Richard A. J. O'Hair

Gas Phase Ion Chemistry of Transition Metal Clusters Production, Reactivity, and Catalysis

This review focuses on the use of mass spectrometry to examine the gas phase ion chemistry of metal clusters. Ways of forming gas phase clusters are briefly overviewed and then the gas phase chemistry of silver clusters is discussed to illustrate the concepts of “magic numbers” and how reactivity can be size dependent. The chemistry of other bare and ligated metal clusters is examined, including mixed metal dimer ions as models for microalloys. Metal clusters that catalyze gas phase chemical reactions such as the oxidation of CO and organic substrates are reviewed. Finally the interface between nanotechnology and mass spectrometry is also considered.

Gas phase acidities of the α amino acids

Abstract The gas phase acidities of 19 α amino acids have been determined using the kinetic method of Cooks and co-workers [R.G. Cooks and T.L. Kruger, J. Am. Chem. Soc., 99 (1977) 1279; S.A. McLuckey, D. Cameron and R.G. Cooks, J. Am. Chem. Soc., 103 (1981) 1313]. The range of gas phase acidities for this class of biomolecules spans from glycine, the least acidic (ΔG°acid = 1402.0 kJ mol−1), to histidine, the most acidic (ΔG°acid = 1356.0 kJ mol−1). For a few, simple amino acids (glycine, alanine, serine, and cysteine), ab initio theory at the HF/6-31 + G*//HF/3-21 (+)G* level was used to calculate gas phase acidities. There is good agreement between the theoretical and experimental acidities with the average deviation being ±8.5 kJ mol−1. Fully optimized structures are reported for these amino acids and their corresponding carboxylates.

The role of nucleophile–electrophile interactions in the unimolecular and bimolecular gas-phase ion chemistry of peptides and related systems†

This account describes the experimental tools (multi-stage mass spectrometric experiments, isotopic and structural labelling, kinetics and theoretical modelling) and physical organic concepts (influence of charge, the intermediacy of ion-molecule complexes, etc.) that can be used to unravel the mechanisms of gas-phase unimolecular and bimolecular ionic reactions of peptides. The role that nucleophile-electrophile interactions play in charge-directed reactions is highlighted for both unimolecular fragmentations (examples are illustrated for protonated sulfur-containing amino acids and peptides) and bimolecular ion-molecule reactions which cleave peptide bonds.

Gold‐Mediated CI Bond Activation of Iodobenzene

Controversy resolved! A combination of gas-phase ion-molecule reactions and theoretical studies confirm bisligated mononuclear Au(I) complexes are unable to undergo oxidative addition of iodobenzene for Sonogashira coupling, but that the ligated gold clusters [Au(3)L(n)](+) (L=Ph(2)P(CH(2))(n)PPh(2); n=3-6) activate the C-I bond. DFT calculations on the transition states show that the linker size n tunes the cluster reactivity.

CH Bond Activation of Methanol and Ethanol by a High‐Spin FeIVO Biomimetic Complex

