Neutralization-reionization Mass Spectrometry Experiments Research Articles

ChemInform Abstract: Neutralization-Reionization Mass Spectrometry Applied to Organometallic and Coordination Chemistry (Update: 1994-1998)

This review presents the results from the last 5 years on neutralization–reionization mass spectrometry (NRMS) studies of metal-containing ions. The results for silicon- and phosphorus-containing species are also discussed. The major aim of NRMS experiments is to generate (in the gas phase) neutrals that cannot be easily produced and/or detected in other experimental conditions. A variety of elusive complexes of coordinatively unsaturated and saturated metal(s), including important reaction intermediates, have been produced by collisional neutralization of their positively or negatively charged counterparts in the mass spectrometer. The detection of these species helps in understanding the mechanisms of reactions in the gas and condensed phases (interstellar chemistry, catalytic transformations, reactions on surfaces, etc.). The reviewed period of time has been characterized by the development of new experimental techniques. These techniques allow studying stability, electronic structure, and reactivity of selected molecules and radicals. The review provides a complete list of metal-, Si-, and P-containing ions that have been studied by using NRMS and related methods prior to 1999. © 1999 John Wiley & Sons, Inc., Mass Spec Rev 18: 87–118, 1999

Pyrazine diradicals, carbenes, ylides, and distonic ions probed by theory and experiment

The [C 4,H 4,N 2] potential energy hypersurface relating to pyrazine and its hydrogen shift isomers has been investigated computationally using hybrid Hartree–Fock/density functional theory and through a variety of tandem mass spectrometry experiments (metastable ion, collision-induced dissociation, and neutralization reionization mass spectrometry). In addition to the conventional pyrazine structure 1, its α-ylide 2, β-ylide 3, and the 1,4-diradical 4 were generated and characterized through neutralization reionization mass spectrometry experiments. Also, the corresponding radical cations 1 ·+ – 4 ·+ were accessible by dissociative electron ionization of the appropriate pyrazine esters. Quantum chemical calculations at the B3LYP/TZVP level of theory reveal that all these species correspond to minima that are separated by significant barriers thus preventing facile isomerization. As an additional, albeit high lying isomer, the 1,3-diradical 5 was computationally identified. In the case of the radical cations the energy differences between the various isomers are much smaller than for the corresponding neutrals; however, pyrazine represents in both cases the most stable species.

Neutralization-reionization mass spectrometry applied to organometallic and coordination chemistry (update: 1994-1998)

This review presents the results from the last 5 years on neutralization–reionization mass spectrometry (NRMS) studies of metal-containing ions. The results for silicon- and phosphorus-containing species are also discussed. The major aim of NRMS experiments is to generate (in the gas phase) neutrals that cannot be easily produced and/or detected in other experimental conditions. A variety of elusive complexes of coordinatively unsaturated and saturated metal(s), including important reaction intermediates, have been produced by collisional neutralization of their positively or negatively charged counterparts in the mass spectrometer. The detection of these species helps in understanding the mechanisms of reactions in the gas and condensed phases (interstellar chemistry, catalytic transformations, reactions on surfaces, etc.). The reviewed period of time has been characterized by the development of new experimental techniques. These techniques allow studying stability, electronic structure, and reactivity of selected molecules and radicals. The review provides a complete list of metal-, Si-, and P-containing ions that have been studied by using NRMS and related methods prior to 1999. © 1999 John Wiley & Sons, Inc., Mass Spec Rev 18: 87–118, 1999

The unimolecular chemistry of quaternary ammonium ions and their neutral counterparts

The quaternary ammonium ions (CH 3) 4N + ( 1 +), (CH 3CH 2) 4N + ( 2 +), C 6H 5N + (CH 3) 3( 3 +), C 6H 5CH 2N + (CH 3) 3 ( 4 +) and C 6H 5CH 2N + (CH 2CH 3) 3 ( 5 +) and the corresponding ammonium radicals 1–5 have been studied in the gas phase by metastable ion (MI) characteristics, collisionally activated dissociation (CAD) and neutralization—reionization mass spectrometry (NRMS). The unimolecular chemistry of the ammonium ions is influenced by the nature of their substituents (R). Based on cations 3 +– 5 +, which carry different types of N +R bonds, preferential dissociation at N +—benzyl > N +—alkyl > N +—phenyl takes place. The two major fragmentation channels observed for all ions are (i) direct cleavage of the N +R bond with charge retention on the species of lowest ionization energy and (ii) formation of an immonium ion by loss of the hydrocarbon RH (from all cations) or RCH 3 (only from 2 + and 5 +). These decompositions proceed after initial, rate-determining isomerization of 1 +–5 + to ion/dipole complexes R 3N + / R or, in case of benzyl substitution, to R +/NR 3. The NRMS experiments reveal that the hypervalent radicals 1–5 are unstable, decaying within ⪡ 0.2 μs by rupture of one of the NR bonds. 3–5 mainly cleave the NR bond which yields the thermodynamically most stable products.

Experimental evidence for the existence of SiCHx (x = 1-3) molecules in the gas phase

The neutral silicon carbon hydrides SiCH{sub x} (x = 1-3) are viable molecules in the diluted gas phase as evidenced by neutralization-reionization mass spectrometry experiments. Thus, previous suggestions concerning conceivable mechanisms for the generation of these molecules in interstellar space are strongly supported.

Neutralization-reionization of CH 4+•: At which stage does fragmentation occur?

Experimental (collision-induced dissociative ionization (CIDI) and neutralization-reionization mass spectrometry (NRMS)) and theoretical (ab initio molecular orbital) procedures were used to examine the origin of the fragmentation observed in NRMS experiments on the methane radical cation. Both theory and experiment indicate that in NRMS experiments involving neutralization by species of high ionization energy (e.g. Hg or Xe), the fragmentation occurs largely at the reionization rather than the neutralization stage. The reverse is true for the strongly exothermic neutralization with Na. Corresponding experiments [C 2H 6] +• ions showed similar behaviour.