Dynamics of octadecylphosphonate monolayers self-assembled on zirconium oxide: A deuterium NMR study

Deuterium NMR spectroscopy has been used to probe the dynamics of deuterated octadecylphosphonate (-1,1-d(2)) (ODPA-d(2)) monolayers on nonporous ZrO2 powder (surface area approximate to 40 m(2)/g) over the temperature range of 200-340 K. At 200 K, a broadened Pake doublet with distinct nonrigid characteristics and a horn splitting of approximately 120 kHz was observed. With increasing temperature, the H-2 spectrum gradually transforms into a relatively narrow and featureless peak. Spectral simulations are performed with the help of plausible motional models, and the results show that, over the whole temperature range studied, the C-1-D bonds have substantial motional freedom with respect to the characteristic H-2 NMR time scale. In addition, at each temperature, a weighted superposition of several simulated line shapes with different rates and site populations is required to account for the observed spectral features, indicating the presence of considerable motional heterogeneity within the ODPA monolayers. The results are further discussed in terms of the known characteristics of the ZrO2 surface and of the conformational transitions in the octadecyl chain.