Neutralization-reionization Mass Spectrometry Research Articles

ChemInform Abstract: Mass Spectrometry and Gas‐Phase Ion Chemistry of Organomanganese Complexes

AbstractReview: 783 refs.

Ligand effects on single-electron transfer of isolated iron atoms in the gaseous complexes [(OC)mFe(OH2)n]+ (m, n = 0–2, m + n = 1, 2)

The properties of neutral and monocationic complexes of iron with one and two water or carbonyl ligands (i.e. FeCO0/+, Fe(OH2)0/+, (OC)Fe(OH2)0/+, and (H2O)Fe(OH2)0/+) are studied by means of wave-function based ab initio theory. Not surprisingly, the addition of ligands to the metal center drastically changes the energetics of the cationic species, whereas the ligation of neutral iron has much smaller effects and the interaction is even repulsive for some neutral quintet states. As a consequence, the associated adiabatic and vertical transitions between the manifold of the neutral and monocationic species experience substantial changes with differences exceeding 1eV in the ionization energies. The results are used to explain the results of earlier studies of (OC)Fe(OH2)0/+ and (H2O)Fe(OH2)0/+ by means of neutralization–reionization mass spectrometry.

Gas-Phase Ion Chemistry and Organic Chemistry: The Story of a Hybrid Six-Sector Mass Spectrometer—The “AutoSpec 6F”

The AutoSpec 6F mass spectrometer is a large, floor standing instrument comprising a pair of commercial EBE geometry (AutoSpec) mass spectrometers coupled in series to provide an hybrid EBE-EBE configuration, (E and B being respectively electrostatic and magnetic sectors.) It was designed in close collaboration between Professor R. Flammang and VG Analytical in Manchester, UK. It was equipped with five collision cells and allowed the recording of high energy CID (collision induced dissociation), MIKES (mass analyzed ion kinetic energy spectrometry) and NRMS (neutralization re-ionization mass spectrometry) data as well as consecutive MSn analyses. The field-free regions between sectors allowed the study of unimolecular decomposition products from long-lived metastable ions. The mass spectrometer became even more versatile when an RF-only quadrupole collision cell was installed between the second and the third electric sector. This allowed the study of associative ion/molecule reactions in the low kinetic energy regime. Bimolecular chemical reactions were performed inside the quadrupole cell when a neutral reagent was introduced and the reaction products were analyzed by high energy CID in the downstream sectors. This paper tells the history and summarizes the capabilities of this versatile instrument.

A Century of Progress in Molecular Mass Spectrometry

The first mass spectrum of a molecule was measured by J.J. Thomson in 1910. Mass spectrometry (MS) soon became crucial to the study of isotopes and atomic weights and to the development of atomic weapons for World War II. Its notable applications to molecules began with the quantitative analysis of light hydrocarbons during World War II. When I joined the Dow Chemical Company in 1950, MS was not favored by organic chemists. This situation improved only with an increased understanding of gaseous ion chemistry, which was obtained through the use of extensive reference data. Gas chromatography-MS was developed in 1956, and tandem MS was first used a decade later. In neutralization-reionization MS, an unusual, unstable species is prepared by ion-beam neutralization and characterized by reionization. Electrospray ionization of a protein mixture produces its corresponding ionized molecules. In top-down proteomics, ions from an individual component can be mass separated and subjected to collision-activated and electron-capture dissociation to provide extensive sequence information.

