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.

Experimental and theoretical study of the decomposition of copper nitrate cluster anions.

Copper nitrate anion clusters Cu(NO3)3(-) and Cu(NO3)2(-) were generated by electrospray ionization and studied with collision-induced dissociation and energy-resolved mass spectrometry. Collision-induced dissociation resulted in three different fragmentation reactions-loss of NO3(-), NO3(•), and NO2(•). The type of fragmentation reaction depends on the oxidation state of the metal. The Cu(NO3)3(-) cluster showed loss of NO3(•) but no loss of NO2(•), whereas the Cu(NO3)2(-) cluster showed loss of NO2(•) but no loss of NO3(•). The fragmentation reactions were studied by theoretical methods. These studies show loss of NO3(•) corresponds to reduction of the metal charge by electron transfer, whereas loss of NO2(•) and metal-oxide bond formation by O(-) abstraction in Cu(NO3)2(-) does not necessarily result in the expected oxidation of the metal.

Heterogeneous substitution effects in chlorocyanomethyl radical and chlorocyanocarbene.

We report a photoelectron-imaging investigation of the chlorocyanomethyl radical (CHClCN) and the corresponding carbene (CClCN). The results are discussed in comparison with the corresponding dichloro- and dicyano-substituted species, focusing on the divergent effects of the halogen and pseudohalogen (CN) substitutions. A cooperative (captodative) interaction of the π-donor Cl and π-acceptor cyano groups favors the increased stability of the CHClCN radical, but a competition of the two substituents is observed in the singlet-triplet splitting of the carbene. The vertical detachment energy (VDE) of CHClCN(-) is determined to be 2.39 ± 0.04 eV, with the broad photoelectron band consistent with the significant geometry change predicted by theory for the detachment transition. The adiabatic electron affinity of CHClCN, EA = 1.86 ± 0.08 eV, is estimated on the basis of the experimental VDE and the computed difference between the VDE and EA values. This result allows the calculation of the bond dissociation energy of chloroacetonitrile, DH298(H-CHClCN) = 87.0 ± 2.7 kcal/mol. Photoelectron imaging of CClCN(-) reveals two main transitions, assigned to the singlet ((1)A') and triplet ((3)A″) states of the CClCN carbene. The respective VDEs are 2.76 ± 0.05 and 3.25 ± 0.05 eV. The experimental results are in good agreement with the theoretically predicted singlet-triplet vertical energy gap at the anion geometry, but inconclusive with regard to the adiabatic singlet-triplet splitting in CClCN. Consistent with the experimental findings, ab initio calculations using the spin-flip approach in combination with the coupled-cluster theory, indicate that the (1)A' and (3)A″ states are nearly degenerate, with the singlet state lying adiabatically only ∼0.01 eV below the triplet.

Communication: photoelectron angular distributions of CH(-) reveal a temporary anion state.

Photoelectron imaging has broadened the scope of traditional photoelectron spectroscopy by combining a simultaneous photoelectron angular distribution, PAD, measurement with kinetic energy analysis. A fundamental understanding of PADs has been largely limited to simple atomic systems. However, a new model has recently been developed that predicts PADs as a function of electron kinetic energy for a simple linear combination of s and p atomic orbitals. We used CH(-) to test this model by acquiring PADs in a photoelectron imaging spectrometer at wavelengths from 600 to 355 nm. The PADs for electron detachment from the HOMO (1π) of CH(-) fit model predictions. However, the PADs associated with detachment from the HOMO-1 (3σ) orbital exhibit anomalous behavior at low electron kinetic energies because of a resonant process that arises from a previously undetected excited state of CH(-).

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.

Photodetachment anisotropy for mixed s-p states: 8/3 and other fractions.

An approximate model for analytical prediction of photoelectron angular distributions in anion photodetachment from mixed s-p states is presented. Considering the dipole-allowed s, p, and d free-electron partial waves, the model describes photodetachment anisotropy in terms of the fractional p character of the initial orbital and the A and B coefficients describing the relative intensities of the p → d to p → s and s → p to p → s channels, respectively. The model represents an extension of the central-potential model to an intermediate regime encompassing varying degrees of s and p contributions to the initial bound orbital. This description is applicable to a broad class of hybrid molecular orbitals, particularly those localized predominantly on a single atom. Under the additional assumption of hydrogenic or Slater-type orbitals, the B/A ratio in photodetachment from a mixed 2s-2p state is shown to equal 8/3. Corresponding fractions are derived for other ns-np mixing cases. The predictions of the model are tested on several anion systems, including NH(2)(-) and CCl(2)(-). The quantitative discrepancies in the latter case are attributed to the breakdown of the central-atom approximation and a mechanism for corresponding corrections is indicated.

