First-principles study of the rotational transitions of H[sub 2] physisorbed over benzene

In the ongoing search for promising compounds for hydrogen storage, novel porous metal-organic frameworks (MOFs) have been discovered recently [M. Eddadoudi, J. Kim, N. L. Rosi, D. Vodak, J. Wachter, M. O'Keeffe, and O. M. Yaghi, Science 295, 469 (2002); N. L. Rosi, J. Eckert, M. Eddadoudi, D. Vodak, J. Kim, M. O'Keeffe, and O. M. Yaghi, Science 300, 1127 (2003)]. Binding sites in these MOFs were deduced from inelastic neutron scattering (INS) spectroscopy of the rotational transitions of the adsorbed molecular hydrogen. In light of this discovery, it is important to have a fundamental understanding of hydrogen adsorption at different sites in this class of MOF materials. As a first step, here we study the case of H(2) adsorbed on benzene as a model of the organic linkers in the microporous crystal. We access the density functional theory results by comparing with correlated ab initio methods, e.g., second-order Møller-Plesset and coupled cluster with noniterative triple excitations. Different approximations for the exchange-correlation potentials were accessed for a set of relevant properties (binding energy, energetically favored configuration, and distance between the adsorbents and adsorbates). In particular, theoretical rotational spectra of the adsorbed H(2) were obtained that could be compared to the experimental INS spectra.