氯原子与几类卤代烷烃氢迁移反应的理论研究/

2019-04-05 14:16:31

反应 CL HCl reactions reg



Cl原子与卤代烷烃之间的氢迁移反应在工业氯化和具有危害性的卤代废弃物的焚烧过程中起重要的作用。在大气化学和环境化学科学领域中也是一类重要的反应,特别是对控制大气中臭氧的浓度有潜在的重要性,与环境相关的Cl催化臭氧的转移有密切的关系。因此研究这类反应的微观反应机制、动力学性质,精确得到反应的速率常数,对保护大气臭氧层和环境有重要的意义。有很多的实验工作测量这些反应的速率常数,但相应的理论研究非常少。而在理论上实现对不同条件下化学反应速率常数的预测是目前理论化学领域中活跃的前沿研究课题。因此,本文利用直接动力学方法,结合密度泛函理论(DFT)和从头算分子轨道理论,对以下氢迁移反应:
Cl + CH4-nFn® CH3-nFn + HCl, n=1-3
Cl + CH4-nCln® CH3-nCln + HCl, n=1-3
Cl + CHCl3-nFn® CCl3-nFn + HCl, n=1-2
Cl + CH4-nBrn® CH3-nBrn + HCl, n=1-3
Cl + CH2BrCl ®CHBrCl + HCl
Cl + CHBrCl2 ®CBrCl2 + HCl
Cl + CH3CCl3 ®CH2CCl3 + HCl
Cl + CH3CF3 ®CH2CF3 + HCl
进行了系统的理论研究,目的是通过研究这些反应的反应路径及速率常数与温度的依赖关系等动力学信息,揭示反应的微观机制。利用GAUSSIAN98程序,在BH&HLYP水平下计算稳定点(反应物、过渡态和产物)的几何和频率,进一步利用内禀反应坐标理论获得反应的最小能量路径,在最小能量路径上选点进行较高水平[QCISD(T)或CCSD(T)]的单点能量校正。结合POLYRATE8.4.1程序,计算了传统过渡态理论速率常数及含小曲率隧道效应校正的改进的正则变分过渡态理论速率常数。主要结果如下:
(1)通过对CH3F + Cl®CH2F + HCl的研究表明:BH&HLYP/6-311G(d,p)水平能够提供精确的几何和频率信息,其结果和QCISD/6-311G(d,p)水平的结果很接近。而在QCISD(T)//BH&HLYP水平进行的单点能量计算能够有效地改进反应路径能量的准确性,以及对获得精确的反应速率常数是必要的。QCISD(T)/6-311+G(2df,2p)//BH&H
LYP水平下计算的含小曲率隧道效应校正的改进的正则变分过渡态理论速率常数和实验值取得较好的一致性,说明这种双水平的密度泛函从头算直接动力学方法适合Cl和卤代烷烃系列氢迁移反应的理论研究。
(2) 反应路径性质:反应过程中,发生了H¢-Cl键成键与C-H¢键断键的协同过程,对振动频率的分析表明C-H¢键的伸缩振动向H¢-Cl键伸缩振动的转变过程所代表反应的振动模式是反应的“反应模式”。这一系列反应均为氢迁移反应。反应的振动绝热基态势能曲线具有双势垒,这种双势垒的产生主要来源于:经典势能曲线(VMEP)具有较低平的入口势垒和零点振动能曲线(ZPE)在过渡态附近的一个相对较大下降的结合,这同Rosenman和McKee对CH3F + Cl®CH2F + HCl的研究结果相一致。对CHBrCl2 + Cl®CBrCl2 + HCl、CHBr3 + Cl®CBr3 + HCl和CHCl3 + Cl®CCl3 + HCl三个反应,振动绝热基态势能曲线在过渡态附近的势垒变成一个类似较短的平台形状。
(3) 在实验测量的温度范围内,含小曲率隧道效应校正的改进的正则变分过渡态理论速率常数和实验值符合得较好。我们的计算为实验研究提供了有利的理论线索。同Rayez等人对Cl同氟代甲烷及氯代甲烷之间的反应在298K下计算的速率常数相比,我们的计算结果更接近实验值。
(4) 小曲率隧道效应只在低温范围对计算反应速率常数起一定的作用,随温度升高其作用越来越小。对CHBrCl2 + Cl®CBrCl2 + HCl、CHBr3 + Cl®CBr3 + HCl和CHCl3 + Cl®CCl3 + HCl三个反应,变分效应对计算反应速率常数有较大的影响,而对其它反应影响较小。产生这种不同变分效应的原因可能是由于这三个反应具有相对更低平的入口势垒的经典势能曲线与在过渡态附近有较大下降的零点振动能曲线的结合,使振动绝热基态势能曲线在反应物区的势垒相对于在过渡态附近的势垒更高的缘故。
(5) 对CH3F+Cl® CH2F + HCl、Cl + CH2F2® CHF2 + HCl和Cl + CHF3® CF3 + HCl三个反应,在反应物中随着F取代H的增加,反应由放热反应逐渐变成吸热反应,反应势垒逐渐升高,反应物中C-H键的强度逐渐增强,其变化趋势与对应反应物中C-H键的裂解能[D(C-H)]逐渐增加的变化趋势相一致。随着F取代的增加,反应速率常数逐渐降低,活化能逐渐升高。F取代效应对这三反应的速率常数的影响主要是表现在活化能的变化上,这同实验的结果相一致。而对Cl与氯代甲烷和溴代甲烷的反应,取代效应与F取代效应正相反,即在反应物中随着Cl或Br取代H的增加,反应的放热逐渐增加,而反应势垒逐渐降低,C-H键的强度逐渐减弱,与对应反应物中C-H键的裂解能[D(C-H)]逐渐降低的变化趋势相一致。
(6) 对Cl + CH3CCl3 ®CH2CCl3 + HCl和Cl + CH3CF3 ®CH2CF3 + HCl两个反应,在BH&HLYP/6-311+G**水平下得到反应的最小能量路径,同时在CCSD(T)/6-311+G(2df,2p)//BH&HLYP水平下,对路径上所选点进行了单点能量校正。计算的含小曲率隧道效应校正的改进的正则变分过渡态理论速率常数和实验值取得较好的一致性。
计算的结果同已知的实验数据进行了充分比较,结果表明这种密度泛函直接动力学方法是可行的,它为理论上实现对大分子体系的反应动力学研究提供了有益的探索。



