Infrared and NMR Spectroscopic Studies of n-Alkanethiols Chemically Grafted on Dimethylzinc-Modified Silica Surfaces

Abstract

Transmission infrared and solid-state nuclear magnetic resonance (NMR) spectroscopies are used to study the surface species formed by sequential reaction of hydroxylated, high-surface-area silica with dimethylzinc (DMZ) and n-alkanethiols of various chain lengths. Reaction of DMZ forms mainly surface methylzinc species. These convert to Zn-bound ethanethiolate surface species by reaction with ethane thiol, evolving nearly identical amounts of methane at each step. Temperature-dependent infrared spectra of these surface thiolate species reveal that they are stable in air and aqueous environments and up to about 400 K in vacuo. The major gas-phase products identified during annealing are diethyl sulfide and ethylene. Longer-chain n-alkanethiols (up to C16) are reacted in benzene solutions and the modified silica powders are characterized by transmission infrared and solid-state NMR spectroscopies. The spectroscopic data demonstrate that the alkanethiolate chains retain their integrity and bind to the surface via the sulfur atom with their “tails” away from the surface. High thiolate coverages (similar to those of well-characterized self-assembled monolayers of thiols on gold and trichlorosilanes on silicon) are obtained in the case of shorter hydrocarbon chains. Carbon−hydrogen−nitrogen analyses and electron microprobe elemental analyses reveal that the density of the thiolate layer decreases with increasing chain length. This is attributed to the highly irregular surface geometry and may be avoided by using planar substrates, thus providing a novel strategy for self-assembly of organic molecules on various hydroxylated surfaces.