The kinetics of formation of self-assembled monolayers (SAMs) on gold generated by the adsorption of 2,2-dipentadecylpropane-1,3-dithiol (d - C17, [CH3(CH2)14]2C[CH2SH]2), 2-pentadecylpropane-1,3-dithiol (m - C17, CH3(CH2)14CH[CH2SH]2), and heptadecanethiol (n -C17, CH3(CH2)16SH) from 1 mM solutions in isooctane were explored. A series of systematic coadsorption studies involving these adsorbates was also examined. Similarly, the kinetics of adsorption of the corresponding disulfides, 4,4-dipentadecyl-1,2-dithiolane (d - C17SS, [CH3(CH2)14]2C[CH2S]2), 4-pentadecyl-1,2-dithiolane (m - C17SS, CH3(CH2)14CH[CH2S]2), and diheptadecyl disulfide (n -C17SS, [CH3(CH2)16S]2), were explored. The kinetics were monitored by optical ellipsometry, contact angle goniometry, and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). For the formation of densely packed SAMs, the data showed two kinetic adsorption regimes:  a fast initial regime in which ∼80−90% of the monolayer was formed, followed by a slower orientational ordering regime in which the alkyl chains became more densely packed and highly crystalline. In contrast, the formation of loosely packed SAMs exhibited a single rapid adsorption regime with little subsequent change. A comparison of the kinetic and coadsorption data generated from the various adsorption studies was interpreted using a steady-state kinetic model involving an initial steady-state physisorption, followed by a chemisorption step, leading ultimately to complete monolayer formation. The relative rates of adsorption in the slow ordering regime were perhaps additionally influenced by the rates of diffusion of the adsorbates through the partially formed monolayer films.

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The adsorption of a series of 1,2-bis(mercaptomethyl)-4,5-dialkylbenzenes (1), spiroalkanedithiols (2), and aliphatic dithiocarboxylic acids (3) on gold yielded new types of self-assembled monolayers (SAMs). The new SAMs were characterized by optical ellipsometry, contact angle goniometry, and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). These results were compared to those obtained from SAMs derived from normal alkanethiols (4) of analogous chain length. Comparisons of ellipsometric thickness and contact angle wettability showed that the new SAMs were well packed and highly oriented. Comparisons of the PM-IRRAS data showed that the SAMs generated from 1 and 2 exhibited slightly less crystallinity than their alkanethiolate-based analogs, while the SAMs generated from 3 exhibited comparable crystallinity to their alkanethiolate-based analogs. Moreover, the SAMs derived from 1 and 2 showed no "odd-even" wettability or PM-IRRAS effects, while those derived from 3 showed remarkably large "odd-even" effects compared to those derived from normal alkanethiols.

Metal nanoparticle catalysts functionalized with small, well-defined organic ligands are important because such systems can provide a spatial control in the catalyst-substrate interactions. This article describes the synthesis, stability, and catalytic property evaluations of four different Pd nanoparticles capped with constitutional isomers of pentanethiolate ligands that have either a straight chain or an alkyl chain with one methyl group at different locations (α, β, or γ from the surface-bound sulfur). The structure and composition analyses of Pd nanoparticles confirm that they have similar average core sizes and organic ligand contents. The presence of methyl group at α position is found to lower the capping ability of short ligands and thus make Pd nanoparticles to lose their colloidal stability during the catalytic reactions. The overall activity and selectivity for hydrogenation and isomerization of pentene and allylbenzene derivatives are investigated for each combination of ligand and substrate. Catalysis results indicate that steric interactions between the ligands on the metal catalyst surface and the alkene substrates are a factor in controlling the activity of Pd nanoparticles. In particular, Pd nanoparticles capped with pentanethiolate isomer having a methyl group at α position exhibit poor and inconsistent catalytic activity due to its low colloidal stability. The presence of a methyl group at β position mildly interferes the adsorption of alkene group on the nanoparticle surface resulting in lower conversion yields. Interestingly, a methyl group at γ position only has a minimal effect on the catalytic property of Pd nanoparticles exhibiting similar catalysis results with Pd nanoparticles capped with straight chain pentanethiolate ligands. This indicates the proximity of steric group at the reactive site controls the nanoparticle activity for surface oriented reactions, such as hydrogenation and isomerization of alkenes in addition to their colloidal stability.

Selective hydrogenation of olefins is an important process in both chemical and pharmaceutical industries. This chapter reviews intriguing catalytic studies accomplished by employing a variety of catalysts such as metal complexes, supported materials, supported metal complexes, and nanosized materials for polyene hydrogenation. In addition, new research area involving unsupported colloidal nanoparticle catalysts, which exhibit an excellent activity and selectivity toward the polyene hydrogenation is introduced. The high activity of colloidal metal nanoparticle catalysts often allows the reactions to be completed under mild conditions, at atmospheric pressure, and room temperature. These colloidal nanoparticle catalysts also offer an advantage of facile separation and multiple recycling without significant losses in activity and selectivity. This chapter provides important fundamental understandings on the influence of chemical environments (solvents, ligands, dopants, etc.) and compositions (metal complex, metals, alloys, etc.) toward the catalytic activity and selectivity of various catalysts in homogeneous, heterogeneous, and semi-heterogeneous conditions. The systematic evaluation discussed in this chapter would pave a way to further develop chemo-, regio-, and stereo-selective catalysts for polyene hydrogenation.