Experimental and theoretical determination of electronic properties in l-dopa

Abstract

(2S)-3-(3',4'-Dihydroxyphenyl)alanine (L-dopa), C 9 H 11 NO 4 , M r =197.19, monoclinic, P2 1 , a=13.619(6), b = 5.232(2), c = 6.062 (3) A, β = 97.56 (4)°, V = 428.191A 3 , Z = 2, D x = 1.529 g cm -3 , D m = 1.515 g cm -3 (T = 293 K), λ(MoKα) = 0.71069 A, μ = 1.2 cm- 1 , F(000) = 208, T=173K, R(F) = 0.017 for 4208 reflections with sin θ/λ < 1.078 A- 1 . The electron distribution has been determined by multipole refinement with the Hansen/Coppens aspherical scattering factor expansion, including multipole terms up to octopoles for C, N and O and up to dipoles for H. The molecular dipole moment was determined as 12(2)D, within an e.s.d. of the ab initio value reported here of 11 D. The bond critical-point properties of the total electron density were determined, giving negative values for ⊇ 2 ρ c consistent with covalent bonds, and are in fair agreement with the ab initio results. An analysis of the hydrogen-bond critical points gave small positive ⊇ 2 ρ values, consistent with ionic, closed-shell interactions between the participating atoms. A set of theoretical structure factors was generated from the ab initio charge distribution and subjected to multipole refinement, to enable a more detailed comparison with experiment.