Hidden rearrangement processes in short-lived negative molecular ions

The study of resonant electron capture by nitrobenzene molecules showed that some fragmentary negative ions are unstable toward electron autodetachment. The measured appearance energy of the neutral component of an [M — H]⁻ ion beam does not agree with the energetics of direct dissociation in a molecular ion. The estimation calculations show that the low appearance energy of [M — H]⁰ neutral components is caused by isomerization of a molecular ion of nitrobenzene to the 2-nitrobenzene structure followed by the formation of a phenoxide ion in the autodetachment state. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 367–370, February, 2006.     

Skeletal rearrangement for water loss from molecular protonated ions of t-butoxycyclohexane

Isotopic labelling and chemical substitution support the proposition that the skeletal rearrangement for water loss from molecular protonated ions of t-butoxycyclohexane involves competition between three reaction pathways. The principal reaction pathway (83%) involves migration of the t-butyl group to the 2-(6-) position of the cyclohexyl ring with reciprocal hydrogen transfer. A second reaction pathway (12%) involves ring contraction followed by reciprocal exchange of the t-butyl group with the 2-(5-) hydrogen atom of the nascent cyclopentyl ring. The third reaction pathway (5%) involves rearrangement of a proton-bound complex to permit ipso attack by isobutene. Stereospecific substitutions indicate that the principal reaction pathway is susceptible to 1,3-diaxial interactions.     

Cyclopropane intermediates in the rearrangement and fragmentation of olefinic molecular ions

Methyl loss from deuterium-labelled molecular ions of 4-methyl-2-pentene, 2-methyl-2-pentene and 1,1,2-trimethylcyclopropane has been investigated for metastable molecular ions and for molecular ions formed by charge exchange with COS⁺˙, XE⁺˙ and CO⁺˙. For metastable ion fragmentation reactions all three compounds exhibit very similar behavior and show specific and essentially equal loss of each of the original methyl groups as well as specific loss of a methyl where the hydrogens derive exclusively from the non-methyl hydrogens of the original molecules. The former results are interpreted in terms of interconversion of the three molecular ions through a ring-opened form of the trimethylcyclopropane molecular ion. The loss of the non-methyl hydrogens as CH₃ is interpreted in terms of isomerization to the 2,3-dimethyl-2-butene structure. With increasing internal energy direct allylic cleavage of the unrearranged methylpentene molecular ions increases in importance while the trimethylcyclopropane molecular ion shows an increased preference for loss of the C(2) methyl group. With increasing internal energy loss of the original non-methyl hydrogens as CH₃ decreases markedly in importance.     

Hydrogen rearrangement in molecular ions of alkyl benzenes: Mechanism and time dependence of hydrogen migrations in molecular ions of 1, 3-diphenylpropane and deuterated analogues

Hydrogen migrations in the molecular ions of 1,3-diphenylpropane, preceding the fragmentations to [C₇H₇]⁺ and [C₇H₈]⁺ ions, have been investigated by use of deuterated derivatives. By comparing the distribution of deuterium labels in the [C₇(H, D)₈]⁺ products from metastable molecular ions with the distribution patterns calculated for various exchange models, it is shown that the H migrations occur by two processes linked by a common intermediate: (i) exchange between hydrogen isotopes at the γ-methylene group and at the ortho positions of the phenyl group: (ii) exchange between hydrogen isotopes at the ortho and orthó positions in the intermediate. In these mechanisms the eight hydrogen isotopes at both benzylic positions and both the ortho and orthó positions of 1,3-diphenylpropane participate in a mutual exchange. A statistical equipartition of the hydrogen isotopes at these eight positions is not reached in metastable molecular ions, however. The distribution pattern of [C₇(H, D)₈]⁺ ions from the deuterium labelled compounds as a function of the mean number n of exchange cycles has been calculated according to this reaction model and compared with experimental results for unstable molecular ions, generated by 70 eV and 12 eV electrons, respectively, and metastable molecular ions. Good agreement is obtained for all compounds and n = 0.4–0.8 for unstable molecular ions and n = 5–8 for metastable ions. Therefore, the hydrogen exchange in the molecular ion of 1,3-diphenylpropane is a rather slow process. These results firmly establish the isomerization reaction involving the conversion of the molecular ion of 1,3-diphenylmethane to the intermediate and hence to the molecular ion of 7-(2-phenylethyl)-5-methylene cyclohexa-1,3-diene and preceding the fragmentations. The postulated intermediate is a true one which corresponds to a s?-complex type ion and which fragments to [C₇H₈]⁺ ions. Surprisingly, no isomerizations of the intermediate by hydrogen shifts within the protonated aromatic system (‘ring walks’) are observed.     

