Résumé
The adsorption of neutral glycine onto amorphous silica was investigated both theoretically and experimentally. DFT calculations were performed at the BLYP-631++G** level using a cluster approach. Several possible configurations involving the formation of H-bonds between glycine and one, two or three silanols, (SiOH) were considered. The most favorable bonding of glycine with one silanol (45 kJ/mol) occurs through the COOH moiety, forming a cycle in which the CO group is H-bond acceptor whereas the acidic OH group is H-bond donor. With two or three silanols, additional H-bonds are formed between the amine moiety and the silanols, leading to an increased adsorption energy (70 and 80 kJ/mol for two and three silanols respectively). Calculated nCO, dHNH and dHCH are sensitive to the adsorption mode. A bathochromic shift of nCO as compared to nCO of free glycine (calculated in the 1755-1790 cm-1 range) is found for glycine in interaction with silanol(s). The more H-bonds of COOH with silanol groups, the higher the bathochromic shift. For dHNH, no shift is found for glycine adsorbed on one and two silanols (where the amine is either not bound or H bond donor), whereas a bathochromic shift is calculated with three silanols when the amine moiety is H-bond acceptor. Experimental FTIR spectra performed at RT for glycine adsorbed at 160°C on Aerosil amorphous silica exhibit bands at 1371, 1423, 1630 and 1699 cm-1. The experimental/calculated frequencies have their best correspondance for glycine adsorbed on two silanols. It is important to note that the forms giving the best correspondence to experimental frequencies are the most stable ones.