Neutralization-reionization Mass Spectra Research Articles

Ionic and neutral mercaptothiocarbonyl: A tandem mass spectrometry and computational study

Dissociative electron ionization (70 eV) of ethyl carbomylmethanedithioate generates m/ z 77 ions of composition [H, C, S 2] +. The collision-induced dissociation (CID) mass spectrum of these ions is consistent with the mercaptothiocarbonyl connectivity HSCS + ( a +). Calculations predict that some of the a + ions may equilibrate with HC(S)S + ( b) + ions prior to dissociation. Neutralization-reionization mass spectrum (NRMS) of these ions confirmed that neutral HSCS is stable in the rarefied gas phase. The relative, dissociation, and isomerization energies for the ions and neutrals studied at B3LYP/6-311G(d,p) and G2/G2(MP2) levels, are used to support and interpret the experimental results.

Selenoketene (H2CCSe)+• and selenoketyl cumulene (HCCSe)+ ions and their neutral counterparts: a tandem mass spectrometric and computational study

AbstractDissociative electron ionization (70eV) of selenophene (C4H4Se) generates m/z 106 ions of composition [H2, C2, 80Se]+• and m/z 105 ions of [H, C2, 80Se]+. From tandem mass spectrometric experiments, Density Functional Theory (DFT) and ab initio calculations, it is concluded that these ions have the structure of selenoketene H2CCSe+• (1a+• )and selenoketyl HCCSe+ (2a+) ions respectively. The calculations predict that selenoketene ion 1a+• is separated by high energy barriers from its isomers selenirene (H$\catcode`@=11 \def\overlsqmatrix#1{\null\vbox{\normalbaselines\m@th \ialign{\hfil$##$\hfil&&\quad\hfil$##$\hfil\crcr \mathstrut\crcr\noalign{\kern-\baselineskip} #1\crcr\mathstrut\crcr\noalign{\kern-\baselineskip}}}} \newdimen\rulwidth \def\downsqbrace#1{\setbox1=\hbox{#1} \rulwidth=\wd1\advance \rulwidth by -6pt \raise3pt\hbox{$\overlsqmatrix{\vrule height5pt\kern-0.5pt\raise5pt\hbox to \rulwidth{\hrulefill}\kern-0.5pt\vrule height5pt\cr\noalign{\vskip-3pt}\hbox{#1}\cr\noalign{\vskip-5pt}}$}} \catcode`@=12 \downsqbrace{{\rm CCHS}}$e)+• 1b+•, ethyne selenol (HCCSeH)+• 1c+•, (CCHSeH)+• 1d+• and (CCSeH2)+• 1e+•. The selenoketyl ion 2a+ is separated by high barriers from its isomers (CCHSe)+ 2b+, and (CCSeH)+ 2c+. Neutralization‐reionization mass spectra (NRMS) of these structurally characterized ions confirmed that the corresponding neutral analogues, selenoketene H2CCSe 1a and selenoketyl radical HCCSe 2a• are stable in the rarefied gas phase. The relative, dissociation, and isomerization energies for selenoketene and selenoketyl ions and neutrals studied at B3LYP/6–31G(d,p) and G2/G2(MP2) levels are used to support and interpret the experimental results. Copyright © 2005 John Wiley & Sons, Ltd.

Generation and characterization of ionic and neutral selenocumulene HC 3Se +/[rad] by tandem mass spectrometry and computational study

Dissociative electron ionization (70 eV) of diphenyl diselenide [(C 6H 5Se) 2] generates m/ z 117 ions of composition [H, C 3, 80 Se ] +. The collision-induced dissociation (CID) mass spectrum of these ions are consistent with the selenocumulene connectivity HCCCSe +. Neutralization–reionization mass spectrum (NRMS) of these structurally characterized ions confirmed that the corresponding neutral analog, hitherto unknown hetero cumulene, HCCCSe, is stable in the rarefied gas phase. The relative, dissociation, and isomerization energies for the ions and neutrals studied at B3LYP/6-31G(d,p) and B3LYP/6-311+G (2d,p) levels are used to support and interpret the experimental results.

Neutralization–reionization of ions produced by electrospray: Instrument design and initial data

A tandem mass spectrometer is described for neutralization–reionization studies of ions produced by electrospray ionization. The instrument consists of a high current electrospray source, an electrodynamic ion funnel, and an octopole ion guide that transport the ions into the high vacuum region of the tandem mass spectrometer. The ions are accelerated to 8250 eV and submitted to collisional neutralization and reionization, followed by deceleration, separation by kinetic energy and/or mass, and detection. The performance of the ionizer and ion transfer elements is optimized as a function of background pressure and the applied rf and dc potentials. Examples are given for neutralization–reionization mass spectra of protonated adenine, heterocycles, and β-alanine- N-methyl amide.

