源自乙醇的C2H5O*自由基生成热的理论预测以及乙氧基中β-C-C键断裂的动力学

Theoretical prediction of the heats of formation of C2H5O* radicals derived from ethanol and of the kinetics of beta-C-C scission in the ethoxy radical.

作者信息

Matus Myrna H, Nguyen Minh Tho, Dixon David A

机构信息

Department of Chemistry, University of Alabama, Tuscaloosa, Alabama 35487-0336, USA.

出版信息

J Phys Chem A. 2007 Jan 11;111(1):113-26. doi: 10.1021/jp064086f.

DOI:10.1021/jp064086f

PMID:17201394

Abstract

Thermochemical parameters of three C(2)H(5)O* radicals derived from ethanol were reevaluated using coupled-cluster theory CCSD(T) calculations, with the aug-cc-pVnZ (n = D, T, Q) basis sets, that allow the CC energies to be extrapolated at the CBS limit. Theoretical results obtained for methanol and two CH(3)O* radicals were found to agree within +/-0.5 kcal/mol with the experiment values. A set of consistent values was determined for ethanol and its radicals: (a) heats of formation (298 K) DeltaHf(C(2)H(5)OH) = -56.4 +/- 0.8 kcal/mol (exptl: -56.21 +/- 0.12 kcal/mol), DeltaHf(CH(3)CHOH) = -13.1 +/- 0.8 kcal/mol, DeltaHf(CH(2)CH(2)OH) = -6.2 +/- 0.8 kcal/mol, and DeltaHf(CH(3)CH(2)O*) = -2.7 +/- 0.8 kcal/mol; (b) bond dissociation energies (BDEs) of ethanol (0 K) BDE(CH(3)CHOH-H) = 93.9 +/- 0.8 kcal/mol, BDE(CH(2)CH(2)OH-H) = 100.6 +/- 0.8 kcal/mol, and BDE(CH(3)CH(2)O-H) = 104.5 +/- 0.8 kcal/mol. The present results support the experimental ionization energies and electron affinities of the radicals, and appearance energy of (CH(3)CHOH+) cation. Beta-C-C bond scission in the ethoxy radical, CH(3)CH2O*, leading to the formation of C*H3 and CH(2)=O, is characterized by a C-C bond energy of 9.6 kcal/mol at 0 K, a zero-point-corrected energy barrier of E0++ = 17.2 kcal/mol, an activation energy of Ea = 18.0 kcal/mol and a high-pressure thermal rate coefficient of k(infinity)(298 K) = 3.9 s(-1), including a tunneling correction. The latter value is in excellent agreement with the value of 5.2 s(-1) from the most recent experimental kinetic data. Using RRKM theory, we obtain a general rate expression of k(T,p) = 1.26 x 10(9)p(0.793) exp(-15.5/RT) s(-1) in the temperature range (T) from 198 to 1998 K and pressure range (p) from 0.1 to 8360.1 Torr with N2 as the collision partners, where k(298 K, 760 Torr) = 2.7 s(-1), without tunneling and k = 3.2 s(-1) with the tunneling correction. Evidence is provided that heavy atom tunneling can play a role in the rate constant for beta-C-C bond scission in alkoxy radicals.

摘要

使用耦合簇理论CCSD(T)计算方法，并采用aug-cc-pVnZ（n = D、T、Q）基组，对源自乙醇的三种C(2)H(5)O自由基的热化学参数进行了重新评估，该基组能使耦合簇能量外推至完全基组极限（CBS极限）。结果发现，甲醇和两种CH(3)O自由基的理论计算结果与实验值在±0.5 kcal/mol范围内相符。确定了一组关于乙醇及其自由基的一致数值：(a)生成热（298 K）：ΔHf(C(2)H(5)OH) = -56.4 ± 0.8 kcal/mol（实验值：-56.21 ± 0.12 kcal/mol），ΔHf(CH(3)CHOH) = -13.1 ± 0.8 kcal/mol，ΔHf(CH(2)CH(2)OH) = -6.2 ± 0.8 kcal/mol，以及ΔHf(CH(3)CH(2)O*) = -2.7 ± 0.8 kcal/mol；(b)乙醇在0 K时的键解离能（BDEs）：BDE(CH(3)CHOH-H) = 93.9 ± 0.8 kcal/mol，BDE(CH(2)CH(2)OH-H) = 100.6 ± 0.8 kcal/mol，以及BDE(CH(3)CH(2)O-H) = 104.5 ± 0.8 kcal/mol。目前的结果支持了自由基的实验电离能和电子亲和能，以及(CH(3)CHOH+)阳离子的出现能。乙氧基自由基CH(3)CH2O中的β-C-C键断裂生成CH3和CH(2)=O，其在0 K时的C-C键能为9.6 kcal/mol，零点校正后的能垒E0++ = 17.2 kcal/mol，活化能Ea = 18.0 kcal/mol，高压热速率系数k(∞)(298 K) = 3.9 s(-1)，其中包括隧道效应校正。后一个值与最新实验动力学数据得到的5.2 s(-1)非常吻合。使用RRKM理论，我们得到了在温度范围（T）为198至1998 K、压力范围（p）为0.1至8360.1 Torr且以N2作为碰撞伙伴时的一般速率表达式k(T,p) = 1.26 x 10(9)p(0.793) exp(-15.5/RT) s(-1)，其中k(298 K, 760 Torr) = 2.7 s(-1)（无隧道效应校正），有隧道效应校正时k = 3.2 s(-1)。有证据表明，重原子隧道效应可能在烷氧基自由基β-C-C键断裂的速率常数中起作用。