Neutralization-reionization Mass Spectrometry Research Articles

Generation of the Elusive meta‐Benzoquinone in the Gas Phase

Ready for matrix isolation? The existence of meta-benzoquinone has been established unambiguously by a combination of neutralization–reionization mass spectrometry and computational studies (see picture). The results suggest that this elusive member of the family of organic biradicals should be accessible in matrix-isolation experiments.

Theoretical Study on Stability and Properties of NC2O Isomers

The structures, energetics, spectroscopies, and stabilities of the doublet NC(2)O radical are explored at density functional theory and ab initio levels. Nine minimum isomers are located connected by 22 interconversion transition states. At the CCSD(T)/6-311+G(2df)//QCISD/6-311G(d)+ZPVE level, the lowest-lying isomer is bent NCCO 1 (0.0 kcal/mol) with (2)A' state followed by bent isomer CNCO 2 (16.7). Two isomers (1 and 2) and another high-lying species CCNO 4 (99.4) with bent structure are considerably stabilized by a barrier of at least 20 kcal/mol. All of the three isomers should be experimentally or astrophysically observable. This result is consistent with their indication of neutralization-reionization mass spectrometry experiments. Also, the calculated spectroscopic properties and bond distances of known NCCO 1 are consistent with recent experimental observations and theoretical studies. The bonding natures of the isomers 1, 2, and 4 are analyzed. Their molecular properties including the heats of formation, adiabatic ionization potentials, and adiabatic electronic affinities are calculated at the higher levels G3//B3LYP, G3(MP2)//B3LYP, QCISD, and CCSD(T) (single-point). Possible formation strategies of the isomers 1, 2, and 4 in laboratory and space are also discussed in detail.

Experimental Detection of Theoretically Predicted N2CO

The missing link in the 28-electron family of open-chain N4, N2CO, and C2O2 molecules has been detected in the gas phase as an isolated, intact N2CO species, whose lifetime exceeds 0.8 μs. The new metastable molecule has been prepared by neutralization–reionization mass spectrometry of labeled N2CO+ ions, which were generated in various N2/CO ionized mixtures (see picture).

Electron-Rich Radicals by Neutralization—Reionization Mass Spectrometry. Generation, Dissociations and Energetics of the Hydrogen Atom Adduct to Acetamide

Protonated acetamide exists as two planar conformers, the more stable anti-form (anti-1(+)) and the syn-form (syn-1(+)), DeltaG(degree) (298) (anti-->syn) = 10.8 kJ mol(-1). Collisional neutralization of 1(+) produces 1-hydroxy-1-amino-1-ethyl radicals (anti-1 and syn-1) which in part survive for 3.7 micros. The major dissociation of 1 is loss of the hydroxyl hydrogen atom (approximately 95%) which is accompanied by loss of one of the methyl hydrogen atoms (approximately 3%) and loss of the methyl group (approximately 2%). The most favorable dissociation of the OH bond is calculated to be only 34 kJ mol(1) endothermic but requires 88 kJ mol(-1) in the transition state. Other dissociations of 1, e.g., loss of one of the amide hydrogens, methyl hydrogens, and loss of ammonia are calculated to proceed through higher- energy transition states and are not kinetically competitive if proceeding from the ground doublet electronic state of 1. The unimolecular dissociation of 1 following collisional electron transfer is promoted by large Franck-Condon effects that result in 8090 kJ mol(-1) vibrational excitation in the radicals. Radicals 1 are calculated to exoergically abstract hydrogen atoms from acetamide in water, but not in the gas phase. The different reactivity is due to solvent effects that favor the products, (.)CH(2)CONH(2) and CH(3)CH(OH)NH(2), over the reactants.

Modeling deoxyribose radicals by neutralization-reionization mass spectrometry. Part 2. Preparation, dissociations, and energetics of 3-hydroxyoxolan-3-yl radical and cation

The title radical ( 1) is generated in the gas-phase by collisional neutralization of carbonyl-protonated oxolan-3-one. A 1.5% fraction of 1 does not dissociate and is detected following reionization as survivor ions. The major dissociation of 1 (∼56%) occurs as loss of the hydroxyl H atom forming oxolan-3-one ( 2). The competing ring cleavages by OC-2 and C-4C-5 bond dissociations combined account for ∼42% of dissociation and result in the formation of formaldehyde and 2-hydroxyallyl radical. Additional ring-cleavage dissociations of 1 resulting in the formation of C 2H 3O and C 2H 4O cannot be explained as occurring competitively on the doublet ground ( X ) electronic state of 1, but are energetically accessible from the A and higher electronic states accessed by vertical electron transfer. Exothermic protonation of 2 also produces 3-oxo-( 1H)-oxolanium cation ( 3 + ) which upon collisional neutralization gives hypervalent 3-oxo-( 1H)-oxolanium radical ( 3). The latter dissociates spontaneously by ring opening and expulsion of hydroxy radical. Experiment and calculations suggest that carbohydrate radicals incorporating the 3-hydroxyoxolan-3-yl motif will prefer ring-cleavage dissociations at low internal energies or upon photoexcitation by absorbing light at ∼590 and ∼400 nm.

