Chapter 8, Harder Questions, Answers
Solvation
- In microsolvation, should thesolvent molecules be subjected to geometry
optimization?
Ideally, the solvent molecules, as well as the solute molecules, should be
subjected to geometry optimization in microsolvation (implicit solvation): in a
perfect calculation all components of the system, in this case the solution, would
be handled exactly. This is feasible for most quantum mechanical (AM1 or PM3,
ab initio, DFT) microsolvation calculations, since these usually use only a few
solvent molecules (see e.g. Chapter 8, [14]). Forcefield (molecular mechanics)
calculations on biopolymers surround the solute with a large number of mole-
cules when implicit solvation is used, and it may not be practical to optimize
these. - Consider the possibility of microsolvation computations with spherical, polar-
izable “pseudomolecules”. What might be the advantages and disadvantages of
this simplified geometry?
The advantages come from geometric simplicity: the orientation of the mole-
cules with respect to the solute does not have to be optimized, nor does the more
ambitious task of solute molecule geometry optimization arise.
The disadvantages stem from the fact that the only solvents that really consist of
spherical molecules are the noble gases. These are used as solvents only in quite
specialized experiments, for example:
(a) Solvent effect on the blue shifted weakly H-bound F 3 CH***FCD 3 complex:
Rutkowski KS, Melikova SM, Rodziewicz P, Herrebout WA, van der Veken
BJ, Koll A (2008) J Mol Struct 880:64
(b) Liquid noble gases as ideal transparent solvents: Andrea RR, Luyten H,
Stufkens DJ, Oskam A (1986) Chemisch Magazine (Den Haag), (January)
23 , 25. (In Dutch)
(c) Depolarization of fluorescence of polyatomic molecules in noble gas sol-
vents: Blokhin AP, Gelin MF, Kalosha I, Matylitsky VV, Erohin NP,
Barashkov MV, Tolkachev VA (2001) Chem Phys 272:69 - In microsolvation, why might just one solvent layer be inadequate?
The essential reason why one (or probably two or three) solvent layers is
not enough is that with, say, one layer the solvent molecules in contact with a
solute molecule are not “distracted’ by an outer layer and so turn their solvating
power on the solute more strongly than if they also had to interact with an outer
solvent layer (see Bachrach SM (2007) Computational organic chemistry.
Wiley-Interscience, Hoboken, NJ, chapter 6). The solute is evidently oversol-
vated. Formally, we can say thatnlayers is sufficient if going tonþ1 layers has
no significant effect on the phenomenon we are studying. Unfortunately, it is not
yet possible to computationally find this limiting value ofnfor higher-level
quantum mechanical calculations.
Answers 647