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The renowned theoretical physicist Victor F. Weisskopf rightly pointed out that a real understanding of natural phenomena implies a clear distinction between the essential and the peripheral. Only when we reach such an understanding - that is to say ­ when we are able to separate the relevant from the irrelevant, will the phenomena no longer appear complex, but intelectually transparent. This statement, which is generally valid, reflects the very essence ofmodelling in the quantum theory of matter, on the molecular level in particular. Indeed, without theoretical models one would be swamped by too many details embodied in intricate accurate molecular wavefunctions. Further, physically justified simplificqtions enable studies of the otherwise intractable systems and/or phenomena. Finally, a lack of appropriate models would leave myriads of raw experimental data totally unrelated and incomprehensible. The present series ofbooks dwells on the most important models of chemical bonding and on the variety of its manifestations. In this volume the electronic structure and properties of molecules are considered in depth. Particular attention is focused on the nature of intramolecular interactions which in turn are revealed by various types ofmolecular spectroscopy. Emphasis is put on the conceptual and interpretive aspects of the theory in line with the general philosophy adopted in the series.



Nuclear Vibrations and Force Constants

While valuable information on chemical bonding can be obtained from studies of the static structure of a bound system, even more understanding can be obtained from the internal restoring forces which control molecular vibrational motions and the mechanical response of molecules to external forces. This chapter deals with the description of such motions in terms of molecular harmonic and anharmonic vibrational force constants. Methods are described for the experimental evaluation of these parameters from spectroscopy, but the primary emphasis is on their computational determination since this approach can give more complete and accurate information for medium-sized polyatomic molecules. Consideration is given to techniques which can provide the highest possible accuracy within the harmonic oscillator approximation for polyatomic molecules and also the progress that has been made in fully anharmonic evaluation of the molecular vibrational potential energy hypersurfaces and the anharmonic energy levels on these surfaces.
James E. Boggs

Some Aspects of the Quantumchemical Interpretation of Integrated Intensities of Infrared Absorption Bands

Integrated Intensities of Infrared Absorption Bands corresponding with fundamental transitions, calculated in the Double Harmonic Approximation and with single determinantal wavefunctions can be analyzed in various ways in order to obtain an insight into the various electronic and vibrational factors determining the intensities. The computational strategy for obtaining dipole moment derivatives, with respect to the normal coordinates governing these intensities, is discussed together with the decomposition in non-local and LMO contributions.
H. P. Figeys, P. Geerlings

The Orbital Concept as a Foundation for Photoelectron Spectroscopy

Chemistry assumes that electrons moving in the potential field of nuclei are crucial to the existence of those forces that define the formation, properties and reaction of chemical compounds. A theoretical approach that adopts this view is the orbital (atomic, AO, or molecular, MO) concept. The orbital concept is concerned with the motion of electrons in the average field of a complicated system of particles (including electrons), and it assumes that the results bear on reality. The success of the method is well documented and the difficulties it encounters, particularly when applied to molecular spectroscopy, have been elaborated [1].
S. P. McGlynn, K. Wittel, L. Klasinc

The Equivalent Bond Orbital Model and the Interpretation of PE Spectra

The low-energy part of the He(Iα) or He(IIα) PE spectrum of a hypothetical molecule M (shown in Fig. 1) consists of four bands, ① to ④, which — for simplicity — we assume not to overlap. The bands ① and ② exhibit resolved vibrational fine structure, whereas the fine structure of ③ and ④ cannot be resolved because of line broadening, of random noise and/or the resolution of the recording, the latter being typically of the order of 20 meV.
Evi Honegger, Edgar Heilbronner

