Direct dynamics study on the reaction of N2H4 with F atom: a hydrogen abstraction reaction?
ABSTRACT We present a systematic direct ab initio dynamics investigation of the reaction between N2H4 and F atom, which is predicted to have three possible reaction channels. The structures and frequencies at the stationary points and the points along the minimum energy paths (MEPs) of all reaction channels were calculated at the UB3LYP/6-31+G(d,p) level of theory. Energetic information of stationary points and the points along the MEPs was further refined by means of the CCSD(T)/aug-cc-pVTZ method. The calculated results revealed that the first two primary channels (N2H4+F-->N2H3+HF) are equivalent and occur synchronously via the formation of a pre-reaction complex with Cs symmetry rather than via the direct H abstraction. The pre-reaction complex then evolves into a hydrogen-bonding intermediate through a transition state with nearly no barrier and a high exothermicity, which finally makes the intermediate further decompose into N2H3 and HF. Another reaction channel of minor role (N2H4+F-->NH2F+NH2) was also found during the calculations, which has the same Cs pre-reaction complex but forms NH2F and NH2 via another transition state with high-energy barrier and low exothermicity. The rate constants of these channels were calculated using the improved canonical variational transition state theory with the small-curvature tunneling correction (ICVT/SCT) method. The three-parameter ICVT/SCT rate constant expressions of k(ICVT/SCT) at the CCSD(T)/aug-cc-pVTZ//UB3LYP/6-31+G(d,p) level of theory within 220-3000 K were fitted as (7.64x10(-9))T (-0.87) exp(1180/T) cm3 mole-1 s-1 for N2H4+F-->N2H3+HF and 1.45x10(-12)(T/298)(2.17) exp(-1710/T) cm3 mole-1 s-1 for N2H4+F-->NH2F+NH2.
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ABSTRACT: In this article we systematically examine the ability of current electronic structure methods to treat transition states whose unrestricted wave functions show significant spin contamination. Three H atom abstraction reactions have been selected as test cases for the study, namely the reactions of trans-N2H2 with H, CH4 with OH, and C2H6 with OH. In each case we calculate the exoergicity, barrier heights, and transition state geometry at 3 to 26 levels of theory. The spin contamination in spin-unrestricted electronic structure calculations of the transition states is in the range of 0.755−0.9. Twelve different kinds of ab initio calculation with electron correlation (UMP2, ROMP2, UMP4, UCCD, UQCISD, UCCSD, UQCISD(T), UCCSD(T), RUCCSD, RCCSD, RUCCSD(T), and RCCSD(T)) are applied with two correlation-consistent basis sets (cc-pVDZ and cc-pVTZ). We conclude that quadratic configuration interaction and coupled cluster methods, even with unrestricted reference states, provide good approximations to transition state geometries and energies.The Journal of Physical Chemistry A 12/1999; 104(3). · 2.77 Impact Factor