Medeiros D J, Blitz M A, James L, Speak T H, Seakins P W
School of Chemistry , University of Leeds , Leeds , LS2 9JT , U.K.
National Centre for Atmospheric Science , University of Leeds , Leeds , LS2 9JT , U.K.
J Phys Chem A. 2018 Sep 20;122(37):7239-7255. doi: 10.1021/acs.jpca.8b04829. Epub 2018 Sep 7.
The reaction of the OH radical with isoprene, CH (R1), has been studied over the temperature range 298-794 K and bath gas pressures of nitrogen from 50 to 1670 Torr using laser flash photolysis (LFP) to generate OH and laser-induced fluorescence (LIF) to observe OH removal. Measurements have been made using both a conventional LFP/LIF apparatus and a new high pressure system. The measured rate coefficient at 298 K ( k = (9.90 ± 0.09) × 10 cm molecule s) in the high pressure apparatus is in excellent agreement with the literature, confirming the accuracy of measurements made with this instrument. Above 700 K, the OH decays were no longer single exponentials due to regeneration of OH from adduct decomposition and the establishment of the OH + CH ⇌ HOCH equilibrium (R1a, R-1a). This equilibrium was analyzed by comparison to a master equation model of reaction R1 and determined the well depth for OH addition to carbon C and C to be equal to 153.5 ± 6.2 and 143.4 ± 6.2 kJ mol, respectively. These well depths are in excellent agreement with the present ab initio-CCSD(T)/CBS//M062X/6-311++G(3df,2p)-calculations (154.1 kJ mol for the C adduct). Addition to the less stable C and C adducts is not important. The data above 700 K also indicated that a minor but significant direct abstraction channel, R1b, was also operating with k = (1.3 ± 0.3) × 10 exp(-3.61 kJ mol/ RT) cm molecule s. Additional support for the presence of this abstraction channel comes from our ab initio calculations and from room-temperature proton transfer mass spectrometry product analysis. The literature data on reaction R1 together with the present data were assessed using master equation analysis, using the MESMER package. This analysis produced a refined data set that generates our recommended k( T, [ M]). An analytical representation of k( T, [ M]) and k( T, [ M]) is provided via a Troe expression. The reported experimental data (the sum of addition and abstraction), k = (9.5 ± 0.2) × 10( T/298 K) + (1.3 ± 0.3) × 10 exp(-3.61 kJ mol/ RT) cm molecule s, significantly extend the measured temperature range of R1.
利用激光闪光光解（LFP）产生羟基自由基（OH）并通过激光诱导荧光（LIF）观测OH的消耗，在298 - 794 K的温度范围以及50至1670托的氮气浴气压力条件下，研究了OH自由基与异戊二烯（CH，R1）的反应。实验使用了传统的LFP/LIF装置和新型高压系统进行测量。在高压装置中测得的298 K时的速率系数（k = (9.90 ± 0.09) × 10 cm³·molecule⁻¹·s⁻¹）与文献值高度吻合，证实了该仪器测量结果的准确性。在700 K以上，由于加合物分解产生的OH再生以及OH + CH ⇌ HOCH平衡（R1a，R - 1a）的建立，OH的衰减不再是单一指数形式。通过与反应R1的主方程模型进行比较分析该平衡，并确定OH加成到碳C₁和C₄上的阱深分别等于153.5 ± 6.2和143.4 ± 6.2 kJ·mol⁻¹。这些阱深与当前的从头算 - CCSD(T)/CBS//M062X/6 - 311++G(3df,2p)计算结果（C₁加合物为154.1 kJ·mol⁻¹）高度一致。加成到较不稳定的C₂和C₃加合物上的情况并不重要。700 K以上的数据还表明，一个较小但显著的直接夺氢通道R1b也在起作用，其速率系数为k = (1.3 ± 0.3) × 10⁻¹² exp( - 3.61 kJ·mol⁻¹/RT) cm³·molecule⁻¹·s⁻¹。对该夺氢通道存在的额外支持来自我们的从头算计算以及室温质子转移质谱产物分析。使用MESMER软件包，通过主方程分析评估了反应R1的文献数据和当前数据。该分析产生了一个经过改进的数据集，据此生成了我们推荐的k(T, [M])。通过Troe表达式给出了k(T, [M])和k(T, [M])的解析表达式。所报道的实验数据（加成和夺氢之和）k = (9.5 ± 0.2) × 10⁻¹² (T/298 K) + (1.3 ± 0.3) × 10⁻¹² exp( - 3.61 kJ·mol⁻¹/RT) cm³·molecule⁻¹·s⁻¹，显著扩展了反应R1的测量温度范围。