The selective and efficient activation of strong organic bonds is one of the major goals in chemistry due to the intense interest in developing more cost-effective and environmentally sustainable routes for the industrial production of chemicals. Many biological enzymes containing metal–oxo active site intermediates, 3] including those that contain a non-heme high-spin (S = 2) Fe=O active-site intermediate, can mediate reactions of relevance to organic synthesis (e.g., C H bond hydroxylation, alcohol oxidation, olefin epoxidation, etc.). As a result, there has been considerable interest in preparing and studying novel Fe=O complexes that can “mimic” these beneficial properties and provide insights into the chemistry of Fe–oxo enzyme active sites. A key challenge is that high-valent Fe–oxo complexes in high-spin states are highly reactive. For example, out of a wide range of synthetic Fe=O complexes that have been reported, only three are high-spin (S = 2) non-heme Fe=O complexes, and these have lifetimes that range from 7 s to 2.2 h at 25 8C. Another approach for studying highly reactive complexes is to generate and investigate such species in the gas phase, where effects of solvent, counterions, and aggregation, which can all lead to degradation of reactive complexes, can either be eliminated, or carefully controlled. Such studies can potentially reveal new types of transition metal mediated reactions, which may uncover important details of reaction mechanisms and direct the development of future condensedphase catalysts. Although there have been numerous gasphase studies of Fe–oxo based ions, and FeO in particular, the chemistry of high-spin non-heme Fe=O complexes, of the types that have only recently been synthesized in the condensed phase, have not been explored in vacuo leaving a considerable gap between the fundamental gasphase Fe–oxo studies and the recent advances in condensedphase high-valent non-heme Fe–oxo coordination chemistry. Herein we report the gas-phase synthesis of the high-spin complex [(bpg)Fe=O] (where bpg is N,N-bis(2-pyridinylmethyl)glycinato ) and its reactions with methanol and ethanol. Electrospray ionization (ESI) of 100 mm solutions of [(bpg)Fe(H2O)OFe(H2O)(bpg)](ClO4)2 [11] dissolved in a 10:90 acetonitrile:CH2Cl2 mixture resulted in the formation of a dominant ion at m/z 320, corresponding to [(bpg)FeOFe(bpg)]. Collision-induced dissociation (CID) of isolated [(bpg)FeOFe(bpg)] (m/z 320) leads to the formation of a population of ions at m/z 328 with a stoichiometry that corresponds to that of [(bpg)FeO], in addition to an ion at m/z 312 corresponding to [(bpg)Fe] (Figure 1a), which is formed through charge separation of the precursor ion [Eq. (1)] in a redox disproportionation reaction.

Gas-phase synthesis of the homo and hetero organocuprate anions [MeCuMe]-, [EtCuEt]-, and [MeCuR]-.

The homocuprates [MeCuMe]- and [EtCuEt]- were generated in the gas phase by double decarboxylation of the copper carboxylate centers [MeCO2CuO2CMe]- and [EtCO2CuO2CEt]-, respectively. The same strategy was explored for generating the heterocuprates [MeCuR]- from [MeCO2CuO2CR]- (R = Et, Pr, iPr, tBu, allyl, benzyl, Ph). The formation of these organocuprates was examined by multistage mass spectrometry experiments, including collision-induced dissociation and ion-molecule reactions, and theoretically by density functional theory. A number of side reactions were observed to be in competition with the second stage of decarboxylation, including loss of the anionic carboxylate ligand and loss of neutral alkene via beta-hydride transfer elimination. Interpretation of decarboxylation of the heterocarboxylates [MeCO2CuO2CR]- was more complex because of the possibility of decarboxylation occurring at either of the two different carboxylate ligands and giving rise to the possible isomers [MeCuO2CR]- or [MeCO2CuR]-. Ion-molecule reactions of the products of initial decarboxylation with allyl iodide resulted in C-C coupling to produce the ionic products [ICuO2CR]- or [MeCO2CuI]-, which provided insights into the relative population of the isomers, and indicated that the site of decarboxylation was dependent on R. For example, [MeCO2CuO2CtBu]- underwent decarboxylation at MeCO2- to give [MeCuO2CtBu]-, while [MeCO2CuO2CCH2Ph]- underwent decarboxylation at PhCH2CO2- to give [MeCO2CuCH2Ph]-. Each of the heterocuprates [MeCuR]- (R = Et, Pr, iPr, allyl, benzyl, Ph) could be generated by the double decarboxylation strategy. However, when R = tBu, intermediate [MeCuO2CtBu]- only underwent loss of tBuCO2-, a consequence of the steric bulk of tBu disfavoring decarboxylation and stabilizing the competing channel of carboxylate anion loss. Detailed DFT calculations were carried out on the potential energy surfaces for the first and second decarboxylation reactions of all homo- and heterocuprates, as well as possible competing reactions. These reveal that in all cases the first decarboxylation reaction is favored over loss of the carboxylate ligand. In contrast, other reactions such as carboxylate ligand loss and beta-hydride transfer become more competitive with the second decarboxylation reaction.

Letter: intercluster chemistry of protonated and sodiated betaine dimers upon collision induced dissociation and electron induced dissociation.

The collision induced dissociation and electron induced dissociation spectra of the [2M + H]+ and [2M + Na]+ clusters of the zwitterionic amino acid, betaine (M), have been examined in a hybrid linear ion trap Fourier transform ion cyclotron resonance mass spectrometer. Intercluster reactions are observed in the collision induced dissociation spectra of [2M + H]+ and [2M + Na]+ and in the electron induced dissociation spectrum of [2M + H]+.