Modeling deoxyribose radicals by neutralization-reionization mass spectrometry. Part 1. Preparation, dissociations, and energetics of 2-hydroxyoxolan-2-yl radical, neutral isomers, and cations

Collisional neutralization of several isomeric C 4H 7O 2 cations is used to generate radicals that share some structural features with transient species that are thought to be produced by radiolysis of 2-deoxyribose. The title 2-hydroxyoxolan-2-yl radical ( 1) undergoes nearly complete dissociation when produced by femtosecond electron transfer from thermal organic electron donors dimethyl disulfide and N,N-dimethylaniline in the gas phase. Product analysis, isotope labeling ( 2H and 18O), and potential energy surface mapping by ab initio calculations at the G2(MP2) and B3-PMP2 levels of theory and in combination with Rice-Ramsperger-Kassel-Marcus (RRKM) kinetic calculations are used to assign the major and some minor pathways for 1 dissociations. The major (∼90%) pathway is initiated by cleavage of the ring C-5O bond in 1 and proceeds to form ethylene and ·CH 2COOH as main products, whereas loss of a hydrogen atom forms 4-hexenoic acid as a minor product. Loss of the OH hydrogen atom forming butyrolactone ( 2, ∼9%) and cleavage of the C-3C-4 bonds (<1%) in 1 are other minor pathways. The major source of excitation in 1 is by Franck-Condon effects that cause substantial differences between the adiabatic and vertical ionization of 1 (5.40 and 6.89 eV, respectively) and vertical recombination in the precursor ion 1 + (4.46 eV). +NR + mass spectra distinguish radical 1 from isomeric radicals 2-oxo-( 1H)oxolanium ( 3), 1,3-dioxan-2-yl ( 9), and 1,3-dioxan-4-yl ( 10) that were generated separately from their corresponding ion precursors.

Modeling deoxyribose radicals by neutralization-reionization mass spectrometry. part 2. preparation, dissociations, and energetics of 3-hydroxyoxolan-3-yl radical and cation

Modeling deoxyribose radicals by neutralization-reionization mass spectrometry. part 2. preparation, dissociations, and energetics of 3-hydroxyoxolan-3-yl radical and cation

Modeling deoxyribose radicals by neutralization-reionization mass spectrometry. Part 1. Preparation, dissociations, and energetics of 2-hydroxyoxolan-2-yl radical, neutral isomers, and cations

Modeling deoxyribose radicals by neutralization-reionization mass spectrometry. Part 1. Preparation, dissociations, and energetics of 2-hydroxyoxolan-2-yl radical, neutral isomers, and cations

Confirmation of the "long-lived" tetra-nitrogen (N4) molecule using neutralization-reionization mass spectrometry and ab initio calculations.

Tetra-nitrogen (N(4)), which has been the subject of recent controversy [Cacace, d. Petris, and Troiani, Science 295, 480 (2002); Cacace, Chem. Eur. J. 8, 3839 (2002); Nguyen et al., J. Phys. Chem. A 107, 5452 (2003); Nguyen, Coord. Chem. Rev. 244, 93 (2003)] as well as of great theoretical interest, has been prepared from the N(4) (+) cation and then detected as a reionized gaseous metastable molecule with a lifetime exceeding 0.8 micros in experiments based on neutralization-reionization mass spectrometry. Moreover, we have used the nature of the charge-transfer reaction which occurs between a beam of fast N(4) (+) ions (8 keV translational energy) and various stationary gas targets to identify the vertical neutralization energy of the N(4) (+) ion. The measured value, 10.3+/-0.5, most closely matches that of the lowest energy azidonitrene (4)N(4) (+)C(s)((4)A(')) ion, resulting in the formation of the neutral bound azidonitrene (3)N(4)C(s)((3)A(")). Neutralization of the global minimum (2)N(4) (+)D( infinity h)((2)Sigma(u) (+)) ion leads to a structure 166 kJ mol(-1) above the dissociation products [N(2)((1)Sigma(g) (+))+N(2)((1)Sigma(g) (+))]; moreover, it was not possible to find a minimum on the (1)N(4) neutral potential energy surface for a covalently bonded structure. Ab initio calculations at the G3, QCISD/6-31G(d), and MP2/AUG-cc-pVTZ levels of theory have been used to determine geometries and both vertical neutralization energies of ions (doublet and quartet) and ionization energies of neutrals (singlet and triplet). In addition, we have also described in detail the EI ion source for the Ottawa VG ZAB mass spectrometer [Holmes and Mayer, J. Phys. Chem. A 99, 1366 (1995)] which was modified for high-pressure use, i.e., for the production of dimer and higher number cluster ions.

