Molecular Mechanics and Dynamics Studies of Chemisorbed Monolayers of Alkanethiolates
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MOLECULAR MECHANICS AND DYNAMIICS STUDIJES OF CHEMISORBED MONOLAYERS OF ALKANETHIOLATES YITZHAK SHNIDMAN,* JAMES E. EILERS,* ABRAHAM ULMAN,** AND HARRELL SELLERS*** *Computational Science Laboratory, Eastman Kodak Company, Rochester, NY 14650 "**Corporate Research Laboratories, Eastman Kodak Company, Rochester, NY 14650 ***Department of Chemistry, South Dakota State University, Brookings, SD 57007 ABSTRACT We study ordering within closely packed monolayers of thiol derivatives self-assembled on gold substrates by chemisorption from solution. We discuss constant stress molecular mechanics and dynamics simulations of alkanethiolate monolayers on Au(1 11) and Au(100) surfaces. In the first case, we use the MM2 force field augmented with classical chemisorption parameters obtained from fitting to results of ab initio quantum mechanical calculations. In the second, the chemisorption to the Au(100) surface was accomplished by using harmonic constraints to restrict the thiolate groups to the square lattice. It is shown that two chemisorption modes of alkanethiolate on Au(1 11) exist, and that both give closely packed ordered monolayers. In the system on Au(100), rearrangement towards close-packing is achieved at the expense of distortions that increase the intrachain elastic energy. INTRODUCTION Self-assembled monolayers (SAMs) are molecular aggregates that are spontaneously formed by the adsorption of amphifunctionalmolecules on a solid surface.' Chemisorption, as well as intrachain (including bond stretches, bends and torsions, as well as nonbonded contributions) 2 and interchain (e.g., van der Waals (vdW), steric, repulsive, and electrostatic) interactions, -5 make significant contributions to the energetics of these systems. The interplay between these interactions, combined with entropic effects, determines both the conformation of the individual chains within the assembly, and their packing and ordering with respect to each other. Unfortunately, this interplay of forces is not well understood, especially in the case of assemblies6 of complex molecules, where different molecular parts prefer different packing arrangements. Such understanding is indispensable for achieving the capability of engineering assemblies with predicted architectures and material properties, starting from known molecular properties. Experiments typically provide isolated pieces of analytical information on different aspects of the system. It is not easy to combine them into a comprehensive microscopic picture of the assembly. Therefore, realistic molecular mechanics (MM) and dynamics (MD) simulations provide an important complement to such experiments and should enable one to gain a detailed, comprehensive understanding of the balance of forces determining conformation and packing at the molecular level. In this paper, we discuss MD simulations of alkanethiolate monolayers on gold surfaces. Chemisorption of alkanethiols on gold gives the Au(I) thiolate (RS-) species probably by RS-H + Aun --+ RS-Au+*Au'
+
H2 ,
(1)
as has been shown both el
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