Aspergillicins A–E: five novel depsipeptides from the marine-derived fungus Aspergillus carneus

A search for new antiparasitic agents from a strain of the fungus Aspergillus carneus isolated from an estuarine sediment collected in Tasmania, Australia, yielded the known terrestrial fungal metabolite marcfortine A (1) as an exceptionally potent antiparasitic agent. This study also yielded a series of new depsipeptides, aspergillicins A-E (2-6) and the known terrestrial fungal metabolite acyl aszonalenin (7). Marcfortine A (1) and acyl aszonalenin (7) were identified by spectroscopic analysis, with comparison to literature data. Complete stereostructures were assigned to aspergillicins A-E (2-6) on the basis of detailed spectroscopic analysis, together with ESIMS analysis of the free amino acids generated by acid hydrolysis, and HPLC analysis of Marfey derivatives prepared from the acid hydrolysate. The peptide amino acid sequence for all aspergillicins was unambiguously assigned by MS(n) ion-trap ESI mass spectrometry.

Structure and Reactivity of the Cysteine Methyl Ester Radical Cation

The structure and reactivity of the cysteine methyl ester radical cation, CysOMe(.+) , have been examined in the gas phase using a combination of experiment and density functional theory (DFT) calculations. CysOMe(.+) undergoes rapid ion-molecule reactions with dimethyl disulfide, allyl bromide, and allyl iodide, but is unreactive towards allyl chloride. These reactions proceed by radical atom or group transfer and are consistent with CysOMe(.+) possessing structure 1, in which the radical site is located on the sulfur atom and the amino group is protonated. This contrasts with DFT calculations that predict a captodative structure 2, in which the radical site is positioned on the α carbon and the carbonyl group is protonated, and that is more stable than 1 by 13.0 kJ mol(-1) . To resolve this apparent discrepancy the gas-phase IR spectrum of CysOMe(.+) was experimentally determined and compared with the theoretically predicted IR spectra of a range of isomers. An excellent match was obtained for 1. DFT calculations highlight that although 1 is thermodynamically less stable than 2, it is kinetically stable with respect to rearrangement.

Gas-phase reactions of aryl radicals with 2-butyne: experimental and theoretical investigation employing the N-methyl-pyridinium-4-yl radical cation

Aromatic radicals form in a variety of reacting gas-phase systems, where their molecular weight growth reactions with unsaturated hydrocarbons are of considerable importance. We have investigated the ion-molecule reaction of the aromatic distonic N-methyl-pyridinium-4-yl (NMP) radical cation with 2-butyne (CH(3)C≡CCH(3)) using ion trap mass spectrometry. Comparison is made to high-level ab initio energy surfaces for the reaction of NMP and for the neutral phenyl radical system. The NMP radical cation reacts rapidly with 2-butyne at ambient temperature, due to the apparent absence of any barrier. The activated vinyl radical adduct predominantly dissociates via loss of a H atom, with lesser amounts of CH(3) loss. High-resolution Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry allows us to identify small quantities of the collisionally deactivated reaction adduct. Statistical reaction rate theory calculations (master equation/RRKM theory) on the NMP+2-butyne system support our experimental findings, and indicate a mechanism that predominantly involves an allylic resonance-stabilized radical formed via H atom shuttling between the aromatic ring and the C(4) side-chain, followed by cyclization and/or low-energy H atom β-scission reactions. A similar mechanism is demonstrated for the neutral phenyl radical (Ph˙)+2-butyne reaction, forming products that include 3-methylindene. The collisionally deactivated reaction adduct is predicted to be quenched in the form of a resonance-stabilized methylphenylallyl radical. Experiments using a 2,5-dichloro substituted methyl-pyridiniumyl radical cation revealed that in this case CH(3) loss from the 2-butyne adduct is favoured over H atom loss, verifying the key role of ortho H atoms, and the shuttling mechanism, in the reactions of aromatic radicals with alkynes. As well as being useful phenyl radical analogues, pyridiniumyl radical cations may form in the ionosphere of Titan, where they could undergo rapid molecular weight growth reactions to yield polycyclic aromatic nitrogen hydrocarbons (PANHs).