Effect of hydration on the kinetics of proton-bound dimer formation: experimental and theoretical study

A kinetic study was performed on the proton-bound dimer formation of cyclopentanone, cyclohexanone, and cycloheptanone at atmospheric pressure with ion mobility spectrometry (IMS) at the temperature range of 30 to 70 °C. Measured rate constants were in the range of 9.5 × 10(-11) to 4.5 × 10(-10) cm(3) s(-1). Rate constants were also calculated using average dipole orientation (ADO) theory employing density functional theory (DFT). Calculated rate constants were in the range of 1.0-5.5 × 10(-9) cm(3) s(-1). The difference between experimental and calculated rate constants was interpreted based on the hydration of the protonated monomers so that water molecules were replaced with a neutral monomer molecule in the process of dimer formation. This process requires activation energy for the formation of dimer and consequently reduces the rate constants. To verify our hypothesis, an effective rate constant (keff) was introduced, which accounted for the energetically activated water-monomer replacement in the dimer formation reactions. A good agreement was observed between the experimental rate constants and calculated keff, confirming the validity of the proposed model in explaining the kinetics of dimer formation in atmospheric pressure.