A Stable Aminothioketyl Radical in the Gas Phase

We report the first preparation of a stable aminothioketyl radical, CH(3)C(•)(SH)NHCH(3) (1), by fast electron transfer to protonated thioacetamide in the gas phase. The radical was characterized by neutralization-reionization mass spectrometry and ab initio calculations at high levels of theory. The unimolecular dissociations of 1 were elucidated with deuterium-labeled radicals CH(3)C(•)(SD)NHCH(3) (1a), CH(3)C(•)(SH)NDCH(3) (1b), CH(3)C(•)(SH)NHCD(3) (1c), and CD(3)C(•)(SH)NHCH(3) (1d). The main dissociations of 1 were a highly specific loss of the thiol H atom and a specific loss of the N-methyl group, which were competitive on the potential energy surface of the ground electronic state of the radical. RRKM calculations on the CCSD(T)/aug-cc-pVTZ potential energy surface indicated that the cleavage of the S-H bond in 1 dominated at low internal energies, E(int) < 232 kJ mol(-1). The cleavage of the N-CH(3) bond was calculated to prevail at higher internal energies. Loss of the thiol hydrogen atom can be further enhanced by dissociations originating from the B excited state of 1 when accessed by vertical electron transfer. Hydrogen atom addition to the thioamide sulfur atom is calculated to have an extremely low activation energy that may enable the thioamide group to function as a hydrogen atom trap in peptide radicals. The electronic properties and reactivity of the simple aminothioketyl radical reported here may be extrapolated and applied to elucidate the chemistry of thioxopeptide radicals and cation radicals of interest to protein structure studies.

A neutralization-reionization and reactivity mass spectrometry study of the generation of neutral hydroxymethylene

Neutral hydroxymethylene HCOH is an important intermediate in several chemical reactions; however, it is difficult to observe due to its high reactivity. In this work, neutral hydroxymethylene and formaldehyde were generated by charge exchange neutralization of their respective ionic counterparts and then were reionized and detected as positive-ion recovery signals in neutralization-reionization mass spectrometry in a magnetic sector instrument of BEE geometry. The reionized species were characterized by their subsequent collision-induced dissociation mass spectra. The transient hydroxymethylene neutral was observed to isomerize to formaldehyde with an experimental time span exceeding 13.9 µs. The vertical neutralization energy of the HCOH(+•) ion has also been assayed using charge transfer reactions between the fast ions and stationary target gases of differing ionization energy. The measured values match the result of ab initio calculations at the QCISD/6-311 + G(d,p) and CCSD(T)/6-311 + + G(3df,2p) levels of theory. Neutral hydroxymethylene was also produced by proton transfer from CH(2) OH(+) to a strong base such as pyridine, confirmed by appropriate isotopic labeling. There is a kinetic isotope effect (KIE) for H(+) versus D(+) transfer from the C atom of the hydroxymethyl cation of ∼3, consistent with a primary KIE of a nearly thermoneutral reaction.

ChemInform Abstract: Gaseous Diphosphorus and Triphosphorus Sulfide: Generation and Identification by Neutralization-Reionization Mass Spectrometry.

Abstract The dissociative ionization by electron impact (70 eV) of tetraphosphorus trisulfide, P4S3, yields [P3S]+ and [PZS]+0 ions whose structures have been investigated by means of Coliisionai Activation (CA) mass spectra. Using the technique of Neutralization-Reionization mass spectrometry (NRMS) it is shown that both ions can be reduced to the corresponding neutral molecules. Thus, triphosphorus sulfide and diphosphorus sulfide are viable molecules in the rarefied gas phase. The results of ab initio MO calculations were used to interpret the experimental findings. Tetrahedral (C3v) and triangular (C2v) structures are proposed for the [P,S]+ and [PZS]+0 ions respectively.

ChemInform Abstract: Structural Characterization of the Atmospherically Important Sulfur Compounds HSO× and SOH× by Charge Reversal and Neutralization-Reionization Mass Spectrometry.

ChemInform Abstract: Structural Characterization of the Atmospherically Important Sulfur Compounds HSO× and SOH× by Charge Reversal and Neutralization-Reionization Mass Spectrometry.

The covalently bound dimer ion HC[dbnd]N[sbnd]C[dbnd]NH[rad]+ and its neutral counterpart

Model chemistry calculations (CBS-QB3 and CBS-APNO methods) and tandem mass spectrometry based experiments indicate that dissociative ionization of 2-methoxy-s-triazine (consecutive losses of CH2O and HCN) yields the elusive covalently bound [H,C,N] dimer ion HCNCNH+, a species of interest in astrochemistry. Neutralization–Reionization Mass Spectrometry (NRMS) experiments indicate that its neutral counterpart, HCNCNH, is a kinetically stable molecule in the rarefied gas-phase.

