Decomposition of Diazomeldrum鈥檚 Acid: A Threshold Photoelectron Spectroscopy Study

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• 作者：Melanie Lang ; Fabian Holzmeier ; Ingo Fischer ; Patrick Hemberger
• 刊名：Journal of Physical Chemistry A
• 出版年：2014
• 出版时间：November 26, 2014
• 年：2014
• 卷：118
• 期：47
• 页码：11235-11243
• 全文大小：487K
• ISSN：1520-5215

文摘

Derivatives of meldrum鈥檚 acid are known precursors for a number of reactive intermediates. Therefore, we investigate diazomeldrum鈥檚 acid (DMA) and its pyrolysis products by photoionization using vacuum ultraviolet (VUV) synchrotron radiation. The threshold photoelectron spectrum of DMA yields an ionization energy (IE) of 9.68 eV. Several channels for dissociative photoionization are observed. The first one is associated with loss of CH₃, leading to a daughter ion with m/z = 155. Its appearance energy AE_0K was determined to be 10.65 eV by fitting the experimental data using statistical theory. A second parallel channel leads to m/z = 69, corresponding to N₂CHCO, with an AE_0K of 10.72 eV. Several other channels open up at higher energy, among them the formation of acetone cation, a channel expected to be the result of a Wolff-rearrangement (WR) in the cation. When diazomeldrum鈥檚 acid is heated in a pyrolysis reactor, three thermal decomposition pathways are observed. The major one is well-known and yields acetone, N₂ and CO as consequence of the WR. However, two further channels were identified: The formation of 2-diazoethenone, NNCCO, together with acetone and CO₂ as the second channel and E-formylketene (OCCHCHCO), propyne, N₂ and O₂ as a third one. 2-Diazoethenone and E-formylketene were identified based on their threshold photoelectron spectra and accurate ionization energies could be determined. Ionization energies for several isomers of both molecules were also computed. One of the key findings of this study is that acetone is observed upon decomposition of DMA in the neutral as well as in the ion and both point to a Wolff rearrangement to occur. However, the ion is subject to other decomposition channels favored at lower internal energies.