Current trends in the development of A. M. Butlerov’s theory of chemical structure

The chemical structure concept developed by A. M. Butlerov supplemented by the views on spatial (J. H. van’t-Hoff and J. A. Le Bel) and electronic (G. Lewis) configurations of molecules constitute the basis of the classical theory of chemical structure. The advent of quantum mechanics and development of the computer chemistry extended and enhanced the conceptual basis of theoretical chemistry, which nevertheless retains its independent value and cannot be reduced to direct physical definitions. The review deals with the evolution of the key concepts of the classical theory of chemical structure and introduction of new notions and approaches to analysis of the structure and reactivity problems, which is associated with the advent of the quantum mechanics and quantum chemistry views and methods.     

Generalization of classical mechanics for nuclear motions on nonadiabatically coupled potential energy surfaces in chemical reactions

Classical trajectory study of nuclear motion on the Born-Oppenheimer potential energy surfaces is now one of the standard methods of chemical dynamics. In particular, this approach is inevitable in the studies of large molecular systems. However, as soon as more than a single potential energy surface is involved due to nonadiabatic coupling, such a naive application of classical mechanics loses its theoretical foundation. This is a classic and fundamental issue in the foundation of chemistry. To cope with this problem, we propose a generalization of classical mechanics that provides a path even in cases where multiple potential energy surfaces are involved in a single event and the Born-Oppenheimer approximation breaks down. This generalization is made by diagonalization of the matrix representation of nuclear forces in nonadiabatic dynamics, which is derived from a mixed quantum-classical representation of the electron-nucleus entangled Hamiltonian [Takatsuka, K. J. Chem. Phys. 2006, 124, 064111]. A manifestation of quantum fluctuation on a classical subsystem that directly contacts with a quantum subsystem is discussed. We also show that the Hamiltonian thus represented gives a theoretical foundation to examine the validity of the so-called semiclassical Ehrenfest theory (or mean-field theory) for electron quantum wavepacket dynamics, and indeed, it is pointed out that the electronic Hamiltonian to be used in this theory should be slightly modified.     

Fast quantum crystallography

Typical contemporary X-ray crystallography delivers the geometries and, at best, the electron densities of molecules or periodic systems in the crystalline phase. Energies, electron momentum densities, and information relating to the pair density such as electron delocalization measures—all crucial to chemistry—are simply missed. Quantum crystallography (QCr) is an emerging line of research aimed at filling this gap by solving the crystallographic problem under the constraints of quantum mechanics. In this way, not only geometries and electron densities become experimentally accessible but also the entire panoply of quantum mechanical properties that are in the output of any quantum chemical software package. However, QCr remains limited to smaller systems (small molecules or small unit cells) due to the exponential bottleneck that plagues quantum mechanical calculations. When combined with a fragmentation technique, termed the “kernel energy method (KEM)”, QCr's reach to larger molecules is extended considerably to almost “any size”, that is, systems of up to many hundreds of thousands of atoms. KEM has made this doable with any chemical model and is capable of providing the entire quantum mechanics of large molecular systems. The smallness of the R-factor adjudicates the accuracy of the quantum mechanics extracted from the crystallography.     

QTAIM as a research programme: a reply to Shahbazian

In this paper I briefly reply to Shant Shahbazian’s comments on my paper “Austere quantum mechanics as a reductive basis for chemistry” and argue that quantum theory of atoms in molecules can be characterised as a research programme in the theories of chemistry. I also explore the areas in which Shahbazian and me agree and disagree.     

