Rigorous ab initio (STO-3G) quantum chemical calculations were performed on various types of pyridinium hydrochloride geometries. The energy-minimized hydrogen-bonded complex is indicated to exist in a single well potential with N-Cl distance of 2.8 Å and N-H distance of 1.4 Å. In accord with the energetics of complex formation, this result indicates that the hydrogen-bonded complex in the gas phase resembles the separated molecules more than the separated ions. The hydrogen- bonding stabilization energy is some 17 kcal/mol. The energy minimized stacked complex places the Cl∼ unsymmetrically, very close to the ortho carbon environment of the ring with distances of the Cl- from the ring atoms being much less than the sum of van der Waals radii. The hydrogen-bonded complex is predicted to be much more stable than the stacked one. The changes in charges and orbital energy levels on complex formation are discussed. A rough estimate of the dispersion energies indicates that these complexes are held together primarily by electrostatic interactions, but that the dispersion interactions are much more significant in the stacked complex. In the gas phase (or inert low dielectric solvents) the hydrogenbonded complex should be the predominant species.
All Science Journal Classification (ASJC) codes
- Colloid and Surface Chemistry