Some aspects of quantum mechanics in chemical theory

Nalewajski, Roman

G. Spinolo, Paolo Ghigna

When compared to classical mechanics (CM), quantum mechanics (QM) clearly appears as a difficult topic to the beginner. However, some knowledge of the basics of QM is a real need also for learners with predominant interest in experimental chemistry. Indeed, QM is the physical theory explaining most fundamental aspects of Chemistry, such as the periodic table of the elements and the chemical bond now called covalent, found by 19th century chemists and for a long time recognized as incompatible with the physical theories of the time (CM and classical electromagnetism). Going beyond the historical point of view, many peculiar aspects of QM and its very language permeate all modern chemistry so that a trustworthy understanding is required to deal with the matter in a sensible way.

Jean Barriol

Volker Termath

Anton Amann, Werner Gans

Abstract Quantum mechanical and classical aspects are of equal importance in chemistry, and theoretical chemistry must be able to deal with this situation. Classical quantities with which a chemist has to deal include, for example, the temperature and the chemical potential of a substance, and chirality, knot‐type and tertiary structures of molecules. Classical concepts of this type are no more consequences of traditional quantum chemistry than is the theory of complex chemical processes (for example adsorption, heterogeneous catalysis) and macroscopic systems. Algebraic quantum mechanics affords a general framework for the discussion of classical quantities and large systems. There are other independent approaches for the theoretical treatment of chemical problems; these can be summarized by using the keywords “fractals”, “chaotic motion”, “quasicrystals” and “knot theory”. The relationships between these four research topics and their connection with algebraic quantum mechanics, as well as their importance for theoretical chemistry, were discussed at a NATO Workshop in 1987 (see Ref. [1]). In an extended theoretical chemistry, the notion of “molecules” will not play a role as central as it does in quantum chemistry. In the theory of large systems in particular, the introduction of complementary approaches appears to be appropriate; the term “molecule” as used by the chemist does not appear either in the description of disordered systems by means of fractals or in the thermodynamic limit of algebraic statistical mechanics.

Andrzej Koleżyński

It is well known from the history of science, that almost all crucial concepts are subject to change during the evolution of scientific theories. There is, however, another (often forgotten or omitted) aspect of this process, i.e. a role of some approximations, necessarily applied to any system of interest, in practically every single calculation carried out within quantum mechanical formalism, which can also result in change of the meaning of some of these concepts. In this paper, the two concepts of great importance in chemistry and physics, namely the concept of an atom, and the concept of a chemical bond are analyzed, both from the point of view of classical chemistry and quantum mechanics. The evolution of their meaning, due to the approximations, resulting in some important differences is highlighted.