tailieunhanh - Ideas of Quantum Chemistry P67

Ideas of Quantum Chemistry P67 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. | 626 12. The Molecule in an Electric or Magnetic Field a b -Ö Q -Ö d 2-r H a----3 0- - Fig. . Explanation of why a dipole moment interacts with the electric field intensity a quadrupole moment with its gradient while the octupole moment does not interact either with the first or with the second. The external electric field is produced by two distant electric charges Q 0 and Q for long distances between them the field in the central region between the charges resembles a homogeneous field and interacts with an object a dipole a quadrupole etc. located in the central region. A favourable orientation of the object corresponds to the lowest interaction energy with Q and Q. Fig. a shows such a low-energy situation for a dipole the charge protrudes towards Q while the charge protrudes towards Q. Fig. b corresponds to the opposite situation energetically non-favourable. As we can see the interaction energy of the dipole with the electric field differentiates these two situations. Now let us locate a quadrupole in the middle c . Let us imagine that a neutral point object has just split into four point charges of the same absolute value . The system lowers its energy by the charges going off the axis because they have increased their distance from the charge Q but at the same time the system energy has increased by the same amount since the charges went off the symmetrically located charge Q. What about the charges The splitting of the charges leads to an energy gain for the right-hand side charge because it approached Q and went off the charge Q but the left-hand side charge gives the opposite energy effect. Altogether the net result is zero. Conclusion the quadrupole does not interact with the homogeneous electric field. Now let us imagine an inhomogeneous field having a non-zero gradient along the axis . both Q charges differ by their absolute values . There will be no energy difference for the minus charges but one of the plus charges will be attracted more .

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