Page 22 - Strategies and Applications in Quantum Chemistry From Molecular Astrophysics to Molecular Engineer

QUANTUM CHEMISTRY: THE NEW FRONTIERS 7
It is worth to remark that the opposite also happens. There is an evolution in the
experimental techniques too, and in some cases this progress makes possible ( or
competitive) the measurement of a quantity formerly available via computations only. One
example is the detailed measurement of the electronic density of a molecule, and of the
related molecular electrostatic potential. The determination of these two observables has
been for many years a task feasible only by quantum-mechanical methods, now the
progresses in the elaboration of diffraction technique measurements makes possible a direct
determination.

I have reported this last example not for the sake of completeness in our discussion, but to
underline a different point. Quantum chemistry, in the work of group I and even more in
the work of group II, put the emphasis on some properties which by tradition are not
object of direct experimental determination. Electron charge distribution and MEP are just
two examples. The use of these quantities by theoreticians has spurred the elaboration of
experimental methods able to measure them. This positive feedback between theory and
experiment is an indication that quantum and experimental chemistry do not live in separate
worlds.

The competition between theory and experiments may be expressed in another way. Is
quantum chemistry able to predict special properties unknown to the experimentalists, or
the existence of compounds not yet synthesized? We are here considering the activity of
group I and the question thus regard a definitive demonstration of the existence (or non-
existence) of a given property or of a given compound; the question must be put in a
different way when addressed to persons belonging to group II. In the present case the
answer is partially positive.

There are several examples in the literature of recent years of convincing numerical
demonstrations that a compound not yet observed has a stable structure. It must be
remarked that these studies usually regard compounds of marginal chemical interest, and
that for innovative problems the quantum approach has always been late with respect to the
experiments. This delay decreases, but it is unlikely to expect that the leadership in the
search of new compounds will be assumed by in-depth calculations.
To substantiate this statement I will quote three examples. In the early sixties the discovery
of noble gases compounds came after the elaboration of the first codes for the ab-initio
calculation of polyatomic molecules: it was not possible however to give at that moment a
serious demonstration of the existence of or related compounds because the technical
means for in-depth calculations were not sufficient. Ten years later there has been a great
fuss about a presumed form of "polymeric water". Several theoreticians tried to
corroborate (or to disprove) that claim. The computational theory was at that time sufficient
to give a reliable description of the water dimer, but completely inadequate to disprove the
existence of that particular state of aggregation. Twenty years later the experimental
discover of a new stable form of carbon, , aroused first a sceptical reaction, then a
widespread interest. A definitive prediction of the stability of via quantum mechanics
was at that time within the possibilities of an efficient computational centre, and it was not
necessary to wait until 1991 to discover the existence of carbon nanotubes. Other more
complex carbon structures are good candidates as potential carriers of new properties, but
there is no indication of resolute efforts of group I in this direction.

Another aspect of quantum chemical activity which we connect with group I is the formal
elaboration of new approaches. At the beginning of the "computational era" (i.e. 30 years
ago) there has been a blossoming of new formulations and new approaches which have
given origin to to computer algorithms constituting the basic structure of today in-depth