Analysis of infrared and Raman spectra calculated by molecular dynamics

Molecular dynamics (MD) is a common and powerful method to simulate chemica l systems at the atomic level. The calculation of vibrational spectra is a less common application of MD simulations. In this work, the interest was f ocused on the analysis of vibrational spectra (infrared and Raman) of polym ers from MD data. For this purpose two methods were developed: the simulati on of polarized LR spectra (IR dichroism) and the spectra of particular int ernal coordinates. Molecular dynamic calculations reproduce the polarized i nfrared spectra with a high accuracy in comparison with experimental data. It is shown that these data tan be obtained from an analysis of the three-d imensional orientation of the molecular dipole moment. The calculated spect rum of any particular coordinate yields a direct correlation between a vibr ational band and the concerned coordinate. We find an excellent agreement b etween these calculated spectra for polyethylene and the potential energy d istribution (PED) derived from NCA. These spectra allow a quick assignment of vibrational bands also for IR or Raman inactive modes, which are found i n power spectrum. Thus, MD is shown to be a method complementary to NCA. (C ) 1999 Elsevier Science B.V. All rights reserved.