Is it possible to have molecules, which are chemisorbed on a surface, but can at the same time exhibit some kind of rotational motion and perhaps even move on the surface? We present such a 'molecular wheel' on semiconductor surfaces.
Fluxional molecules are ideal choice for this purpose. Here we consider a special class of fluxional molecules known as 'ring whizzers'. These are very simple molecules, which are not rigid and undergo intra-molecular rearrangements very easily. The rearrangement leaves the structure of the molecules unchanged, but does change the arrangement of the atoms in the molecule, and is usually studied using dynamic NMR spectroscopy. In particular, ring whizzers are ring containing organometallic compounds for which the M-C bond, keep on changing at ordinary temperatures. We first study the experimentally known ring whizzers Cyclopentadienyltrimethylsilane [(η1-C5H5)Si(CH3)3] and Cyclopentadienyl-trimethylgermane [(η1-C5H5)Ge(CH3)3] and Cyclopentadienyltrimethyl-stannane [(η1-C5H5)Sn(CH3)3] in detail, theoretically and find the barriers to their rotational motion. Then we ask: what would happen if the cyclopentadienyl ring is bonded to an M atom, which is a part of an M surface?.
Our calculations show that cyclopentadienyl, adsorbed to Si, Ge or Sn surfaces (co-adsorbed with H, so that all neighboring sites are blocked, can exhibit interesting spinning motion due to their fluxional behavior. The motion involves movement of the point of attachment around the ring, resulting in a wheel like motion of the molecule, though there is no net motion of the molecule in space (see the figure). Our studies predict the activation energy for this to be rather low [~13.5 kcal/mol for Si(111), ~11.0 kcal/mol for Ge(111) surface and ~6.0 kcal/mol for Sn (111) surface], and hence it should be possible to observe this experimentally.
The calculations were done for a cluster containing 13 M atoms, using GAUSSIAN 94 using the B3LYP hybrid-correlation-functional.