Jiang Kezhi, Bian Gaofeng, Qiu Huayu, Pan Yuanjiang, Lai Guoqiao
Department of Chemistry, Zhejiang University, Hangzhou, 310036, China.
J Phys Chem A. 2009 Jan 29;113(4):697-706. doi: 10.1021/jp808273m.
The gas-phase pyrolysis of ketonic hydrazones from S-methyl dithiocarbazate R(1)R(2)C=N-NHC(=S)SCH(3) (R(1), R(2) = alkyl or aryl) was investigated by online GC-MS pyrolysis and theoretical calculation. Both of these pyrolytic products, ascribed to methanethiol and the corresponding N-isothiocyanate imines, were detected in the total ion chromatography (TIC) of GC-MS. Calculation results exhibit two stable configurational structures for reactants (Re), which can interconvert with relatively low barriers (<78 kJ/mol). DFT calculations showed that the two unimolecular pyrolytic processes, a direct 1,2-elimination of CH(3)SH for syn-Re and a two-step reaction pathway for trans-Re involving tautomer interconversion followed by decomposition of CH(3)SH, are competitive in the reaction. Both syn-Re and trans-Re exhibit lower critical energies in the propagation step of the radical pyrolysis than that in the unimolecular pyrolysis process (187.76 kJ/mol via 131.91 kJ/mol for syn-Re, and 159.15 kJ/mol via 98.92 kJ/mol for trans-Re). However, much more energy is needed to excite the compound to produce the methylthio radical, with 262.03 and 253.60 kJ/mol for syn-Re and trans-Re, respectively. Therefore, the unimolecular pyrolysis rather than the radical one occurs in the condition of this study.
通过在线气相色谱-质谱联用热解技术和理论计算，研究了S-甲基二硫代氨基甲酸盐R(1)R(2)C=N-NHC(=S)SCH(3)（R(1), R(2) = 烷基或芳基）酮腙的气相热解反应。在气相色谱-质谱联用仪的总离子色谱图（TIC）中检测到了这两种热解产物，分别归属于甲硫醇和相应的N-异硫氰酸酯亚胺。计算结果表明反应物（Re）存在两种稳定的构型结构，它们可以以相对较低的势垒（<78 kJ/mol）相互转化。密度泛函理论计算表明，两种单分子热解过程在反应中相互竞争，一种是顺式-Re的直接1,2-消除CH(3)SH，另一种是反式-Re的两步反应途径，包括互变异构转化，随后分解CH(3)SH。顺式-Re和反式-Re在自由基热解的传播步骤中均表现出比单分子热解过程更低的临界能量（顺式-Re通过131.91 kJ/mol为187.76 kJ/mol，反式-Re通过98.92 kJ/mol为159.15 kJ/mol）。然而，激发该化合物产生甲硫基自由基需要更多的能量，顺式-Re和反式-Re分别为262.03和253.60 kJ/mol。因此，在本研究条件下发生的是单分子热解而非自由基热解。
