Computational and NMR study of quaternary ammonium ion conformations in solution

Abstract

Conformations around C–N bonds at the quaternary centre in tetraalkylammonium ions in water solution are investigated. Structures of Me₄N⁺, Et₄N⁺, n-Pr₄N⁺, n-Bu₄N⁺, and n-Pe₄N⁺ are calculated using quantum mechanical HF and DFT methods together with the PCM solvent model. Relative solvation free energies of tetraalkylammonium ions are further estimated from microscopic molecular dynamics free energy perturbation simulations using the Gromos-87 and Amber-95 force fields. The predicted free energy difference in solution between two stable conformations of Et₄N⁺, D_2d and S₄, is 0.6–1.0 kcal mol⁻¹ (in favour of D_2d), which is in quantitative agreement with the recent Raman spectroscopy results. The energies of the g⁺g⁻ conformations of Et₄N⁺ are 3.6–4.0 kcal mol⁻¹ higher. The ions with longer hydrocarbon chains show quite similar energy gap between D_2d and S₄. The torsion barrier for a two-step interconversion between the D_2d and S₄ structures is 9.5 kcal mol⁻¹ (HF/6-31G(d) calculations). The computational results are augmented by NMR measurements of the Et₄N⁺–I⁻ salt in aqueous solution, which predict a symmetric structure of Et₄N⁺ in water. However, the D_2d and S₄ conformers are not discernible due to presumably high similarity of chemical shifts. The calculated conformational energetics in solution together with previously observed D_2d, S₄ and high-energy g⁺g⁻-type structures of Et₄N⁺, n-Pr₄N⁺, and n-Bu₄N⁺ in the solid state indicate that the carbon chain conformations at the quaternary ammonium centre sensitively depend on the actual microenvironment.