Infrared and NMR spectroscopic studies of n-alkanethiols chemically grafted on dimethylzinc-modified silica surfaces

Transmission infrared and solid-state nuclear magnetic resonance (NMR) spec troscopies are used to study the surface species formed by sequential react ion of hydroxylated, high-surface-area silica with dimethylzinc (DMZ) and n -alkanethiols of various chain lengths. Reaction of DMZ forms mainly surfac e methylzinc species. These convert to Zn-bound ethanethiolate surface spec ies by reaction with ethane thiol, evolving nearly identical amounts of met hane at each step. Temperature-dependent infrared spectra of these surface thiolate species reveal that they are stable in air and aqueous environment s and up to about 400 K in vacuo. The major gas-phase products identified d uring annealing are diethyl sulfide and ethylene. Longer-chain n-alkanethio ls (up to C-16) are reacted in benzene solutions and the modified silica po wders are characterized by transmission infrared and solid-state NMR spectr oscopies. The spectroscopic data demonstrate that the alkanethiolate chains retain their integrity and bind to the surface via the sulfur atom with th eir "tails" away from the surface. High thiolate coverages (similar to thos e of well-characterized self-assembled monolayers of thiols on gold and tri chlorosilanes on silicon) are obtained in the case of shorter hydrocarbon c hains. Carbon-hydrogen-nitrogen analyses and electron microprobe elemental analyses reveal that the density of the thiolate layer decreases with incre asing chain length. This is attributed to the highly irregular surface geom etry and may be avoided by using planar substrates, thus providing a novel strategy for self-assembly of organic molecules on various hydroxylated sur faces.