Richard D. Bowen

Alkane eliminations from ions in the gas phase

A review of reactions involving the elimination of an alkane from a variety of even and odd electron ions in the gas phase is presented. Particular attention is focused on the mechanisms of these reactions and the role of ion–neutral complexes. Alkane eliminations from open shell species derived by ionisation of ketones, ethers, alcohols, amines, alkanes and alkyl-substituted cycloalkanes are considered, together with ethane loss from the distonic ion CH2=O+CH2CH2•. These reactions are, without obvious exception, complex-mediated. Elimination of alkanes from an assortment of closed shell ions, including protonated acetaldehyde, dialkylmethylene immonium ions, onium ions, quaternary ammonium cations, alkoxide anions, enolate anions and deprotonated amines are also discussed. These reactions tend to be complex-mediated when heterolytic bond cleavages are possible. When such cleavages are not energetically feasible, alkane eliminations can take place by asynchronous, concerted mechanisms. Several factors affect the relative rates of alkane elimination compared to those of competing alkyl radical loss in the dissociations of these radical-cations; these factors include the internal energy of the ion, the size of the ionic and neutral partners, the initial orientation of these components and the binding energy of the complex. In addition, primary and secondary isotope effects influence both alkyl radical and alkane elimination. When alkanes are eliminated from radical-cations, reaction between the partners is often quickly overtaken in importance by direct separation of the components if the internal energy is raised either by increasing the energy of the ionisation process or by prior isomerisation over an energy barrier. This trend demonstrates that the attractions between the partners are easily overcome by quite small amounts of excess energy in the system. Typical binding energies between the ionic and non-polar neutral partners lie in the range 0–30 kJ mol−1 at distances at which covalent forces cease to be significant. Illustrative examples are given to show how studies of reactions involving ion–neutral complexes offer a means of obtaining information on the dynamics of short range interactions between ions and neutrals, including the motions of the partners relative to each other.

Raman spectroscopic and structural studies of indigo and its four 6,6'-dihalogeno analogues.

The Raman and electron impact mass spectra of synthetic indigo and its four 6,6-dihalogeno analogues are reported and discussed. The influence of varying the halogen on these Raman spectra is considered. Particular emphasis is laid on distinguishing indigo from 6,6-dibromoindigo and differentiating between the dihalogenocompounds, so as to develop protocols for determining whether artefacts are coloured with dyes of marine or terrestrial origin and whether such artefacts are dyed with genuine Tyrian Purple or with dihalogenoindigo substitutes that do not contain bromine. The value of even low resolution electron impact mass spectrometry in a forensic context as a means of identifying authentic 6,6-dibromoindigo and distinguishing it from its dihalogenoanalogues is emphasised

The mechanism of alkyl radical elimination from ionised γγ-disubstituted alkenyl methyl ethers

The mechanism of alkyl radical loss from ionised alkenyl methyl ethers containing two γ-alkyl substituents, R1R2C=CHCH2OCH3+•, has been studied by investigating the collision-induced dissociation spectra of the resultant C n H2n–1O+ oxonium ions (n = 5–7). Comparison of these spectra with one another and those of reference ions generated by dissociative ionisation of secondary allylic alkenyl methyl ethers indicates that expulsion of a γ-alkyl group occurs without isomerisation of the heavy atom skeleton via an allylic rearrangement. This finding is consistent with the occurrence of two consecutive 1,2-H shifts in R1R2C=CHCH2OCH3+•, followed by γ-cleavage of the ionised enol ether, R1R2CHCH=CHOCH3+•, to give R1CH=CHCH=OCH3+ or R2CH=CHCH=OCH3+. Thus, CH3CH2CH2(CH3CH2)C=CHCH2OCH3+• loses C2H5• and C3H7• to yield CH3CH2CH2CH=CHCH=OCH3+ and CH3CH2CH=CHCH=OCH3+, respectively.

Reactions of metastable ionized γ, γ-dialkylallyl methyl ethers: relative rates for elimination of alkyl radicals by formal γ-cleavage

Abstract The reactions of seven metastable ionised allylic alkenyl methyl ethers of general structure, R1R2C= CHCH2OCH3+ [R1=CH3, R2=C2H5, n-C3H7, n-C6H13, or iso-C3H7; R1=C2H5, R2=n-C3H7 or iso-C3H7; R1 = n-C3H7, R2 = n-C4H9] with two different γ-alkyl substituents are reported and discussed. Two common reactions are observed: loss of a molecule of methanol or elimination of an alkyl radical. The second fragmentation is interpretable by a mechanism in which two consecutive 1,2-hydrogen shifts lead to an ionised enol ether, R1R2CHCH ue5fb CHOCH3+, which then undergoes γ-cleavage to give either R1CH ue5fb CHCH ue5fb O+CH3 or R2CH ue5fb CHCH ue5fb O+CH3. Competition experiments reveal that the relative rate of alkyl radical loss from these metastable radical-cations follows the order (CH3)2CH· > CH3CH2· > CH3CH2CH2· ≥ CH3CH2CH2CH2·⪢ CH3· Metastable ionised ethers containing a γ-isopropyl group differ from their isomers with an n-propyl substituent in expelling propane at an appreciable rate. A unique process apparently involving loss of a neutral with a mass of 74 Da is observed from metastable CH3CH2CH2CH2CH2CH2(CH3)C ue5fb CHCH2OCH3+, this fragmentation is attributed to consecutive elimination of two species, C3H6 followed by CH3OH, rather than direct expulsion of C4H10O.

