John L. Holmes

Neutral products of ion fragmentations: HCN and HNC identified by collisionally induced dissociative ionization

Abstract Collisionally induced dissociative ionization of HCN and HNC permits their identification as neutral products in the fragmentations of ionized pyridine and aniline respectively.

Metastable ion studies (VIII): An analytical method for deriving kinetic energy release distributions from metastable peaks

Abstract Using a simple five-parameter mathematical model, a method is presented for obtaining, analytically, the distribution of kinetic energy, T, released in a metastable decomposition, n(T) vs. T, directly from the experimental curve. The method can be applied to both (pseudo) Gaussian and dish-topped peaks. The equations n(T) vs. T have been integrated analytically in both cases to yield expressions for T(average) in terms of the five parameters of the mathematical model. All the metastable peaks examined thus far are described accurately by the model. The resulting T-distribution curves can be sensitive indicators of the presence of two fragmentation pathways in the metastable decompositions.

The mass spectrometry of neutral species produced from mass-selected ions by collision and by charge exchange. Experiments with tandem collision gas cells

The neutralisation—reionisation mass spectra (NRMS) of a number of organic cations have been measured. Neutralisation was achieved by electron transfer from Xe atoms in the first of two collision cells in the second field-free region of a double focussing mass spectrometer of reversed geometry. Reionisation took place by collisional excitation with He atoms in the second cell; ions passing beyond the first cell were prevented from entering the second by an ion beam deflector electrode. The NRMS of benzene and acetic acid molecular ions were similar to the normal electron impact (EI) mass spectra and collisional activation (CA) mass spectra of the two molecules. Acetic acid molecules generated from the unimolecular dissociation of the proton bound dimer [CH3COOH]2H+ had a collision-induced dissociative ionisation (CIDI) mass spectrum closely similar to that derived from EI. Effects arising from neutral species generated by normal collisionally induced fragmentations of the ions in the first cell were shown to be small. The significant differences between the EI, CA and NRMS of methyl formate and its CIDI mass spectrum, derived similarly from [HCOOCH3]2H+, were considered indicative of the presence of rearranged molecular ions among those sampled by CA, some, but not all, of which produced stable neutral species. The NRMS of the acetyl, carboxyl and formyl cations were measured under conditons of good energy resolution. The observations were interpreted in terms of the dissociation characteristics of the intermediate free radicals, each of which underwent sufficient collisional energisation during the electron transfer from Xe, to decompose by simple bond cleavages. For , the results were in qualitative agreement with recent estimates for the energy barriers for the competing reactions yielding and .

Heats of formation of organic radicals from appearance energies

Abstract The heats of formation of eighteen organic radicals have been measured from appearance energies for fragmentations in which the daughter ion structure and heat of formation are known. The agreement with values obtained from kinetic measurements is generally good.

Isomeric distonic and H-bridged [C2H6O]+ radical cations

Abstract Ab initio molecular-orbital calculations (SD Cl/6-31G★★ confirm that the most stable [C2H6O]+ isomer is the distonic ion [CH2CH2OH2]+. Mass spectrometric measurements give ΔH0f, 298 = 732 ± 5 kJ mol−1. This ion interconverts with a yet-unidentified isomer, [CH2CH⋯H⋯OH2]+, a hydrogen-bridged water/ethene radical cation complex. The latter ion lies in a shallow well, 38 kJ mol−1 above [CH2CH2OH2]+ and 41 kJ mol−1 below the dissociation into [C2H4]+ and H2O. Above the interconversion barrier the isomers behave like ethene-ion/water-dipole complexes in which the dipole can move all around the ion. This behaviour may relate to the very small kinetic energy release in the dissociation.

Fragmentation rate constants and appearance energies for reactions having a large kinetic shift and the energy partitioning in their metastable decomposition

Abstract Appearance energies (AE) of primary daughter ions for reactions possessing marked kinetic shifts have been measured using energy selected electrons and metastable peak observations. The time scales and limiting threshold rate constants corresponding to these measurements have been estimated by comparisons with results from photoion-photoelectron coincidence experiments. The dependence of the fraction of excess internal energy, E ‡, released as kinetic energy of the products, T , on the magnitude of E ‡ is described. In general, the fraction increases with decrease of E ‡ and cannot adequately be described by the empirical equation of Haney and Franklin, E ‡ = 0.44 T × (number of vibrational degrees of freedom).

Metastable ion studies II. Computer assisted interpretation of the shapes of metastable peaks

Abstract Metastable peak shapes have been computed for the AEI-GEC MS902S mass spectrometer. The effects of instrument parameters, the masses of the precursor (m1) and daughter (m2) ions and the magnitude of the kinetic energy released upon fragmentation have been considered. The results confirm that dish-topped metastable peaks do not arise from single-valued energy releases, but from the release of a range of energies. Preliminary calculations for the distribution of released energies are presented for the fragmentation C3H7+ → C3H5+ + H2. The results show that Th, the energy release calculated from the peak width at the “horns” or upper extremities is close to the minimum energy released. This, in turn, is a minimum value for the reverse activation energy. The depth of dish, D, was also computed for 63 wide-ranging values of single-valued energy release (Ti) and of the mass quotient {(m1−m2)/m2}. Experimental values of D agreed well with those computed using Ti = Th. The significance of this agreement is discussed. A Monte-Carlo method for evaluating metastable peak shapes is also presented.

The collisionally induced dissociations of the carboxyl and formate positive and negative ions

Abstract The formate positive ion, [HCOO]+, is produced by the collisionally induced charge reversal of the corresponding negative ion. Those [HCOO]+ ions having insufficient energy (

Stabilization energy and positional effects in halogen-substituted alkyl ions

Abstract The heats of formation, ΔH0f, of a series of halogen-substituted alkyl ions have been determined by appearance energy measurements with energy selected electrons. Halogen substitution at the formal charge site significantly reduces the heterolytic bond strength D[R+H−], the effect being not greatly different for Cl, Br and I. The β-substituted halo-ethyl ions clearly show stabilization resulting from the formation of bridged halonium ions but this effect is absent in the propyl and butyl analogues.

Correlating thermochemical data for gas-phase ion chemistry

Abstract This article presents a brief review of correlation schemes which relate ionization energies, ionic heats of formation, proton affinities and heterolytic bond strengths with some simple property of the ion, most commonly its size. The schemes reviewed here have mainly been developed for homologous series and simple atom or group substitutions at charge-bearing sites. The utility of such schemes is illustrated by appropriate examples.