甲酸和氯原子與分子反應機制及動力學研究

Abstract

本論文藉由Gaussian 09W氣相反應模擬軟體，計算氣相下HCOOH及HOCO與Cl及Cl2的反應，找出可能的反應路徑及反應中可能存在之中間產物和最終產物，並計算出重要低能障路徑之反應速率。 在HCOOH + Cl2反應中，trans-HCOOH + Cl2 → HCOOCl + HCl之反應能障為52.2 kcal mol-1，吸收15.1 kcal mol-1的能量形成HCOOCl + HCl，反應速率常數在溫度 100 - 2000 K下表示式為9.30x10E(-21)T^(2.07)exp(-24404/T) cm3 molecule-1 s-1，在298 K下速率常數為3.59x10E(-51) cm3 molecule-1 s-1。 在HOCO + Cl2反應中，放熱30.5 kcal mol-1形成穩定錯合物ClCOOH--Cl，之後越過能障5.3 kcal mol-1形成Cl + HCl + CO2，反應速率常數在溫度298 – 2000 K下之表示式為1.73x10E(-17)T^(1.55)exp(893/T) cm3 molecule-1 s-1; 在298 K下速率常數為2.37x10E(-12) cm3 molecule-1 s-1。 在HCOOH + Cl反應中，trans-HCOOH和氯原子克服2.1 kcal mol-1能障生成穩定cis-HOCO與HCl，並釋放出2.1 kcal mol-1的能量; 另外一個路徑為克服10.3 kcal mol-1的能量產生HCO2與HCl之錯合物，並吸收9.0 kcal mol-1的能量。由計算結果可推測HCOOH + Cl較易生成HOCO + HCl，此路徑反應速率在溫度298 K為1.7x10E(-13)cm3 molecule-1 s -1，和實驗觀測結果1.8x10E(-13) cm3 molecule-1 s-1 吻合。 在HOCO和氯原子反應中，可生成穩定中間物 chloroformic acid (ClCOOH)放熱約80.0 kcal mol-1，再越過24.0 kcal mol-1能量可生成HCl與CO2並釋放出約100.0 kcal mol-1，此路徑反應速率在溫度298 K下為4.1x10E(-11) cm3 molecule -1s -1，和實驗觀測結果4.8x10E(-11) cm3 molecule -1 s-1一致。

The photochemical reactions between chlorine molecules and formic acid are chain reactions which proceed at least partially via chlorofomic acid and the final products detected experementally are carbon dioxide and hydrogen chloride. In order to understand the mechanism of the chain reaction, the Gaussian code was utilized to simulate the reactions of Clx (x=1, 2) with HCOOH and HOCO at the CCSD(T)/aug-cc-pVTZ//BHandHLYP/6-311++G(3df,2p) level of theory. The reactions involved are HCOOH + Cl¬2, HOCO + Cl2, HCOOH + Cl and HOCO + Cl. The HCOOH + Cl2 reaction forms HCOOCl + HCl, with a barrier height of 52.2 kcal mol-1. The rate constant predicted with the transition-state theory is 9.30x10E(-21)T^(2.07)exp(-24404/T) cm3 molecule-1 s-1 in the temperature range from 100 to 2000 K. In the HOCO + Cl2 reaction, HCl + CO2 + Cl are produced via a chlorine atom with a statble complex of chloroformic acid, which fragment with a barrier of 5.3 kcal mol-1. The rate constant predicted with the transition-state theory is 1.73x10E(-17)T^(1.55)exp(893/T) cm3 molecule-1 s-1 in the temperature range from 298 to 2000 K. The hydrogen abstraction from HCOOH by Cl atom is considered as the key process in the chain reaction. In this reaction channel, the barrier height for formation of HOCO + HCl was found to be 2.1 kcal mol-1 and the predicted rate constant is 1.7x10E(-13) cm3 molecule-1 s-1 at 298 K, which is in good agreement with an experimental value of 1.8x10E(-13) cm3 molecule-1 s-1. In the HOCO + Cl reaction, the Cl atom can form a stable complex(ClCOOH) with HOCO, which lies -79.7 kcal mol-1 relative to the reactants; the complex decomposed readily into HCl + CO2 via a transition state with a barrier of 23.4 kcal mol-1. The predicted rate constant is 4.1x10E(-11) cm3 molecule-1 s-1 at 298 K, which is in good agreement with an experimental value of 4.8x10E(-11) cm3 molecule-1 s-1.