Abstract: Both conformers(C_i,D_3d) of 18-crown-6 have been studied by Langevin mode method under four viscidities of solvent:0.0,3.0×10^-4,6.0×10^-4 Pa·s(about the viscidity of methol at room temperature),9.0×10^-4 Pa·s(about the viscidity of water at room temperature).For viscidity of 9.0×10^-4 Pa·s,both Oseen method and Rotne-Prage method were used in the calculation.The results showed that the viscidities of solvent greatly influenced on the properties of 18-crown-6,especially the distribution of the vibrational frequency and the librational angle of the bond vector.The effects of nondiagonal elements of the frictional matrix must be considered by using Oseen or Rotne-Prage method when the viscidity of solvent is large.At the same time,the results also show that Langevin mode method is a very effectivce model for interpreting molecular motion in solution.

Key words: Langevin mode, 18-Crown-6, Vibrational analysis, Correlation function