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Wyznaczenie elektrycznego momentu dipolowego kationu ARH+

Molski M.

Wiadomości Chemiczne

2006

[Z] 60, 5-6

423-439

Determination of the electric dipolar moment of a molecule is one of the most important procedures applied to characterize the molecule spectral activity, type of the chemical bonds between atoms forming the molecule and its geometry. Electric dipolar moment can be determined by making use of the Stark or Zeeman effects, quantum-mechanical ab initio calculations or from highly resolved MW and IR rotation-vibrational spectra using spectroscopic methods. Each of the methods mentioned has advantages and disadvantages. For example, the Stark method cannot be applied to molecular ions as under external electric field they are accelerated towards the wall of the discharge tube. In 1955 Townes et al., developed the method of determination of the dipolar moment using the Zeeman effect. This approach has been generalized in 1987 by Laughlin et al., and applied to ArH+ cation. They obtained the values: 1.4(4) D, 1.59(40) D and 3.0(6) D, which seriously differed from the ab initio result 2.2(1) D provided by Rosmus in 1979. The additional ab initio calculations performed by Pyykko et al., and then by Geertsen and Scuseria in 1989, confirmed the correctness of the result obtained earlier by Rosmus. In such circumstances only the application of a third independent method could provide a correct value of the electric dipolar moment of ArH+. Following this suggestion, Molski in 2001 applied the spectroscopic method to evaluate dipolar moment of ArH+ from highly resolved MW and IR spectra including pure rotational and vibration-rotational lines. The result obtained 2.12(55) D confirmed that ab initio calculations provided a reliable value of the dipolar moment of ArH+, whereas the experimental values obtained by Laughlin et al., were less accurate. The supplementary calculations performed for KrH+ and HeH+ indicate that the method of Laughlin et al. does not produce reliable values of the electric dipolar moment for diatomic ions, where as for polyatomic molecules this method is reliable.