TY - JOUR
T1 - Ab initio MO calculations for the reactions of NH2 with H2, H2O, NH3 and CH4
T2 - Prediction of absolute rate constants and kinetic isotope effects
AU - Mebel, A. M.
AU - Moskaleva, L. V.
AU - Lin, Ming-Chang
PY - 1999/4/2
Y1 - 1999/4/2
N2 - 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.
AB - 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.
KW - Amino radical
KW - G2M method
KW - Hydrogen abstraction
KW - Rate constant
KW - Transition state theory
UR - http://www.scopus.com/inward/record.url?scp=0033515406&partnerID=8YFLogxK
U2 - 10.1016/S0166-1280(98)00423-0
DO - 10.1016/S0166-1280(98)00423-0
M3 - Article
AN - SCOPUS:0033515406
SN - 0166-1280
VL - 461-462
SP - 223
EP - 238
JO - Journal of Molecular Structure: THEOCHEM
JF - Journal of Molecular Structure: THEOCHEM
ER -