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

The bicoordinated dihydroxyphosphenium ion P(OH) 2 + ( 1 +) 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 + forming PO + ions at m/ z 47 involved another isomer, OPOH 2 + ( 2 +), for which the optimized geometry showed a long POH 2 bond. Dissociative 70-eV electron ionization of diethyl phosphite produced mostly 1 + together with a less stable isomer, HP(O)OH + ( 3 +). Ion 2 + is possibly co-formed with 1 + upon dissociative 70-eV electron ionization of methylphosphonic acid. Neutralization-reionization of 1 + confirmed that P(OH) 2 · ( 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 OH bonds in 1. The other is an isomerization to 3 followed by cleavage of the PH bond to form OPOH 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)/6–311+G(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.

Tautomerization and Dissociation of Dimethyl Phosphonate Ions(CH3O)2P(H)=O•+:Theory and Experiment in Concert

The unimolecular gas phase chemistry of the title ion,(CHThe identity of the ions was probed by tandem mass spectrometry methods. These include MI (metastable ion) or CID (collision induced dissociation) spectra, consecutive MI/CID and CID/CID spectra, NRMS (neutralization-reionization mass spectra), NR/CID and CIDI (collision induced dissociative ionization) spectra, time-resolved CID spectra and deuterium labelling. The energetics of the CHTheory and experiment yield a consistent potential energy profile that describes the isomerization and low energy dissociation chemistry of ions

A Tandem Mass Spectrometry Study of [C,H2,X2,Si]+• Ions (X = H, D, Cl) and the Generation of Their Neutral Counterparts

Two [Si,C,H4]+• isomers, having similar stabilities and a relatively low barrier for their interconversion, have been separately generated in mass spectrometric experiments. CH2SiD2+• and D2-CH3SiH+• ions were produced from the same ionic precursor, namely ionized 1,1-dideutero-silacyclobutane (D2-I). Dissociation of the latter in the ion source resulted in the silaethene structure, whereas metastable dissociation gave rise to the dideuterated methylsilylene. Appearance energy measurements confirmed the formation of the two [Si,C,H2,D2]+• isomers, whose heats of formation were estimated to be 1017 and 1038 kJ mol−1 for CH3SiH+• and CH2SiH2+•, respectively. Both ion-source and metastable dissociations of ionized D2-I resulted in [Si,C,H3,D]+• and [Si,C,H4]+• ions possessing the methylsilylene structure. The collision-induced dissociation (CID) and neutralization–reionization (NR) mass spectra of CH2SiD2+• and D2-CH3SiH+• ions were distinctively different. The characteristic dissociation channel for the CH2SiD2 structure was the loss of CH2. Some dissociations of the ionized silaethene involved the formation of the methylsilylene intermediate. However, CH3SiH+• ions produced by this isomerization appeared to be unstable and did not survive neutralization–reionization. As a result, the CID mass spectra of CH2SiD2+• ions prior to neutralization and surviving the NR event were almost identical. Strong recovery signals in the NR mass spectra were consistent with the intrinsic stability of neutral silaethene and methylsilylene. The interconversion of these structures generated by neutralization of their positively-charged counterparts seems to be very unlikely. The CID, NR and NR/CID mass spectra of [Si,C,H2,Cl2]+• ions produced from various precursors and in different time frames were indistinguishable. They corresponded to the CH2SiCl2 connectivity of the atoms. The heat of formation for CH2SiCl2+• ions was estimated to be ∼732 kJ mol−1, based on appearance energy measurements. The presence of a strong recovery signal in the NR mass spectra was consistent with the formation of stable neutral 1,1-dichlorosilaethene in the gas phase on the microsecond time frame.