Oxygen cluster anions revisited: solvent-mediated dissociation of the core O4(-) anion.

The electronic structure and photochemistry of the O(2n)(-)(H(2)O)(m), n = 1-6, m = 0-1 cluster anions is investigated at 532 nm using photoelectron imaging and photofragment mass-spectroscopy. The results indicate that both pure oxygen clusters and their hydrated counterparts with n ≥ 2 form an O(4)(-) core. Fragmentation of these clusters yields predominantly O(2)(-) and O(2)(-)·H(2)O anionic products, with the addition of O(4)(-) fragments for larger parent clusters. The fragment autodetachment patterns observed for O(6)(-) and larger O(2n)(-) species, as well as some of their hydrated counterparts, indicate that the corresponding O(2)(-) fragments are formed in excited vibrational states (v ≥ 4). Yet, surprisingly, the unsolvated O(4)(-) anion itself does not show fragment autodetachment at 532 nm. It is hypothesized that the vibrationally excited O(2)(-) is formed in the intra-cluster photodissociation of the O(4)(-) core anion via a charge-hopping electronic relaxation mechanism mediated by asymmetric solvation of the nascent photofragments: O(4)(-) → O(2)(-)(X(2)Π(g)) + O(2)(a(1)Δ(g)) → O(2)(X(3)Σ(g)(-)) + O(2)(-)(X(2)Π(g)). This process depends on the presence of solvent molecules and leads to vibrationally excited O(2)(-)(X(2)Π(g)) products.

O(-) + acetaldehyde reaction products: search for singlet formylmethylene, a Wolff rearrangement intermediate.

The mass-resolved anionic products of the reaction of O(•-) with acetaldehyde, H(3)CCHO, are studied using photoelectron imaging. The primary anionic products are vinoxide, H(2)CCHO(-), formylmethylene anion, HCCHO(•-), and ketenylidene anion, CCO(•-). From photoelectron spectra of HCCHO(•-), the electron affinity of triplet (ground state) formylmethylene (1.87 ± 0.02 eV) and the vertical detachment energy corresponding to the first excited triplet state (3.05 eV) are determined, but no unambiguous assignment for singlet HCCHO could be made. The elusive singlet is a key intermediate in the Wolff rearrangement, resulting in formation of ketene. The fast rearrangement associated with a large geometry change upon photodetachment to the singlet surface may be responsible for the low intensity of the singlet compared to the triplet bands in the photoelectron spectrum. The title reaction also yields CCO(•-), whose formation from acetaldehyde is novel and intriguing, since it requires a multistep net-H(4)(+) abstraction. A possible mechanism is proposed, involving an [H(2)CCO(•-)]* intermediate. From the measured electron affinities of HCCHO (above), H(2)CCHO (1.82 ± 0.01 eV), and CCO (2.31 ± 0.01 eV), several new thermochemical properties are determined, including the C-H bond dissociation energies and heats of formation of several organic molecules and/or their anions. Overall, the reactivity of O(•-) with organic molecules demonstrates the utility of this anion in the formation of a variety of reactive intermediates via a single process.

Photoelectron spectroscopic study of the oxyallyl diradical.

