The 1,4-phenylenediisocyanide dimer: gas-phase properties and insights into organic self-assembled monolayers.

The 1,4-phenylenediisocyanide (PDI) dimer serves as an intriguing case of the substituted benzene dimer, as well as a prototype system for self-assembled monolayers of organic isocyanide complexes. Structures and binding energies are explored using recently developed dual-basis second-order Møller-Plesset perturbation theory energies and gradients. The structures are dictated by a combination of dispersion and electrostatics, a combination not properly treated with local or gradient-corrected density functionals. The PDI dimer binds more than twice as strongly as unsubstituted benzene dimers in several configurations, and greater directional specificity between parallel-displaced and T-shaped structures is observed. A rotated-parallel structure is the predicted lowest-energy, gas-phase configuration, in which the isocyanide ligands are staggered on the monomers. Relevant potential energy curves of the dimer are also presented, and insights into PDI monolayer formation on metal surfaces are explored via simple two-body models. Based on the adsorbate interaction alone, a high-coverage configuration and non-vertical tilt are predicted to be favorable, although the total binding for PDI in these configurations is still insufficient to form ordered monolayers, a result consistent with previous experimental findings. Additional phenyl rings (biphenyldiisocyanide, triphenyldiisocyanide) significantly stabilize the interaction and provide the additional dispersion necessary for an ordered monolayer.