Novel tandem quadrupole-acceleration-deceleration mass spectrometer for neutralization-reionization studies

The bicoordinated dihydroxyphosphenium ion P(OH) 2 ϩ (1 ϩ ) was generated specifically by charge-exchange dissociative ionization of triethylphosphite and its connectivity was confirmed by collision induced dissociation and neutralization-reionization mass spectra. The major dissociation of 1 ϩ forming PO ϩ ions at m/z 47 involved another isomer, O¢POOH 2 ϩ (2 ϩ ), for which the optimized geometry showed a long POOH 2 bond. Dissociative 70-eV electron ionization of diethyl phosphite produced mostly 1 ϩ together with a less stable isomer, HP(O)OH ϩ (3 ϩ ). Ion 2 ϩ is possibly co-formed with 1 ϩ upon dissociative 70-eV electron ionization of methylphosphonic acid. Neutralization-reionization of 1 ϩ confirmed that P(OH) 2 ⅐ (1) was a stable species. Dissociations of neutral 1, as identified by variable-time measurements, involved rate-determining isomerization to 2 followed by fast loss of water. A competitive loss of H occurs from long-lived excited states of 1 produced by vertical electron transfer. The A and B states undergo rate-determining internal conversion to vibrationally highly excited ground state that loses an H atom via two competing mechanisms. The first of these is the direct cleavage of one of the OOH bonds in 1. The other is an isomerization to 3 followed by cleavage of the POH bond to form O¢POOH as a stable product. The relative, dissociation, and transition state energies for the ions and neutrals were studied by ab initio and density functional theory calculations up to the QCISD(T)/6 -311ϩG(3df,2p) and CCSD(T)/aug-cc-pVTZ levels of theory. RRKM calculations were performed to investigate unimolecular dissociation kinetics of 1. Excited state geometries and energies were investigated by a combination of configuration interaction singles and time-dependent density functional theory calculations. (J Am Soc Mass Spectrom 2002, 13, 250 -264)

Section: Methodsmentioning

confidence: 99%

Generation and characterization of ionic and neutral P(OH)₂^+/. in the gas phase by tandem mass spectrometry and computational chemistry

Rapole

Srinivas

Bhanuprakash

et al. 2002

“…In case that there are precursor ions at adjacent masses (m p1 and m p2 ) forming a common fragment m f , the latter is transmitted through the energy filter lens if the difference in the product ion kinetic energies (⌬eV) is comparable to or smaller than the filter energy bandpass width ⌬T pass Ϸ 40°eV°(eq°2)° [19].°For°example,°product°ions°at°m/z 57 produced from 7-keV precursor ions of m/z 100 and 101 will have (⌬eV) ϭ 40 eV and thus be transmitted and appear in the NR mass spectrum at m/z 57.…”

Section: Methodsmentioning

confidence: 99%

“…Neutralization-reionization (NR) mass spectra were measured on a tandem quadrupole acceleration-deceleration mass spectrometer described previously [19]. Ions were generated in the gas-phase in standard electron impact (EI) or chemical ionization (CI) ion sources [12].°The°source°conditions°were°as°follows:°emission current, 500 A for EI and 1 mA for CI.…”

Section: Methodsmentioning

confidence: 99%

See 1 more Smart Citation

Simple b ions have cyclic oxazolone structures. A neutralization-reionization mass spectrometric and computational study of oxazolone radicals

Chen

Turecček

2005

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 ϩϩ 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 Ͼ 10 over an 80 -300 kJ mol U pon low-energy collisions, gas-phase peptide ions fragment by peptide bond cleavages that result in the formation of ions containing Nterminal residues (b-series ions) and C-terminal residues (y-ion series) [1]. Based on both ion dissociations and computational studies of ion structure and energetics, it is generally accepted that y ions correspond to truncated protonated peptides [2]. In contrast, there has been some recent disagreement regarding b ions for which as many as four different structures have been proposed, e.g., (1) open chain acylium; (2) protonated oxazolone; (3) protonated diketopiperazine; and (4) immonium ions formed from oxazolone structures [3]. Quantum chemical calculations at various levels of theory (Hartree-Fock, Møller-Plesset, and density functional theory) prefer protonated oxazolone structures for b-ions that are usually the most stable or the only stable isomers [4,5]. The formation of protonated oxazolone structure for b-ions is also consistent with the mechanism of peptide ion dissociations that was elucidated by detailed ab initio calculations on model diand tripeptides [6], as discussed in detail in a recent authoritative review [3]. The fragmentation mechanism (Scheme 1) involves proton migration to the amide nitrogen followed by a nucleophilic attack at the amide carbonyl by the neighboring carbonyl oxygen from the N-terminal site. This forms an intermediate with an N-protonated oxazolone ring that can eliminate a molecule of the neutral C-terminal-truncated peptide to form the b-ion [4]. Alternative structures and mechanisms for b-ion formation have been proposed for peptides having polar groups in side chains of amino acid residues, His, Glu, Asn, Lys, and Arg, if these were at N-terminal sites of the peptide bond to be cleaved [7].Experimental support for oxazolone b-ion structures was provided by CAD spectra of model systems. For example, Yalcin et al. showed that the CAD spectrum of the b ion from protonated C 6 H 5 CO-Gly-Gly-OH matched that of protonated 2-phenyloxazol-5-one [4]. The reported low-energy CAD spectra showed only two fragment ions due to loss of CO and formation of C 6 H 5 CO ϩ [4]. Evidence for oxazolone-like neutral counterparts of y-typ...

“…Neutralization-reionization mass spectra were measured on a tandem quadrupole acceleration-deceleration mass spectrometer equipped with electron impact (EI) and chemical ionization (CI) ion sources [28]. Liquid samples were degassed by several freeze-pumpthaw cycles and introduced into the ion source from a glass liquid introduction system at room temperature and 2-3 ϫ 10 Ϫ6 torr partial pressure.…”

Section: Measurementsmentioning

confidence: 99%

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

Shetty

Sadı́lek

Chen

et al. 2004

The title radical (1) is generated in the gas-phase by collisional neutralization of carbonylprotonated 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 (ϳ56%) 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 ϳ42% 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 2 H 3 O and C 2 H 4 O 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 ϳ590 and