Ideas of Quantum Chemistry P80 shows how quantum mechanics is applied to chemistry to give it a theoretical foundation. The structure of the book (a TREE-form) emphasizes the logical relationships between various topics, facts and methods. It shows the reader which parts of the text are needed for understanding specific aspects of the subject matter. Interspersed throughout the text are short biographies of key scientists and their contributions to the development of the field. | 756 13. Intermolecular Interactions Fig. . Bacteriophage T represents a supramolecular construction that terrorizes bacteria. The hexagonal head contains a tightly packed double helix of DNA the virus genetic material wrapped in a coat build of protein subunits. The head is attached to a tube-like molecular connector built of 144 contractible protein molecules. On the other side of the connector there is a plate with six spikes protruding from it as well as six long kinked legs made of several different protein molecules. The legs represent a landing aparatus which using intermolecular interactions attaches to a particular receptor on the bacterium cell wall. This reaction is reversible but what happens next is highly irreversible. First an enzyme belonging to the monster makes a hole in the cell wall of the bacterium. Then the 144 protein molecules contract probably at the expense of energy from hydrolysis of the ATP molecule adenosine triphosphate - a universal energy source in biology which the monster has at its disposal. This makes the head collapse and the whole monster serves as a syringe. The bacteriophage s genetic material enters the bacterium body almost in no time. That is the end of the bacterium. Perturbational method has limited applicability - at long intermolecular separations what is called the polarization approximation may be used - at medium distances a more general formalism called the symmetry adapted perturbation theory may be applied - at short distances of the order of chemical bond lengths perturbational approaches are inapplicable. One of the advantages of a low-order perturbational approach is the possibility of dividing the interaction energy into well defined physically distinct energy contributions. In a polarization approximation approach the unperturbed wave function is assumed as a product of the exact wave functions of the individual subsystems QQ a Q B Q. The corresponding zero-order energy is eQQ Ea q E b Q. Then the .
