Ab Initio Study of Hydrogen Abstraction Reactions

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Abstract

Ab initio electronic structure calculations have been carried out on six hydrogen abstraction reactions of the form, X−H + Y• → X• + H−Y, where X, Y = CH3, NH2, and OH. Geometric structures for the reactants, reactant complexes, transition states, product complexes, and products of each reaction have been gradient optimized at both the UMP2 and DFT(B3LYP) theory levels using the 6-311++G(2d,p) basis set. The character of each stationary state as a minimum or saddle point was determined by a harmonic force field calculation. PMP2 and CCSD(T) energies were also calculated at the UMP2 optimized geometries. CCSD(T) geometry optimizations were carried out for selected cases to resolve differences in results between UMP2 and DFT. The calculated reaction energies, barrier heights, and weak complex stabilities are compared to experiment, where possible, and among the different theory levels. The geometric structures and wave function properties are compared between the UMP2, CCSD(T), and DFT(B3LYP) methods. In general, all the methods predict reaction energies to within several kcal/mol of experiment. Calculated DFT(B3LYP) activation barriers are usually equal to or smaller than fitted Arrehenius equation activation energies (Ea), with the gap increasing as X and Y are more electronegative. The DFT(B3LYP) activation barrier for the CH4 + CH3• reaction is, therefore, in the best agreement with experiment. The UMP2, PMP2//UMP2, and CCSD(T)//UMP2 methods give activation energies that are somewhat greater than Ea, as expected. The geometric structures of the three exchange reaction (X ≠ Y) transition states (TS) generally agree reasonably well between the UMP2 and DFT(B3LYP) methods, with active site bond length differences reflecting variations in the calculated exothermicities of the reactions. The UMP2 and DFT(B3LYP) methods give different conformations for the NH3 + OH• transition state, and CCSD(T) endorses the DFT results. For the H2O + OH• reaction UMP2 gives the symmetric structure as an energy minimum. DFT(B3LYP) predicts the symmetric structure to be the TS, in agreement with the optimized CCSD(T) result. For the NH3 + NH2• reaction the UMP2 transition state is symmetric but with an unusually low imaginary reaction path frequency, while the DFT(B3LYP) frequency is reasonable. CCSD(T)//UMP2 and DFT(B3LYP) hydrogen-bonding energies for the reaction and product complexes are very similar and generally smaller than the UMP2 and PMP2//UMP2 results. The equilibrium geometric structures at the UMP2 and DFT(B3LYP) levels generally agree reasonably well. However, for the NH2···H−OH product complex DFT(B3LYP) gives a planar conformation, while UMP2 predicts a Cs nonplanar configuration.

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