L-Prolinium l-proline halogenides: a comparison of the respective chloride, bromide and iodide salts

Abstract

• # Chloride, bromide and iodide salts with l -prolinium l -proline dimeric cation were studied • # Tetragonal chloride and bromide are isostructural, while iodide is orthorhombic • # Short O-H…O hydrogen bonds of dimeric cations are reflected in the infrared spectra • # Isostructural chloride and bromide are not SHG-active in agreement with Kleinman condition Structures and vibrational spectra of l -prolinium l -proline chloride (I), l -prolinium l -proline bromide (II) and l -prolinium l -proline iodide (III) were analyzed at 100K, 200K and room temperature (RT). The salts (I) and (II) are isostructural and crystallize in the tetragonal space group P 4 1 2 1 2, Z=8 at RT and in orthorhombic P 2 1 2 1 2 1 , Z=8 at 100K and 200K. The salt (III) is orthorhombic, space group P 2 1 2 1 2 1 , Z=4 at all three temperatures. The O⋅⋅⋅O distances in l -prolinium l -proline dimeric cations are equal to 2.464(3) Å (I), 2.456(3) Å (II) and 2.456(4) Å (III) at RT. The presence of strong hydrogen bonds in the crystal structures is reflected in the infrared spectra. The influence of symmetry on second harmonic activity is discussed.