Nitrogen Adsorption on Graphite: Defying Physisorption

The adsorption of a nitrogen molecule at the graphite surface can be considered a paradigm of molecular physisorption [1]. The binding of N$_2$ can be phenomenologically described in terms of a competition between quadrupole--quadrupole and van der Waals dispersion energies. Of particular interest is the relative stability of the so-called ``in-plane'', ``out-of-plane'' and ``pin-wheel'' monolayer structures, in which the nitrogen molecules alternate between parallel and perpendicular configurations on the surface. By combining state-of-the-art electronic structure methods, such as dispersion-corrected density-functional theory and M{\o}ller-Plesset second-order perturbation theory along with high-level coupled cluster [CCSD(T)] calculations, we are able to gain quantitative insight into the adsorption mechanism of N$_2$@graphite and achieve very good agreement with experimental desorption enthalpy. We challenge the commonly held view of a closed-shell adsorbed N$_2$ molecule, finding a noticeable charge-density polarization for nitrogen in a perpendicular configuration on the surface. We map out the N$_2$@graphite potential energy surface as a function of sliding and orientation and discuss the influence of quantum zero-point energy for different adsorption sites. [1] D. Marx and H. Wiechert, Adv. Chem. Phys. {\bf 95}, 213 (1996).