1H Fast MAS NMR Studies of Hydrogen-Bonding Interactions in Self-Assembled Monolayers

Abstract

The structures formed by the adsorption of carboxyalkylphosphonic acids on metal oxides were investigated by ¹H fast magic angle spinning (MAS), heteronuclear correlation (HETCOR), and ¹H double-quantum (DQ) MAS solid-state NMR experiments. The diacids HO₂C(CH₂)_nPO₃H₂ (n = 2, 3, 11, and 15) were adsorbed on TiO₂ and two types of ZrO₂ powders having average particle sizes of 20, 30, and 5 nm, respectively. Carboxyalkylphosphonic acids bind selectively via the phosphonate group, forming monolayers with pendant carboxylic acid groups. Whereas dipolar coupled P−OH protons are detected on TiO₂, there are only isolated residual P−OH groups on ZrO₂, reflecting the relative binding strengths of phosphonic acids on these two substrates. From a comparative ¹H MAS NMR study with an analogous monolayer system, HO₂C(CH₂)₇SH coated gold nanoparticles, the hydrogen-bonding network at the monolayer/air interface is found to be quite disordered, at least for SAMs deposited on nonplanar substrates. Whereas only hydrogen-bonded homodimers occur in the bulk diacids, hydrogen bonding between the carboxylic and phosphonic acid groups is present in multilayers of the diacids on the ZrO₂ nanopowder.