Chlorination of ammonia and aliphatic amines by Cl2: DFT study of medium and substituent effects

The mechanism of chlorination of ammonia and aliphatic amines by Cl₂ was studied by quantum‐chemical calculations using a series of DFT functionals. Three different reaction pathways were considered for the reaction between Cl₂ and NH₃ in the gas phase. Several intermediates and transition state structures, not described earlier, were located on the corresponding potential energy surface. It is calculated that the reaction field effects (SCIPCM) on the chlorination is much less pronounced than the effect of a specific solvent interaction which was modeled by an explicit water molecule. It is also found that the calculated energy barrier and the reaction free energy of the chlorination of different amines are dependent on the alkyl‐substituent effects. With increase in the basicity of amine, the chlorination reaction becomes more feasible. Calculated geometries of intermediates and overall reaction energetics are significantly influenced by the method for a treatment of electron correlation (DFT vs. MP2), and by the fraction of HF exchange (χ) in DFT functionals. With increase in the χ in the corresponding functional, the DFT results approach those obtained at the MP2 level, and are closer to experimental values, as well. Copyright © 2008 John Wiley & Sons, Ltd.