structural and vibrational properties of the two enantiomers of etodolac: A complete assignment of the vibrational spectra

The structural and vibrational properties of the R- and S-enantiomers of etodolac in this chapter were predicted by combining the available experimental vibrational spectra (FTIR-Raman) with ab-initio calculations based on density functional theory (DFT). For complete assignment of the infrared and Raman spectra, DFT calculations were combined with Pulaýs Scaled Quantum Mechanics Force Field (SQMFF) methodology to fit the theoretical frequency values to the experimental ones. A complete assignment of the 120 normal vibrational modes for etodolac was performed. The presence of an equal mixture of the enantiomers in the racemate is inferred from the absence of bands corresponding to CH2 and CH3 symmetric stretching modes in the Raman spectrum. The presence of the S-enantiomer in the racemate is evidenced by Raman bands at 810 and 434 cm-1 and by IR bands at 1478, 1300, 1262 and 1147 cm-1. IR bands at 2869, 1282 and 988 cm-1 and a Raman band at 579 cm-1 indicated the presence of the R-enantiomer of etodolac in the racemate. The structural properties of both enantiomers, such as bond orders, possible charge-transfer and topological properties of the indole and pyran rings, were studied by means of the Natural Bond Orbital (NBO) and Atoms in Molecules (AIM) theories and HOMO-LUMO investigation. These studies reveal the high stability of the R-enantiomer relative to the S-enantiomer.