Microsolvation of glycine by silanol ligands: A DFT study

The interaction of glycine with SiOH silanols, which cover the surface of amorphous silica are modeled and investigated by means of the DFT B3LYP/6-31++G** method. The neutral and zwitterion form of glycine are considered. Cooperative hydrogen bonding, with one or several silanol groups, are studied. Different scenarii are modeled: adsorption from the gas phase at 0 and 430 K and adsorption of the microsolvated glycine. It is shown that hydroxylated silica does not act to stabilize the zwitterion form of glycine and thus cannot be considered as a "solid solvent": glycine remains more stable in the neutral state when bonded up to 3 silanols. In contrast, if water molecules are adsorbed on the silanols, they act as co-stabilizors of the zwitterionic state. Two distinct domains exist in the (TP) phase diagram: in the first one, glycine is stabilized on silanols and in the second, glycine is more stable in the gas phase. Vibration frequencies are calculated and compared with experimental ones. (c) 2006 Elsevier B.V. All rights reserved.