Dynamics of the Reaction of O(³P) Atoms with Alkylthiol Self-assembled Monolayers

Abstract

We have studied the dynamics of the reactions of O( 3 P) atoms with alkylthiol self-assembled monolayers (SAMs). Superthermal O( 3 P) atoms, with a fairly broad distribution of laboratory-frame kinetic energies (mean ) 16 kJ mol -1 , fwhm ) 26 kJ mol -1 ), were generated by 355 nm photolysis of NO 2 introduced at a low pressure above the SAM surface. Nascent OH V′ ) 0 products were detected by laser-induced fluorescence. SAMs of two different alkyl chain lengths, C 6 and C 18 , were studied. The existence of SAM layers, and their robustness under our experimental conditions during the relevant measurement period, were confirmed by scanning-tunneling microscopy (STM). Reaction at the SAM surface was verified as the authentic source of the hydroxyl radicals using a perdeuterated C 6 D 13 -SAM sample. The OH appearance profiles as a function of photolysis-probe delay, and the rotational-state distributions at their peaks, were compared with those of liquid squalane (C 30 H 62 , 2,6,10,15,19,23-hexamethyltetracosane). The reactivity of the SAMs and of squalane was found to be comparable. We conclude that the O( 3 P) atoms must be able to access the more reactive secondary hydrogen atoms along the alkyl chains of the SAMs. We find no perceptible differences in reactivity or product energy disposal between the two SAM chain lengths. Both produce a substantial fraction of the OH with relatively high velocities, which must result from direct, impulsive reaction. There is also a slower component, with velocities consistent with a thermal, trapping-desorption mechanism. The proportion of this component appears to be lower for SAMs than for squalane. This would be compatible with the expected greater smoothness of the SAM surface at the molecular scale. We find little evidence for significant rotational excitation of the OH products, although the details of any correlation between translational and rotational energy release require further investigation. We compare our results with the limited available prior theoretical modeling of O( 3 P) + SAM systems.

Determining the composition of the vacuum–liquid interface in ionic-liquid mixtures

“…Our previous work on reactions of O( 3 P) with thioalkyl self-assembled monolayers, for example, has shown the reduced overall reactivity of the SAMs due to the preferential exposure of the less-reactive terminal methyl group. 73,74 Nevertheless, this effect appears to be quite modest for…”

Ionic Liquid–Vacuum Interfaces Probed by Reactive Atom Scattering: Influence of Alkyl Chain Length and Anion Volume

“…To account for this effect, the experimental density profiles have been compared to simulated density profiles with thermal OH Maxwell-Boltzmann speed distributions corresponding to different values of the assumed temperature, accounting through a Monte Carlo method for geometric and other experimental broadening effects, as we have described previously. 48 Profiles corresponding to values of T between 300 and 2200 K, in intervals of 100 K, were simulated and the closest fit to each experimental profile was selected. Translational temperatures for RTILs typically fell between 1300 K and 1700 K, while squalane had values of T ~ 1200 K. The required density-flux transformation was then carried out rigorously on the simulated profiles.…”

“…The LIF-based approach has been applied to squalane (2,6,10,15,19,23-hexamethyltetracosane), widely used as a standard in this field, and other long-chain hydrocarbons and to related self-assembled monolayers. [43][44][45][46][47][48][49][50] The molecular beam approach with mass spectrometric detection was also first applied to squalane and extended to other related surfaces. 9,51-54 5 RAS-LIF detects exclusively the OH products of hydrogen abstraction as they scatter into the gas phase.…”

Experimental and theoretical studies of the interaction of gas phase nitric acid and water with a self-assembled monolayer

“…Support for this hypothesis is found in the impact of HNO 3 on the order/disorder of the monolayer. Decreases in the intensity of C-H stretching vibrations for the SAM in the 2800-3000 cm À1 region can indicate either reaction with removal of some of the SAM, or increased disorder 54,55,102,103,106 in the organic monolayer. Although it is possible that HNO 3 reacts with the unsaturated portion of the SAM, peaks due to expected reaction products (e.g., organic nitrates or carbonylcontaining compounds) were never observed in the IR spectra.…”