Coordination chemistry of vitamin C. Part IV. The effects of the alkali metal ions Li + Na +, K +, NH 4+, Rb + and Cs + on the solid and solution structure of L-ascorbic acid

Abstract

The interaction of L-ascorbic acid with the alkali metal ions has been studied in aqueous solution at pH 6–7. The solid salts of Li-ascorbate·2H 2O, Na-ascorbate·H 2O, K-ascorbate·H 2O, NH 4-ascorbate, Rb-ascorbate·2H 2O, and Cs-ascorbate·2H 2O have been isolated and characterizes by 13C NMR and Fourier Transform infrared spectroscopy. Spectroscopic evidence suggested that in aqueous solution, the alkali metal ions are in the form of M(H 2O) n + with negligible amounts of direct metal-ascorbate interaction. In the solid state, the Na and K salts are similar and different from those of the Li-, NH 4, Rb-, and Cs-salts. The NH 4 cations are hydrogen bonded to the ascorbate anion donor groups, stabilizing the NH 4-ascorbate salt. The Li ions bind to the ascorbate anion O-3 or O-1, O-2, and to H 2O molecules, forming four-coordinated metal ions. The Na and K ions are in the vicinity of the O-5, O-6, and O-1, O-2, as well as O-3 and O-6 of the four different acid anions, resulting in six-coordinated metal ions. The Rb and Cs ions bind to the O-3, O-2, and O-5, O-6, as well as O-1 atoms of different ascorbate anions and to H 2O molecules, forming eight-coordination metal cations. The ascorbate anion uses most of its donor atoms to form three-dimensional polymeric structures around these metal ions. The H-bonding network of the free acid will be rearranged upon ionization. The binding arrangement of the C-1 = O group of the acid anion would be similar in the Li, Na, and K salts and differ from those of the NH 4-, Rb-, and Cs-ascorbates.