The photoelectron spectrum of the oxyallyl (OXA) radical anion has been measured. The radical anion has been generated in the reaction of the atomic oxygen radical anion (O(•-)) with acetone. Three low-lying electronic states of OXA have been observed in the spectrum. Electronic structure calculations have been performed for the triplet states ((3)B(2) and (3)B(1)) of OXA and the ground doublet state ((2)A(2)) of the radical anion using density functional theory (DFT). Spectral simulations have been carried out for the triplet states based on the results of the DFT calculations. The simulation identifies a vibrational progression of the CCC bending mode of the (3)B(2) state of OXA in the lower electron binding energy (eBE) portion of the spectrum. On top of the (3)B(2) feature, however, the experimental spectrum exhibits additional photoelectron peaks whose angular distribution is distinct from that for the vibronic peaks of the (3)B(2) state. Complete active space self-consistent field (CASSCF) method and second-order perturbation theory based on the CASSCF wave function (CASPT2) have been employed to study the lowest singlet state ((1)A(1)) of OXA. The simulation based on the results of these electronic structure calculations establishes that the overlapping peaks represent the vibrational ground level of the (1)A(1) state and its vibrational progression of the CO stretching mode. The (1)A(1) state is the lowest electronic state of OXA, and the electron affinity (EA) of OXA is 1.940 ± 0.010 eV. The (3)B(2) state is the first excited state with an electronic term energy of 55 ± 2 meV. The widths of the vibronic peaks of the X̃ (1)A(1) state are much broader than those of the ã (3)B(2) state, implying that the (1)A(1) state is indeed a transition state. The CASSCF and CASPT2 calculations suggest that the (1)A(1) state is at a potential maximum along the nuclear coordinate representing disrotatory motion of the two methylene groups, which leads to three-membered-ring formation, i.e., cyclopropanone. The simulation of b̃ (3)B(1) OXA reproduces the higher eBE portion of the spectrum very well. The term energy of the (3)B(1) state is 0.883 ± 0.012 eV. Photoelectron spectroscopic measurements have also been conducted for the other ion products of the O(•-) reaction with acetone. The photoelectron imaging spectrum of the acetylcarbene (AC) radical anion exhibits a broad, structureless feature, which is assigned to the X̃ (3)A'' state of AC. The ground ((2)A'') and first excited ((2)A') states of the 1-methylvinoxy (1-MVO) radical have been observed in the photoelectron spectrum of the 1-MVO ion, and their vibronic structure has been analyzed.

Photodissociation of nitromethane cluster anions.

Three types of anionic fragments are observed in the photodissociation of nitromethane cluster anions, (CH(3)NO(2))(n)(-), n=1-6, at 355 nm: NO(2)(-)(CH(3)NO(2))(k), (CH(3)NO(2))(k)(-), and OH(-) (k

Electronic structure and spectroscopy of oxyallyl: a theoretical study.

Electronic structure of the oxyallyl diradical and the anion is investigated using high-level ab initio methods. Converged theoretical estimates of the energy differences between low-lying electronic states of oxyallyl (OXA) as well as detachment energies of the anion are reported. Our best estimates of the adiabatic energy differences between the anion (2)A(2) and the neutral (3)B(2) and (3)B(1) states are 1.94 and 2.73 eV, respectively. The (1)A(1) state lies above (3)B(2) vertically, but geometric relaxation brings it below the triplet. The two-dimensional scan of the singlet (1)A(1) potential energy surface (PES) reveals that there is no minimum corresponding to a singlet diradical structure. Thus, singlet OXA undergoes prompt barrierless ring closure. However, a flat shape of the PES results in the resonance trapping in the Franck-Condon region, giving rise to the experimentally observable features in the photoelectron spectrum. By performing reduced-dimensionality wave packet calculations, we estimated that the wave packet lingers in the Franck-Condon region for about 170 fs, which corresponds to the spectral line broadening of about 200 cm(-1). We also present calculations of the photodetachment spectrum and compare it with experimental data. Our calculations lend strong support to the assignment of the photoelectron spectrum of the OXA anion reported in Ichino et al. (Angew. Chem., Int. Ed. Engl. 2009, 48, 8509).

Photoelectron imaging of NCCCN(-): The triplet ground state and the singlet-triplet splitting of dicyanocarbene.

The photoelectron spectra of NCCCN(-) have been measured at 355 and 266 nm by means of photoelectron imaging. The spectra show two distinct features, corresponding to the ground and first excited states of dycianocarbene. With support from theoretical calculations using the spin-flip coupled-cluster methods, the ground electronic state of HCCCN is assigned as a triplet state, while the first excited state is a closed-shell singlet. The photoelectron band corresponding to the triplet is broad and congested, indicating a large geometry change between the anion and neutral. A single sharp feature of the singlet band suggests that the geometry of the excited neutral is similar to that of the anion. In agreement with these observations, theoretical calculations show that the neutral triplet state is either linear or quasilinear (X (3)B(1) or (3)Sigma(g) (-)), while the closed-shell singlet (a (1)A(1)) geometry is strongly bent, similar to the anion structure. The adiabatic electron binding energy of the closed-shell singlet is measured to be 3.72+/-0.02 eV. The best estimate of the origin of the triplet band gives an experimental upper bound of the adiabatic electron affinity of NCCCN, EA

Photoelectron imaging of cyanovinylidene and cyanoacetylene anions.