Odds and Trends: Recent Developments in the Chemistry of Odorants

Fragrance chemistry is, together with the closely related area of flavor chemistry, one of the few domains, if not the only one, in which chemists can immediately experience structure–activity relationships. This review presents structure–odor correlations and olfactophore models for the main odor notes of perfumery: “fruity”, “marine”, “green”, “floral”, “spicy”, “woody”, “amber”, and “musky”. New trendsetters and so‐called captive odorants of these notes are introduced, and recent activities and highlights in fragrance chemistry are summarized. The design of odorants, their chemical synthesis, and their use in modern perfumery is discussed. Our selection is guided and illustrated by creative fragrances, and features new odorants which encompassed current trends in perfumery. New odorants for grapefruit and blackcurrant, for galbanum, and leafy top notes are presented. Compounds with fashionable marine, ozonic, and aquatic facets are treated, as well as new odorants for classical lily‐of‐the‐valley, rose, and jasmine accords. Compounds with sweet and spicy tonalities are also discussed, as are the most recent developments for woody notes such as sandalwood and vetiver. We conclude with musky and ambery odorants possessing uncommon or unusual structural features. Some odor trends and effects are illustrated by microencapsulated fragrance samples, and areas where there is need for the development of new synthetic materials and methodologies are pointed out. Thus, chemists are invited to explore fragrance chemistry and participate in the design and synthesis of new odorants. This review gives the latest state of the art of the subject.     

A Classical and Quantum Chemical Analysis of Gaseous Heat Capacity

Physical chemistry is considered to be a scientifically abstract and mathematically intensive course in the undergraduate chemistry curriculum. To most students, the physical chemistry course involves a semester that deals with macroscopic properties and another that deals with microscopic evaluations of chemical systems. They often fail to see the importance of statistical mechanics in making the connection between the content of the two semesters. In this paper, we propose a computational exercise that complements a simple physical chemistry experiment that can be used to understand the chemical basis of a macroscopic property such as the heat capacity of gases using microscopic (classical and quantum) mechanics. Students are given the opportunity to use (1) computational chemistry software to calculate the contributions of translational, rotational, and vibrational motion to the energy of molecules; (2) a graphing program to study the linear and nonlinear dependence of energy on temperature; (3) classical, quantum, and statistical mechanical theory to verify experimental data; (4) regression analysis to approximate the heat capacity constant of simple gases from energy calculations.     

Fundamentals of Silicon Chemistry: Molecular States of Silicon-Containing Compounds

Silicon and its compounds have made possible the design of new materials, which, from computers to space travel, have helped to shape the technology of our 20th century. Conversely, the demands of new technology have stimulated the fast development of silicon chemistry as part of the “renaissance” of inorganic chemistry. This article uses selected examples of predominantly organosilicon compounds to discuss in simplified terms the measurement and assignment of suitable spectroscopic “molecular fingerprints” as well as the resulting benefit for the preparative chemist. The comparison of “equivalent” states of “chemically related” molecules is emphasized, based on perturbation arguments and supporting quantum-chemical models. Special attention is given to the relation between structure and energy, which allows us to understand and to predict the connectivity between and the spatial arrangement of silicon “building blocks”, the energy-dependent electron distribution over the effective nuclear potentials of a molecular framework, and, especially, the partly considerable effects of “silicon substituents” on molecular properties. Future-directed extensions and applications include polysilane band structures, Rydberg states of chromophores containing silicon centers, redox reactions and ion-pair formation of silicon-substituted π systems, and molecular dynamic phenomena in solution or on thermal fragmentation in the gas phase. The main objective is a set of clear and practical rules for interpreting measurements and planning experiments.     

Vibrational spectrum of a 1D oscillator: The quantum,the Wigner,and the classical ways

Infrared spectra have been used in many chemical applications, and theoretical calculations have been useful for analyzing these experimental results. While quantum mechanics is used for calculating the spectra for small molecules, classical mechanics is used for larger systems. However, a systematic understanding of the similarities and differences between the two approaches is not clear. Previous studies focused on peak position and relative intensities of the spectra obtained by various quantum and classical methods, but here, we included “absolute” intensities in the evaluation. The infrared spectrum of a one-dimensional (1D) harmonic oscillator (HO) and Morse oscillator were examined using four treatments: quantum, Wigner, truncated Wigner, and classical microcanonical treatments. For a 1D HO with a linear dipole moment function (DMF), the quantum and Wigner treatments give nearly the same spectra. On the other hand, the truncated Wigner underestimates the fundamental transition's intensity by half. In the case of cubic DMF, the truncated Wigner and classical methods fail to reproduce the relative intensity between the fundamental and second overtone transitions. Unfortunately, all the Wigner and classical methods fail to agree with the quantum results for a Morse oscillator with just 1% anharmonicity.     

