Survivor ion

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. 

The enol of glycine (4) has been generated by neutralization with trimethy-lamine of the corresponding cation-radical (4+ ) prepared by dissociative ionization of isoleucine (Scheme 3) [71]. The +NR+ mass spectrum of 4+ showed a substantial survivor ion attesting to the stability of isolated 4. In contrast, the survivor ion from glycine (5) is much less stable and appears as a very minor peak in the +NR+ mass spectrum of 5, in spite of the fact that neutral 5 is thermodynamically more stable than 4. 

The enol of acetamide (6) is also quite stable as an isolated species, as documented by the dominant survivor ion in the +NR+ mass spectrum (Fig. 6). Compared to 6, the thermodynamically more stable neutral acetamide (7) shows less... 

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. 

Ion 16+ was distinguished from an isomeric H0-S(H)=0+ structure by CAD of a D-labeled derivative that showed loss of both OH and OD, compatible with the structure having two OH groups. +NR+ of 16 showed a substantial survivor ion that attested to the stability of the isolated molecule, as contrasted to the elusiveness of sulfurous acid in the condensed phase [77]. 

Dihydroxysulfane, 17, is another elusive acid that has been generated by NRMS [78].The precursor cation-radical, 17+, was obtained by dissociative ionization of dimethylsulfate according to Scheme 6. Upon NR, acid 17 gave an abundant survivor ion showing that the intermediate sulfane was a stable molecule. Collisional activation of neutral 17 caused only minor dissociation by elimination of water, further attesting to the considerable stability of the isolated molecule. 

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]. 

An interesting class of elusive neutral species is heterocyclic ylids, as represented by the so-called Hammick intermediate that was postulated 65 years ago to explain the accelerated decarboxylation of 2-picolinic acid [146, 147]. An analogous dissociation takes place in ionized 2-picolinic acid in the gas-phase and was employed to generate the pyridine ion isomer 35+ (Scheme 13) [148]. Collisional neutralization of 35+ with A/,AT-dimethylaniline produced neutral ylid 35, which can also be represented as a singlet a-carbene (Scheme 13). Ylid 35 showed a survivor ion in the +NR+ mass spectrum, which was further char-... 

In particular, 36+ showed a weak survivor ion and dissociation products by multiple losses of H atoms. The low relative abundance of the survivor ion was explained by density-functional theory calculations that showed that neutral 36 was intrinsically unstable and underwent ring closure to form another high-energy intermediate, norcaradiene imine (37), which is 260 kj mol-1 less stable than benzonitrile (Scheme 14) [154]. 

In addition to short-lived molecules that were assigned to the structure classes discussed above, there are various interesting intermediates that are mentioned here separately. Nitrosomethane (38), which is the less stable tautomer of formaldoxime, was generated by collisional reduction of the stable cation-radical and characterized by NRMS [155,156]. The precursor cation for 38 was produced by three different reactions, e.g., elimination of OH upon exothermic protonation of nitromethane [156], electron-induced loss of O from nitromethane [155, 156], and electron-induced CH20 extrusion from ethyl nitrite [156] (Scheme 15). Nitrosomethane gives rise to a moderately abundant survivor ion in the +NR+ mass spectrum and does not undergo unimolecular isomerization to any of its more stable tautomers. 

The lifetimes of hypervalent radicals have been found to depend rather dramatically on isotope substitution. For example, dimethyloxonium, (CH3)2OH, dissociates completely on a 1 -ps time scale when formed by collisional reduction of the stable cation (CH3)2OH+. By contrast, (CH3)2OD furnishes an abundant survivor ion in the +NR+ mass spectrum that is evidence that the deuterated hypervalent radical is metastable [178,179]. From the time scale of the NR measurements and the survivor ion relative intensities one can estimate that (CH3)2OH dissociates >5 times faster than (CH3)2OD. Similar isotope effects have been observed for CH3OH [180], C2H5OH [181], and hypervalent ammonium radicals, e.g., CH3NH [182], (CH3)2NH [60], (CH3)3NH [183], and [pyrrolidinium] [184], which are metastable only as deuterated species. 

For example, the lowest-energy dissociation of 61 to form H2N-CH-OH and CO requires 129 kj mol-1 threshold energy, but must overcome a 310 kj mol-1 barrier for the rate-determining isomerization to H2N-CH(OH)-CO [260]. a-Radicals derived from methyl glycinate (62) and glycine AT-methylamide (63) are somewhat less stable, but still show fractions of undissociated survivor ions in the +NR+ mass spectra. This increased reactivity is supported by theoretical calculations that show additional dissociation channels, e.g., elimination of CH3OH from 62 and elimination of CO from 63 [260]. 

The interpretation of NR mass spectra is not always a simple task. Survivor ions, if present, can be easily recognized by their position on the mass or energy scale. They also appear as very sharp peaks unlike all other ions in NR mass spectra. Ions other than survivor ions may originate from different sources. 

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,...

As a rule, complexes with even-electron ligands, such as solvates, alkene, benzene complexes, do not produce survivor ions. This may be reversed, however, by changing the experimental conditions (target gas, experimental time frame, etc.). Sometimes, adding a ligand increases the stability of complexes. For example, neutral LFeCO showed an unexpectedly higher stability compared with FeL. ... 

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