Jan E. Szulejko

Protomers: formation, separation and characterization via travelling wave ion mobility mass spectrometry

Travelling wave ion mobility mass spectrometry (TWIM-MS) with post-TWIM and pre-TWIM collision-induced dissociation (CID) experiments were used to form, separate and characterize protomers sampled directly from solutions or generated in the gas phase via CID. When in solution equilibria, these species were transferred to the gas phase via electrospray ionization, and then separated by TWIM-MS. CID performed after TWIM separation (post-TWIM) allowed the characterization of both protomers via structurally diagnostic fragments. Protonated aniline (1) sampled from solution was found to be constituted of a ca. 5:1 mixture of two gaseous protomers, that is, the N-protonated (1a) and ring protonated (1b) molecules, respectively. When dissociated, 1a nearly exclusively loses NH(3) , whereas 1b displays a much diverse set of fragments. When formed via CID, varying populations of 1a and 1b were detected. Two co-existing protomers of two isomeric porphyrins were also separated and characterized via post-TWIM CID. A deprotonated porphyrin sampled from a basic methanolic solution was found to be constituted predominantly of the protomer arising from deprotonation at the carboxyl group, which dissociates promptly by CO(2) loss, but a CID-resistant protomer arising from deprotonation at a porphyrinic ring NH was also detected and characterized. The doubly deprotonated porphyrin was found to be constituted predominantly of a single protomer arising from deprotonation of two carboxyl groups.

A pulsed electron beam, variable temperature, high pressure mass spectrometric re-evaluation of the proton affinity difference between 2-methylpropene and ammonia

Abstract A new constructed pulsed electron beam high pressure mass spectrometer which incorporates a reverse geometry, BE, sector mass spectrometer has been used to examine proton transfer equilibria as a function of temperature for a series of bases intermediate in proton affinity between 2-methylpropene and ammonia. The data obtained support the recent conclusion of Meot-Ner and Sieck (J. Am. Chem. Soc., 113 (1991) 448) that the proton affinity of ammonia should be revised upward by some 4 kcal mol−1. Examination of clustering equilibria at high temperatures for strongly bound proton bound dimer species establishes that temperatures are accurately measured in the new apparatus. Thermochemial data obtained for the association of NH+4 with 2-methylpropene suggest that the proton affinity of t-butylamine must also be revised upward by nearly 8 kcal mol−1. Use of the capability of the reverse geometry configuration of the mass spectrometer to examine collision-induced dissociation of mass-selected ions establishes that the cluster of NH+4 with 2-methylpropene has the same structure as protonated t-butylamine. Consideration of the kinetics of three-body association reactions reveals that temporal profiles in high pressure mass spectrometry experiment may have to be examined for up to 10 ms to ensure that true thermal equilibrium is established in clustering reactions.

A pulsed ionization high-pressure mass spectrometric study of methyl cation transfer and methyl cation-induced clustering in dimethyl ether-acetone mixtures

Abstract A newly constructed pulsed ionization high-pressure mass spectrometer has been used to examine the methyl cation transfer equilibria between the methylated adducts of acetone and dimethyl ether and the parent compounds. The data indicate that dimethyl ether is a stronger base toward CH+3 than is acetone, a reversal from the proton affinity order. Clustering equilibria of (CH3)2COCH+3 with the acetone and dimethyl ether have also been examined. The thermochemical data obtained show that the adducts are covalently bound species with considerable restriction of internal rotations. New thermochemical data for ΔHof and So for all species observed are reported.