On the description of high-resolution experiments in molecular physics

We argue that all high-resolution experiments on small molecules some of which currently “belong” to chemistry and others to physics, should be brought together and discussed in a unified way using the best theory available, namely quantum mechanics, and that preconceptions about “molecular structure” should be avoided in this area of physical science.     

Chemical thermodynamics and theoretical models

This paper presents a brief discussion of the relationship between classical thermodynamics and theoretical models of chemical systems. Applications of thermodynamics to real chemical systems and to related theoretical models concerning “complexes in nonelectrolyte solutions” and “substituent and solvent effects in organic chemistry” are described, with particular emphasis on research that my colleagues and I have done in these areas. This paper concludes with a brief autobiographical section on “people and places”.     

Few-body quantum and many-body classical hyperspherical approaches to reactions and to cluster dynamics

The hyperspherical method is a widely used and successful approach for the quantum treatment of elementary chemical processes. It has been mostly applied to three-atomic systems, and current progress is here outlined concerning the basic theoretical framework for the extension to four-body bound state and reactive scattering problems. Although most applications only exploit the advantages of the hyperspherical coordinate systems for the formulation of the few-body problem, the full power of the technique implies representations explicitly involving quantum hyperangular momentum operators as dynamical quantities and hyperspherical harmonics as basis functions. In terms of discrete analogues of these harmonics one has a universal representation for the kinetic energy and a diagonal representation for the potential (hyperquantization algorithm). Very recently, advances have been made on the use of the approach in classical dynamics, provided that a hyperspherical formulation is given based on “classical” definitions of the hyperangular momenta and related quantities. The aim of the present paper is to offer a retrospective and prospective view of the hyperspherical methods both in quantum and classical dynamics. Specifically, regarding the general quantum hyperspherical approaches for three- and four-body systems, we first focus on the basis set issue, and then we present developments on the classical formulation that has led to applications involving the implementations of hyperspherical techniques for classical molecular dynamics simulations of simple nanoaggregates.     

How Chemistry and Physics Meet in the Solid State

To make sense of the marvelous electronic properties of the solid state, chemists must learn the language of solid-state physics, of band structures. An attempt is made here to demystify that language, drawing explicit parallels to well-known concepts in theoretical chemistry To the joint search of physicists and chemists for understanding of the bonding in extended systems, the chemist brings a great deal of intuition and some simple but powerful notions. Most important among these is the idea of a bond, and the use of frontier-orbital arguments. How to find localized bonds among all those maximally delocalized bands? Interpretative constructs, such as the density of states, the decomposition of these densities, and crystal orbital overlap populations, allow a recovery of bonds, a finding of the frontier orbitals that control structure and reactivity in extended systems as well as discrete molecules.     

Early impact of quantum physics on chemistry: George Hevesy’s work on rare earth elements and Michael Polanyi’s absorption theory

After Heitler and London published their pioneering work on the application of quantum mechanics to chemistry in 1927, it became an almost unquestioned dogma that chemistry would soon disappear as a discipline of its own rights. Reductionism felt victorious in the hope of analytically describing the chemical bond and the structure of molecules. The old quantum theory has already produced a widely applied model for the structure of atoms and the explanation of the periodic system. This paper will show two examples of the entry of quantum physics into more classical fields of chemistry: inorganic chemistry and physical chemistry. Due to their professional networking, George Hevesy and Michael Polanyi found their ways to Niels Bohr and Fritz London, respectively, to cooperate in solving together some problems of classical chemistry. Their works on rare earth elements and adsorption theory throws light to the application of quantum physics outside the reductionist areas. They support the heuristic and persuasive value of quantum thinking in the 1920–1930s. Looking at Polanyi’s later oeuvre, his experience with adsorption theory could be a starting point of his non-justificationist philosophy.     

