Ab initio calculation of the band structure of some boron polymers

Ab initio calculations using a STO-3G basis set have been performed on the polymer systems (HBX)_n where X = Be, BH, CH₂, NH, and O. Energy band diagrams and accompanying density-of-states plots have been obtained. The highest filled orbital of (HBNH)_n and (HBO)_n occurs at the X-point and possesses character while the - framework orbital at the X- point is the highest filled level for (HBBe)_n, (HBBH)_n, and (HBCH₂)_n. The conduction band for all five species has symmetry and the band gap of the (HBX)_n species increases in the order X = Be < BH < NH < O < CH₂. An estimate of the energy of polymerisation of the (HBX)_n systems suggests that HBNH is particularly stabilised by polymerisation. The electron distribution in (HBBe)_n shows a - electron drift towards boron, while in the other four systems the net electron transfer is directed away from boron. There is significant electron back-donation to boron in (HBO)_n and (HBNH)_n.