Charge-density analysis of hydrogen-bonded complexes of glycine by the maximum entropy method

Charge-density analysis of hydrogen-bonded complexes of glycine is undertaken in order to elucidate the chemical nature of very strong hydrogen bonds present in these complexes. We have chosen two glycine complexes triglycine sulphate (TGS) and Bis (glycinium) oxalate (DGO) which are known to exhibit very strong hydrogen bonding. Electron density maps demonstrate the covalent nature of very strong O–H...O bond found in DGO and TGS complexes. Strong hydrogen bonds in TGS have significant covalent character. This charge-density analysis confirmed the existence of the bifurcated acceptor hydrogen bond which is responsible for the coupling between the two components of structural disorder found at high temperatures (T > 49 °C) in TGS crystals, namely disorder of amino group of one of the glycine ions (GI) and the hydrogen atom disorder between the carboxylic group of other two glycines GII and GIII. charge density studies have demonstrated that hydrogen bonds are more convincingly reproduced by the model independent maximum entropy method (MEM) than by multipole models, hence we have used MEM procedure in our charge-density analysis.