Discovery of the new metastable HONF. radical.

The new radical HONF has been detected in the gas phase by neutralization-reionization mass spectrometry (NRMS). The radical has been identified and directly characterized as a gaseous isolated species, having a lifetime of at least 1 microsecond and a linear cis-trans structure of H-O-N-F connectivity. Detection of this molecule, which is highly unstable towards the dissociation into HF and NO and kinetically sufficiently stable to be observed, represents an advance in the search for high-energy species.

Transient Intermediates of Chemical Reactions by Neutralization—Reionization Mass Spectrometry

AbstractFor Abstract see ChemInform Abstract in Full Text.

Discovery of two high-energy N2O2 isomers.

Two N2O2 isomers containing N2/O2 and NO/NO subunits, respectively, were detected by neutralization-reionization mass spectrometry (NRMS) as metastable species with lifetimes exceeding 1 µs.

Cover Picture: Experimental Detection of the H2NO3 Radical (ChemPhysChem 10/2003)

AbstractThe cover pictures shows…︁‥the mass spectrum of nitrogen hydroxide oxide, the H2NO3. radical, which was discovered by neutralization reionization mass spectrometry (NRMS) and characterized as a relatively long‐lived (over 1 μs) metastable species. The discovery required the availability of the appropriate charged precursor that fortunately had previously been prepared by this group (1989) as a result of their sustained interest in gas‐phase ion chemistry of nitrates, nitrites and related molecules. Cacace et al. found that protonation of nitric acid by strong Brønsted acids yields, in addition to the H2ONO2+ ion, smaller amounts of the less stable (HO)2NO+ isomer, namely charged nitrogen hydroxide oxide. This result has allowed the present discovery by NRMS (yellow trace), whereas NRMS of the H2ONO2+ ion does not result in detectable H2NO3. neutral species (green trace). The importance of nitrogen hydroxide oxide stems from its relevance to a variety of research areas, from the reduction of the NO32− anion, to atmospheric chemistry, radiolysis of nuclear‐waste solutions, the charge‐transfer mechanism of aromatic nitration, etc. Find out more in the communication by Cacace et al. on pp. 1128–131.

Experimental detection of the H2NO3 radical.

It's in the air. The authors detected by neutralization–reionization mass spectrometry the nitrogen hydroxide oxide 3, which is relevant to atmospheric chemistry and a key intermediate in the NO32− reduction, as a gaseous radical species with a lifetime of 1 μs. The charged precursor utilized was the high-energy isomer 2 from the protonation of HNO3, whereas the more stable isomer 1 dissociates upon neutralization.

A mass spectrometry study of tirapazamine and its metabolites: insights into the mechanism of metabolic transformations and the characterization of reaction intermediates

Tandem mass spectrometry methods were used to study the sites of protonation and for identification of 3-amino-1,2,4-benzotriazine 1,4-dioxide ( 1, tirapazamine), and its metabolites (3-amino-1,2,4-benzotriazine 1-oxide ( 3), 3-amino-1,2,4-benzotriazine 4-oxide ( 4), 3-amino-1,2,4-benzotriazine ( 5), and a related isomer 3-amino-1,2,4-benzotriazine 2-oxide ( 6). Fragmentation pathways of 3 and 5 indicated the 4-N-atom as the most likely site of protonation. Among the N-oxides studied, the 4-oxide ( 4) showed the highest degree of protonation at the oxygen atom. The differences in collision-induced dissociation of isomeric protonated 1-, 2- and 4-oxides allowed for their identification by LC/MS/MS. Gas phase and liquid phase protonation of tirapazamine occurred exclusively at the oxygen in the 4-position. A loss of OH radical from these ions ( 2 +) resulted in ionized 3. Neutralization-reionization mass spectrometry (NR MS) experiments demonstrated the stability of the neutral analogue of protonated tirapazamine in the gas phase in the μs time-frame. A significant portion of the neutral tirapazamine radicals ( 2) dissociated by loss of hydroxyl radical during the NR MS event, which indicates that previously proposed mechanisms for redox-activated DNA damage are reasonable. The activation energy for loss of hydroxyl radical from activated tirapazamine ( 2) was estimated to be ∼14 kcal mol −1. Stable neutral analogues of [ 3 + H] + and [ 5 + H] + ions were also generated in the course of NR MS experiments. Structures of these radicals were assigned to the molecules having an extra hydrogen atom at one of the ring N-atoms. Quantum chemical calculations of protonated 1, 3, 4 and 5 and the corresponding neutrals were performed to assist in the interpretation of experimental results and to help identify their structures.

