Department of Chemistry, Acadia, University, Wolfville, Nova Scotia B4P 2R6, Canada.
J Phys Chem A. 2010 Jan 14;114(1):291-8. doi: 10.1021/jp908168u.
The rate coefficient for the reaction of atomic bromine with 1,4-dioxane was measured from approximately 300 to 340 K using the relative rate method. Iso-octane and iso-butane were used as reference compounds, and the experiments were made in a bath of argon containing up to 210 Torr of O(2) at total pressures between 200 and 820 Torr. The rate coefficients were not affected by changes in pressure or O(2) concentration over our range of experimental conditions. The ratios of rate coefficients for the reaction of dioxane relative to the reference compound were put on an absolute basis by using the published absolute rate coefficients for the reference reactions. The variation of the experimentally determined rate coefficients with temperature for the reaction of Br with 1,4-dioxane can be given by k(1)(exp)(T) = (1.4 +/- 1.0) x 10(-11)exp[-23.0 +/- 1.8) kJ mol(-1)/(RT)] cm(3) molecule(-1) s(-1). We rationalized our experimental results in terms of transition state theory with molecular data from quantum chemical calculations. Molecular geometries and frequencies were obtained from MP2/aug-cc-pVDZ calculations, and single-point energies of the stationary points were obtained at CCSD(T)/CBS level of theory. The calculations indicate that the 1,4-dioxane + Br reaction proceeds in an overall endothermic addition-elimination mechanism via a number of intermediates. The rate-determining step is a chair-to-boat conformational change of the Br-dioxane adduct. The calculated rate coefficients, given by k(1)(calc)(T) = 5.6 x 10(-11)exp[-26.6 kJ mol(-1)/(RT)] cm(3) molecule(-1) s(-1), are in very good agreement with the experimental values. Comparison with results reported for the reactions of Br with other ethers suggests that this multistep mechanism differs significantly from that for abstraction of hydrogen from other ethers by atomic bromine.
用相对速率法测定了 300 到 340 K 下溴原子与 1,4-二氧六环的反应速率系数。异辛烷和异丁烷被用作参考化合物，实验在总压为 200 到 820 托，其中含有高达 210 托 O(2)的氩浴中进行。在我们的实验条件范围内，速率系数不受压力或 O(2)浓度变化的影响。通过使用参考反应的已发表的绝对速率系数，将相对于参考化合物的二氧六环反应的速率系数比值置于绝对基础上。Br 与 1,4-二氧六环反应的实验确定的速率系数随温度的变化可以表示为 k(1)(exp)(T) = (1.4 +/- 1.0) x 10(-11)exp[-23.0 +/- 1.8) kJ mol(-1)/(RT)] cm(3) molecule(-1) s(-1)。我们根据过渡态理论，用量子化学计算得到的分子数据来解释我们的实验结果。分子几何形状和频率是从 MP2/aug-cc-pVDZ 计算中获得的，并且在 CCSD(T)/CBS 理论水平上获得了稳定点的单点能量。计算表明，1,4-二氧六环 + Br 反应通过多个中间体以总体吸热加成消除机制进行。速率决定步骤是 Br-二氧六环加合物的椅式到船式构象变化。计算得到的速率系数 k(1)(calc)(T) = 5.6 x 10(-11)exp[-26.6 kJ mol(-1)/(RT)] cm(3) molecule(-1) s(-1)，与实验值非常吻合。与 Br 与其他醚反应的报道结果进行比较表明，这种多步机制与溴原子从其他醚中提取氢的机制有很大的不同。
