Electron ionization rearrangements

If a charge exchange process, A + + B- A -f- B +, occurs when the distance between the two particles is large, we expect that no transfer of translational energy takes place in the reaction and that the same selection rules govern the ionization as in spectroscopic transitions. This means that if the molecule B is in a singlet state before the ionization, the ion B + will be formed in a doublet state after ionization of one electron without rearrangements of any other electrons, at least for small molecules. 

Most of these processes are very fast. Ionization happens on the low femtosecond timescale, direct bond cleavages require between some picoseconds to several tens of nanoseconds, and rearrangement fragmentations usually proceed in much less than a microsecond (Fig. 5.3 and Chap. 2.7). Finally, some fragment ions may even be formed after the excited species has left the ion source giving rise to metastable ion dissociation (Chap. 2.7). The ion residence time within an electron ionization ion source is about 1 ps. [9]... 

The mass spectra of various /3-diketonates have been extensively studied.762,763 The availability of ionization energies, measured by mass spectra, has led to a fairly detailed debate as to the nature of the orbital from which the first electron ionizes.764 Gas phase rearrangements,765 often involving fluorine migration to the metal, have been observed. The SIMS spectrum of [Cr(acac)3] is particularly simple,766 yielding the base peak [Cr(acac)2]+. 

The structure of diphosphallenic radical cations, generated from the allene ArP=C=PAr by electrochemical oxidation, has been examined using EPR spectroscopy. Ab initio calculations including correlation effects at the MP2 and MCSCF levels have determined that two rotamers exist compatible with Jahn-Teller distortion of the allene.146 Anodically generated radical cations of alkyl phosphites [(RO P] and silylphosphites [(RO)2POSiMe3] reacted with alkenes by initial attack at the C=C bond followed by electron transfer, deprotonation, and elimination of an alkyl or trimethylsilyl cation to form identical alkyl phosphate adducts.147 The electron ionization-induced McLafferty rearrangement of n-hexylphosphine afford the a-distonic radical cation CTEPH, the distinct reactivity of which suggests there is no... 

When neopentyl bromide is boiled in ethanol, it gives only a rearranged substitution product. This product results from a methyl shift (represented by the symbol CH ), the migration of a methyl group together with its pair of electrons. Without rearrangement, ionization of neopentyl bromide would give a very unstable primary carbocation. 

Using electron ionization, the molecular radical cation is formed in the source. This radical cation fragments into a radical and a cation with an even number of electrons or, through rearrangements or multiple steps, into a neutral molecule and a new odd-electron cation. The latter are often easily recognized in the spectrum because their mass is even in the absence of a nitrogen atom. 

Electron ionization (25 eV) mass spectra of picolinic esters from Go-octadenoic acid and ante-isononadecanoic acid. The fragment ion at mfz 151 corresponds to a McLafferty rearrangement product. Reproduced (modified) from Harvey D.J., Biomed. Mass Spectrom., 9,33,1982, with permission. 

Sample ionization. Requirements for sample ionization are much more severe in MS/MS than in GC/MS. For MS/MS, the ionization method should create one ion for each component, and the structure of the ion should be the same as that of the neutral surrogate. Electron ionization usually does not fulfill these requirements, since the ions formed often include those from rearrangement reactions, and the degree of fragmentation is excessive. Chemical ionization provides the requisite single ion for each component of the matrix in the form of the quasimolecular ion (MFH)+. 

Koopmans theorem cannot be applied to this molecule- The SCF values are still in a rather poor agreement with experiment (partly owing to a small basis set used), but the order of ionization potentials is correct. The breakdown of the Koopmans theorem may be elucidated by a different extent of the electron reorganization involved in electron ionizations from the respective molecular orbitals. For ligand orbitals e or a2u little rearrangement upon ioniza-... 

The presence of heteroatoms with non-bonding n electrons favors the localization of charges. When the radical and/or the charge are localized, this influences the fragmentation and the reaction mechanisms can be classified as sigma electron ionizations, localized charge initiations, radical site initiations, and rearrangements. 

The rules indicated above seem to not have much connection to the fragmentation results obtained in pyrolysis. For example, Stevenson s rule, the charge site ionization mechanism, and the sigma electron ionization mechanism are not applicable to pyrolysis products, as the molecules in pyrolysis are not ionized. On the other hand, the a cleavage and certain rearrangements may be similar for the two processes. Also the fact that small molecule elimination is favored in mass spectrometry makes possible that, with a certain frequency, pyrolysis products are similar to mass fragments obtained in mass spectrometry. 

Gas-phase unimolecular rearrangements of a series of substituted cyclopropyl anions 14, generated by electron ionization of the respective neutral reagents, to the corresponding allyl anions 15 were found to have ring-opening barriers in the range 19 to >36 kcal mol ... 

A radical-cation Cope rearrangement of 2,5-diphenylhexa-l,5-dienes under electron ionization conditions (by mass spectrometry at 70 eV) has been described to occur in the gas phase. The reaction directionality differs from that in a thermal transformation. The rearrangement of hexamethyl-Dewai-benzene 410 into hexamethylbenzene (equation 156) as well as the closure of the bridged hexahydrodiene 411 into the so-called birdcage hydrocarbon 412 proceed during hemin-catalyzed epoxidation via a radical cation intermediate (equation 157)224. These processes are Cope-like rearrangement because two double bonds are separated by one CHt group in 410 and by three -hybridized C-atoms in 411. 

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