McAnoy Andrew Michael, Paine Martin Robert Lloyd, Blanksby Stephen James
Human Protection and Performance Division, Defence Science and Technology Organisation, 506 Lorimer St, Fishermans Bend, Victoria, 3207, Australia.
Org Biomol Chem. 2008 Jul 7;6(13):2316-26. doi: 10.1039/b803734e. Epub 2008 Apr 30.
The gas phase degradation reactions of the chemical warfare agent (CWA) simulant, dimethyl methylphosphonate (DMMP), with the hydroperoxide anion (HOO(-)) were investigated using a modified quadrupole ion trap mass spectrometer. The HOO(-) anion reacts readily with neutral DMMP forming two significant product ions at m/z 109 and m/z 123. The major reaction pathways correspond to (i) the nucleophilic substitution at carbon to form CH(3)P(O)(OCH(3))O (m/z 109) in a highly exothermic process and (ii) exothermic proton transfer. The branching ratios of the two reaction pathways, 89% and 11% respectively, indicate that the former reaction is significantly faster than the latter. This is in contrast to the trend for the methoxide anion with DMMP, where proton transfer dominates. The difference in the observed reactivities of the HOO(-) and CH(3)O(-) anions can be considered as evidence for an alpha-effect in the gas phase and is supported by electronic structure calculations at the B3LYP/aug-cc-pVTZ//B3LYP/6-31+G(d) level of theory that indicate the S(N)2(carbon) process has an activation energy 7.8 kJ mol(-1) lower for HOO(-) as compared to CH(3)O(-). A similar alpha-effect was calculated for nucleophilic addition-elimination at phosphorus, but this process--an important step in the perhydrolysis degradation of CWAs in solution--was not observed to occur with DMMP in the gas phase. A theoretical investigation revealed that all processes are energetically accessible with negative activation energies. However, comparison of the relative Arrhenius pre-exponential factors indicate that substitution at phosphorus is not kinetically competitive with respect to the S(N)2(carbon) and deprotonation processes.
使用改进的四极杆离子阱质谱仪研究了化学战剂（CWA）模拟物甲基膦酸二甲酯（DMMP）与过氧氢根阴离子（HOO⁻）的气相降解反应。HOO⁻阴离子很容易与中性的DMMP反应，形成质荷比为109和123的两个主要产物离子。主要反应途径对应于：（i）在碳上的亲核取代，在一个高度放热的过程中形成[CH₃P(O)(OCH₃)O]⁻（质荷比109），以及（ii）放热的质子转移。两条反应途径的分支比分别为89%和11%，这表明前一个反应明显比后一个反应快。这与甲醇根阴离子与DMMP反应的趋势相反，在后者的反应中质子转移占主导。观察到的HOO⁻和CH₃O⁻阴离子反应活性的差异可被视为气相中α-效应的证据，并且在B3LYP/aug-cc-pVTZ//B3LYP/6-31+G(d)理论水平的电子结构计算支持了这一点，该计算表明与CH₃O⁻相比，HOO⁻的S(N)2（碳）过程的活化能低7.8 kJ mol⁻¹。对于磷上的亲核加成-消除也计算出了类似的α-效应，但这个过程——溶液中CWA过水解降解的一个重要步骤——在气相中未观察到与DMMP发生。理论研究表明，所有过程在能量上都可以通过负活化能实现。然而，相对阿仑尼乌斯指前因子的比较表明，磷上的取代在动力学上相对于S(N)2（碳）和去质子化过程没有竞争力。
