Molecular dynamics analysis of the dipole moment and conformational properties of 2-(acetyloxy)ethyl-2-(2-naphthyl)acetate

Abstract

A sample of 2-(acetyloxy)ethyl-2-(2-naphthyl)acetate (ANA), model compound of the side group of poly(2-{[2-(2-naphthyl)-acetyl]oxy}ethyl acrylate) (PNAEA), has been synthesized and its mean-squared dipole moment measured in dioxane solutions at 30 °C, providing an experimental result of 〈µ²〉 = 5.28 D². Molecular dynamics simulations, performed with the Amber force field, the DL_–POLY package and the charge distribution provided by MOPAC, gives a theoretical value 〈µ²〉 = 5.69 D², in good agreement with experience. The analysis of the MD trajectories indicates that the preferred orientations for the C^ar–CH₂ bond are those in which the plane defined by the pair of bonds C^ar–CH₂–C* is roughly perpendicular to the aromatic group (i.e.ϕ ≈ ±90°) while trans orientation of the CH₂–C* bond is strongly disfavoured (by ca. 1.5 kcal mol^–1) versus gauche. In the O–CH₂–CH₂–O segment, the O–CH₂ bonds show a strong preference for trans while CH₂–CH₂ prefers gauche and does not require any kind of adjustment a posteriori to account for the so called ‘gauche effect’. Thus, the first-order conformational energies representing the stability of gauche versus trans are E_σ ≈ 1.2, E_σ1 ≈ –0.9 kcal mol^–1 respectively for O–CH₂ and CH₂–CH₂. Second-order interactions E_ω and E_γ respectively controlling the stability of opposite (i.e. g^±g^±) and identical (i.e. g^±g^±) combinations of gauche states over the pair of bonds O–CH₂–CH₂ are both negative (ca. E_ω ≈ –1, E_γ ≈ –0.6 kcal mol^–1). Values of the mean-squared dipole moment computed by adding contributions produced by two ester groups with this set of conformational energies are in very good agreement with the results obtained by the actual MD simulations.