Halide effect in L(+) glutamic acid halogen acid salts

L(+) glutamic acid halogen acid salts [HOOC(CH ₂) ₂CH(NH ₂)COOH·H X], with X = Cl, Br, I, and mixed crystals of the hydrochloride and hydrobromide were studied by crystallographic, optical, and calorimetric methods. All three salts are isostructural, crystallizing in the orthorhombic space group P2 ₁2 ₁2 ₁. The linear polarizability follows the general scheme I > Br > Cl but the specific refractive energy contributions of the halides to the refractive indices are less than predicted by factors of 12 and 2, respectively. The first nonlinear polarizability follows the trend Cl = Br > I and is ascribed to the electronic nature of the nonlinear optical polarizability of the hydrogen bonds. The anomalous behavior of the hydroiodide is attributed to the different molecular spatial arrangement.