Glycine radicals in the gas phase †‡

2-Glycyl radical (1), 2-glycyl methyl ester (2), and 2-glycyl N-methylamide radicals (3) were generated in the gas phase by collisional neutralization of the corresponding cations and studied by neutralization–reionization (NR) mass spectrometry and ab initio and density functional theory calculations. Stable fractions were found for 1–3 that did not dissociate on the 4 µs timescale of the experiment. The major unimolecular dissociations observed were cleavages of the CO–X bonds (X = OH, OCH3, and NHCH3) to form aminoketene, and cleavages of the C–CO bond to form aminocarbene, which gave rise to prominent fragments in the NR mass spectra. In addition, hydrogen rearrangements occurred in 1–3 and an unusual methyl migration took place in 3. G2(MP2) bond dissociation energies are reported for 1 and 1+. Combined B3LYP and PMP2/6-311+G(2df,p) calculations were used to provide bond dissociation energies for 2, 2+, 3 and 3+.

Letter: False recovery signals in neutralization–reionization mass spectra

Letter: False recovery signals in neutralization–reionization mass spectra

A tandem mass spectrometry study of [C 6,H 5,O] + ions

Tandem mass spectrometric methods and appearance energy measurements have been used to identify and characterize four isomeric [C 6,H 5,O] + ions. The phenoxy cation, C 6H 5O +, has a ΔH f o = 207 kcal mol −1, while the meta- and para-HOC 6H + 4 ions have ΔH f o values at ca. 220 kcal mol −1; the ortho isomer is higher in energy by 5–10 kcal mol −1. Much lower in energy is the conjugated cyclopentadienylcarbonyl ion, estimated ΔH f o ≈ 161 kcal mol −1. The meta- and para-HOC 6H + 4 ions could not be distinguished by mass spectrometric methods, but the ortho isomer differs in its metastable ion characteristics, having a significantly different kinetic energy release for CO loss. Collisional activation mass spectrometry allowed distinction between the ions: phenoxy, hydroxyphenyl and C 5H 5CO +. Greater differences between the isomers were observed in their neutralization-reionization mass spectra.

Protonation of isomeric methoxyhexenes. Effects of double bond⋯H +⋯OCH 3 interaction

Protonated methoxyhexenes exist as hydrogen-bonded structures in the gas phase. The hydrogen bonding facilitates exchange between the proton of the ether group and the hydrogen atoms of the hexenyl chain that precedes dissociation by loss of methanol. The position of the double bond has an effect on the formation of cyclic intermediates and the mechanisms for hydrogen transfer in metastable ions and following collisional activation. Collisional neutralization of protonated methoxyhexenes results in complete dissociation of the intermediate radicals by cleavage of the CH 2-O and O-H bonds, but not the O-CH 3 bond. A mechanism for intramolecular hydrogen atom trapping by the double bond in the radicals is suggested. Collisionally activated dissociation and neutralization-reionization mass spectra of isomeric dimethylsilyloxyhexene cations provide little structural information as to the position of the double bond.

Distonic ion .CH2CH2SCH2+ and the isomeric trimethylene and propylene sulfide radical cations. Assessment of structures and reactivities via decomposition and redox reactions.

The distonic radical ion .CH2CH2SCH2+ (1+.), generated by dissociative electron ionization of 1,4-dithiane or 1,4-thioxane, is identified in the gas phase by its collisionally activated dissociation (CAD), neutralization -reionization (+NR+) and charge-reversal (+CR-) mass spectra. The unimolecular chemistry of 1+. is shown to be substantially different from that of the isomeric, ring-closed, trimethylene sulfide ion (2+.). Hence, a substantial isomerization barrier must separate 1+. from the thermodynamically more stable 2+.. Charge permutation (i.e. charge-stripping, +NR+ and +CR-) are far superior, compared to collision-induced fragmentation, for distinguishing 1+. from 2+., mainly because the oxidized (1++ and 2++) and reduced forms (1 and 2 as well as 1-. and 2-.) of these cations have much lower tendencies for isomerization that 1+. and 2+. themselves. The diradical .CH2CH2SCH2. (1), formed by neutralization of 1+., is found to exist as a bound species, requiring appreciable activation energies for both decomposition to CH2CH2 plus SCH2 and ring-closure to 2. The dissociations and redox reactions of the propylene sulfide ion (3+.) are also assessed in this study and clearly indicate that 3+. is a stable C3H6S+. isomer. Further, the C3H6S+. ions from thiane, 1,3-dithiane and 2-methyl-1,3-dithiane are characterized based on their combined CAD, +NR+ and +CR- spectra. The two 1,3-dithianes produce ionized trimethylene sulfide, 2+.. In contrast, thiane gives rise to a C3H6S+. isomer other than 1+.(-3t); the data strongly suggest that this isomer is the 1-propene-1-thiol radical cation, namely CH3CH = CH-SH+..