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

The covalently bound HC [tbnd]N dimer ions HC [dbnd]N−N [dbnd]CH [rad]+ and HC [dbnd]N−C( [dbnd]N)H [rad]+ are stable species in the gas-phase, but the neutral counterparts are not

Tandem mass spectrometry based experiments and computational chemistry (CBS-QB3/APNO methods) indicate that ionized s-tetrazine eliminates N 2 to yield the elusive linear HC N dimer ion HC N–N CH + ( 1). The ion is stable in the rarefied gas-phase but, as it is generated with considerable internal energy, it readily interconverts with HC N–C( N)H + ( 2), via a 1,2-HCN shift. This more stable HCN dimer ion was generated independently by loss of HCN from ionized s-triazine. Whereas the HN C dimer ion HN C C NH + has a (kinetically) stable neutral counterpart, theory and experiment (neutralization–reionization mass spectrometry) agree that the HC N dimer ions 1 and 2 do not.

The covalently bound HNC dimer ion HN [dbnd]C [dbnd]C [dbnd]NH [rad]+ has a kinetically stable neutral counterpart

Neutralization-reionization mass spectrometry (NRMS) and computational chemistry (CBS-QB3/APNO methods) have been used to show that HN C C NH (ethenediimine) and its isomer H 2N–C–C N (aminocyanocarbene) are generated as kinetically stable molecules in the rarefied gas-phase by one electron reduction of their ionic precursors. One route to the very stable ion HN C C NH + involves loss of HC N from the hydrogen-bridged radical cation [ HN C ⋯ H 2 N – C – C N ] + via a remarkable quid-pro-quo catalysis in which both the ion and the neutral undergo isomerization.

Gas-phase chemistry of alkylcarbonate anions and radicals

Alkylcarbonate anions and radicals ROCOO −/ (R = H, CH 3, C 2H 5, i-C 3H 7, and t-C 4H 9) are investigated in the gas phase by means of mass spectrometry and ab initio calculations. Structural parameters and energies are obtained at the MP2/6-311++G(3df,3pd)//MP2/6-311++G(d,p) level of theory. Standard enthalpies of formation for the anions and radicals are determined via atomization energies and isodesmic reactions using the CBS-Q method. Further, alkylcarbonate anions are probed by metastable ion and collisional activation experiments, and the chemistry of the neutral radicals is investigated by charge-reversal and neutralization-reionization mass spectrometry. Although decarboxylation dominates the unimolecular reactivity of the species for both charge states, some other interesting features are observed, particularly for the anions, such as the formation of the CO 3 − radical anion or the presence of ionic fragments formed via hydrogen atom transfer.

Cytosine neutral molecules and cation–radicals in the gas-phase: Structures, energetics, ion chemistry, and neutralization–reionization mass spectrometry