Dissociation and rearrangement of molecular ions of acetals in the field mass spectrometer

Theoretical and Experimental Chemistry -     

Oxygen rearrangement processes in mass spectrometry

The recent publication¹ concerning oxygen rearrngement process in the mass spectra of aromatic and unsturated esters parallels an investigation by the auther on similar compounds. Some further definitive aspects of this study are reported here.     

Evidence for the Amino–Claisen rearrangement occurring in the molecular ions of N-allylaniline

One of the most intense peaks in the mass spectrum of N-allylaniline is at m/z 106 (97%). High resolution analysis and collision-induced dissociation studies confirm that this peak contains mostly [C₇H₈N]⁺ ions having the anilinomethene structure, but also a small contribution is seen from [C₈H₁₀]⁺ ions which result from the loss of the elements of HCN from molecular ions, following an Amino–Claisen rearrangement. The occurrence of a thermal rearrangement in the sample molecules cannot, however, be completely ruled out. Studies on metastable molecular ions of N-allylaniline and collision-induced dissociation of the m/z 106 ions formed from these show that, in the case of molecular ions with energies closer to threshold, the rearrangement reaction competes much more effectively with the direct cleavage reaction.     

Amino Phosphate Monoesters: A Convenient Source of 2-Alkylamino-3-methoxy-3-phenylpropionates via Aziridinium Ions

The synthesis of a series of amino ethers was carried out in a two-step method involving phosphorylation of the corresponding amino alcohols and subsequent double displacement via an aziridinium intermediate. The new products were obtained in good yields with excellent regioselectivity.     

Unimolecular rearrangement of α-silylcarbenium ions. An ab initio study

The unimolecular rearrangements of hydrogen, methyl and phenyl groups at the Si atom in α-silylcarbenium ions have been investigated using an ab initio molecular orbital method. MP2/6–31 + G^*//HF/6–31G^* calculations predict that all three groups migrate from the Si to an adjacent C_α with no energy barrier. Thus, the silicenium ion is the only stable species in each potential energy surface. The conformation of the benzylsilicenium ion, (C₆H₅)CH₂−SiH₂⁺, indicates that the phenyl ring is significantly bent toward the silyl cationic center in order to interact with the vacant 3p(Si⁺) orbital. In contrast to MP2 results, Hartree-Fuck calculations (both HF/3–21G^* and HF/6–31G^* levels) predict small energy barriers for 1,2-migrations of H and Me (1.4 kcal mol⁻¹ for H migration, and 1.5 kcal mol⁻¹ for Me migration, respectively, at the HF/6–31G^* level). This difference provides convincing evidence that the incorporation of electron correlation is of particular importance in describing the potential energy surface for the rearrangement of α-silylcarbenium ions to silicenium ions. The results of the calculations have also been applied to the possible rearrangement mechanism of α-chlorosilanes to chlorosilanes, assuming that the experimental conditions are favorable toward the generation of ionic species. Various factors which may govern the migratory aptitudes of various R groups, i.e. (1) activation energies, (2) overall reaction energies and (3) the conformational preference of reactants have been investigated. The calculated activation energy obtained, namely the energy for the generation of the silicenium ion and the C⁻¹ ion from an α-chlorosilane, is consistent with the experimental migratory aptitude in the gas phase observed in mass spectrometers.     

Stability of molecular ions

Physical Institute, Leningrad State University. Translated from Zhurnal Strukturnoi Khimii, Vol. 32, No. 5, pp. 133–134, September–October, 1991.     

