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 (T,P) 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.