The molecular view haunted chemists for a long time. Since this vision promised an outline of chemical reality in terms of basic building blocks, complementary approaches were neglected. Nowadays, the molecular program of chemistry has arrived at its successful termination so that the theoreticians find themselves challenged by new ways of looking at familiar facts of chemistry.Our modern theoretical first principles allow a precise mathematical definition of Bohr’s notion of complementarity and enforce a pluralism in looking at the nature of things. In particular, complementarity is revealed with conspicious clarity everywhere in chemistry. The complementary aspects of classical and quantal descriptions of molecules and the complementarity between molecules and chemical substances are discussed. The discourse is concluded with a hint of how chemistry could help in learning complementary ways of thinking and to place complementarity into a wider perspective.
The molecular view haunted chemists for a long time. Since this vision promised an outline of chemical reality in terms of basic building blocks, complementary approaches were neglected. Nowadays, the molecular program of chemistry has arrived at its successful termination so that the theoreticians find themselves challenged by new ways of looking at familiar facts of chemistry.Our modern theoretical first principles allow a precise mathematical definition of Bohr’s notion of complementarity and enforce a pluralism in looking at the nature of things. In particular, complementarity is revealed with conspicious clarity everywhere in chemistry. The complementary aspects of classical and quantal descriptions of molecules and the complementarity between molecules and chemical substances are discussed. The discourse is concluded with a hint of how chemistry could help in learning complementary ways of thinking and to place complementarity into a wider perspective.
Aspects of the methodology and language of chemistry and physics are discussed. Chemistry defines the chemical properties of any substance from the results of its interaction with other substances using the logic of relations. Therefore, describing the properties of substances means using sets of different ideas, including ones that are opposite in meaning. Consequently, depending on the nature of reagents with respect to which properties are established, substances show chemical dualism. This dualism was established in chemistry long before the discovery of wave–particle dualism, to understand which N. Bohr proposed the complementarity principle in 1927. The methodology of natural sciences corresponds to the principle of complementarity and the need to use it to understand the world and record the results in the linguistic reality of several languages.
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.
This introductory chapter provides an overview of quantum mechanics, which is a subject of central importance in chemistry, and is by nature a mathematical theory. The chemist's approach to the understanding of matter and its chemical transformations is to take a microscopic view, connecting experimental observation with the properties of constituent molecules. However, at the microscopic level, atoms, molecules, and their constituent particles do not obey the familiar laws of mechanics that pertain to the everyday macroscopic world. For example, the existence of molecular line spectra indicates that molecules can be found only at certain well-defined energies, unlike objects at our own scale whose energy appears to be continuously variable. This book aims to explain the fundamentals of quantum mechanics from the point of view of chemistry; describe the areas of chemistry where quantum mechanics is most important; and show how quantum mechanics is applied to chemical problems.
The concept of atomic and molecular orbitals has been a fundamental pillar in modern chemistry, shedding light on the structures and reactivity of chemical compounds. This article examines the evolution and significance of orbital theory, its applications in chemistry, and the ongoing debate about the existence of orbitals from both physics and chemistry perspectives. Philosophical aspects related to the ontology of orbitals are explored, emphasizing the complex interplay between mathematical abstractions and tangible reality. The multifaceted nature of orbitals, their role in quantum mechanics, and their implications for understanding the quantum realm are discussed. While the debate surrounding the ontological status of orbitals remains ongoing, it highlights the profound nature of inquiries into the fundamental essence of reality. This exploration underscores the significance of continuous research and discourse in advancing our understanding of these fundamental constituents of the quantum world.
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.
Ninety years ago in 1927, at an international congress in Como, Italy, Niels Bohr gave an address which is recognized as the first instance in which the term "complementarity", as a physical concept, was spoken publicly [1], revealing Bohr's own thinking about Louis de Broglie's "duality". Bohr had very slowly accepted duality as a principle of physics: close observation of any quantum object will reveal either wave-like or particle-like behavior, one or the other of two fundamental and complementary features. Little disagreement exists today about complementarity's importance and broad applicability in quantum science. Book-length scholarly examinations even provide speculations about the relevance of complementarity in fields as different from physics as biology, psychology and social anthropology, connections which were apparently of interest to Bohr himself (see Jammer [2], Murdoch [3] and Whitaker [4]). Confusion evident in Como following his talk was not eliminated by Bohr's article [1], and complementarity has been subjected to nine decades of repeated examination ever since with no agreed resolution. Semi-popular treatments [5] as well as expert examinations [6-9] show that the topic cannot be avoided, and complementarity retains its central place in the interpretation of quantum mechanics. However, recent approaches by our group [10-13] and others [14-20] to the underlying notion of coherence now allow us to present a universal formulation of complementarity that may signal the end to the confusion. We demonstrate a new relationship that constrains the behavior of an electromagnetic field (quantum or classical) in the fundamental context of two-slit experiments. We show that entanglement is the ingredient needed to complete Bohr's formulation of complementarity, debated for decades because of its incompleteness.