The association reaction between C2H and 1-butyne: a computational chemical kinetics study

Abstract

The potential energy surfaces (PES) for the reaction of the C(2)H radical with 1-butyne (C(4)H(6)) have been studied using the CBS-QB3 method. Density functional B3LYP/cc-pVTZ and M06-2X/6-311++G(d,p) calculations have also been performed to analyze the reaction energetics. For detailed theoretical calculation on the total reaction mechanism, the initial association reactions on more and less substituted C atoms of 1-butyne are treated separately followed by a variational transition state theory (VTST) calculation to obtain reaction rates. The successive unimolecular reactions from the association reaction complexes are subjected to Rice-Ramsperger-Kassel-Marcus (RRKM) calculations for reaction rate constants and product branching ratios. The calculated rate constants in the temperature range 70-295 K for both the association reactions are found to be highly temperature dependent at low temperatures, which is contrary to the experimental findings of temperature independent association rates. We have explained this observation with the help of variational nature of the transition states, and we found a "loose" transition state at low temperatures. The calculated product branching ratios for the unimolecular reactions generally agree with the available experimental data, although some channels show a significant method dependency and therefore the correlation with experiment is lost to some extent. Our detailed reaction energetics calculations confirm that the C(2)H + C(4)H(6) reaction proceeds without an entrance barrier and leads to the important products ethynylallene + CH(3), 1,3-hexadiyne + H, 3,4-hexadiene-1-yne + H, 2-ethynyl-1,3-butadiene + H, 3,4-dimethylenecyclobut-1-ene + H and fulvene + H exothermic by 25-75 kcal mol(-1), with strong dependence of the product distribution on the association mode of C(2)H with C(4)H(6), making these reactions fast under low temperature conditions of Titan's atmosphere. Therefore this study can provide a detailed picture of the complex hydrocarbon formation mechanism in the upper atmosphere.

Computational Study on Thermochemical Properties for Perhalogenated Methanols (CX3OH) (X = F, Cl, Br)

“…Therefore, it was difficult to test the accuracy of the Δ f H 0 results for these compounds. It is well known that the use of composite methods usually leads to accurate energetics, especially for reactions involving small molecular systems, with an error that does not usually exceed 6 kJ mol –1 . Hence, the accuracy of the HF, MP2, and B3LYP results is tested by comparing the calculated Δ f H 0 values for CCl 3 OH and CBr 3 OH at these levels of theory with the G3MP2 and CBS-QB3 values.…”

A DFT study on the reaction pathway involved in the metal‐ion‐templated synthesis of benzo‐21‐crown‐7(B21C7) from catechol and hexaethylene glycol ditosylate in the presence of base

“…[ 49 ] This popular hybrid functional is reported to produce excellent results for thermochemistry and reaction kinetics. [ 50 ] The split‐valence triple zeta basis set 6‐311G has been used with added polarization on heavy atoms, as well as on H atoms (6‐311G**). True energy minima have been verified by the absence of any imaginary frequency, whereas the verifications of TS are done by the presence of a single imaginary frequency.…”

Abstraction and addition kinetics of C2H radicals with CH4, C2H6, C3H8, C2H4, and C3H6: CVT/SCT/ISPE and hybrid meta-DFT methods

“…23 The structural parameters of reactants, pre-reactive complexes (PRCs), transition states, and products were optimized using the hybrid meta exchange correlation functional, called the M06-2X 24,25 method. [29][30][31] By employing the POLYRATE 2008 program 32 and GAUSSRATE 2009A, 33 theoretical rate coefficients were calculated over the wide temperature range of 150-5000 K using the canonical variational transition state theory [34][35][36] (CVT) with the small-curvature tunneling 37,38 (SCT) method. All the reactants, pre-reactive complexes and products were identified with zero imaginary frequency (number of imaginary (NIMG) = 0), and TSs were identified with one imaginary frequency (NIMG = 1).…”

“…Similar types of methodology used for the kinetic studies of this class of reactions are available in the literature. [29][30][31] By employing the POLYRATE 2008 program 32 and GAUSSRATE 2009A, 33 theoretical rate coefficients were calculated over the wide temperature range of 150-5000 K using the canonical variational transition state theory [34][35][36] (CVT) with the small-curvature tunneling 37,38 (SCT) method. The CVT rate coefficient for temperature T is expressed as…”