Through-space and Through-bond Interactions as Mirrored in Photoelectron Spectra

Useful and transparent orbital models for rationalizing intramolecular interactions between identical functional groups are discussed in some detail. A seminal idea of Hoffmann, Imamura and Hehre to dissect interactions between molecular fragments into through-bond and through-space is found to possess a high interpretive power. A simple perturbational approach employing semilocalized molecular orbitals suffices for most purposes if information at the qualitative level is desired. Quantitative theoretical data are conveniently produced by the method of Heilbronner and Schmelzer. It is gratifying and intellectually pleasing that theoretical results are in good agreement with PES observations. Thus one can say that orbital models represent a useful means for treating and understanding long-range interactions within molecules themselves. They give a theoretical framework to the empirical knowledge which relates properties of molecular systems to the features of their constituent fragments.
Mirjana Eckert-Maksić

Penning Ionization — The Outer Shape of Molecules

Important intermolecular interaction processes involving electron transfer, energy transfer, molecular collisions, and chemical reactions take place through a close contact of molecules, since a short-range interaction, such as an exchange-interaction, is responsible for most of the above processes. In order to elucidate such processes, it is interesting and seems almost necessary to study the outer characteristics of molecules by introducing an experimental probe which provides information on the frontier properties of molecules. As shown below in detail, Penning ionization electron spectroscopy (PIES) is a sensitive method for probing electron densities as well as potential energy surfaces at the very frontier of the molecule where the molecule is attacked by the reagent.
Koichi Ohno, Yoshiya Harada

The Meaning and Distribution of Atomic Charges in Molecules

Various methods of representing changes in electron densities upon formation of chemical bonds are briefly discussed. Virtues and shortcommings of describing charge (re)distribution in terms of sets of numbers, which can be identified as gross atomic electron populations, are thoroughly considered subsequently. Experimental techniques yielding atomic charges are critically assessed and it is concluded that ESCA spectroscopy gives the most straightforward and transparent insight into distribution of atomic monopoles in molecular systems. Applications of the atomic charge concept in rationalizing a number of physical and chemical properties of molecules are described in some detail. It is concluded that properly defined atomic charges possess a grain of truth and their interpretive power is stressed. Finally, prospects of future developments are briefly sketched.
Karl Jug, Zvonimir B. Maksić

Electron Spectroscopy for Chemical Analysis (ESCA) — Basic Features and their Model Description

The conceptual framework of the inner-core binding energy shifts in molecules is considered. It is shown that the basic facets of electron spectroscopy for chemical analysis (ESCA) are well understood and can be interpreted by simple physical models based on classical electrostatics. This transparent and intuitively appealing approach provides a rationale for characteristic ESCA fingerprints of atoms in chemical environments and offers the underlying principle for empirical additivity of ESCA shifts exhibited by functional groups. The role of the final state after photoionization has been completed is discussed in extenso. Finally, the importance of ESCA in atomic charge analysis, determination of gas-phase acidities and basicities and its interplay with other spectroscopic techniques (PES, NMR, MW and Mössbauer) is briefly discussed.
Zvonimir B. Maksić

Experimental Momentum-Space Chemistry by (e, 2e) Spectroscopy

The notion that chemists might benefit by looking at molecular orbitals and chemical bonding phenomena from the complementary momentum-space perspective was first suggested by Coulson and Duncanson some half a century ago. With the rapid development of (e, 2e) spectroscopy in the past two decades, electron momentum distributions of individual ionic states in the valence shell can be measured direclty to high precision. In addition to accessing the nature, symmetry and parentage of a characteristic orbital in a particular ionic state, the experimental momentum distribution provides a direct and stringent evaluation of the quality of ab-initio self-consistent-field wavefunctions. This has provided a powerful technique to investigate chemical bonding in the laboratory. A phenomenological look at (e, 2e) spectroscopy in its first two decades of development will be presented with a special emphasis on its applications in quantum chemistry. Some general observations regarding the use of (e, 2e) data in electronic structure determination and wavefunction evaluation will be made. A summary of momentum-space chemical concepts will be given in order to provide qualitative understanding of the observed features in the experimental spherically averaged momentum density using the three-dimensional density topography of calculated wavefunctions. Finally, a bibliographical update of recent (e, 2e) literature will be provided.
K. T. Leung

