Quantum chemistry study of the van der Waals dimers of benzene, naphthalene, and anthracene:: Crossed (D2d) and parallel-displaced (C2h) dimers of very similar energies in the linear polyacenes

A quantum chemistry study of the ground-state structures and binding energies of the van der Waals dimers of benzene, naphthalene, and anthracene has been made at the MP2/6-31G and MP2/6-31+G levels of theory. For naphthalene and anthracene, the calculations yield two low energy dimers of very similar energies: D-2d (crossed) and C-2h (parallel-displaced). Conformers, analogous to the T-shaped dimer of benzene, are less stable than the crossed and the parallel-displaced dimers. BSSE-corrected MP2/6-31+G//MP2/6-31G binding energies of the crossed and parallel-displaced dimers are,respectively, 15.77 and 15.65 kJ/mol for naphthalene and 36.40 and 31.84 kJ/mol for anthracene. The fully optimized MP2/6-31G structures of the parallel-displaced dimer of naphthalene has a horizontal displacement of 1.5 Angstrom along the short axes of the monomers and a vertical displacement of 3.5 Angstrom. The corresponding horizontal and vertical separations for the parallel-displaced (PD) dimer of anthracene are 1.1 and 3.6 Angstrom, respectively. For the crossed dimers, the vertical separation of the aromatic rings is 3.6 Angstrom for naphthalene and 3.3 Angstrom for anthracene. The greater binding energy and the shorter horizontal displacement of the anthracene PD dimer relative to the naphthalene PD dimer are consistent with the stronger dispersion interactions expected of a larger dimer. The greater stability and smaller vertical separation of the crossed dimer of anthracene relative to that of naphthalene can also be attributed to the disparity in the dispersion interactions.