The structure of C 5H 5RFe + (R = F, Cl, Br, I, O, OH, OCH 3, C 6H 5, H) ions in the gas phase and the generation of their neutral counterparts by neutralization-reionization mass spectrometry

The structure of C5H5FeR(+·) ions was studied by tandem mass spectromerry that included the neutralization-reionization (NR) method. Halogen-containing species (R = F, Cl, Br, I) showed fragmentation that was consistent with a structure that has the cyclopentadienyl ring and R as separate ligands at the metal atom (structure A). This structure also was identified for C5H5FeO(+) and CpFeOH(+·) ions, but these species also easily isomerized to metal-cyclopentadiene structure, RC5H5Fe(+·) (B), followed by hydrogen rearrangement(s) and CO loss. B was the dominant structure of C5H5FeH(+·) and C5H5FeC6H 5 (+·) ions. All ions that have structure A showed recovery signals in their NR mass spectra that indicated that their stable neutral counterparts were generated. The NR mass spectra also provided complementary information about the structure of ions before neutralization and after reionization.

Reactions of ionized dibutyl ether

The reactions of ionized di-n-butyl ether are reported and compared with those of ionized n-butyl sec-butyl and di-sec-butyl ether. The main fragmentation of metastable (CH3CH2CH2CH2)2O+. is C2H5⋅ loss (˜85%), but minor amounts (2–4%) of CH3⋅, C4H7⋅, C4H9⋅, C4H10 and C4H10O are also eliminated. In contrast, C2H5⋅ elimination is of much lower abundance (20 and 4%, respectively) from metastable CH3CH2CH2CH2OCH(CH3)CH2CH3+. and [CH3CH2(CH3)CH]2O+., which expel mainly C2H6 and CH3⋅ (35–55%). Studies on collisional activation spectra of the C6H13O+ oxonium ions reveal that C2H5⋅ loss from (CH3CH2CH2CH2)2O+. gives the same product, (CH3CH2CH2CH2 +OCHCH3) as that formed by direct cleavage of CH3CH2CH2CH2OCH(CH3)CH2CH3+.. Elimination of C2H5⋅ from (CH3CH2CH2CH2)2O+. is interpreted by means of a mechanism in which a 1,4-H shift to the oxygen atom initiates a unidirectional skeletal rearrangement to CH3CH2CH2CH2OCH(CH3)CH2CH3+., which then undergoes cleavage to CH3CH2CH2CH2+OCHCH3 and C2H5⋅. Further support for this mechanism is obtained from considering the collisional activation and neutralization-reionization mass spectra of the (C4H9)2O+. species and the behaviour of labelled analogues of (CH3CH2CH2CH2)2O+.. The rate of ethyl radical loss is suppressed relative to those of alternative dissociations by deuteriation at the γ-position of either or both butyl substituents. Moreover, C2H5⋅ loss via skeletal rearrangement and fragmentation of the unlabelled butyl group in CH3CH2CH2CH2OCH2CH2CD2CH3+. occurs approximately five times more rapidly than C2H4D⋅ expulsion via isomerization and fission of the labelled butyl substituent. These findings indicate that the initial 1,4-hydrogen shift is influenced by a significant isotope effect, as would be expected if this step is rate limiting in ethyl radical loss.

A tandem mass spectrometry study of the gas phase RSC 6H 5Cr +L (R = H, C 6H 5; L = arene) and C 6H 5SSC 6H 5Cr + ions

Ion-molecule reactions of chromium containing ions with arylsulfides have been studied in the gas phase and their products have been characterized by tandem mass spectrometry. C 6H 5SH and (C 6H 5) 2S react as typical aromatic compounds and give rise to Cr +C 6H 5SR′ and RC 6H 5Cr +C 6H 5SR′ [R = H, CH 3, CH(CH 3) 2; R′ = H, C 6H 5] ions. Metastable ion mass spectra of the latter species show that the metal is more strongly bound to diphenylsulfide than to alkylbenzenes. C 6H 5SSC 6H 5 reacts with chromium-containing ions to form only Cr +(C 6H 5SSC 6H 5). The decomposition characteristics of this ion and, in particular, the presence of a recovery signal in the neutralization-reionization mass spectrum are in keeping with the formation of a 1,2-dithia[2]cyclophane complex ion, which rearranges into a structure(s) that contains CrS bond(s). No evidence was found for metal atom insertion

A neutralization—reionization (NR) case study for cationic iron complexes with simple ligands: NR mass spectra of Fe(C 2H 4) + and Fe(CO) +

