Quantum chemistry study of the van der Waals dimers of benzene, naphthalene, and anthracene: Crossed (D-2d) and parallel-displaced (C-2h) dimers of very similar energies in the linear polyacenes

A quantum chemistry study of the ground-state structures and binding energi es 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 simil ar energies: D-2d (crossed) and C-2h (parallel-displaced). Conformers, anal ogous to the T-shaped dimer of benzene, are less stable than the crossed an d the parallel-displaced dimers. BSSE-corrected MP2/6-31+G//MP2/6-31G bindi ng 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 anthr acene. The fully optimized MP2/6-31G structures of the parallel-displaced d imer 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. Th e corresponding horizontal and vertical separations for the parallel-displa ced (PD) dimer of anthracene are 1.1 and 3.6 Angstrom, respectively. For th e crossed dimers, the vertical separation of the aromatic rings is 3.6 Angs trom for naphthalene and 3.3 Angstrom for anthracene. The greater binding e nergy and the shorter horizontal displacement of the anthracene PD dimer re lative to the naphthalene PD dimer are consistent with the stronger dispers ion interactions expected of a larger dimer. The greater stability and smal ler vertical separation of the crossed dimer of anthracene relative to that of naphthalene can also be attributed to the disparity in the dispersion i nteractions.