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Methyl 3-Methyl-2H-azirine-2-carboxylate Photochemistry Studied by Matrix-isolation FTIR and DFT Calculations

机译:用基质分离FTIR和DFT计算研究3-甲基-2H-叠氮基-2-羧酸甲酯的光化学

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摘要

The aliphatic 2H-azirine, methyl 3-methyl-2H-azirine-2-carboxylate (MMAC), has been synthesized and its monomeric form investigated by IR spectroscopy in an argon matrix, at 10 K, as well as theoretically (DFT/B3LYP/6-311++G(d,p)). Two low-energy conformers of MMAC (Ct and Cc) were found in the matrix, both exhibiting the cis conformation around the C−O bond but differing in the arrangement around the C−Cα bond. The two conformers were photoreactive upon in situ broadband UV excitation (λ > 235 nm), yielding nitrile ylide (P1) and ketene imine (P2) type products, which resulted from cleavage of the C−C or C−N bond, respectively. The kinetics of the reactions leading to the formation of P1 and P2 are of first order, with the processes being favored when the reactant is in the Cc conformation. Very interestingly, the C−N bond photocleavage, which is unusual for aliphatic 2H-azirines, was found to be preferred over the generally favored in 2H-azirines C−C bond breakage. This behavior is attributed to the presence in the molecule of the electron-withdrawing methoxycarbonyl substituent, which accelerates the intersystem crossing toward the T1 triplet state and, in this way, favors the C−N bond cleavage. In addition to the primary photoprocesses leading to formation of P1 and P2, secondary photoprocesses leading to the decarboxylation and decarbonylation of P2 have been also observed.
机译:已经合成了脂肪族2H-叠氮基,3-甲基-2H-叠氮基-2-羧酸甲酯(MMAC),并在10 K以及理论上在氩气基质中通过红外光谱研究了其单体形式(DFT / B3LYP / 6-311 ++ G(d,p))。在基质中发现了两个低能量构象的MMAC(Ct和Cc),均在C-O键周围表现出顺式构象,但在C-Cα键周围的排列不同。这两个构象异构体在原位宽带UV激发下(λ> 235 nm)发生光反应,分别产生腈基内酯(P1)和乙烯酮亚胺(P2)型产物,这分别是由C-C或C-N键的断裂引起的。导致形成P1和P2的反应动力学是一级的,当反应物呈Cc构型时,该过程特别有利。非常有趣的是,发现对于脂肪族2H-叠氮基不常见的C-N键光解比通常在2H-叠氮基C-C键断裂中更受欢迎。该行为归因于吸电子的甲氧羰基取代基在分子中的存在,该基团加速了系统间的交叉,并朝着T1三重态迁移,并因此促进了CN键的裂解。除了导致P1和P2形成的主要光过程外,还观察到导致P2脱羧和脱羰基的二次光过程。

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