Low-energy, low-temperature mass spectra. Part 17: selected aliphatic amides

The 12 eV, 350 K electron impact ionisation mass spectra of 28 aliphatic amides of diverse structure are reported and discussed. All the spectra contain strong molecular ion signals, often corresponding to the base peak, thus providing unequivocal molecular mass information. Structural information is also accessible from specific primary fragmentations associated with the substituent(s) attached to the carbon and nitrogen atom of the amide group. Alkene elimination from the ionised amides takes place most readily from the substituent attached to the carbon atom via a McLafferty rearrangement, but it also occurs to a lesser degree from some ionised amides with no γ-hydrogen atom. In contrast, alkyl radical loss is usually associated with fission of an N-alkyl group, typically by apparent α-cleavage. Other primary fragmentations of significance include expulsion of an alkenyl radical derived from an N-alkyl substituent. In one such process, the eliminated alkenyl radical has one carbon atom fewer than the initial N-alkyl substituent; this fragmentation is preferentially associated with the N-neopentyl group and, to a lesser degree, the N-isobutyl group. A related process involves loss of an alkenyl radical with the same number of carbon atoms as the N-alkyl group; this reaction occurs for all four ionised N-butylformamides, but is most favoured for the isobutyl and t-butyl isomers. Both these processes entail double hydrogen transfer with the formation of an exceptionally stable protonated amide as the ionic product. Further dissociation of the primary fragment ions to give secondary fragment ions occurs only relatively rarely. The differences in the spectra of isomeric amides in which only the structure of an N-alkyl group is varied are often quite pronounced, especially in the case of n-butyl and isobutyl substituents, thus enhancing the analytical value of these low-energy, low-temperature mass spectra.

Raman spectroscopic study of allyl methyl ether (3-methoxy-1-propene), CH2CHCH2OCH3, and some isotopically labelled analogues

Abstract Fourier-transform Raman spectra of allyl methyl ether, CH 2 ue5fbCHCH 2 OCH 3 , three deuterated derivatives and one 13 C derivative have been obtained. Comparison of the spectra of the deuterated and protiated compounds in conjunction with polarization data has enabled full vibrational assignments to be made for the carbon-hydrogen modes and the 13 C data have identified some skeletal modes of the CO and CC bonds. As a result of the data obtained from the deuterated compounds in particular, some initial suggestions have been revised for tentative literature assignments of molecules of biological interest. In other cases, confirmation of existing assignments has been made.

The structure and mechanism of formation of C5H9O+ from ionized phytyl methyl ether

The recent proposal that ionized phytyl methyl ether [C16H33(CH3)C=CHCH2OCH3+·] undergoes an allylic rearrangement to ionized isophytyl methyl ether [CH2=CHC(C16H33)(CH3)OCH3+·] before elimination of an alkyl radical is discussed. Both literature precedent and new results in which the structure of the [M-C16H33·]+ fragment ion is established by comparison of its collision-induced dissociation mass spectrum with the spectra of isomeric C5H9O+ ions of known structure are inconsistent with this proposal. The forma Hon of CH3CH=CHCH=O+CH3 by loss of a γ-alkyl substituent without skeletal isomerization rather than CH2=CHC(CH3)=O+CH3 after allylic rearrangement is explained in terms of a mechanism that involves two 1,2-H shifts, followed by σ-cleavage of the resultant ionized enol ether, C16H33(CH3)CH-CH=CHOCH3+·.