Negative ions of cyanoacetylene and cyanovinylidene are generated simultaneously via the competing 1,1-H(2)(+) and 1,2-H(2)(+) abstraction channels of O(-) reaction with acrylonitrile. The two stable isomeric forms of the anion, CCHCN(-) and HCCCN(-), are separated by a large (approximately 2 eV) potential energy barrier. Their photodetachment provides access to both the reactant and the product sides of the neutral cyanovinylidene --> cyanoacetylene rearrangement reaction, predicted to involve only a very small barrier. Using photoelectron imaging spectroscopy at 532 and 355 nm, the adiabatic electron affinity of the reactive intermediate :C horizontal lineCHCN (X(1)A'), is determined to be 1.84 +/- 0.01 eV. The photoelectron spectrum of CCHCN(-) exhibits a vibrational progression attributed to the excitation of the CCH bending mode. The observed spectral features are reproduced reasonably well using a Franck-Condon simulation under the parallel-mode approximation. In contrast to unsubstituted acetylene, cyanoacetylene has a stable anionic state, which is adiabatically weakly bound, but has an experimentally determined vertical detachment energy of 1.04 +/- 0.05 eV. This measurement, along with the broad, structureless photoelectron spectrum of HCCCN(-) (with no identifiable origin), reflects the large geometry difference between the w-shaped structure of the anion and the linear equilibrium geometry of HCCCN.

Infrared spectroscopy of hydrated bicarbonate anion clusters: HCO3(-)(H2O)(1-10).

Infrared multiple photon dissociation spectra are reported for HCO(3)(-)(H(2)O)(1-10) clusters in the spectral range of 600-1800 cm(-1). In addition, electronic structure calculations at the MP2/6-311+G(d,p) level have been performed on the n = 1-8 clusters to identify the structure of the low-lying isomers and to assign the observed spectral features. General trends in the stepwise solvation motifs of the bicarbonate anion can be deduced from the overall agreement between the calculated and experimental spectra. The most important of these is the strong preference of the water molecules to bind to the negatively charged CO(2) moiety of the HCO(3)(-) anion. However, a maximum of four water molecules interact directly with this site. The binding motif in the most stable isomer of the n = 4 cluster, a four-membered ring with each water forming a single H-bond with the CO(2) moiety, is retained in all of the lowest-energy isomers of the larger clusters. Starting at n = 6, additional solvent molecules are found to form a second hydration layer, resulting in a water-water network bound to the CO(2) moiety of the bicarbonate anion. Binding of a water to the hydroxyl group of HCO(3)(-) is particularly disfavored and apparently does not occur in any of the clusters investigated here. Similarities and differences with the infrared spectrum of aqueous bicarbonate are discussed in light of these trends.

Laboratory observation of the valence anion of cyanoacetylene, a possible precursor for negative ions in space.

Valence anions of cyanoacetylene, HCCCN(-), are synthesized by the 1,2-H(2) (+) abstraction reaction of O(-) with acrylonitrile, H(2)C=CHCN, while the competing 1,1-H(2) (+) channel of the same reaction yields the cyanovinylidene anions, CCHCN(-). The key to the formation of the elusive, adiabatically weakly bound HCCCN(-) is the bent -C=C-C[triple bond]skeleton of the reactant. The photoelectron spectrum of HCCCN(-), measured by means of photoelectron imaging at 532 nm, consists of a broad structureless band with a vertical detachment energy of 1.04+/-0.05 eV. The observed anions are stable counterparts of the low-lying anionic resonances of cyanoacetylene, which may contribute (by way of dissociative attachment) to the formation of carbon-rich and CN-containing negative ions in extraterrestrial environments.

Low-lying electronic states of CH(3)NO(2) via photoelectron imaging of the nitromethane anion.

Negative-ion photoelectron imaging at 532, 392, 355, and 266 nm is used to assign several low-lying electronic states of neutral nitromethane CH(3)NO(2) at the geometry corresponding to the anion equilibrium. The observed neutral states include (in the order of increasing binding energy) the X (1)A(') ground state, two triplet excited states, a (3)A(") and b (3)A("), and the first excited singlet state, A (1)A("). The state assignments are aided by the analysis of the photoelectron angular distributions resulting from electron detachment from the a(') and a(") symmetry molecular orbitals and the results of theoretical calculations. The singlet-triplet (X (1)A(')-a (3)A(")) splitting in nitromethane is determined as 2.90(+0.02)/(-0.07) eV, while the vibrational structure of the band corresponding to the formation of the a (3)A(") state of CH(3)NO(2) is attributed to the ONO bending and NO(2) wagging motions excited in the photodetachment of the anion.