The Transition State for Carboxylic Acid Deprotonation on Cu(100)

Abstract

The kinetics of acid deprotonation on the Cu(100) surface have been studied using four different fluorinated carboxylic acids (CF2HCO2H, CF3CO2H, CF2HCF2CO2H, and CF3CF2CO2H). All four acids adsorb molecularly on Cu(100) at 90 K but deprotonate during heating to form carboxylates on the surface at temperatures below 300 K. Temperature-programmed reaction spectroscopy and X-ray photoemission spectroscopy were used to verify that the acids deprotonate on the Cu(100) surface. Work function measurements were used to study the deprotonation kinetics during heating and to estimate the activation energy barriers ( ) to deprotonation. The nature of the transition state to acid deprotonation on Cu(100) was probed by analysis of linear free energy relationships (LFER) or correlations of with the acid substituent constants, σF. The field reaction constant, ρF, or the slope of the LFER was found to be ρF = −52 ± 13 kJ/mol in the limit of zero coverage and ρF = −19 ± 2 kJ/mol at a coverage of 1/2 ML. These values of ρF can be compared to the value of ρF = −104 kJ/mol for acid deprotonation in the gas phase. This comparison suggests that the transition state for acid deprotonation on Cu(100) must be anionic with respect to the reactant (RCO2H(ad) ↔ [RCO2δ-···Hδ+]‡ → ).