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 ZPh(4)(0, +/-1) (Z = B, C, N) molecules have been optimized, in S-4 and D-2d 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 S-4 symmetry, but the difference of the energies of the S-4 and the D-2d conformations is estimated to amount to only similar to1 kcal mol(-1), with a barrier to concerted rotational reorientation of the phenyl rings of similar to5.0 kcal mol(-1) for BPh4-, similar to6.4 kcal mol(-1) for CPh4, and similar to7.9 kcal mol(-1) for NPh4+. A redetermination of the crystal structure of CPh4 from -60degreesC X-ray data agrees with the results of previous determinations at ambient temperature; the CPh4 molecule in the crystal is slightly compressed along the S-4 axis relative to the free molecule, otherwise the geometries of the two are essentially the same. Correlations between the optimized geometric parameters of the ZPh(4)(0, +/-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.