Dimethyl Peroxide Radical Cation: A New Theoretical and Experimental Approach to the C₂H₆O^•+₂ Potential Energy Surface

Abstract

Abstract:The structure and the unimolecular fragmentations of the metastable dimethyl peroxide radical cation have been investigated by mass spectrometric and isotopic labeling methods as well as high-level a b initio calculations. In line with the theoretical results, neutralization-reionization and charge rcvcrsal experiments suggest that ionized dimethyl peroxide bears a CH,OOCH;+ connectivity. In the cation the 0-0 bond dissociation energy is larger than that of the neutral counterpart; in contrast, thc C -0 bond strength is slightly and that of the C--H bond significantly reduced upon ionization. These energetic changes upon one-electron oxidation of' CH,OOCH, are also reflected in the N R and CR mass spectra of CH,OOCH;+. Further, for metastable CH,OOCHlj+ two major fragmentation pathways are observed :

Intermediacy of Proton‐Bound Dimers and Ion/Dipole Complexes in the Unimolecular Decompositions of Dialkyl‐Peroxide Radical Cations: Evidence for a coupled proton and hydrogen‐atom transfer

“…Finally, the mass-spectral differences of 2 ' 4 + , as compared to 1' and 5+, are traced back to structural features and reaction energetics. Before doing so, we note, in passing, in agreement with theoretical and experimental results [6] obtained for ionized CH,OOCH,, that the peroxide cation radicals most likely have the connectivity of the neutral peroxides. Thus, it is justified to use the peroxidic structures of 1'-5+ as starting points for the discussion of their unimolecular decays.…”

“…') 2, For an application of nine different ionization techniques in the structural analysis of organic peroxides, see For a combined mass-spectrometric and computational analysis of the unimolecular decompositions of CH,OOCHF,see [6]. [21. Results and Discussion.…”

Gas‐phase basicities of polyfunctional molecules. Part 2: Saturated basic sites

“…Several reports were devoted to the study of the gas‐phase reactivity of protonated peroxides. Schalley et al (1997) described in detail the unimolecular fragmentations of protonated methyl hydroperoxide and dimethyl peroxide in a combined experimental and theoretical study. Protonation of the analytes was achieved by H 2 chemical ionization.…”

“…It has been shown (Van Driel et al, 1985; Schalley et al, 1997) that protonation of CH 3 OOCH 3 under CH 4 or H 2 chemical ionization induces, to some extent, the formation of ${\rm CH}_{{\rm 3}} {\rm (OH)OCH}_{{\rm 3}}^{{\rm + }} $ ion even if dissociation mostly proceeds via a methanol/formaldehyde proton bound complex. Schalley et al (1997) also estimated its proton affinity by DFT calculations and obtained a value of 748.5 kJ/mol, while a value of 754 kJ/mol is obtained at the G3 level. An experimental measurement was performed by using the equilibrium method with an FT‐ICR instrument (Aschi et al, 1998).…”

Charge permutation reactions in tandem mass spectrometry

“…68,71 Positive ion charge inversion has also been explored using a tandem quadrupole 28 and multiple-sector mass spectrometers. 72,73 Although not investigated as extensively as negative ion charge inversion owing to the low yields resulting from the small cross-section of this process, 2 positive ion charge inversion has been demonstrated to be a useful tool in the structural study of some ions of interest. It has been reasoned that the nature of the rapid conversion process, at least for the one-step mechanism, leads to the product negative ions maintaining essentially the same structure as the precursor positive ions.…”