Scattering, Accommodation, and Trapping of HCl in Collisions with a Hydroxylated Self-Assembled Monolayer

Abstract

Time-of-flight molecular beam scattering techniques are used to explore the energy exchange, thermal accommodation, and residence time of HCl in collisions with an OH-terminated self-assembled monolayer. The monolayer, consisting of 16-mercapto-1-hexadecanol (HS(CH(2))(16)OH) self-assembled on gold, provides a well-characterized surface containing hydroxyl groups located at the gas-solid interface. Upon colliding with the hydroxylated surface, the gas-phase HCl is found to follow one of three pathways: direct impulsive scattering, thermal accommodation followed by prompt desorption, and temporary trapping through HO--- HCl hydrogen bond formation. For an incident energy of 85 kJ/mol, the HCl transfers the majority, >80%, of its translational energy to the surface. The extensive energy exchange facilitates thermalization, leading to very large accommodation probabilities on the surface. Under the experimental conditions used in this work, over 75% of the HCl approaches thermal equilibrium with the surface before desorption and, for a 6 kJ/mol HCl beam, nearly 100% of the molecules that recoil from the surface can be described by a thermal distribution at the temperature of the surface. For the molecules that reach thermal equilibrium with the surface prior to desorption, a significant fraction appear to form hydrogen bonds with surface hydroxyl groups. The adsorption energy, determined by measuring the HCl residence time as a function of surface temperature, is 24 +/- 2 kJ/mol.

Experimental evidence that halogen bonding catalyzes the heterogeneous chlorination of alkenes in submicron liquid droplets

“…From molecular beam scattering experiments, k des is expected to decrease with temperature following an Arrhenius relationship: as temperature decreases, desorption time lengthens and k des becomes smaller. 61 The temperature dependence of k sol is more ambiguous in prior studies. In some studies, 59,60,62 k sol is thought to be weakly temperature dependent, varying much more slowly than desorption due to the smaller activation energy for solvation.…”

“…This is consistent with the delayed desorption of gas when forming interfacial hydrogen-bonds, as reported by Morris and coworkers. 61 Thus, any decrease in k des , due to halogen-bonding, will enhance both the chlorination rate of Sqe at the interface as well as solvation rate of Cl 2(ads) , which together would account for the accelerated kinetics.…”

Energy transfer and chemical dynamics at solid surfaces: The special role of charge transfer

“…Analysis of scattering angular and TOF distributions and their dependence on initial conditions, for example initial translational energy, can be used to distinguish between trapping/desorption and direct scattering events [180][181][182] an approach that has recently been extended to atomic and molecular scattering from liquid [183][184][185][186][187] as well as selfassembled monolayer surfaces [188][189][190][191][192][193][194][195]. In similar way, Eley-Rideal reactive events can be immediately distinguished from those that follow a Langmuir-Hinshelwood mechanism [196][197][198][199][200][201][202].…”

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

“…[5][6][7] Fairbrother and co-workers have also examined the destruction of alkyl, fluorinated, and X-raymodified SAMs by a thermalized mixture of atomic and molecular oxygen (O( The first such experiment was carried out by Naaman and coworkers 10 using molecular beams of He, Ar, NO, and O 2 from alkyl and fluorinated SAMs and time-of-flight mass spectrometry (TOF-MS) to detect the scattered species. Subsequent work by Siebener and co-workers [11][12][13][14][15] and, most comprehensively, by Morris and co-workers [16][17][18][19][20][21][22][23][24][25][26] has investigated the effects of varying the identity of the impinging gas, impact angle, final angle, initial energy, monolayer temperature, SAM terminal group, alkyl chain length, and metal substrate (hence, surface density). This has led to a wealth of information on factors affecting energy transfer at the gas-SAM interface.…”