An ab initio study of the properties of some hydride dimers

Abstract

The properties of the homodimers of the covalent hydrides of the elements from lithium to fluorine and from sodium to chlorine have been computed by means of ab initio molecular orbital theory. These properties show some interesting discontinuities, illustrating the significant differences between the natures of the dimers of the hydrides of the metals and metalloids of groups 1–3, and those of the electronegative non-metals of groups 5–7. The dimers of the group 1–3 hydrides are very strongly bound, featuring two bridging hydrogen atoms per dimer molecule, in structures similar to that of the well-known diborane species. Those of the hydrides of groups 6 and 7 are stabilized by conventional, almost linear, hydrogen bonds. The same is true of the ammonia dimer, but that of phosphine contains two equivalent pairs of bridging hydrogen atoms in a very weakly bound, doubly bifurcated hydrogen-bonded aggregate. The methane and silane dimers are found to be barely bound at all.