Handbook of Solvents - George Wypych - ChemTech - Ventech!

2.2 Molecular design of solvents 37
proach the properties of the molecule. If the molecular system in question consists of an isolated
free molecule, then it is “molecular design”. If the properties are of complex
macroscopic nature, then it is “material design”. Problem remains in the intermediate between
the above two. Because fundamental properties shown by the ensemble of molecules
are not always covered properly by the above two types of design. This is because the molecular
design is almost always based on quantum chemistry of free molecule and the material
design relies too much on empirical factor at the present stage. When we proceed to
molecular ensemble (mainly liquid phase), as the matter of fact, we must use statistical mechanics
as the basis of theoretical approach.
Unfortunately, statistical mechanics is not familiar even for the large majority of
chemists and chemical engineers. Moreover, fundamental equations in statistical mechanics
cannot often be solved rigorously for complex systems and the introduction of approximation
becomes necessary to obtain useful results for real systems. In any theoretical
approach for molecular ensemble, we must confront with so-called many-body problems
and two-body approximations must be applied. Even in the frameworks of this approximation,
our knowledge on the intermolecular interaction, which is necessary in statistical mechanical
treatment is still poor.
Under such a circumstance, numerical method should often be useful. In the case of
statistical mechanics of fluids, we have Monte Carlo (MC) simulation based on the Metropolis
scheme. All the static properties can be numerically calculated in principle by the MC
method.
Another numerical method to supplement the MC method should be the numerical integration
of the equations of motion. This kind of calculation for simple molecular systems
is called molecular dynamics (MD) method where Newton or Newton-Euler equation of
motion is solved numerically and some dynamic properties of the molecule involved can be
obtained.
These two methods are invented, respectively, by the Metropolis group (MC, Metropolis
et al., 1953) 1 and Alder’s group (MD, Alder et al., 1957) 2 and they are the molecular
versions of computer experiments and therefore called now molecular simulation. 3 Molecular
simulation plays a central role in “molecular ensemble design”. They can reproduce
thermodynamics properties, structure and dynamics of a group of molecules by using high
speed supercomputer. Certainly any reasonable calculations on molecular ensemble need
long computer times, but the advance in computer makes it possible that this problem becomes
gradually less serious.
Rather, the assignment is more serious with intermolecular interaction potential used.
For simple molecules, empirical model potential such as those based on Lennard-Jones potential
and even hard-sphere potential can be used. But, for complex molecules, potential
function and related parameter value should be determined by some theoretical calculations.
For example, contribution of hydrogen-bond interaction is highly large to the total interaction
for such molecules as H2O, alcohols etc., one can produce semi-empirical
potential based on quantum-chemical molecular orbital calculation. Molecular ensemble
design is now complex unified method, which contains both quantum chemical and statistical
mechanical calculations.
2.2.2 FROM PREDICTION TO DESIGN
It is not new that the concept of “design” is brought into the field of chemistry. Moreover,
essentially the same process as the above has been widely used earlier in chemical engineer-