Proton affinities of aldehyde molecules determined from the forward and backward gas-phase proton transfer reactions in a selected ion flow-drift tube†

Abstract

Proton affinity (PA) and gas-phase basicity (GB) are important thermodynamic properties that provide insights into ion–molecule interactions. Aldehydes play a significant role in biology, the environment, and industry, but their PAs remain unknown for those with more than 5 C atoms. This study aims to experimentally determine PAs and GBs of hexanal, heptanal, and octanal using pentanal as a reference. A selected ion flow drift tube (SIFDT) was used to study proton transfer reactions among all possible combinations of protonated and neutral molecules from this set. Rate coefficients (k), equilibrium constants (K), and effective temperatures (T_eff) were used to calculate Gibbs free energy changes (ΔG) and enthalpy changes (ΔH). PAs and GBs were then determined relative to the known values of pentanal. Experimental PAs were found to increase with aldehyde chain length: pentanal 796.6 kJ mol⁻¹ < hexanal 809.6 kJ mol⁻¹ < heptanal 813.4 kJ mol⁻¹ < octanal 824.0 kJ mol⁻¹. Theoretical enthalpies and entropies were obtained via density functional theory (DFT) B3LYP/6-311++G(d,p) with D4 dispersion correction for both open and bent protonated structures, allowing comparison with experimental data. The theoretical calculations for open structures underestimate the observed PAs, while the bent structures align more closely with experimental trends, indicating that larger protonated aldehydes may have bent and cyclic shapes. These findings contribute to bridging the gaps in knowledge about protonated aldehydes, providing a better understanding of their ion chemistry.