Gas-phase cytosine molecules and cation–radicals represent a complex system of several nearly isoenergetic tautomers within each group. Computational methods differ in ordering the relative enthalpies of neutral cytosine tautomers. At our highest level of theory, CCSD(T)/aug-cc-pVTZ calculations find an enol form, anti-2-hydroxy-4-aminopyrimidine ( 2), to be the most stable neutral tautomer in the gas-phase, followed by its rotamer, syn-2-hydroxy-4-aminopyrimidine ( 3), the canonical oxo-form, 4-amino-1,2-dihydropyrimidin-2( 1H)-one ( 1), imino-forms, 2-oxo-4-iminodihydro( 1H, 3H)pyrimidine ( 4 and 5), and another oxo-form, 4-amino-dihydropyrimidin-2( 3H)-one ( 6). Other tautomers, such as anti-anti, syn-syn and syn-anti-2-hydroxy-4-iminodihydro( 3H, 4H)pyrimidines ( 7– 9), are less stable. The adiabatic ionization energies of the major cytosine tautomers have been calculated to be 8.71, 8.64, 8.62, 8.58, 8.64, and 8.31 eV for 1, 2, 3, 4, 5, and 6, respectively. Cytosine cation–radicals show very close relative energies that increase in the order of 6 + (most stable) < 2 + ≈ 3 + < 4 + ≈ 7 + ≈ 1 + < 5 +. In addition, distonic ions having radical centers at C-5 ( 10 +) and C-6 ( 11 + are found as low-energy isomers of 1 +– 7 +. Metastable cytosine cation–radicals undergo ring-cleavage dissociations by eliminations of CO (major) and HN C O (minor). The energetics of these and other higher-energy dissociations, including the pertinent transition states, have been established by high-level ab initio and density functional theory calculations and plausible mechanisms have been proposed. Collisional neutralization of cytosine cation–radicals with trimethylamine and dimethyldisulfide as electron donors forms stable molecules that are detected as cation–radicals following collisional reionization. The dissociations observed upon neutralization–reionization mainly include ring-cleavages followed by loss of N C O, HN C O, and formation of C 2H 3N, C 2H 2N, and CO neutral fragments that are assigned to ion dissociations following reionization.

Modeling Deoxyribonucleic Acid and Ribonucleic Acid Damage in the Gas Phase

This short review outlines the tandem mass spectrometric methods for the generation and analysis of transient nucleobase radicals relevant to deoxyribonucleic acid and ribonucleic acid damage. Radical hydrogen atom adducts to uracil, adenine, cytosine and N-methylcytosine were generated by femtosecond electron transfer to the corresponding gas-phase cations in fast beams at 8 keV kinetic energy. Radical unimolecular dissociations were monitored by product analysis following collisional ionization to cations or anions using neutralization-reionization mass spectrometry. The radical energetics and dissociation kinetics were further analyzed by mapping the potential energy surfaces by high-level ab initio calculations in combination with Rice-Remsberger-Kassel-Marcus calculations of unimolecular rate constants. This first- principles-based approach allows one to model radical dissociations occurring from doublet ground electronic states of radical intermediates, assign reaction mechanisms and derive quantitative branching ratios for dissociation channels that are in agreement with experiments. Theoretical analysis also provides distinction between radical dissociations occurring on the ground and excited electronic state potential energy surfaces. Specific characterization of excited state dissociations of nucleobase and other polyatomic radicals remains a challenging topic for both experimentalists and computational chemists.

Simple b Ions Have Cyclic Oxazolone Structures. A Neutralization-Reionization Mass Spectrometric and Computational Study of Oxazolone Radicals

The 2-methyloxazol-5-on-2-yl radical (3) and its deuterium labeled analogs were generated in the gas-phase by femtosecond electron-transfer and studied by neutralization-reionization mass spectrometry and quantum chemical calculations. Radical 3 undergoes fast dissociation by ring opening and elimination of CO and CH(3)CO. Loss of hydrogen is less abundant and involves hydrogen atoms from both the ring and side-chain positions. The experimental results are corroborated by the analysis of the potential energy surface of the ground electronic state in 3 using density functional, perturbational, and coupled-cluster theories up to CCSD(T) and extrapolated to the 6-311 ++ G(3df,2p) basis set. RRKM calculations of radical dissociations gave branching ratios for loss of CO and H that were k(CO)/k(H) > 10 over an 80-300 kJ mol(-1) range of internal energies. The driving force for the dissociations of 3 is provided by large Franck-Condon effects on vertical neutralization and possibly from involvement of excited electronic states. Calculations also provided the adiabatic ionization energy of 3, IE(adiab) = 5.48 eV and vertical recombination energy of cation 3(+), RE(vert) = 4.70 eV. The present results strongly indicate that oxazolone structures can explain fragmentations of b-type peptide ions upon electron capture, contrary to previous speculations.