The use of kinetic isotope effects for the determination of internal energy distributions in isolated transient species in the gas phase

Isotope effects on the loss of H and D are used to estimate the internal energy distribution in transient radicals produced by collisional electron transfer in the gas phase, as studied by neutralization–reionization mass spectrometry (NRMS). Experimental intermolecular and intramolecular isotope effects are compared to those calculated by RRKM theory on ab initio potential energy surfaces and convoluted with energy distribution curves. Fitting parameters in trial energy distribution curves provides widths and maxima that are compared with the energetics of ion and neutral formation. For collisional neutralization of 2-hydroxypyridinium, 3-hydroxypyridinium, and C(OH) 3 + cations by electron transfer from polarizable molecular targets, the most probable internal energy of the resulting transient radical is expressed as a simple sum of the precursor ion internal energy and the Franck–Condon energy acquired by vertical electron capture. This simple formula allows one to predict internal energies in transient neutral systems where a complete kinetic analysis of isotope effects is difficult or impractical.

Charged and neutral NO3 isomers from the ionization of NOx and O3 mixtures.

Mass spectrometric techniques have been utilized in conjunction with theoretical methods to detect and characterize new species formed upon ionization of gaseous mixtures containing ozone and an NOx oxide. NO5+ as well as isomeric NO4+ and NO3+ ions have been identified. Moreover, utilization of neutralization reionization mass spectrometry (NRMS) has provided strong evidence for, if not a conclusive demonstration of, the existence of a new NO3 isomer, in addition to the long-known trigonal radical, as a gaseous species with a lifetime in excess of approximately 1 microsecond.

From N2 and O2 to N4 and O4: Pneumatic Chemistry in the 21st Century

AbstractFor Abstract see ChemInform Abstract in Full Text.

Lactone enols are stable in the gas phase but highly unstable in solution.

2-Hydroxyoxol-2-ene (C(5)-1), the enol tautomer of gamma-butyrolactone, was generated in the gas phase as the first representative of the hitherto elusive class of lactone enols and shown by neutralization-reionization mass spectrometry to be remarkably stable as an isolated species. Ab initio calculations by QCISD(T)/6-311+G(3df,2p) provided the enthalpies of formation, proton affinities, and gas-phase basicities for gaseous lactone enols with four- (C(4)-1), five- (C(5)-1), and six-membered rings (C(6)-1). The acid-base properties of C(4)-C(6) lactones and enols and reference carboxylic acid enols CH(2)=C(OH)(2) (3) and CH(2)=C(OH)OCH(3) (4) were also calculated in aqueous solution. The C(4)-C(6) lactone enols show gas-phase proton affinities in the range of 933-944 kJ mol(-)(1) and acidities in the range of 1401-1458 kJ mol(-)(1). In aqueous solution, the lactone enols are 15-20 orders of magnitude more acidic than the corresponding lactones, with enol pK(a) values increasing from 5.6 (C(4)-1) to 14.5 (C(6)-1). Lactone enols are moderately weak bases in water with pK(BH) in the range of 3.9-8.1, whereas the lactones are extremely weak bases of pK(BH) in the range of -10.5 to -17.4. The acid-base properties of lactone enols point to their high reactivity in protic solvents and explain why no lactone enols have been detected thus far in solution studies.

The Generation of Low-Valence Tin Derivatives, RSn(I), in the Gas-Phase by Neutralization—reionization Mass Spectrometry

Neutralization-reionization mass spectrometry (NRMS) was applied to the generation and characterization of low-valence Sn(I) derivatives. The observation of recovery signals in the NR mass spectra of RSn+ ions (R=H, Cl, Br, CH3, C2H, C6H5) demonstrated that their neutral counterparts are stable species in the gas-phase with a lifetime of at least 5 μs. According to quantum chemical calculations, a favorable Franck–Condon factor may contribute to the stability of RSn neutrals generated in the NR event. The experimental results for tin acetylide and phenyltin are the first examples of the generation of these previously unknown molecular species.