Hydrogen rearrangement in molecular ions of alkyl benzenes: Appearance potentials and substituent effects on the formation of [C7H8]+ ions

The mechanism of the formation of [C₇H₈]⁺ ions by hydrogen rearrangement in the molecular ions of 1-phenylpropane and 1,3-diphenylpropane has been investigated by looking at the effects of CH₃O and CF₃ substituents in the meta and para positions on the relative abundances of the corresponding ions and on the appearance energies. The formation of [C₇H₈]⁺ ions from 1,3-diphenylpropane is much enhanced at the expense of the formation of [C₇H₇]⁺ ions by benzylic cleavage, due to the localized activation of the migrating hydrogen atom by the γ phenyl group. A methoxy substituent in the 1,3-diphenylpropane, exerts a site-specific influence on the hydrogen rearrangement, which is much more distinct than in 1-phenylpropane and related 1-phenylalkanes, the rearrangement reaction being favoured by a meta methoxy group. The mass spectrum of 1-(3-methoxyphenyl)-3-(4-trideuteromethoxyphenyl)-propane shows that this effect is even stronger than the effect of para methoxy groups on the benzylic cleavage. From measurements of appearance potentials it is concluded that the substituent effect is not due to a stabilization of the [C₇H₇X]⁺ product ions. Whereas the [C₇H₇]⁺ ions are formed directly from molecular ions of 1-phenylpropane and 1,3-diphenylpropane, the [C₇H₈]⁺ ions arise by a two-step mechanism in which the s? complex type ion intermediate can either return to the molecular ion or fragment to [C₇H₈]⁺ by allylic bond cleavage. Obviously the formation of this s? complex type ion, is influenced by electron donating substituents in specific positions at the phenyl group. This is borne out by a calculation of the ΔH_f values of the various species by thermochemical data. Thus, the relative abundances of the fragment ions are determined by an isomerization equilibrium of the molecular ions, preceding the fragmentation reaction.     

Concerning the Beckmann rearrangement of aryl heteryl ketoxime ions

The possible Beckmann rearrangement of aryl heteryl ketoxime ions has been investigated by studies on the 70 eV mass spectra, metastable ion spectra and collision induced dissociation spectra of pairs of isomeric ketoximes and corresponding amides. Aryl cycloalkyl ketoximes were also studied, and the molecular ions of these and the corresponding amides showed no evidence of interconversion. The introduction of the heterocyclic moieties made little change to this situation although some novel rearrangements were observed.     

Kinetic energy release during CO loss by rearrangement of α-benzoylcarbenium ions

Primary α-benzoylcarbenium ions (a) and tertiary α-benzoyldimethylcarbenium ions (b) are obtained by chemical ionization of ω-hydroxyacetophenone and its dimethyl derivative, respectively. Both α-acylcarbenium ions decompose by a rearrangement reaction and subsequent loss of a CO molecule. The kinetic energy released during this process by metastable ions a and b has been determined under different experimental conditions. The kinetic energy released during the CO elimination from the tertiary α-benzoyldimethylcarbenium ions b is independent of the experimental conditions and gives rise to dish-topped peaks with T₅₀=440±20 meV in the MIKE spectra of b. In contrast to this the kinetic energy releases and the peak-shapes in the MIKE spectra of the primary α-benzoylcarbenium ion a varies with the experimental conditions. The mechanism of this rearrangement reaction is discussed, and it is shown by a MNDO calculation of the heats of formation of the relevant ions that the different characteristics of the kinetic energy release during the fragmentation of primary and tertiary carbenium ions can be attributed to different types of reaction energy profiles.     

Polarizability of negative molecular ions

Theoretical and Experimental Chemistry -     

Microwave spectroscopy of molecular ions

In this paper a brief overview of the research in microwave spectroscopy of molecular ions done at the University of Wisconsin will be given, with major emphasis on work done in the past year. Five molecular ions (CO⁺, HCO⁺, HNN⁺, HCS⁺, and HOC⁺) have been studied in this work, and all of them have also been detected by radioastronomy. Molecular structures (r_s for HCO⁺, HOC⁺, and HNN⁺ and r_e for HCO⁺) have been determined and important dynamical information has been obtained from pressure broadened linewidths (Δν/P), Doppler shifts, and relative intensity data. In particular the Δν/P values have been shown to correspond to the Langevin cross-section, indicating the monopole-induced dipole interaction is the pertinent intermolecular force.