Handbook of Solvents - George Wypych - ChemTech - Ventech!

2.1 Solvent effects on chemical systems 25
Figure 2.1.10. Intramolecular and intermolecular proton transfer in glycine, leading from the neutral form to the
zwitterionic one.
Table 2.1.2. Relative energies (in Kcal/mol) for the complexes formed between a
molecule of water with the neutral glycine (NE·H 2O), the zwitterion (ZW·H 2O), and the
state of transition (TS·H 2O), as well as for the neutral glycine system and independent
molecule of water (NE+H 2O), obtained with a base HF/6-31+G**
NE·H2O TS·H2O ZW·H2O NE+H2O
0 29.04 16.40 4.32
the supermolecule calculation, the intramolecular mechanism will still continue as the preferred
in the autoionization of the glycine. The model of supermolecule has been useful to
us, along with the competition of other models, 54,55 to shed light on the mechanism by which
an aminoacid is autoionized in aqueous solution.
If the calculations are done again but this time including the presence of the solvent by
means of a continuum model (implemented by means of a dielectric constant = 78.4; for the
water), the situation becomes different. 55,58-60 Now it is the conformation closest to the zwitterionic
(II) which is the most stable, by some 2.7 Kcal/mol. This could be explained on the
basis of the greater dipolar moment of the structure (6.3 Debyes for the conformation II
compared to 1.3 of the conformation I).
The next step was to reproduce the formation of the zwitterion starting with the most
stable initial structure in the midst of the solvent (II). Using the continuum model, we observe
how the structure II evolves towards a state of transition which, once overcome, leads