INTRODUCTIONMany of us who are involved in teaching a special-topic graduate course may have the experience that it is difficult to find suitable references, especially reference materials put together in a suitable text format. Presently, several excellent book series exist and they have served the scientific community well in reviewing new developments in physical chemistry and chemical physics. However, these existing series publish mostly monographs consisting of review chapters of unrelated subjects. The modern development of theoretical and experimental research has become highly specialized. Even in a small subfield, experimental or theoretical, few reviewers are capable of giving an in-depth review with good balance in various new developments. A thorough and more useful review should consist of chapters written by specialists covering all aspects of the field. This book series is established with these needs in mind. That is, the goal of this series is to publish selected graduate texts and stand-alone review monographs with specific themes, focusing on modern topics and new developments in experimental and theoretical physical chemistry. In review chapters, the authors are encouraged to provide a section on future developments and needs. We hope that the texts and review monographs of this series will be more useful to new researchers about to enter the field. In order to serve a wider graduate student body, the publisher is committed to making available the monographs of the series in a paperbound version as well as the normal hardcover copy. Cheuk-Yiu Ng PREFACEThe Born-Oppenheimer adiabatic approximation represents one of the cornerstones of molecular physics and chemistry. The concept of adiabatic potential-energy surfaces, defined by the Born-Oppenheimer approximation, is fundamental to our thinking about molecular spectroscopy and chemical reaction dynamics. Many chemical processes can be rationalized in terms of the dynamics of the atomic nuclei on a single BornOppenheimer potential-energy surface. Nonadiabatic processes, that is, chemical processes which involve nuclear dynamics on at least two coupled potential-energy surfaces and thus cannot be rationalized within the BornOppenheimer approximation, are nevertheless ubiquitous in chemistry, most notably in photochemistry and photobiology. Typical phenomena associated with a violation of the Born-Oppenheimer approximation are the radiationless relaxation of excited electronic states, photoinduced unimolecular decay and isomerization processes of polyatomic molecules. During the last few decades, we have witnessed a change of paradigms in nonadiabatic chemistry. First, the remarkable advances achieved in femtosecond laser technology and time-resolved spectroscopy have revealed that the radiationless decay of excited electronic states may take place much faster than previously thought. The traditional theory of radiationless decay processes, developed in the sixties and seventies of the last century, cannot explain electronic decay occurring on ...
To understand the intermolecular interactions between chalcogen centers (O, S, Se, Te), quantum chemical calculations on pairs of model systems were carried out. For the oxygen derivatives, one of the components of the supermolecules consists of dimethyl ether, while the second component is either dimethyl ether (1) or ethynyl methyl ether (2) or methyl cyanate (3). The model calculations were also extended to the sulfur (4-6), selenium (7-9), and tellurium congeners (10-12). The MP2/SDB-cc-pVTZ, 6-311G level of theory was used to derive the geometrical parameters and the global energies of the model systems. A detailed analysis based on symmetry adapted perturbation theory (SAPT) reveals that induction and dispersion forces contribute to the bonding in each case. For 1-3 the electrostatic energy also contributes to the intermolecular bonding, but not for 4-12. The NBO analysis reveals that the interaction in the dimers 1-3 is mainly due to weak hydrogen bonding between methyl groups and chalcogen centers. Similar hydrogen bonding is also found in the case of 4 and to a lesser extent in 5 and 7. For the aggregates with heavier centers the chalcogen-chalcogen interaction dominates, and hydrogen bonding only plays a minor role. Electron-withdrawing groups on the chalcogen centers increase the interaction energy and reduce the intermolecular distance dramatically. The one-electron picture of an interaction between the lone pair of the donor and the chalcogen carbon sigma orbital allows a qualitatively correct reproduction of the observed trend.
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