DFT computational and experimental study of indole continuum solvation

Abstract

A combined DFT and experimental study of the structure and the harmonic vibrational frequencies of free and solvated indole was performed. The DFT study included full geometry optimizations, as well as numerical harmonic vibrational analyses at BLYP/6-31G** level of theory. The solvent influence was included via the SCRF methodology (within the Onsager model). The FTIR spectra of indole solutions in CCl 4 were recorded and compared with the theoretical results. The BLYP formalism was shown to be very suitable for vibrational assignments without scaling in the 1600–1000 cm −1 frequency range. The computed higher frequency modes (3500–3000 cm −1) overestimate the experimental values, and scaling with a constant factor of about 0.98 is required. The continuum solvation has only a subtle influence on both the structure and harmonic force field. However, especially for the frequency range in which no scaling is needed, the gradient-corrected form of the DFT methodology is much more useful for vibrational assignments than the standard HF procedures that include scaling. On the basis of these more accurate DFT calculations, several reassignments were suggested in the vibrational spectra of the vapour specimen (compared to the HF/3-21G ones), which improved the convergence of the subsequent force fields fitting.