The chemistry of some low energy C5H9O+ oxonium ions

Abstract The reactions of the low energy oxonium ions CH3CHue605CHue5f8C+(H)OCH3, b1+, CH2ue605CHue5f8C+(CH3)OCH3, b2+, and CH2ue605C(CH3)ue5f8C+(H)OCH3, b3+, are reported and compared with those of their 1,3-H shift isomers CH3CHue605CHCH2OCH2+, b4+, CH2ue605CHCH(CH3)OCH2+, b5+, and CH2ue605C(CH3)CH2OCH2+, b6+. The metastable ion (MI) spectrum of the C4H6OCH3+ ions b1+– b3+ is dominated by loss of CH2O. Elimination of C3H6, which is associated with a composite metastable peak, is also significant from b2+ and b3+. From (multiple) collision experiments and analysis of D- and 13C-labeled isotopologues, it follows that b1+–b3+ do not readily interconvert. Loss of CH2O is proposed to involve a 1,5-H shift followed by a (dipole-assisted) 1,3-H shift into an energy-rich ion-neutral complex (INC) [C4H7+/CH2ue605O]. Loss of CH2O from b4+–b6+ may also occur via an INC. This reaction is associated with a very small kinetic energy release, indicating that it generates the most stable C4H7+ ion, CH2ue605CHC+(H)CH3, at the thermochemical threshold. However, this process is only prominent for ions b5+, which also undergo a facile loss of H2O, via rearrangement in the INC [C4H7+/CH2ue605O], to yield C5H7+ (most probably the cyclopentenyl cation). Loss of C3H6 and CO dominates the MI spectra of b4+ and b6+ and these reactions, which also occur from b5+, are proposed to take place from 1,2-H shift isomers of the cyclic counterparts of b4+–b6+. The behavior of b1+–b3+ and b4+–b6+ differs considerably from their C4H7O+ homologues, CH2ue605CHue5f8C+(H)OCH3, a1+, and CH2ue605CHCH2OCH2+, a2+. Differences in the dissociation characteristics of the CnH2n−2OCH3+ species (n = 3–5) are discussed in terms of the energetics of the products that may be formed. ΔHf (298 K) values for the key ions in this study were obtained from CBS-QB3 calculations and thermochemical estimates.

The influence of the size and structure of a spectator alkyl group on the relative rates of alkyl radical elimination from ionised tertiary amines

The relative rates of alkyl radical elimination by α-cleavage of ionised amines of general structure, CH3CH2CH2(CH3CH2)CHN(CH3)R+• or CH3CH2CH2CH2(CH3CH2CH2)CHN(CH3)R+•, where R = n-C n H2n + 1 (n = 1–10, 12 or 14), iso-C5H11, CH2CH(CH3)C2H5, neo-C5H11 or CH2CH2C(CH3)3, are reported. The size of the spectator alkyl group, R, affects the ratio of ethyl and propyl radical loss from metastable ionised amines containing a 3-hexyl group; the slight preference for expelling the smaller ethyl radical increases initially before gradually falling as n increases from 1 to 14. In contrast, the degree of branching in isomeric pentyl or hexyl spectator alkyl groups has very little effect on the ratio of ethyl and propyl radical loss. For faster reactions occurring in the ion source, there is at most a marginal variation in the ratio of ethyl to propyl radical loss as n increases. In general, the kinetic energy release which accompanies loss of either radical increases slowly with n, but remains small, as would be expected if the reaction occurred without appreciable reverse critical energy. Similar trends are found for the relative rates of propyl and butyl radical elimination from ionised amines containing a 4-octyl group.

The chemistry of ionized ethyl glycolate, HOCH 2 CO 2 C 2 H 5 •+ , and its enol isomer, HOCH=C(OH)OC 2 H 5 •+ . Formation of ionized and neutral trihydroxyethylene

The unimolecular reactions of ionized ethyl glycolate, HOCH2CO2C2H5•+, and its enol isomer, HOCH=C(OH)OC2H5•+, have been studied by a variety of techniques including 2H-, 13C- and 18O-labelling experiments, kinetic energy release information, analysis of collision-induced dissociation and neutralization–reionization mass spectra and thermochemical measurements. The metastable ionized enol eliminates C2H4 essentially exclusively by β-hydrogen transfer to give (HO)2C=CHOH•+, which itself expels H2O with very high selectivity (∼99%). The metastable ionized keto isomer also eliminates C2H4, but minor amounts (∼5%) of competing fragmentations resulting in expulsion of H2O, HOCO• and HCO• are also observed. Moreover, in this case, loss of C2H4 no longer involves specific β-hydrogen transfer. This behavior is interpreted in terms of rearrangement of the ionized keto form via a 1,5-H shift to the distonic ion HOCH2C+(OH)OCH2CH2•, which undergoes a further 1,5-hydrogen shift to form HOCH=C(OH)OC2H5•+, from which C2H4 is lost to give (HO)2C=CHOH•+. The chemistry of these C4H8O3•+ species, in which unidirectional tautomerism of HOCH2CO2C2H5•+ to HOCH=C(OH)OC2H5•+ via two 1,5-H shifts is important, contrasts sharply with the behavior of the lower homologues, HOCH2CO2CH3•+ and HOCH=C(OH)OCH3•+, for which the analogous tautomerism via 1,4-H shifts does not occur. The mechanism for loss of C2H4 from HOCH=C(OH)OC2H5•+ is essentially the same as that observed for ionized phenyl ethyl ether in that the reaction proceeds via an exothermic proton transfer in an ethyl cation–radical complex. Neutralization–reionization experiments show that neutral trihydroxyethylene in the gas phase is a remarkably stable species which does not tautomerize to glycolic acid, HO–CH2–COOH. Using G2(MP2) theory, the enthalpy of formation, ΔHf298, of the neutral molecule (most stable conformer) was found to be −449.4 kJ mol−1 while that of the corresponding ion was calculated to be 299.6 kJ mol−1.