Bonding in Condensed Phases

On the other hand, one has to take into account the influence of the surrounding which must induce an irreversible evolution of the H-bond system when its fast mode is excited the fast mode may be directly damped by the medium that is the direct relaxation mechanism. It may be also damped through the slow mode to which it is anharmonically coupled, that is the indirect relaxation mechanism. A schematical illustration of these two damping mechanism is given in Fig. 2. Of course, the role played by damping must be more important for H bonds in condensed phase. 

The tetrafluorides of selenium and tellurium are similar to SF4 both structurally (in the gaseous molecules) and in terms of reactivity, ffowever, the bonding in condensed phases is different and leads to several types of structures. Selenium tetrafluoride can be prepared by combining the elements, but it can also be obtained by the reaction of Se02 with SF4 at high temperature. The tellurium compound can be prepared by the reaction of SeF4 and Te02 at 80 °C. 

There are five chapters in Part I Introduction to quantum theory, The electronic structure of atoms, Covalent bonding in molecules, Chemical bonding in condensed phases and Computational chemistry. Since most of the contents of these chapters are covered in popular texts for courses in physical chemistry, quantum chemistry and structural chemistry, it can be safely assumed that readers of this book have some acquaintance with such topics. Consequently, many sections may be viewed as convenient summaries and frequently mathematical formulas are given without derivation. 

The classic technique used to study the hydrogen bond in condensed phases, both in solution and the solid state. The hydrogen bond is manifested by a decrease in the value of the A-H stretching (Aj ah) with respect to the free A-H value, together with a broadening of this vibrational band. 

As an example of hydrogen-bonded molecules which play the role of a bath in relaxation of the excited i (A-H) vibrational level can be the dimers of phosphinic acids studied in the gas phase at high temperatures [43]. In Fig. 8 the results of investigations of profiles of broad IR bands ascribed to i (OH) vibrations with characteristic Hadzi s ABC trio are shown. These spectra are very similar to those recorded for strong hydrogen bonds in condensed phases [44]. 

Normally reduced considerably by hydrogen bonding in condensed phases. 

Bonding in Condensed Phases 31 Fig. 1.25. Unit cell for NaCl... 

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