Low-Energy Collisions of Pyrazine and d₆-Benzene Molecular Ions with Self-Assembled Monolayer Surfaces:  The Odd−Even Chain Length Effect

Abstract

The low-energy reactive collisions of two independent probe ions, pyrazine and d 6-benzene, illustrate an odd−even hydrocarbon chain length effect for a wide range of hydrocarbon self-assembled monolayer (SAM) surfaces. SAM surfaces prepared from alkanethiols ranging from CH3(CH2)10SH to CH3(CH2)17SH chemisorbed to polycrystalline gold are shown to exhibit an odd−even effect where the orientation of the terminal methyl group determines the reaction behavior of the thin film. X-ray photoelectron spectroscopy shows the surfaces to be homogeneously covered and provides evidence for the presence of a thiolate (Au−SR) on the SAM surface. When 30 eV incident ions were used, the extent of hydrogen addition to the incident probe ion was larger for odd carbon chain length SAM surfaces when compared to the even chain length films. For an odd chain length SAM surface, the terminal methyl group exposes a C−H bond perpendicular to the surface, increasing hydrogen addition reactivity for the probe ions. The low-energy probe ions also reacted with the surfaces to form alkyl addition products. The alkyl addition products also showed an odd−even effect. In this case, the even chain length SAM surfaces were more reactive than the odd chain length surfaces. For an even SAM surface, the terminal C−C bond is oriented quasi-perpendicular to the Au surface, allowing more direct access to the carbon atom on the terminal methyl group and increasing its reactivity.

Even−Odd Orientation and Chain-Length Effects in the Energy Exchange of Argon Collisions with Self-Assembled Monolayers

“…17,18 Their work shows that energy transfer to a surface is markedly greater for a monolayer-covered surface than it is for a bare gold surface and that penetration into the monolayer, even for greater than 10 eV ions, is limited to the topmost two-to-three layers of carbon atoms. Wysocki and coworkers, 13 and Bernasek et al, 14 have also investigated the reaction dynamics of ion-surface scattering using SAMs. This set of work has provided important insight into how end-group orientation and the overall chain-length of a monolayer influences the ion-surface reaction dynamics.…”

“…The energy exchange dynamics of gas−surface collisions involving atomic gases and organic surfaces have been the subject of recent studies. − However, the often complex nature of organic surfaces has limited the number of detailed investigations into gas−surface energy transfer. A viable strategy for addressing this challenge is to employ self-assembled monolayers (SAMs) of alkanethiols on gold to create stable, well-ordered, and reproducible organic surfaces.…”

“…In addition to this work, several investigators including Cooks et al have explored the characteristics of high-energy ionic collisions on model organic surfaces using a variety of ω-functionalized SAMs. , Hanley and co-workers have combined experimental and computational approaches to study the energy transfer of SiMe 3 + on a 6-carbon alkanethiol SAM. , Their work shows that energy transfer to a surface is markedly greater for a monolayer-covered surface than it is for a bare gold surface and that penetration into the monolayer, even for greater than 10 eV ions, is limited to the topmost two-to-three layers of carbon atoms. Wysocki and co-workers, and Bernasek et al, have also investigated the reaction dynamics of ion-surface scattering using SAMs. This set of work has provided important insight into how end-group orientation and the overall chain-length of a monolayer influences the ion-surface reaction dynamics.…”

Odd‐Even Effects in Ion‐Beam‐Induced Desorption of Biphenyl‐Substituted Alkanethiol Self‐Assembled Monolayers

“…CH 3 or CF 3 ) of aliphatic SAMs/Au(111). [4][5][6] Thus, a possibility of using low-energy molecular ion beams for analysis of the molecule-vacuum interface in SAMs was clearly demonstrated.…”

The potential impact of biogenic emissions of isoprene on urban chemistry in the United Kingdom

“…In particular, the odd‐even effect was observed in the reaction of low‐energy pyrazine and [D 6 ]benzene molecular ions with the terminal group (e.g. CH 3 or CF 3 ) of aliphatic SAMs/Au(111) 4–6. Thus, a possibility of using low‐energy molecular ion beams for analysis of the molecule–vacuum interface in SAMs was clearly demonstrated.…”