The hydrogen abstraction reactions from halogen-substituted hydrocarbons by chlorine atom attacks in the gas phase play an important role in the processes of industrial chlorination and in the incineration of hazardous halogenated wastes. These reactions are also important for the atmospheric chemistry and environmental chemistry. They have potential importance in controlling the ozone concentration in atmospheric chemistry, especially in response to environmental concern related to the Cl-catalyzed removal of stratospheric ozone. So the studies on reaction mechanism and investigation on the rate constants of these reactions play significant role for envirenmental and stratospheric ozone protection. There have many experimental investigations available in the literature concerning rate constants for these reactions, but the theoretical studies are little. Furthermore, the study and determination of rate constants for a specific chemical reaction has always been one of the main research fields in chemistry. It is one of the most active subjects to predict the rate constants in different conditions theoretically. In this thesis, ab initio and density functional theory direct dynamics methods have been used to take a systematic theoretical study on the following hydrogen abstraction reactions:
Cl + CH4-nFn® CH3-nFn + HCl, n=1-3
Cl + CH4-nCln® CH3-nCln + HCl, n=1-3
Cl + CHCl3-nFn® CCl3-nFn + HCl, n=1-2
Cl + CH4-nBrn® CH3-nBrn + HCl, n=1-3
Cl + CH2BrCl ®CHBrCl + HCl
Cl + CHBrCl2 ®CBrCl2 + HCl
Cl + CH3CCl3 ®CH2CCl3 + HCl
Cl + CH3CF3 ®CH2CF3 + HCl
The main object is to provide accurate results for the reaction path and the temperature dependence relation of rate constants, in order to explore the reaction mechanism of these reactions. By means of the Gaussian98 program, at the BH&HLYP level, the geometries and frequencies of the stationary points(reactant, transition state, and prroduct) are calculated. The minimum energy path (MEP) is calculated from s=-2.0 to 2.0(amu)1/2bohr at the same level by intrinsic reaction coordinate (IRC) theory. Furthermore, selected points along the MEP, the force constant matrices as well as the harmonic vibrational frequencies are obtained. In order to gain more accurate information of energy, the energies of the selected points on the MEP are refined at QCISD(T)//BH&HLYP(or CCSD(T) //BH&HLYP) level. By POLYRATE-Version 8.4.1 program, the conventional transition state theory(TST) and improved canonical variational transition state theory with small-curvature tunneling correction(ICVT/SCT) are applied to obtain the reaction rate constants. The main results can be summarized as follows:
(1) The study on the reaction CH3F + Cl®CH2F + HCl indicates that the BH&HLYP/6-311G(d,p) level of theory can give the accurate geometries and frequencies, which are in good agreement with the results by QCISD/6-311G(d,p) level. The sigle point energy calculated at the QCISD(T)//BH&HLYP level can provide more accurate information on energy of the reaction path. This is indispensable to calculate accurately of the reaction rate constants. The ICVT/SCT rate constants calculated at the QCISD(T)/6-311+G(2df,2p)//BH&HLYP level are in good agreement with the experimetal values. The results of this study show that the dual level DFT direct dynamics method is appropriated for the theoretical study on the hydrogen abstraction reactions for Cl atoms with halogen-substituted alkyls.
