Temperature Dependence Of The Reaction Kinetics Of C₂(a³Л_u &X¹Σ_g⁺) Radical

This dissertation presents a temperature dependence study on the gas phase reactions of the C2(a3Πu & X1Σg+) radicals with a selected series of saturated alkanes, alcohols, alkenes, alkyne and some inorganic molecules by means of pulsed laser photolysis/laser-induced fluorescence technique. The C2(a3Πu & X1Σg+) radicals were produced by laser photolysis of C2Cl4 and can be readily detected via the well-known Swan (d3Πg-a3Πu) and Mulliken (D1EU-X1Eg) bands, respectively. From the analysis of the relative concentration-time behavior of C2(a3Πu & X1Σg+) under pseudo first-order conditions, the pseudo-first-order rate constant(k′) were determined; Linear least-squares fit of k′versus [reactant] therefore yields the bimolecular rate constant. The mechanisms for these reactions were discussed based on the experimental results and the theoretical calculation. Then the difference of the reactivity of the C2(X1Σg+) and C2(a3Πu) was discussed in detailed at different temperatures. The main results are summarized as follows:1. Temperature dependence of the reactions of C2(a3Πu & X1Σg+) with alkanes: The bimolecular rate constants for the reactions of C2(a3Πu) with C2H6, C3H8, n-C4H10, i-C4H10, n-C6H14 and c-C6H12 have been measured over the temperature range 298-673 K. Pronounced negative temperature dependences of the rate constants for the reactions of C2(a3Πu) with n-C4H10,i-C4H10, and n-C6H14 have been detected below-373 K, which is mainly attributed to steric hindrance of the more reactive secondary or tertiary C-H bonds by less reactive CH3 groups. We found that only secondary and tertiary C-H bonds contribute significantly to the observed negative temperature effect. In addition, our experimental results showing kt(i-C4H10)>ks(C3H8,n-C4H10, and n-C6H14)～10kp support the mechanism of hydrogen abstraction in the reactions of C2(a3Πu) with the saturated alkanes. We also measured the bimolecular rate constants for the reactions of C2(X1Σg+) with CH4, C2H6, C3H8,n-C4H10 and i-C4H10 over the temperature range 298-573 K. A weakly positive temperature dependence was observed for all reactions studied. Since the rate constants approached the gas-kinetic limit and the bimolecular rate constants for the reactions of C2(X1Σg+) with n-C4H10 and i-C4H10 were almost identical at each corresponding temperature, we draw a conclusion that the reactions of C2(X1Σg+) with the saturated alkane molecules proceed via an insertion mechanism.2. Temperature dependence of the reactions of C2(a3Πu & X1Σg+) with alcohol molecules:We measured the bimolecular rate constants for the reactions of C2(a3Πu) with methanol isotopomers, C2H5OH, 1-C3H7OH,2-C3H7OH and 1-C4H9OH over the temperature range 293-673 K. The observed deuterium kinetic isotope effects of the reactions of C2(a3Πu) with methanol, along with the positive temperature dependences of the reactions of C2(a3Πu) with alcohol molecules, allow us to reach a conclusion that the reactions of C2(a3Πu) with alcohol molecules in 293-673 K proceed via a site-specific hydrogen abstraction mechanism, that is, H-atom abstraction from the alkyl rather than hydroxyl site dominates reactivity. Absolute rate coefficients for the reactions of the C2(X1Σg+) with CH3OH, C2H5OH 1-C3H7OH and I-C4H9OH were determined over the temperature range 298-573 K. Because the rate constants approached the gas-kinetic limit along with the large difference between C2(X1Σg+) and C2(a3Πu) reaction rates, we thought that C2(X1Σg+) reacts with alcohol molecules by an insertion mechanism.3. Temperature dependence of the reactions of C2(a3Πu & X1Σg+) with alkenes and alkyne:We measured the bimolecular rate constants for the reactions of C2(a3Πu) with C2H4, C3H6,2-C4H8 and C2H2 over the temperature range 298-673 K and C2(X1Σg+) with C2H4 and C2H2 over the temperature range 293-573 K. Because the pσorbital is single occupied in C2(a3Πu) and empty in C2(X1Σg+), C2(a3Πu) reaction rates are smaller than C2(X1EΣg+) reaction rates at each corresponding temperature and reactant. The observed bimolecular rate constants along with the negative temperature dependences of these reactions allow us to reach a conclusion that the reactions of C2(a3Πu & X1Σg+) with alkenes and alkyne in the temperature range 298-673 K proceed via an addition mechanism. 4. Temperature dependence of the reactions of C2(a3Πu) with some inorganic molecules:Absolute rate coefficients for the reactions of the C2(a3Πu) with H2S, CS2, NO and so on were determined over the temperature range 293-673 K. The reaction of C2(a3Πu) with H2S proceeds mainly via hydrogen abstraction mechanism, and C2(a3Πu) reacts with CS2 and NO by an addition mechanism.