Neutralization-reionization mass spectra

Figure 7 Neutralization-reionization mass spectra of (A) HOS" and (B) HSO". Modified ZAB four-sector mass spectrometer of BEBE configuration (B = magnet sector E = electric sector). Oxygen used as collision gas used in each collision cell (see Figure 5). The pressure of the gas in each cell was adjusted so the reduction in the main beam is 20% for each collision event. Reproduced with permission of ACS from Goldberg N and Schwarz H (1994) Accounts of Chemical Research 27 347-352.

FIGURE 4. Neutralization/reionization mass spectrum of [M — C2H4]+ ion of 1-nitropropane15... 

Figure 1 Neutralization-reionization mass spectrum of CpFe Br and CID mass spectrum of survivor ions (reproduced by permission of Elsevier from J. Am. Soc. Mass Spectrom., 1995, 6, 1143-1153 1995,...

Figure 8 Neutralization-reionization mass spectrum of N=C-C=C-0 Details as in the legend to Figure 7. Reproduced with permission of Elsevier from Muedas CA, Sulzle D and Schwarz H (1992) International Journal of Mass Spectrometry and Ion Processes Z R17-R22.

Evidence for the formation of 34 (R = Ph) was provided by neutralization reionization mass spectrometry and more directly by the matrix isolation and spectroscopic investigations on 34 (R = Ph) in an argon matrix at 12 K. The UV spectrum of 34 (R = Ph) exhibits characteristic bands at X = 364, 386, 404, 420, 440, 470 and 502 nm, resembling those of the electronic spectrum of anthracene, but with the expected bathochromic shifts. If one irradiates into the maximum at X = 502 nm, all bands shown in the spectrum disappear completely within 5 minutes. The vanishing of these characteristic bands can again be explained by the photoisomerization of silaanthracene 34 (R = Ph) to the corresponding Dewar valence isomer. 

In neutralization-reionization mass spectrometric experiments on CH2Si+ formed by electron-impact dissociative ionization of ClCH2SiH3, Srinivas, Stilzle and Schwarz found evidence for the formation of a viable neutral molecule whose fragmentation pattern and collisional activation mass spectrum were in accord with a H2C=Si structure422. These authors suggested that their experiments supported electron-capture by CH2Si+" as a mechanism for the formation of H2C=Si in interstellar space. Various models have predicted that H2C=Si is one of the most abundant forms of silicon in dense interstellar clouds423. 

Figure 10 (A) Neutralization-reionization ( NR ) spectrum of /7-butoxide anions using 02for neutralization and reionization. (B) Charge reversal ("CR ) spectrum of / -butoxide anions using O2. (C) -NIDD spectrum of / -butoxide anions. Reprinted with permission of Wiley-VCH from Hornung G, Schalley CA, Dieterle M, Schroder D and Schwarz H (1997) A study of the gas-phase reactivity of neutral alkoxy radicals by mass spectrometry a-cleav-ages and Barton-type hydrogen migrations. Chemistry a European Journal Z 1866-1883.

Figure Bl.7.7. Summary of the other collision based experiments possible with magnetic sector instruments (a) collision-mduced dissociation ionization (CIDI) records the CID mass spectrum of the neutral fragments accompanying imimolecular dissociation (b) charge stripping (CS) of the incident ion beam can be observed (c) charge reversal (CR) requires the ESA polarity to be opposite that of the magnet (d) neutiiralization-reionization (NR) probes the stability of transient neutrals fonned when ions are neutralized by collisions in the first collision cell. Neutrals surviving to be collisionally reionized in the second cell are recorded as recovery ions in the NR mass spectrum.

The basic NR mass spectrum contains information on the fraction of undissociated (survivor) ions and also allows one to identify dissociation products that are formed by purely unimolecular reactions. NRMS thus provides information on the intrinsic properties of isolated transient molecules that are not affected by interactions with solvent, matrix, surfaces, trace impurities, radical quenchers, etc. However, because collisional ionization is accompanied by ion excitation and dissociation, the products of neutral and post-reionization dissociations overlap in the NR mass spectra. Several methods have been developed to distinguish neutral and ion dissociations and to characterize further short lived neutral intermediates in the fast beam. Moreover, collisionally activated dissociation (CAD) spectra have been used to characterize the ions produced by collisional reionization of transient neutral intermediates [51]. This NR-CAD analysis adds another dimension to the characterization of neutral intermediates, because it allows one to uncover isomerizations that do not result in a change of mass and thus are not apparent from NR mass spectra alone. 

