Theoretical Investigation on Mechanism, Thermochemistry, and Kinetics of the Gas-phase Reaction of 2-Propargyl Radical with Formaldehyde

Abstract

Gas-phase mechanism and kinetics of the reactions of the 2-propargyl radical(H₂CCCH), an important intermediate in combustion processes, with formaldehyde were investigated using ab initio molecular orbital theory at the coupled-cluster CCSD(T)//B3LYP/6-311++G(3df, 2p) method in conjunction with transition state theory(TST), variational transition state theory(VTST) and Rice-Ramsperger-Kassel-Marcus(RRKM) calculations for rate constants. The potential energy surface(PES) constructed shows that the H₂CCCH+HCHO reaction has six main entrances, including two H-abstraction and four additional channels, in which the former is energetically more favorable. The H-abstraction channels slide down to two quite weak pre-complexes COM-01(−9.3 kJ/mol) and COM-02(−8.1 kJ/mol) before going via energy barriers of 71.3(T0/P1) and 63.9 kJ/mol(T0/P2), respectively. Two post-complexes, COM-1(−17.8 kJ/mol) and COM-2(−23.4 kJ/mol) created just after coming out from T0/P1 and T0/P2, respectively, can easily be decomposed via barrier-less processes yielding H₂CCCH₂+CHO(P1, −12.4 kJ/mol) and HCCCH₃+CHO(P2, −16.5 kJ/mol), respectively. The additional channels occur initially by formation of four intermediate states, H₂CCCHCH₂O(I1, 1.1 kJ/mol), HCCCH₂CH₂O(I3, 4.5 kJ/mol), H₂CCCHOCH₂(I4, 10.2 kJ/mol), and HCCCH₂OCH₂(I6, 19.1 kJ/mol) via energy barriers of 66.3, 59.2, 112.2, and 98.6 kJ/mol at T0/1, T0/3, T0/4, and T0/6, respectively. Of which two channels producing I4 and I6 can be ignored due to coming over the high barriers T0/4 and T0/6, respectively. The rate constants and product branching ratios for the low-energy channels calculated show that the H₂CCCH+HCHO reaction is almost pressure-independent. Although the H₂CCCH+HCHO→I1 and H₂CCCH+HCHO→I3 channels become dominant at low temperature, however, they are less competitive channels at high temperature.

Keywords

Reaction mechanism / Propargyl radical / Formaldehyde / Potential energy surface(PES) / B3LYP / CCSD(T) / Rice-Ramsperger-Kassel-Marcus(RRKM) / Kinetics

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