A theoretical study of regio and stereoselectivity nitration of thymol and carvacrol using DFT approach

This work is a detailed theoretical study of the nitration aromatic substitution reactions of thymol and carvacrol. In this process, a mixture of nitric and sulfuric acids is used to produce the nitronium agent NO2+. The computational calculations were performed at the Gaussian 09 using the DFT approaches. The first task of the computing study was to determine the optimized geometry of these compounds using the Becke three-parameter hybrid exchange functional and the Lee-Yang-Parr correlation functional (B3LYP) with the 6-311+G (d, p) basis set. Geometry optimization calculations have been carried out to find the global and local minima for reactants and intermediates, respectively, and to locate the saddle points for the transition states. The vibration frequencies have been calculated in order to check the character of the stationary points obtained after the geometry optimization. It is expected only positive frequencies for reactants, intermediates and products, but only one negative imaginary frequency for transition states.The analysis of the nucleophilic fk- Fukui Function and Pk- Parr functions allows characterising the C4 carbon atom as the most nucleophilic center of thymol and carvacrol, in clear agreement with the regioselectivity obtained. Calculation of activation energies, analysis of the potential energy surface and the Gibbs free energy indicates that this reaction takes place through a two-step mechanism