Ab initio MO calculations for the reactions of NH₂ with H₂, H₂O, NH₃ and CH₄: Prediction of absolute rate constants and kinetic isotope effects

Ab initio calculations for the NH₂ + H₂, NH₂ + CH₄, NH₂ + NH₃, and NH₂ + H₂O reactions are performed using the G2M method. Obtained energy barriers and molecular parameters are used for the TST calculations of absolute rate constants. The G2M method is shown to predict the activation energies within 2-3 kcal/mol. Adjustment of the calculated energy barrier by such an amount usually leads to good agreement between observed and predicted rate constants. Fitted three-parameter expressions for theoretical rate constants of the title reactions are obtained and recommended for practical applications. The calculations show significant isotope effects for the NH₂ + RH reactions. The primary isotope effect in the reactions of NH₂ with D₂, CD₄, ND₃, and D₂O is due to the absence of tunnelling and the increase of the quantum mechanical barriers because of differences in zero-point energies and results in the significant (up to 25 times) decrease of the reaction rates, especially, at low temperatures. The secondary isotope effect observed for the ND₂ reactions is opposite; the rate constants become slightly higher (up to two times at 300 K), since the quantum chemical barriers decrease.