Assignment of Vibrational Spectra of 1,10-Phenanthroline by Comparison with Frequencies and Raman Intensities from Density Functional Calculations

Abstract

NIR−FT-Raman spectra of 1,10-phenanthroline were recorded of crystalline material and three different solutions (CD3OD, CHCl3, and CS2) in the wavenumber range 100−1800 cm-1. FT-IR spectra of the solid material were obtained for the range 400−1800 cm-1. A complete assignment of the experimental vibrational spectra (IR and Raman) of 1,10-phenanthroline is given on the basis of calculated frequencies and Raman intensities obtained from the DFT(BP86) harmonic force field. A satisfactory agreement of the harmonic BP86 wavenumbers with the fundamental experimental ones is found. This good agreement is due to a systematic error compensation for BP86 harmonic force fields. Raman intensities were calculated within the double harmonic approximation with different basis sets in order to investigate whether moderately sized triple-ζ basis sets with comparatively few polarization functions can properly describe nonresonant Raman scattering. A satisfactory agreement with experimental data for all in-plane vibrations is already obtained with a standard valence triple-ζ basis plus polarization functions. However, for out-of-plane modes, we found a pronounced dependence of frequencies and intensities on the basis set size, which can be well understood by comparison with the results obtained with a larger basis set. The Sadlej basis set, which is recommended for the calculation of electrical properties, leads to deviations of up to 90 cm-1 for out-of-plane vibrations. Since 1,10-phenanthroline is hygroscopic and can hardly be obtained in water-free form, particular attention is paid to the influence of bound solvent molecules, like water and methanol, on the vibrational spectrum. Model systems with single solvent molecules attached to the isolated phenanthroline molecule were studied. It was found that the wavenumber shifts induced are generally small.