Christoph A. Schalley

Mass spectrometric approaches to the reactivity of transient neutrals

During the past few years, Neutralisation–Reionisation Mass Spectrometry (NRMS) has developed from a method for the generation and structural characterisation of elusive and highly reactive neutral molecules to a useful tool for probing their chemical reactivity. Three major principles can be distinguished: (i) peak shape analysis, (ii) activation of the neutrals by collisions or light, and (iii) variation of the neutrals’ lifetimes. Several methodological approaches are discussed in conjunction with illustrating examples for the chemical reactivity of transient neutrals.

A neutralization-reionization mass spectrometric study of alkyl hydroperoxide cation radicals and four distinguishable [C,H3,O2]+ isomers

Abstract The cation radicals of simple alkyl hydroperoxides ROOH+ (R = CH3, C2H5, iso-C3H7, tert-C4H9) have been studied by various mass spectrometric techniques, and neutralization-reionization (NR) methods in particular. Collisional activation (CA) of the beam of fast neutrals before reionization (NCR) and collision experiments with the mass-selected survivor ions (NR/CA) demonstrate that the peroxide Oue5f8O bond remains intact in ionized methyl hydroperoxide. In the series of hydroperoxides, the abundances of the survivor ions decrease with α-substitution, which can be traced back to increasing contributions of [ R + OOH ] ion/dipole complexes to the parent ion beam. For the elucidation of ion structures, +NR− experiments are shown to be particularly helpful; in addition, they also provide insight into the bonding situations in hydroperoxides. Unimolecular loss of H from metastable ions (MI) of CH3OOH+ leads to hydroperoxy methyl cations, CH2OOH+, which are characterized by their MI/CA mass spectra. For comparison, four distinguishable [C,H3,O2]+ isomers (CH2OOH+, CH3OO+, HC(OH)2 , and H3O+·CO) have been generated and examined by MI, CA, and NR experiments. With the exception of the proton-bound species H3O+·CO, the corresponding neutral [C,H3,O2]. radicals exist as well and do not interconvert into each other. In addition, HOCH2O. radicals can be probed by −NR+ experiments with HOCH2O− anions. Unimolecular and collision-induced decomposition of CH2OOH+ gives rise inter alia to a composite [C,H2,O]+ peak, which consists of a narrow gaussian and a broad, dish-topped component. The narrow component vanishes in the +NR+ experiment.

Mass spectrometry as a tool to probe the gas-phase reactivity of neutral molecules

Neutralization-reionization (NR) and charge-reversal (CR) mass spectrometric experiments can be combined to investigate the reactivity of neutrals generated in high energy collisions. Provided that the species under study exists as anion, neutral, and cation, the reactions of neutral molecules can be distinguished from those of projectile and recovery ions by taking neutral and ion decomposition difference (NIDD) mass spectra. The scope and limitation of this approach are discussed in detail for several diatomic species and selected polyatomic molecules. The NIDD spectra of tightly bound diatomics exhibit only minor signals which can be interpreted within the framework of vertical electron transfer processes and their Franck-Condon factors. More significant features arise for systems with weaker bonds in either the neutral or one of the charged states, for example, the different oxidation states of chlorine and hypochlorite, i.e. Cl2+·/Cl2/Cl2+ and ClO+/ClO·/ClO−, respectively. The NIDD spectra of some polyatomic compounds demonstrate the performance of the method for the elucidation of the neutrals structures and reactivities. The [C,H,O2]−/·/+ system is studied in detail and may serve as a model system for prototype carboxylate and acylium ions, HCOO− and HOCO+, respectively. The NIDD spectra of peroxide molecular cations reveal the intrinsic features of peroxidic systems in their different oxidation states, i.e. preferential Oue5f8O bond rupture in the neutral species as compared to a favorable Cue5f8O bond cleavage in the ionic systems. The gas-phase reactivity of small alkoxy radicals is dominated by α-cleavages, and Barton-type 1,5-hydrogen migrations are observed for larger alkoxy radicals. Application of NIDD to probe the neutral species generated by electron detachment from the ·CH2COO− distonic ion reveals the potential of the method to study diradicals. Finally, two C2N3 isomers are discussed as extreme examples in which the neutrals do not exhibit any distinct reactivity under the experimental conditions chosen. B.V.