The two faces of chemistry: can they be reconciled?

Shortly before his death, Richard Bader commented in this Journal on the dichotomy that exists within chemistry and between chemists. We believe that the dichotomy results from different goals and objectives inherent in the chemical disciplines. At one extreme are designers who synthesize new molecules with interesting properties. For these chemists, the rationale underpinning molecular synthesis is far less important than the end product—the molecules themselves. At the other extreme are the chemists who seek a fundamental understanding of molecular properties. We suggest that the Quantum Theory of Atoms in Molecules, by virtue of the rich hierarchical structure inherent in the theory, offers a bridge through which to unite these two groups. However, if there is to be reconciliation, it falls to the theorists to develop “quantum mechanically” correct tools and concepts useful to the synthetic and applied chemist.     

Methods and Applications of Quantum Chemistry. Part I: Physical and Mathematical Basis

After an introduction to the fundamental concepts of quantum mechanics it is shown how to describe a chemical system in the language of theoretical physics. The equations which one obtains can not, in general, be solved in closed form. Approximate numerical methods that furnish sufficiently accurate solutions for many problems of interest are, however, available. The most important of these are presented briefly, and are given physically visualizable interpretations as far as this is possible. Special attention is devoted to more modern methods of quantum chemistry, to the basis assumptions underlying them, their scope, and their limitations. Treatment of the theory of the chemical bond is reserved for Part II.     

Quantum chemistry simulation on quantum computers: theories and experiments

It has been claimed that quantum computers can mimic quantum systems efficiently in the polynomial scale. Traditionally, those simulations are carried out numerically on classical computers, which are inevitably confronted with the exponential growth of required resources, with the increasing size of quantum systems. Quantum computers avoid this problem, and thus provide a possible solution for large quantum systems. In this paper, we first discuss the ideas of quantum simulation, the background of quantum simulators, their categories, and the development in both theories and experiments. We then present a brief introduction to quantum chemistry evaluated via classical computers followed by typical procedures of quantum simulation towards quantum chemistry. Reviewed are not only theoretical proposals but also proof-of-principle experimental implementations, via a small quantum computer, which include the evaluation of the static molecular eigenenergy and the simulation of chemical reaction dynamics. Although the experimental development is still behind the theory, we give prospects and suggestions for future experiments. We anticipate that in the near future quantum simulation will become a powerful tool for quantum chemistry over classical computations.     

我国理论化学的两位先驱者——孙承谔与王守竞

孙承谔先生是国际知名的化学家,其重要的工作是将过渡态理论成功应用于真实的化学反应速率常数的计算.作为中国理论化学的开拓者和教育家,他为我国化学事业的发展做出了杰出贡献.王守竞先生将新诞生的量子力学应用于氢分子和不对称陀螺的研究,取得了国际公认的成果.之后,又投身应用科技,出任中央机器厂首任总经理,是我国机械工业的奠基人.     

Atoms and bonds in molecules and chemical explanations

The concepts of atoms and bonds in molecules which appeared in chemistry during the nineteenth century are unavoidable to explain the structure and the reactivity of the matter at a chemical level of understanding. Although they can be criticized from a strict reductionist point of view, because neither atoms nor bonds are observable in the sense of quantum mechanics, the topological and statistical interpretative approaches of quantum chemistry (quantum theory of atoms in molecules, electron localization function and maximum probability domain) provide consistent definitions which accommodate chemistry and quantum mechanics.