Theoretical Parameters of NMR Spectroscopy

The theory of magnetic shielding and nuclear spin-spin coupling in nuclear magnetic resonance (NMR) spectroscopy is reviewed. For each of the parameters, we review first the physical basis starting with the Hamiltonians for the relevant interactions. Then, we present a noncomprehensive review of the methods used for numerical calculations. The emphasis is on general characterization of the various approaches and on the nonempirical work, but the semiempirical and empirical work judged as the most important by the reviewers is also quoted. Some representative numerical results are presented.
Jozef Kowalewski, Aatto Laaksonen

Theoretical Approaches to ESR Spectroscopy

Atoms and molecules which contain unpaired electrons possess magnetic moments due to the electron’s intrinsic spin and, in cases where the total spin S ≠ 0, to the electron’s orbital angular momentum. Experimental techniques which measure the interaction of the intrinsic spin with an external magnetic field fall into the category of “electron spin resonance” (ESR) spectroscopy. Techniques which deal with orbital angular momemtum are often differentiated through the use of the label “electron paramagnetic resonance” (EPR) spectroscopy. In this article we shall use the term ESR to refer to both.
David Feller, Ernest R. Davidson

Rovibrational Averaging of Molecular Electronic Properties

The measured value of a molecular electronic property is a rovibrational average for a given v, J state or a thermal average. These electronic properties may exhibit measurable isotope effects and are found to be v, J-state-dependent and temperature dependent. Rovibrational effects are intimately related to both the molecular potential surface and the electronic property surface. When both surfaces are available it becomes possible to correct the observed values of the property for rovibrational effects and thereby to elicit the value of the property at the equilibrium geometry of the molecule, for comparison with the ab initio theoretical value. Alternatively, temperature coefficients and isotope shifts may be calculated from both theoretical surfaces, for direct comparison with experiment. In some favourable cases it is possible to obtain an empirical estimate of the sensitivity of the property to a change in geometry from a measurement of a temperature coefficient or an isotope effect. A review of the theory and its applications to the nuclear magnetic shielding, spin-rotation constant, nuclear hyperfine constant, electric field gradient, spin-spin coupling, dipole moment, magnetizability, electric dipole polarizability and hyperpolarizabilities reveals some interesting general trends.
Cynthia J. Jameson

Properties of Molecules in Excited States

Quantum chemical calculations for organic molecules in excited states are considerably more complex than for ground state species, due to the fact that much fewer empirical data about excited state geometries are available and that geometry optimization on the CI level, which is a prerequisite for all calculations of excited state properties, is by no means simple. In this paper we discuss
  • qualitative models, which may be helpful in locating minima on excited state potential surfaces
  • methods and results for calculations of excited state properties at various levels of sophistication; particular emphasis is laid on sudden polarization phenomena, cis-trans isomers of acetylene in its excited states, the pyramidalization of the carbonyl group, excited states of aromatic compounds (TICT states) and
  • theoretical treatments of radiative and non-radiative deactivation of excited states.
M. Klessinger, T. Pötter

Semiclassical Interpretation of Intramolecular Interactions

Within the range of energies belonging to the domain covered by chemistry, there are interaction phenomena involving material systems which may vary enormously in size and internal composition. In many cases the basic unit in which the phenomenon occurs is very large and composite, in other cases it is very simple, indeed atomic, in nature. For chemists, however, the center of attention remains the molecule, even if the bewildering variety of structures and properties encountered poses a continuous challenge to anyone wrestling with a chemical problem, whatever his principal interest or goal may be.
J. Tomasi, G. Alagona, R. Bonaccorsi, C. Ghio, R. Cammi

The Analysis of Potential Energy Surfaces in Terms of the Diabatic Surface Model

In this paper, we summarize the salient features of the Diabatic Surface model, which represents a theoretical instrument for analysing a Potential Energy Surface (PES). In particular this model provides information about the global properties of a PES and gives a rationalization of the main features of a critical point (e.g. intermediates, transition states, etc.), such as its origin, existence and index.
Fernando Bernardi, Massimo Olivucci, Michael A. Robb


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