Diaminocarbene, H2N-C-NH2 (26), was prepared by collisional reduction of the corresponding cation-radical that was in turn generated by dissociative ionization of aminoguanidine [102]. Carbene 26 gives an abundant survivor ion in the +NR+ mass spectrum and is clearly distinguished from its more stable isomer formamidine. Amino(hydroxy)carbene, H2N-C-OH, has also been prepared by NRMS [103]. Hydroxy-thiohydroxy-carbene cation-radical, HO-C-SH+ (27+ ), is formed somewhat unexpectedly by ethylene elimination from ionized S-ethylthioformate and O-ethylthioformate instead of the expected thioformic acid. Carbene ion 27+ was characterized by a +NR+ mass spectrum that showed a dominant survivor ion of reionized carbene [104]. The energetics of neutral and ionic HO-C-SH have been addressed by ab initio calculations [105]. Di-(thiohydroxy)carbene, HS-C-SH, is also known [106]. 

Radical ions, including that from 2-methylthiirane, were studied by mass spectrometry <1996RCM235>. Ion CsHsS (+90) loses more CHj to afford an ion with miz 59 and it also affords more CH3 + than isomers 91 and 92. The major dissociation product of 90 was C3HS+. The neutralization/reionization spectrum of 90 showed a large recovery peak, as expected, indicating that the intermediate, propylene sulfide, is a stable molecule. The neutralization/ reionization spectrum of the radical anion of propylene cpisulfide did not give a CsH S- recovery peak, indicating that the anion of propylene episulfide is unstable. 

In the mass spectrum of oxazole, one of the four major peaks, w/z 42, corresponds to loss of HCN. Using neutralization-reionization MS, it was concluded that this ionic species was an oxirene radical cation (27) <89JA44i>. Oxazoles in general show sequential loss of CO and a nitrile. Frag-... 

The stability and unimolecular reactivity of the neutral generated in the neutralization step are characterized by the mass spectrum arising after reionization. The presence of a recovery peak in this spectrum provides evidence that the neutral intermediate has survived intact i.e. undissociated) for microsecond(s). Whether it has retained the connectivity of the precursor ion is judged by comparison of the NR spectrum to the CAD spectrum of the precursor ion or the NR spectra of other, usually stable and known isomers. Both these strategies are presented below with representative examples. 

In a special extension of this technique, the mass-analyzed primary ions are neutralized to form a beam of dissociating neutrals, whose products are reionized to form an NR mass spectrum (McLafferty et al. 1980a Wesdemiotis and McLafferty 1987 Holmes 1989 McLafferty 1990). Such dissociation of the... 

Neutralisation-reionization of the fragment (NfR) at m/z 143 from the compound (1). The inset shows the CID spectrum. The NR spectrum is the same as the CID one, except for m/z 66. This shows that this m/z 66 ion is produced during the neutralization process and reionized. Both spectra confirm the (2) structure for the m/z 143 fragment. It is possible that structure (3) is an intermediate in this process. Reproduced (modified) from Flammang R., Laurent S., Flammang-Barbieux M. and Wentrup C., Rapid Comm. Mass Spectrom., 6, 667, 1992, with permission. 

The +NR+ spectrum of 8 showed a small survivor ion, but differed substantially from the spectra of other C2H5NO isomers, e.g., 6, 7, AT-methylamino(hy-droxy)carbene (9), and N-methylformamide (10). The low intensity of survivor ions in the NR mass spectra of enol imines is due to Franck-Condon effects in collisional reionization that result in vibrational excitation of the resulting cation radical followed by dissociation. Franck-Condon effects were studied for collisional ionization of acetimidic acid, CH3C(OH)=NH, which was one of the neutral dissociation products of 1 -hydroxy- 1-methylamino-l-ethyl radical, a hydrogen atom adduct to AT-methylacetamide [37]. The cation-radical dissociates extensively upon reionization, and the dissociation is driven by a 74 kj mol-1 Franck-Condon energy acquired by vertical ionization. 

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