ON THE FORMATION OF THE CARBON DIOXIDE ANION RADICAL CO2-. IN THE GAS PHASE

Abstract Although carbon dioxide is well known to have a negative electron affinity, the CO2− · anion radical can be generated in several types of mass spectrometric experiments, such as collisional activation of carboxylate ions or double electron transfer to CO2+ ·. In particular, it is shown that vibrational excitation of the precursor cations increases the yield of the bound anion radicals in charge inversion experiments. Combined application of experimental and theoretical means indicates that for bent geometries, the 2A1 state of the carbon dioxide anion radical is stable against electron detachment in the μs timescale of the experiments. In a chemical sense, the CO2− · anion radical can be regarded as an activated carbon dioxide unit in which the C–O bonds are weakened and the carbon center exhibits distinct radical character. Thus, CO2− · constitutes a new type of distonic anion in which the charge and the unpaired electron are located in different symmetry planes.

On the cleavage of the peroxide OO bond in methyl hydroperoxide and dimethyl peroxide upon protonation

Abstract The unimolecular decays of protonated methyl hydroperoxide and dimethyl peroxide have been studied by tandem mass spectrometric techniques in combination with isotopic labeling as well as computational methods. The potential-energy surfaces calculated at the BECKE3LYP/6-311++G∗∗ level of theory are in good agreement with the experimental findings. The decomposition of the protonated peroxides can be described by a general mechanistic scheme which involves rearrangement to proton-bridged complexes, i.e. [CH 2 O-H-OH 2 ] + and [CH 2 O-H-O(H)CH 3 ] + , respectively. When formed unimolecularly via rearrangement of the protonated peroxides, these complexes are rovibrationally highly excited; consequently, their fragmentations are affected remarkably as compared to proton-bound complexes of lower internal energy which are independently generated from the corresponding alcohol and carbonyl compounds in a chemical ionization plasma. For methyl hydroperoxide, both oxygen atoms can be protonated, giving rise to two isomeric cations with rather similar heats of formation but entirely different fragmentation behaviors. Cleavage of the Oue5f8O bond in dimethyl peroxide upon protonation results in proton- as well as methyl-cation-bridged intermediates, e.g. [CH 2 O-H-O(H)CH 3 ] + and [CH 2 O-CH 3 -OH 2 ] + .

Mass‐Spectrometric Experiments together with Electronic Structure Calculations Support the Existence of the Elusive Ammonia Oxide Molecule and Its Radical Cation

Mass-spectrometric experiments were combined with ab initio calculations to explore the cationic and neutral [H3,N,O]☆+/0 potential energy surfaces and relevant anionic species. The calculations predict the existence of three stable cationic and neutral [H3,N,O]☆+/0 isomers, i.e. ammonia oxide H3NO☆+/0 (1☆+/0), hydroxylamine H2NOH☆+/0 (2☆+/0) and the imine-water complex HNOH2☆+/0 (3☆+/0). Hydroxylamine 2 represents the most stable isomer on the neutral surface (Erel = 0), and the metastable isomers 1 (Erel = 24.8 kcal mol–1) and 3 (Erel = 61.4 kcal mol–1) are separated by barriers of 49.5 kcal mol–1 and 64.2 kcal mol–1, respectively. Adiabatic ionization of 2 (IEa = 9.15 eV) yields 2☆+, which is 21.4 kcal mol–1 more stable than 1☆+ and 36.4 kcal mol–1 more stable than 3☆+. The barriers associated with the isomerizations of the cations are 58.6 kcal mol–1 for 2☆+ 1☆+ and 71.4 kcal mol–1 for 2☆+ 3☆+. Collisional activation (CA) and unimolecular decomposition (MI) experiments allow for a clear distinction of 1☆+ from 2☆+. Besides, neutralization/reionization (NR) experiments strongly support the gas-phase existence of the long-sought neutral ammonia oxide.