Computational and NMR study of quaternary ammonium ion conformations in solutionстатья
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Аннотация:Conformations around C–N bonds at the quaternary centre in tetraalkylammonium ions in water solution are
investigated. Structures of Me 4 N + , Et 4 N + , n-Pr 4 N + , n-Bu 4 N + , and n-Pe 4 N + 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 Et 4 N + , D 2d and S 4 , 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 Et 4 N + are 3.6–4.0 kcal mol 1 higher. The ions with longer hydrocarbon chains show quite
similar energy gap between D 2d and S 4 . The torsion barrier for a two-step interconversion between the D 2d and
S 4 structures is 9.5 kcal mol 1 (HF/6-31G(d) calculations). The computational results are augmented by NMR
measurements of the Et 4 N + –I salt in aqueous solution, which predict a symmetric structure of Et 4 N + in water.
However, the D 2d and S 4 conformers are not discernible due to presumably high similarity of chemical shifts.
The calculated conformational energetics in solution together with previously observed D 2d , S 4 and high-energy
g + g -type structures of Et 4 N + , n-Pr 4 N + , and n-Bu 4 N + in the solid state indicate that the carbon chain
conformations at the quaternary ammonium centre sensitively depend on the actual microenvironment.