Stepwise solvation of halides by alcohol molecules in the gas phase

Abstract The gas phase equilibrium clustering reactions X−(ROH)n + ROH ⇌ X−(ROH)n+1 (X = F, Cl, Br, I; R = CH3, CH3CH2, (CH3)2CH, (CH3)3C; n = 0, 1, 2) have been investigated by using pulsed-ionization high pressure mass spectrometry (PHPMS). From the corresponding van’t Hoff plots the standard enthalpies (ΔHn,n+1O) and entropies (ΔSn,n+1O) were obtained, which are discussed in terms of the radii of the halides, the geometry of the alcohol molecules, the number of alcohol molecules, and molecular properties such as polarizability and gas phase acidity. The observed enthalpy trends can be explained on the basis of ion-dipole, ion-induced dipole, and dipole-dipole interactions within the clusters. The observed entropy trends are qualitatively discussed in terms of hindered rotations and low frequency intermolecular vibrations. In general, where available, there is good agreement between the present data and literature values obtained by various experimental techniques. In addition to the experiments, both density functional theory (DFT) calculations at the B3LYP/6–311+G(d,p) level of theory and G2 level calculations have been performed on a number of selected systems to test these methods for obtaining energetic data and to gain more insight into the structures of the investigated clusters.

Deuterium isotope effects on gas phase ion-molecule hydrogen-bonding interactions: Alcohol-alkoxide and alcohol-chloride adduct ions

Abstract Fourier Transform Ion Cyclotron Resonance (FT-ICR) and High Pressure Mass Spectrometric (HPMS) measurements of the deuterium isotope effect and kinetics of adduct ion formation have been used to probe the nature of the potential describing the motion of the hydrogen in gas phase ion-molecule hydrogen-bonding interactions. Hydrogen-bonding systems reported in this paper are alkoxide ion and chloride ion solvated by one molecule of alcohol, ROH●OR- and ROH●Cl-. Significant differences in the isotope effects were observed for the two systems. These differences are explained on the basis of the differing hydrogen bond strengths of the adduct ions, and the ability of the chloride ion to partake in multiple site, or chelate, interactions. In addition, HPMS studies of the kinetics of the reaction of CH3O− with CH3OH reveal that a double minimum potential energy surface may be appropriate for describing the adduct ion formation. These experimental studies have been supplemented by ab initio calculations to determine adduct ion structures as well as to permit statistical thermodynamic calculations of the isotope effect.

Fourier transform ion cyclotron resonance investigation of the deuterium isotope effect on gas phase ion/molecule hydrogen bonding interactions in alcohol—fluoride adduct ions

Abstract Fourier transform ion cyclotron resonance measurements of the deuterium isotope effect have been used to probe the nature of the potential describing the motion of the hydrogen in gas phase ion/molecule hydrogen bond interactions. The hydrogen bonding system studied is fluoride ion solvated by one molecule of aliphatic alcohol, ROH · F−. No variation of the isotope effect with alcohol acidity was observed for unsubstituted aliphatic alcohols. Arguments are presented which imply that the internal rotation of the neutral alcohols plays an important role in the determination of the overall isotope effect, as well as the more commonly considered zero-point energy effects.

Metastable and collision-induced fragmentation studies and thermochemistry of isomeric C4H11Si+ ions and their adducts with C4H12Si silanes

Metastable Ion (MI) and collision-induced dissociation (CID) mass spectra for all isomeric even-electron [C(4)H(12)Si - H](+) ions were recorded and compared. Deuterium labeling experiments indicated that most precursors give rise to silylium ions. Silylium ions with two or more methyl groups are found to lose C(2)H(4) after isomerization via a straightforward hydrogen transfer to the appropriate ethylsilylium ion. Similarly, all isomeric propyl- and butyl-containing silylium ions are found to lose C(2)H(4) by rearrangement preceding dissociation. In the CI source of the mass spectrometer many of the silylium ions are found to cluster with the parent neutral silane present in the source to give stable [M - H](+)+M adduct ions. The MI and CID spectra of these adduct ions were also obtained. In the MI spectra of all adducts, except i-BuSiH(3), only the starting silylium ion is observed. Under CID conditions generation of silylium ions dominates. Deuterium labeling studies show that this dissociation may be accompanied by some rearrangement, in particular for the adducts from i-BuSiH(3). High-pressure mass spectrometric clustering equilibrium measurements were also carried out to determine the enthalpies and entropies of binding of the silylium ions to the neutral silanes. These measurements yield insight into the effects of various alkyl group substitutions on the association thermochemistry in these adducts. Copyright 2000 John Wiley & Sons, Ltd.