Handbook of Solvents - George Wypych - ChemTech - Ventech!

2.1 Solvent effects on chemical systems 23
Figure 2.1.7. The Solvent Excluding Surface of a particular solute is the envelope of the volume excluded to the
solvent considered as a whole sphere that represents its charge density.
ertheless, when studying systems in solution, the goodness of the results grows with the
number of solvent molecules included in the calculation, but so also does the computational
effort. Because of this, the main limitation of the supermolecule model is that it requires
computational possibilities which are not always accessible, especially if it is desired to
carry out the quantum mechanical calculations with a high level of quality. This problem is
sometimes resolved by severely limiting the number of solvent molecules which are taken
into account. However, this alternative will limit our hopes to know what is the global effect
of the solvent over the molecules of solute, specifically the far reaching interactions solute-solvent.
Where the supermolecule model is effective is in the analysis of short distance
interactions between the molecules of the solute and those of the solvent, provided a sufficient
number of solvent molecules are included in the system being studied to reproduce the
effect being studied and provided that the calculation can be tackled computationally. 49 In
this manner the supermolecule model takes the advantage over other models of modelling
the solute-solvent interaction, which is the case of the continuum models. Recent advances
are removing the boundaries between the different computational strategies that deal with
solvent effects. A clear example is given by the Car-Parrinello approach where a quantum
treatment of the solute and solvent molecules is used in a dynamic study of the system. 50
2.1.3.5 Application example: glycine in solution
A practical case to compare the advantages obtained with the utilization of the different
models can be found in the autoionization of the aminoacids in aqueous solution. The chemistry
of the aminoacids in an aqueous medium has been at the forefront of numerous studies,
51-57 due to its self-evident biological interest. Thanks to these studies, it is well known
that, whilst in the gas phase the aminoacids exist as non-autoionized structures, in aqueous
solution it is the zwiterionic form which is prevailing (Figure 2.1.8).
This transformation suggests that when an aminoacid molecule is introduced from
vacuum into the middle of a polar medium, which is the case of the physiological cellular
media, a severe change is produced in its properties, its geometry, its energy, the charges
which its atoms support, the dipolar molecular moment, etc., set off and conditioned by the
presence of the solvent.
Utilizing Quantum Mechanics to analyze the geometry of the glycine in the absence of
solvent, two energy minima are obtained (Figure 2.1.9), differentiated essentially by the rotation
of the acid group. One of them is the absolute energy minimum in the gas phase (I),
and the other is the conformation directly related with the ionic structure (II): The structure
which is furthest from the zwiterionic form, I, is 0.7 Kcal/mol more stable than II, which