Quantum chemistry of cocaine and its isomers II: Spectroscopy

Abstract

We investigate the potential of spectroscopic methods to distinguish enantiomers, with the example of the naturally occurring r-cocaine and its s-cocaine enantiomer. We compute conventional spectra in the infrared and ultraviolet frequency range by applying density functional theory and validate the results by comparing them to experimental data available for r-cocaine. Using the B3LYP functional, the calculated IR frequencies and intensities agree well with experimental spectra, and the corresponding normal modes have been analyzed. 1H and 13C NMR experimental chemical shifts have been well reproduced using the gauge independent atomic orbital method. However, In the UV/Vis range, the M06-L functional shows a significant improvement compared to B3LYP.The enantiomeric behavior of the r-cocaine and s-cocaine isomers cannot be reproduced unless the conformational effects are considered. Consequently, we compute a weighted average spectra for the ten lowest conformations up to 3.39 kcal/mol. Simulated Vibrational Circular Dichroïsm (VCD) and Electron Circular Dichroïsm (ECD) spectra of both isomers exhibit spectra with pronounced features of identical magnitude, making their identification unambiguous.