MOLECULAR-STRUCTURE OF MONOLAYERS FROM THIOL-TERMINATED POLYIMIDE MODEL COMPOUNDS ON GOLD .2. MOLECULAR-DYNAMICS SIMULATIONS

Extensive molecular dynamics simulations of organic thiol monolayers s elf-assembled on gold were performed. Interaction parameters obtained from quantum mechanical calculations were used for the sulfur-metal bo nd, The first simulations concerned C-20 alkane monolayers for compari son with experiment Then dynamics simulations of a (4-mercaptophenyl)p hthalimide (4-MPP) self-assembled monolayer on a Au(111) surface were performed. The structure and dynamics of the interphase were simulated at several surface densities and for two bonding geometries, one with the sulfur atom sp hybridized and one with sp(3) hybridization. The c omputed tilt angle for a high-density monolayer was 23 degrees for the sp-hybridized case, in good agreement with the experiments presented in the previous paper. The nature of the sulfur-metal bond strongly af fected the observed tilt angle, and the angle increased with decreasin g surface density. The distribution of rotation angles of the imide ri ng was broad. The molecules of the interphase exhibited substantial th ermal fluctuations at room temperature but did not freely rotate.