Como a Mecânica Quântica Transformou Sonhos em Realidade

Resumo

As the atomic theory of matter became more established and accepted by the majority of the scientific
community, chemists began to dream about the possibility of understanding the physical and chemical
properties of different substances in terms of their microscopic composition. Identifying the constituent
elements of a substance and their respective proportions allowed chemists to characterize it by a “chemical
formula”. It did not take long for chemists to realize that substances could have different properties even
though they had the same “chemical formula”. This observation led chemists to the conclusion that it
was not enough to identify the constituent elements of a substance. It was essential to determine how
the atoms of those elements were arranged spatially. Thus, chemists began to realize that the physical
and chemical properties of a substance were closely related to its “molecular structure”. The dream
now seemed less impossible and more fascinating: how to arrange a certain group of atoms spatially in
order to obtain a molecule with specific physical and chemical properties? In this context, Molecular
Modeling is a subject as old as post-Lavoisier chemistry. However, in addition to knowing the spatial
arrangement of the atoms in a molecule, it was necessary to know how these atoms are connected, that
is, what the “chemical structure” of the molecule was and what kept them “connected”. In other words,
what was the origin of the “chemical bond”. The end of the century before last and the first quarter of
the last century were the stage for a fantastic revolution in our knowledge of the atomic structure of
matter. Benefiting from this development, chemists began to have at their disposal powerful instruments
for the microscopic analysis of the macroscopic properties of chemical systems, quantum chemistry
and classical and quantum statistical mechanics. Finally, in the last forty years, with the availability of
personal computers and high-performance workstations, combined with high-resolution graphics terminals
and sophisticated multifunctional “software”, the dream began to become reality and the development
of what we now call Quantum Chemistry had an enormous impact on Chemistry. However, in order to
have a clear measure of the extent of this impact, it is essential to reconstruct the environment in which
Chemistry found itself at the end of the 19th century, with all its difficulties. Only then will the reader
be able to understand the incredible change brought about by the application of quantum mechanics to
Chemistry. This reconstruction will be done by following the evolution of three concepts that form the
pillars of modern chemistry, molecular structure, chemical structure and chemical bonding.
Keywords: Chemical formula; molecular structure; chemical structure; chemical bonding; quantum
mechanics; quantum interference.