Imperfections in the aqueous lattice of clathrates, as detected by electron trapping

Abstract

X-ray analysis has been used to elucidate the structure of hydrates, especially of aqueous clathrates, but it has not given information on imperfections occuring in crystals. Other methods, sensitive to the disturbance of structural periodicity, have to be applied to detect and identify the defects. Pulse radiolysis shows identical, intense transient spectra in a group of hydrates which exhibit certain common features. The spectrum (λmax = 620 nm) is tentatively ascribed to the electron trapped in a single water molecule vacancy in the aqueous moiety of the crystal. The pre-existing specific defect is formed during the crystallization process. The optical absorption spectrum is similar to the hydrated electron spectrum in liquid water or aqueous solutions. The nature of the traps is deduced from the known structure of clathrates, in which small anions substitute for one water molecule and may be displaced. Among investigated salt hydrates, amine hydrates and true clathrate hydrates, not all compounds show the ability to trap electrons. The present paper discusses the conditions for the occurrence of water molecule vacancies in the aqueous moiety of the clathrate and the general aspects of the observed phenomenon. A need to investigate imperfections by other methods is also stressed, although such alternatives are not yet available.