Norvège, avec a = 3,693(1),b = 12,221(2), etc = 2,865(1) Å a été rafinée à partir de données de diffraction aux rayons-X d'un seul cristal (MoKα; 2θ maximum = 120°) à une valeur Rw de 0,032 dans les groupes d'espacement Amam et A21am. Dans les deux cas on trouve le proton occupant les positions de type (x, 0,038, 0), où x = 0,104 et 0,0396, à à peu près 0,75 Å de l'atome d'oxygène donateur, correspondant à la présence de chaînes de liens d'hydrogène assymétriques avec une polarité au hasard parallèles à l'axe a, et indiquant que le groupe d'espacement éffectif est Aman. Aucune densité d’électron significative n'a été observée ni au centre (0,0,0) ni àla position ( 1/4, y, 0), correspondant aux liens d'hydrogène symmétriques et bifurqués, respectivement, entre les couches d'octaèdres.

Des calculs de l’énergie totale d'une portion représentative de la structure de la boéhmite en tant que fonction de l'orientation hydroxyl, utilisant des orbites moléculaires semi-empiriques de type Hückel, s'accordent avec la distribution de densité d’électrons observée en suggérant une préférence pour (1) l'hybridisation sp3, plutôt que sp2, des orbites d'atome d'oxygène donateur, et (2) des liens d'hydrogène assymétriques plutôt que symmétriques. Le désordre des positions de l'atome d'hydrogène dans des chaînes individuelles de liens d'hydrogène a été montré être beaucoup moins probable à température ambiante que le désordre entre des chaînes adjacentes totalement cohérentes. [D.J.]

Keywords

Aluminum hydroxideBoehmiteCrystal structureElectron densityHydrogen positionsMolecular orbitalsSingle crystal
Type
Research Article
Information
Clays and Clay Minerals , Volume 29 , Issue 6 , December 1981 , pp. 435 - 445
Copyright
Copyright © 1981, The Clay Minerals Society

Footnotes

1

For those more recent studies in which a unit-cell definition has been used corresponding to the standard Cmcm setting of space group D2h 17, the results will be transformed into the original and more commonly used Amam setting.

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