Experimental and theoretical proton affinity of limonene

Abstract

Gas-phase basicity (GB) and proton affinity (PA) of limonene were derived from measurements of proton transfer equilibria carried out by high pressure pulsed electron beam source mass spectrometry. Experimental GB and PA are 842±5 kJ mol^-1 and 875±5 kJ mol^-1, respectively. The proton affinity of C₁₀H₁₆ is compared to abinitio (HF and MP2) and density functional predictions for the protonation energy. Theoretical calculations based on density functional theory are in very good agreement with experimental results. Our best theoretical predictions for the enthalpy of protonation range from 869.6 kJ mol^-1 (B3PW91/6-31G*) to 873.9 kJ mol^-1 (BLYP/6-31G*). Theoretical calculations also suggest an important reorganisation of the molecular structure and charge distribution of limonene after protonation.