The insertion of amines and alcohols into proton bound dimers: a density functional study

Abstract

Insertion complexes of various bases with the protonated acetonitrile and acetone dimers have been studied using density functional methods including exact exchange contributions. The insertion mechanism has been investigated using an intrinsic reaction coordinate calculation. For some thermochemical quantities, calibration studies using larger basis sets and coupled cluster methods have been carried out. We find that B3LYP/cc-pVDZ will somewhat overestimate association energies due to basis set incompleteness error, which is partially compensated by an opposite error in the correlation treatment. B3LYP/4-21G will yield qualitatively correct structures, which PM3 and HF/4-21G generally do not, yielding instead asymmetric insertion complexes. The insertion energy increases with increasing proton affinity of the inserting base, while the association energy between the protonated central base and the ligands decreases. For the insertion complexes of the acetone dimer, the conformational equilibrium shifts from syn-syn to syn-anti with increasing proton affinity. The re geometries of protonated acetone dimer and its complexes are found to exhibit slight deviations from their intuitive symmetry that the calculations predict will be absent in the r0 geometries. Computed association and switching enthalpies are in very good agreement with experiment, while proton transfer enthalpies fare less well due to the change in hydrogen bond number involved. Geometries and vibrational frequencies for all structures considered are available as Supporting Information to the paper.