Formation of a Spin-Forbidden Product, (1)[MnO4](-), from Gas-Phase Decomposition of (6)[Mn(NO3)3](.).

The manganese nitrate complex, [Mn(NO3)3](-), was generated via electrospray ionization and studied by tandem quadrupole mass spectrometry. The complex is assumed to decompose into [MnO(NO3)2](-) by elimination of NO2(•). The [MnO(NO3)2](-) product undergoes elimination of NO2(•) to yield [MnO2(NO3)](-), or elimination of NO(•) to yield [MnO3(NO3)](-). Both [MnO2(NO3)](-) and [MnO3(NO3)](-) yield [MnO4](-) via the transfer of oxygen atoms from the remaining nitrate ligand. The mechanism of permanganate formation is interesting because it can be generated through two competing pathways, and because the singlet ground state is spin-forbidden from the high-spin sextet [Mn(NO3)3](-) precursor. Theory and experiment suggest [MnO2(NO3)](-) is the major intermediate leading to formation of [MnO4](-). Theoretical studies show crossing from the high-spin to low-spin surface upon neutral oxygen atom transfer from the nitrate ligand in [MnO2(NO3)](-) allows formation of (1)[MnO4](-). Relative energy differences for the formation of (1)[MnO4](-) and (1)[MnO3](-) predicted by theory agree with experiment.

Fragmentation of [Ni(NO3)3](-): A Study of Nickel-Oxygen Bonding and Oxidation States in Nickel Oxide Fragments.

Gas-phase nickel nitrate anions are known to produce nickel oxide nitrate anions, [NiOx(NO3)y](-) upon fragmentation. The goal of this study was to investigate the properties of nickel oxide nitrate complexes generated by electrospray ionization using a tandem quadrupole mass spectrometer and theoretical calculations. The [Ni(NO3)3](-) ion undergoes sequential NO2(•) elimination to yield [NiO(NO3)2](-) and [NiO2(NO3)](-), followed by elimination of O2. The electronic structure of the nickel oxide core influences decomposition. Calculations indicate electron density from oxygen is delocalized onto the metal, yielding a partially oxidized oxygen in [NiO(NO3)2](-). Theoretical studies suggest the mechanism for O2 elimination from [NiO2(NO3)](-) involves oxygen atom transfer from a nitrate ligand to yield an intermediate, [NiO(O2)(NO2)](-), containing an oxygen radical anion ligand, O(•-), a superoxide ligand, O2(•-), and a nitrite ligand bound to Ni(2+). Electron transfer from superoxide partially reduces both the metal and oxygen and yields the energetically favored [NiO(NO2)](-) + O2 products.

Gas-Phase Fragmentation of Aluminum Oxide Nitrate Anions Driven by Reactive Oxygen Radical Ligands.

Gas-phase metal nitrate anions are known to yield a variety of interesting metal oxides upon fragmentation. The aluminum nitrate anion complexes, Al(NO3)4(-) and AlO(NO3)3(-) were generated by electrospray ionization and studied with collision-induced dissociation and energy-resolved mass spectrometry. Four different decomposition processes were observed, the loss of NO3(-), NO3(•), NO2(•), and O2. The oxygen radical ligand in AlO(NO3)3(-) is highly reactive and drives the formation of AlO(NO3)2(-) upon loss of NO3(•), AlO2(NO3)2(-) upon NO2(•) loss, or Al(NO2)(NO3)2(-) upon abstraction of an oxygen atom from a neighboring nitrate ligand followed by loss of O2. The AlO2(NO3)2(-) fragment also undergoes elimination of O2. The mechanism for O2 elimination requires oxygen atom abstraction from a nitrate ligand in both AlO(NO3)3(-) and AlO2(NO3)2(-), revealing the hidden complexity in the fragmentation of these clusters.

Fragmentation of Cr(NO3)4(-): Metal Oxidation upon O(•-) Abstraction.

The decomposition of chromium nitrate anion, Cr(NO3)4(-), was investigated by tandem mass spectrometry. The major fragments correspond to sequential elimination of NO2(•) via O(•-) abstraction from each nitrate ligand to yield CrOn(NO3)(4-n)(-), n = 1-4, products. The metal is oxidized upon the first three O(•-) abstraction reactions to yield the fully oxidized Cr(VI), closed-shell, CrO3(NO3)(-) fragment. A CrO4(-) fragment was detected, but the metal is not further oxidized upon the fourth O(•-) abstraction. Experiment and theory indicate the first three O(•-) abstraction reactions are low energy processes, but the formation of CrO4(-) is considerably higher in energy. Theoretical studies show the 3d electrons in chromium are removed by O(•-) for CrOn(NO3)(4-n)(-), n = 1-3, to yield oxo, O(2-) ligands, but the electron density is replaced by donation from π bonds involving the oxygen lone pairs. Theory predicts a decrease in metal charge for each O(•-) abstraction, opposite the trend expected for oxidation, due to π electron donation from the oxygen atoms.

Fragmentation of deprotonated polyethylene glycols, [PEG-H]-.

RATIONALE: Polyethylene glycols (PEGs) are soluble molecules utilized in a wide range of applications. Mass spectrometry and fragmentation patterns of positively charged PEG oligomers are well-known, but decomposition mechanisms of the deprotonated ions have not been studied. METHODS: Deprotonated PEGs were generated by electrospray ionization of PEG in water/acetonitrile. Collision-induced dissociation (CID) experiments were carried out in a tandem mass spectrometer. The anions were studied using a tandem mass spectrometer to carry out CID experiments. A series of small PEG oligomers, with 1 to 8 monomer units, were studied in order to monitor size-dependent effects on fragmentation reactions. RESULTS: Because deprotonated PEG ions have a unique charge site, their dissociation pathways can easily be monitored. The ions fragment by loss of C2H4O monomer units, with an alternating intensity pattern that suggests the loss of an even number of monomer units is favored. Smaller oligomers and oligomer fragments also yielded fragments corresponding to H2 elimination and H2O loss. H2 elimination occurs by the generation of a hydride ion which deprotonates an alcohol upon leaving, while dehydration appears to be a charge-remote process. CONCLUSIONS: The fragmentation of deprotonated PEG is dominated by intramolecular S(N)2 reactions involving the terminal oxide anion.

The sequential dissociation of protonated polyethylene glycols.

Early investigations of protonated polyethylene glycol fragmentation suggested the dissociation mechanism includes both direct and sequential processes. Experiments designed to study the proposed mechanisms of sequential dissociation are absent from the literature. In order to obtain additional experimental details about the fragmentation reactions, the dissociation of protonated polyethylene glycol was studied by energy-dependent collision-induced dissociation (CID). Key fragment ions were separated by mass differences corresponding to the loss of single monomer units. Several fragment ions were also generated by in-source fragmentation and studied by CID. These experiments indicate the primary ions undergo sequential dissociation by the loss of either one or two monomer units. The results suggest that at least two different mechanisms must be considered to explain the sequential dissociation of protonated polyethylene glycols. The reaction involving the elimination of two subunits suggests the loss of a six-membered 1,4-dioxane product, while the elimination of a single subunit involves the loss of acetaldehyde by a 1,2-hydride shift rearrangement.