Crystal chemistry of tetraradial species. Part 10. Tilting at windmills: conformations of the tetraphenyl species ZPh40, ±1 (Z = B, C, N)

The equilibrium geometries of the isoelectronic ZPh40, ±1 (Z = B, C, N) molecules have been optimized, in S4 and D2d symmetries, at the HF/6-31G(d,p) and B3LYP/6-31G(d,p) levels, with the phenyl rings constrained to be planar. The ground-state conformations of all three species are shown to be of S4 symmetry, but the difference of the energies of the S4 and the D2d conformations is estimated to amount to only ~1 kcal mol–1, with a barrier to concerted rotational reorientation of the phenyl rings of ~5.0 kcal mol–1 for BPh–4, ~6.4 kcal mol–1 for CPh4, and ~7.9 kcal mol–1 for NPh+4. A redetermination of the crystal structure of CPh4 from –60°C X-ray data agrees with the results of previous determinations at ambient temperature; the CPh4 molecule in the crystal is slightly compressed along the S4 axis relative to the free molecule, otherwise the geometries of the two are essentially the same. Correlations between the optimized geometric parameters of the ZPh40, ±1 molecules have been examined and analyzed in detail, as have the substitution-induced changes in the geometry of the phenyl ring. The results confirm and strengthen the conclusions from the statistical studies by Domenicano et al. on the geometry of benzene rings monosubstituted by atoms of elements from the first row of the Periodic system.Key words: tetraphenylborate, tetraphenylmethane, tetraphenylammonium, ab initio molecular geometries.