The role of bonding electrons in intermolecular forces; solid acetylene

Abstract

Acetylene crystallizes in orthorhombic Cmca below 133 K and in cubic Pa3 above. We calculate the lattice energy Φ, using pairwise potentials as the terms in two sums, E _HFSCF, the Hartree-Fock self-consistent field MO energy with 6–31 G* basis and E _disp the dispersion energy. In the latter a more extended basis was used to determine the locations and polarizabilities of the localized MOs in the molecule. The triple bond has by far the largest polarizability, that of the atomic cores being too small to contribute to E _disp. Both structures were relaxed by steepest descent and do not change greatly. The results, without corrections for basis set superposition errors (BSSE), were Φ for Pa3, -24·3 kJ mol^-1, for Cmca, -23·5 kJ mol^-1 (experimental, -21·8 kJ mol^-1), the calculated order of stability of the two structures thus being the reverse of that observed. When BSSE corrections were applied in the Pa3 calculations, too large a unit cell dimension and too small a lattice energy were obtained. The better agreement obtained for both structures without BSSE corrections could be fortuitous. Further work, using a larger and better basis set (which will entail more extensive computation) is required.