The authors have treated the relaxation and the dynamics of the (111) surfaces of the fluorides CaF2 and SrF2 in the framework of shell models, in which the short-range interactions are represented by Born-Mayer potentials. The relaxation in the outer layer of the crystal, consisting of three closely spaced F-Ca/SrF layers, is significant. The calculated normal vibrations of a 19-layer slab are presented in 3-dimensional diagrams of the normalized surface squared amplitude, which provide a very direct and pictorial display of the surface localization of vibrational states. For CaF2 the calculations are compared with the HREELS data of Longueville et at. (1990).

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Because of their possible role as model compounds for the structural units of carbon nitride C,N,, the preparation, structural chemistry, and some representative reactions of tetra-(amino)methanes have been (re)investigated. In the crystal, C(NMe2), (1) has a molecular geometry close to DZd symmetry as proposed by theoretical calculations using state-of-theart density tunctional methods. The coordination of the central carbon atom is distorted tetrahedral and the configuration of the nitrogen atoms is strongly pyramidal, as opposed to almost planar in the tetra(amin0)silanes. Tetra(pyrro1idi-ny1)methane has a similar core structure, with all heterocyclic substituents in an envelop conformation flexible in solution.Tetra(piperidiny1)methane is more rigid in solution, owing to a more congested structure, with much higher inversion barriers for the six-membered rings. Hydrolysis of 1 leads to MezNH and hexamethylguanidinium hydroxide, and treatment of 1 with HAuCl,(aq) affords crystalline [C(NMe,)3]+ AuCl;, the structure of which has also been determined. Compound 1 is a strong nucleophile and can be used as an aminating agent, converting e.g. halosilanes into dimethylaminosilanes, with the guanidinium cation as the leaving group. The experimental results are discussed in the light of recent predictions regarding bulk carbon nitrides.

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