C S 2 O + and CS2O in the gas phase: An experimental and computational study

The CS2O+ ion and CS2O molecule were prepared and structurally characterized by mass spectrometric techniques as isolated species in the gas phase. The theoretical analysis, performed by B3LYP and CCSD(T) computational methods, predicted different CS2O+ isomers, SSCO+, O(CS2)+, SCSO+, SCOS+ and S(COS)+, and structurally related singlet and triplet CS2O. Experiment and theory agree in identifying the obtained CS2O+ ions as a mixture of SCSO+ and SCOS+ isomers. CS2O neutral species, prepared by neutralization-reionization mass spectrometry, were directly characterized as intact, long-lived species with a lifetime tau > or =2 micros.

Adenine Radicals in the Gas Phase: An Experimental and Computational Study of Hydrogen Atom Adducts to Adenine

The elusive hydrogen atom adduct to the N-1 position in adenine, which is thought to be the initial intermediate of chemical damage, was specifically generated in the gas phase and characterized by neutralization-reionization mass spectrometry. The N-1 adduct, 1,2-dihydroaden-2-yl radical (1), was generated by femtosecond electron transfer to N-1-protonated adenine that was selectively produced by electrospray ionization of adenine in aqueous-methanol solution. Radical 1 is an intrinsically stable species in the gas phase that undergoes specific loss of the N-1-hydrogen atom to form adenine, but does not isomerize to the more stable C-2 adduct, 1,2-dihydroaden-1-yl radical (5). Radicals 1 that are formed in the fifth and higher electronically excited states of DeltaE > or = 2.5 eV can also undergo ring-cleavage dissociations resulting in expulsion of HCN. The relative stabilities, dissociation, and transition state energies for several hydrogen atom adducts to adenine have been established computationally at highly correlated levels of theory. Transition state theory calculations of 298 K rate constants in the gas phase, including quantum tunnel corrections, indicate the branching ratios for H-atom additions to C-8, C-2, N-3, N-1, and N-7 positions in adenine as 0.68, 0.20, 0.08, 0.03, and 0.01, respectively. The relative free energies of adenine radicals in aqueous solution point to the C-8 adduct as the most stable tautomer, which is predicted to be the predominating (>99.9%) product at thermal equilibrium in solution at 298 K.

Unimolecular Fragmentation of CH3NH2: Towards a Mechanistic Description of HCN Formation

AbstractThe mechanism of the consecutive fragmentation of methylamine, CH3NH2, is studied by means of neutralization‐reionization mass spectrometry (NRMS), labeling experiments, and calculations employing density functional theory. It is shown that under the conditions of NRMS the fragmentations proceed by a radical mechanism that involves four distinct X–H bond cleavages (X = C, N). In the first step, a hydrogen atom is eliminated from the methyl group; next, homolytic cleavage of an N–H bond occurs. The so‐formed CH2NH intermediate then, in competition, releases a hydrogen atom either from the nitrogen or the carbon part, resulting in the formation of the radicals H2CN· and HCNH·, respectively. The reaction sequence is terminated by yet another homolytic bond cleavage to produce HCN. An alternative pathway proceeding by an initial 1,1‐elimination of H2 from the methyl group of CH3NH2, followed by a 1,1‐elimination of H2 from the amino function, is also considered, but found to be more energy‐demanding. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2005)

The Generation of Neutral Derivatives of Low-Valence Iron, RFe(I), by Netralization—Reionization Mass Spectrometry

Neutralization-reionization mass spectrometry is applied for the generation of low-valence iron derivatives. Neutralization of Rfe(+) (R=H, F, Cl, Br, I, CN, OH, NH(2), acac, C(6)H(5)) ions resulted in the corresponding neutrals having lifetimes of at least 5 micros. Atom connectivities in RFe ions and neutrals were elucidated by a variety of tandem mass spectrometry techniques. The present study provides the first experimental evidence of the intrinsic stability of neutral IFe, NCFe, acacFe and C(6)H(5)Fe.