DFT study of hydrogen bonding between metal hydroxides and organic molecules containing N, O, S, and P heteroatoms: clusters vs. surfaces

Abstract

Hydrogen bonds between either a water molecule or metal hydroxides and small organic molecules with functional groups that contain N, O, S, or P heteroatoms were analyzed using DFT calculations to shed some light on the question of whether hydroxylated nanoparticles and surfaces can be stabilized with organic molecules via hydrogen bonding interactions. Two different models of metal hydroxides were used, that is, small discrete clusters and periodic slab models of surfaces, where Al(OH)3 and Cu(OH)2 served as model systems. For small discrete cluster models, formula units of Al(OH)3 and Cu(OH)2 were taken, whereas for extended surface models, boehmite-AlOOH(010) and Cu(OH)2(001) surfaces were used. According to our results, the Cu(OH)2 cluster is usually a better H-bond acceptor and donor than the water molecule, whereas the Al(OH)3 cluster prefers to either act as an H-bond donor or to form two H-bonds, one as an H-bond donor and the other as an H-bond acceptor. Among the considered organic molecules with functional groups containing N, O, S, or P heteroatoms, imidazole and (CH3)2POOH form the strongest H-bonds; the two molecules are very good H-bond acceptors as well as H-bond donors. These two molecules were also used to analyze hydrogen bonding with the boehmite-AlOOH(010) and Cu(OH)2(001) surfaces. The comparison between the surface and small-cluster calculations reveals that although cluster calculations can give reasonable estimates of adsorption energy provided that all formed H-bonds are properly accounted for (which is not always trivial), there are nevertheless structural intricacies—such as additional H-bonds with second-neighbor OH groups that may form on surfaces—that cannot be captured with small clusters. The more realistic aqueous conditions were also analyzed using the continuum solvation model. They not only influence the properties of H-bonds that are usually shorter than in vacuum but also induce deprotonation of adsorbed molecules, as observed for (CH3)2POOH on a Cu(OH)2 surface.