乙炔基自由基与水蒸气反应的脉冲激光光解及量子化学-统计速率研究

Carl Shaun A, Nguyen Hue Minh Thi, Elsamra Rehab M I, Nguyen Minh Tho, Peeters Jozef

Department of Chemistry, University of Leuven, Belgium.

J Chem Phys. 2005 Mar 15;122(11):114307. doi: 10.1063/1.1861887.

The rate coefficient of the gas-phase reaction C(2)H + H(2)O-->products has been experimentally determined over the temperature range 500-825 K using a pulsed laser photolysis-chemiluminescence (PLP-CL) technique. Ethynyl radicals (C(2)H) were generated by pulsed 193 nm photolysis of C(2)H(2) in the presence of H(2)O vapor and buffer gas N(2) at 15 Torr. The relative concentration of C(2)H radicals was monitored as a function of time using a CH* chemiluminescence method. The rate constant determinations for C(2)H + H(2)O were k(1)(550 K) = (2.3 +/- 1.3) x 10(-13) cm(3) s(-1), k(1)(770 K) =(7.2 +/- 1.4) x 10(-13) cm(3) s(-1), and k(1)(825 K) = (7.7 +/- 1.5) x 10(-13) cm(3) s(-1). The error in the only other measurement of this rate constant is also discussed. We have also characterized the reaction theoretically using quantum chemical computations. The relevant portion of the potential energy surface of C(2)H(3)O in its doublet electronic ground state has been investigated using density functional theory B3LYP6-311 + + G(3df,2p) and molecular orbital computations at the unrestricted coupled-cluster level of theory that incorporates all single and double excitations plus perturbative corrections for the triple excitations, along with the 6-311 + + G(3df,2p) basis set [(U)CCSD(T)6-311 + + G(3df,2p)] and using UCCSD(T)6-31G(d,p) optimized geometries. Five isomers, six dissociation products, and sixteen transition structures were characterized. The results confirm that the hydrogen abstraction producing C(2)H(2)+OH is the most facile reaction channel. For this channel, refined computations using (U)CCSD(T)6-311 + + G(3df,2p)(U)CCSD(T)6-311 + + G(d,p) and complete-active-space second-order perturbation theory/complete-active-space self-consistent-field theory (CASPT2/CASSCF) [B. O. Roos, Adv. Chem. Phys. 69, 399 (1987)] using the contracted atomic natural orbitals basis set (ANO-L) [J. Almlof and P. R. Taylor, J. Chem. Phys.86, 4070 (1987)] were performed, yielding zero-point energy-corrected potential energy barriers of 17 kJ mol(-1) and 15 kJ mol(-1), respectively. Transition-state theory rate constant calculations, based on the UCCSD(T) and CASPT2/CASSCF computations that also include H-atom tunneling and a hindered internal rotation, are in perfect agreement with the experimental values. Considering both our experimental and theoretical determinations, the rate constant can best be expressed, in modified Arrhenius form as k(1)(T) = (2.2 +/- 0.1) x 10(-21)T(3.05) exp[-(376 +/- 100)T] cm(3) s(-1) for the range 300-2000 K. Thus, at temperatures above 1500 K, reaction of C(2)H with H(2)O is predicted to be one of the dominant C(2)H reactions in hydrocarbon combustion.

采用脉冲激光光解 - 化学发光（PLP - CL）技术，在500 - 825 K的温度范围内通过实验测定了气相反应C₂H + H₂O→产物的速率系数。在15托的H₂O蒸气和缓冲气体N₂存在下，通过对C₂H₂进行193 nm脉冲光解产生乙炔基（C₂H）。使用CH*化学发光方法监测C₂H自由基的相对浓度随时间的变化。对于C₂H + H₂O反应，速率常数测定结果为k₁(550 K) = (2.3 ± 1.3)×10⁻¹³ cm³ s⁻¹，k₁(770 K) =(7.2 ± 1.4)×10⁻¹³ cm³ s⁻¹，k₁(825 K) = (7.7 ± 1.5)×10⁻¹³ cm³ s⁻¹。还讨论了该速率常数唯一其他测量值中的误差。我们还使用量子化学计算从理论上对该反应进行了表征。利用密度泛函理论B3LYP6 - 311++G(3df,2p)以及在无限制耦合簇理论水平上的分子轨道计算（该理论包含所有单重和双重激发以及三重激发的微扰校正），结合6 - 311++G(3df,2p)基组[(U)CCSD(T)6 - 311++G(3df,2p)]，并使用UCCSD(T)6 - 31G(d,p)优化几何结构，研究了处于双重电子基态的C₂H₃O势能面的相关部分。表征了五种异构体、六种解离产物和十六种过渡结构。结果证实，生成C₂H₂ + OH的氢提取是最容易发生的反应通道。对于该通道，使用(U)CCSD(T)6 - 311++G(3df,2p)(U)CCSD(T)6 - 311++G(d,p)以及使用收缩原子自然轨道基组（ANO - L）[J. Almlof和P. R. Taylor，J. Chem. Phys.86, 4070 (1987)]的完全活性空间二阶微扰理论/完全活性空间自洽场理论（CASPT2/CASSCF）[B. O. Roos，Adv. Chem. Phys. 69, 399 (1987)]进行了精确计算，分别得到零点能量校正后的势能垒为17 kJ mol⁻¹和15 kJ mol⁻¹。基于UCCSD(T)和CASPT2/CASSCF计算（其中还包括H原子隧穿和受阻内旋转）的过渡态理论速率常数计算结果与实验值完全吻合。综合我们的实验和理论测定结果，在300 - 2000 K范围内，速率常数最好以修正的阿伦尼乌斯形式表示为k₁(T) = (2.2 ± 0.1)×10⁻²¹T³.⁰⁵ exp[-(376 ± 100)/T] cm³ s⁻¹。因此，在温度高于1500 K时，预计C₂H与H₂O的反应是烃类燃烧中C₂H的主要反应之一。