Theoretical study on the rate constants for the C₂H₅ + HBr → C₂H₆ + Br reaction

Abstract

The reaction C(2)H(5) + HBr --> C(2)H(6) + Br has been theoretically studied over the temperature range from 200 to 1400 K. The electronic structure information is calculated at the BHLYP/6-311+G(d,p) and QCISD/6-31+G(d) levels. With the aid of intrinsic reaction coordinate theory, the minimum energy paths (MEPs) are obtained at the both levels, and the energies along the MEP are further refined by performing the single-point calculations at the PMP4(SDTQ)/6-311+G(3df,2p)//BHLYP and QCISD(T)/6-311++G(2df,2pd)//QCISD levels. The calculated ICVT/SCT rate constants are in good agreement with available experimental values, and the calculate results further indicate that the C(2)H(5) + HBr reaction has negative temperature dependence at T < 850 K, but clearly shows positive temperature dependence at T > 850 K. The current work predicts that the kinetic isotope effect for the title reaction is inverse in the temperature range from 200 to 482 K, i.e., k(HBr)/k(DBr) < 1.

Detailed benchmark ab initio mapping of the potential energy surfaces of the X + C2H6 [X = F, Cl, Br, I] reactions

“…Comparing our new benchmark data to previous results 14,16,17,22,25,[28][29][30]33,35,[37][38][39][40][41] of the literature, where only the main H-abstraction channel was investigated for the X + C 2 H 6 [X = F, Cl, Br, I] reactions, we find considerable inaccuracies regarding the barrier heights, even their signs for X = F and Cl.…”

“…Besides the main H-abstraction reaction path and the higher-energy H-substitution studied also for halogen + methane reactions, in the X + C 2 H 6 [F, Cl, Br, I] reactive system methyl-substitution appears as a new mechanism and is expected to occur at lower energies then H-substitution. For these reactions so far only the H-abstraction channel (X = F, Cl) [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] or its reverse reaction (X = Br, I) [34][35][36][37][38][39][40][41] have been investigated.…”

“…the HX + C 2 H 5 -X + C 2 H 6 reactions have been the subject of both theoretical and experimental investigations due to their supposedly non-Arrhenius behavior. [34][35][36][37][38][39][40][41] For the HBr + C 2 H 5 -Br + C 2 H 6 reaction theoretical studies using the CCSD(T)/cc-pVTZ method as highest level of theory revealed a loosely-bound adduct in the entrance channel and a tightly-bound TS very close in energy and both being below the reactant asymptote. 35,38,39 The HI + C 2 H 5 -I + C 2 H 6 reaction has also been studied both by experimental 34,40 and theoretical 40,41 techniques.…”

Reaction Dynamics of CH3+ HBr → CH4+ Br at 150-1000 K

“…7,10,[12][13][14]17,18 A particularly significant aspect of the temperature dependence is that some simple reactions involving radicals proceed via negative activation energies at low temperatures (< 500 K). 7,[12][13][14]17,18 One such reaction is CH 3 + HBr → CH 4 + Br, which is a prototypical system for free radical reactions and the subject of the present paper. In recent years, chemical reactions involving bromine and its derivatives such as HBr, BrO, HOBr and CH 3 Br have become an important subject in studying the delicate balance of atmospheric molecules and atoms, in particular those in the Arctic polar region, [19][20][21][22][23] even though their concentrations are low.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] The common characteristics of these reactions are the abstraction of a hydrogen atom by a radical in a highly exo-ergic environment with a low or negligible activation energy barrier and rate constants showing weak temperature dependence. 7,10,[12][13][14]17,18 A particularly significant aspect of the temperature dependence is that some simple reactions involving radicals proceed via negative activation energies at low temperatures (< 500 K). 7,[12][13][14]17,18 One such reaction is CH 3 + HBr → CH 4 + Br, which is a prototypical system for free radical reactions and the subject of the present paper.…”