(2) The property of the reaction path: In the reaction process, these reaction has a cooperative process of C-H¢ bond breaking and H¢-Cl bond associating. From analysis of the vibration frequency, it is shown that the “reactive mode” is the connecting the frequency of the C-H′ stretching of reactant with the frequency of the H′-Cl stretching vibration of HCl. By analyzing the reaction path of these reactions, it is shown that the ground-state vibrational adiabatic potential energy curves for these have two barriers, a situation similar to the analogous reaction CH3F+Cl studied by Rosenman and McKee. The ground-state vibrational adiabatic potential curve (VaG ) has two barriers, and the nonregular VaG shape might be attributed to the combination of two different factors: the low energy barrier and the relatively large early drop of the zero point energies(ZPE) prior to the saddle point zone. But the two barriers behavior is not remarkable for the three reactions CHBrCl2+Cl, CHBr3 + Cl and CHCl3 + Cl.
(3) Our calculated ICVT/SCT rate constants are generally in good agreement with the experimental values in the measured temperature range. For the reactions of Cl with fluoromethanes and chloromethanes, compared with one previous theoretical investigation of room temperature rate constants at a lower level by Rayez et al, our calculated rate constants are much closer to the experimental values.
(4) The small curvature tunneling effect is only important in the lower temperature range for these reactions and becomes small with increase of temperature. The variational effect is large on the rate constants for the reactions CHBrCl2 + Cl®CBrCl2 + HCl, CHBr3 + Cl®CBr3 + HCl and CHCl3 + Cl®CCl3 + HCl and is small for the other reactions of our sduties. The cause of a large variational effect may be attributed to the large early drop in the ZPE profile combined with a low, broad classical barrier profile (VMEP).
(5) For the three reactions Cl+CH3F ® CH2F + HCl, Cl + CH2F2® CHF2 + HCl and Cl + CHF3® CF3 + HCl, the increase of the classical barrier height for the H-abstraction on the order of CH3F (6) For the reactions Cl + CH3CCl3 ®CH2CCl3 + HCl and Cl + CH3CF3 ®CH2CF3 + HCl, the minimum energy paths are calculated at the BH&HLYP/6-311+G(d,p)level. In order to obtain more accurate information on energy, the energies of the selected points on the MEP are refined by CCSD(T)/6-311+G(2df,2p)//BH&HLYP level. The ICVT/SCT rate constants calculated at the CCSD(T)/6-311+G(2df,2p)//BH&HLYP level are in good agreement with the experimental values.
The calculated results are fully compared with the experimental values, and the results show that the DFT direct dynamics method is reliable and feasible. This research provides a useful exploration for the dynamics study on the system with larger molecular size.