The cationic iron(I) complexes Fe(C 2H 4) + and Fe(CO) + were examined by neutralization—reionization mass spectrometry (NRMS). Whereas the NR mass spectrum of the carbonyl complex exhibits a signal corresponding to the reionized neutral molecule Fe(CO), for the neutral Fe(C 2H 4) complex no recovery signal is observed; rather, in the course of the experiment the complex dissociates to Fe and C 2H 4. An explanation for this seemingly contradictory behaviour of structurally related metal complexes in a NR process is provided by ab initio MO calculations, in which the geometries of Fe(C 2H 4) + ( 4B 2), Fe(C 2H 4) ( 5B 2), Fe(CO) + ( 4Σ −, Fe(CO) ( 3Σ −, and Fe(CO) ( 5Σ −) have been fully optimized at the QCISD(T) level of theory. From the theoretical results, a neutralization-reionization scheme for organometallic ions MX + emerges which considers the effects caused by curve-crossing from a bound state to a repulsive ground-state asymptote of the neutral building blocks M and X. Thus, even for bound organometallic complexes MX, recovery signals in the NR mass spectrum can only be detected if the internal energy deposited in MX in the vertical electron-transfer reaction MX + → MX is too small to permit this curve-crossing. If dissociation occurs on the time scale of the NR experiments, the spectrum exhibits features of both the metal M and the ligand X, thus revealing the structural properties of the (organic) ligand X bound to the metal ion.

Efficiency of collision‐induced ionization of fast neutrals: Effects of neutral internal energy and neutral translational energy

AbstractThe ionization efficiencies of fast beams of CO and C2H4, with oxygen as target gas were shown to increase with increasing translational energy and to decrease with increase in internal energy. The latter may arise from a greater probability of homolysis in the neutral projectile species; it is not likely to arise from increased ionization of the target. Experimental parameters (and their validity) relating to the measurement of collision‐induced dissociative ionization mass spectra and which are also applicable to the reionization step in neutralization—reionization mass spectra were investigated.

The generation of OC 2O ·+ and OC 2O and a study of ionized OC 3O and C 2O by tandem mass spectrometry

Three of the less common oxides of carbon, OC 2O, OC 3O and C 2O, have been studied by tandem mass spectrometric methods. Neutralization—reionization (NR) mass spectrometry provided evidence for the possible generation of (hitherto unobserved) stable OC 2O molecules having a lifetime of at least 1 μs. Ionized OC 3O and C 2O both produced stable neutral species in their NR mass spectra in keeping with their known chemistry. The heats of formation of OC 2O ·+ and C 2O ·+ were measured to be 940 ± 10 and 1412 ± 5 kJ mol −1, the latter in good agreement with earlier work.

Iminopropadienones, RN:C:C:C:O. Theory and experiment

Ab initio molecular orbital calculations at the MP2/6-31G* level have been used to examine the structures and infrared spectra of a new class of compounds, the iminopropadienones, RN=C=C=C=O (R = H, CH3, and Ph). The agreement between calculated and experimental IR spectra of PhNCCCO, Ph15NCCCO, and PhNCC13CO is excellent. Inclusion of electron correlation is found to be important for the correct prediction of the relative intensities of the cumulenic stretching vibrations. All three iminopropadienones are predicted to have a slightly bent NCCCO backbone (∠CCC ≈ 176°). As with carbon suboxide, these cumulenes are calculated to have an extremely flat CCC bending potential. The parent compound, HNCCCO (4a) is calculated to be thermodynamically stable toward dissociations into (i) HNCC + CO, (ii) HNC + CCO, (iii) HN + CCCO, and (iv) H• + NCCCO•. Rearrangement of 4a to the more stable cyanoketene isomer requires a sizeable barrier of 402 kJ mol-1 [G2(MP2)]. The calculated stability of 4a is consistent with its experimental observation in neutralization-reionization mass spectrum. The adiabatic ionization energy and heat of formation of HNCCCO are predicted to be 9.51 eV and 175 kJ mol-1, respectively.

Collisional cooling effects on the charge reversal and neutralization—reionization mass spectra of the negative cluster ion of sulfur dioxide (SO 2) .−2

The charge reversal and neutralization—reionization mass spectra of the sulfur dioxide cluster anion (SO 2) 2 .− 2 are strongly affected by the ion-source pressure conditions and two possible origins for the collisional cooling effects are discussed.