Communications: Chain and double-ring polymeric structures: Observation of AlnH3n+1− (n = 4–8) and Al4H14−

Abstract

A pulsed arc discharge source was used to prepare gas-phase, aluminum hydride cluster anions, AlnHm−, exhibiting enhanced hydrogen content. The maximum number of hydrogen atoms in AlnHm− species was m = 3n+1 for n = 5–8, i.e., AlnH3n+1−, and m = 3n+2 for n = 4, i.e., Al4H14−, as observed in their mass spectra. These are the most hydrogen-rich aluminum hydrides to be observed thus far, transcending the 3:1 hydrogen-to-aluminum ratio in alane. Even more striking, ion intensities for AlnHm− species with m = 3n+1 and m = 3n+2 hydrogen atoms were significantly higher than those of nearby AlnHm− mass peaks for which m<3n+1, i.e., the ion intensities for AlnH3n+1− and for Al4H14− deviated from the roughly bell-shaped ion intensity patterns seen for most AlnHm− species, in which m ranges from 1 to 3n. Calculations based on density functional theory showed that AlnH3n+1− clusters have chain and/or double-ring polymeric structures.