L. Friedman

A study of the formation of high molecular weight water cluster ions (m/e<59 000) in expansion of ionized gas mixtures

Mass spectrometric determinations have been made of size distributions of cluster ions produced by expansion of ionized mixtures of water and various carrier gases. Measurements span a range up to mass to charge ratios of 59 000. Dilute water–nitrogen carrier gas mixtures producing cluster masses up to 12 000 gave size distributions which are first order in water concentration. This observation is explained by the relatively large value of the ion–dipole associative collision cross sections which determined the rate of growth of cluster ions in the mass range studied. Studies in different mass carrier gases show that cluster ion size distributions are largely determined by the cross section for, and number of, associative collisions taking place during the expansion process. Estimates of associative collision efficiency have been made for water and alcohol cluster ion growth on expansion. Evidence has been obtained for steric inhibition of efficient alcohol collision processes compared to relatively effic...

Intramolecular Isotope Effects in the HD—Rare Gas Ion—Molecule Reactions

The ion—molecule reactions of HD+—He, HD+—Ne, Ar+–HD, and Kr+–HD have been studied and the intramolecular isotope effects determined over a range of kinetic energies of the reactant ion. The intramolecular isotope effects were found to vary with ion kinetic energy, indicating energy transfer to internal degrees of freedom in the reaction products. Three types of isotope effects are proposed to account for experimental data. An atom or ion transfer mechanism which gives rise to a higher probability of formation of XH+ over XD+ is the dominant factor at high ion kinetic energies. At low energies the unimolecular decomposition of a triatomic intermediate is assumed to form preferentially XD+. For the HD+—He and HD+—Ne reactions evidence is obtained indicating dissociation of the ionic product with an associated isotope effect favoring the XH+.

Study of the Mass Spectra of the Lower Aliphatic Alcohols

Mass spectra and appearance potentials have been determined for methanol, ethanol, and propanol. These data are used in semiempirical application of the statistical theory of mass spectra. The theory facilitates assignment of plausible mechanisms for the production of the various ion fragments. It is possible to obtain computed spectra in agreement with experimental data using an internally consistent set of structural parameters.

Hydrogen—Helium Ion—Molecule Reactions

The reactions producing HeH+ from gaseous mixtures of H2 and He subjected to electron impact in the ion source of a mass spectrometer have been studied. Ionization efficiency curves show practically all of the HeH+ to be formed by reaction of H2+ and He. Phenomenological cross sections were compared with values for the formation of H3+ from the H2+–H2 present in the ion source. The maximum value of the ratio of rate constants for HeH+ and H2+ formation computed from the experimental data was 28% of the theoretical ratio. If the heat of the endothermic reaction between H2+ and He is supplied from internal energy in H2+, then the rate constant should be computed in terms of the fraction vibrationally excited H2+ with sufficient internal energy to meet the needs of the reaction rather than total H2+ in the ion source. An estimate of the population of the excited vibrational state in H2+ produced from H2 by electron impact gives approximately 29% with enough energy to react. The translational energy dependenc...

ISOTOPIC HYDROGEN ION MOLECULE REACTIONS

The reactions of isotopic hydrogen‐molecule ions with isotopic hydrogen molecules have been investigated using a mass spectrometer ion source as the reaction chamber. No significant differences were detected in a study of the velocity distributions of the product and reactant ions. Rate constants for the formation of H3+, D3+, and H2D++D2H+ were obtained from the reactions of H2+–H2, D2+—D2, and HD+—HD, respectively, which were in excellent agreement with theory. Isotopic fractionation was observed in the production of H2D+ and D2H+ from mixtures of H2 and D2, HD and H2, HD and D2. The isotope effects are qualitatively consistent with a more rapid formation of lower zero‐point energy intermediates.

Tandem Mass‐Spectrometer Study of D3O+ and Solvated Derivatives

The mass spectra of heavy water produced by electron impact on neutral gas at pressures up to 1 torr have been studied in a tandem mass spectrometer. The enthalpies of hydration of solvated D3O+ as well as the deuteron affinity of D2O were studied by measurement of kinetic‐energy thresholds for the dissociation of the respective ions in collision with helium. Evidence was obtained for excited states in D3O+, D5O2+, D7O3+, D9O4+, etc., which survived for time periods in excess of 10−4 sec. Efforts to de‐excite these ions with neutral D2O gas molecules in the high‐pressure source were rarely successful, indicating difficulties in the interpretation of data based on the assumption of thermodynamic equilibrium in such ion sources. Values for the heat of association of two and three neutral heavy‐water molecules were deduced from the threshold data. Relative cross sections for the attenuation of various ionic species were measured. The significant differences in the attenuation cross section point up problems ...

Mass Spectrum of Lithium Iodide

The mass spectrum of lithium‐iodide vapor produced from salt heated in an oven in the ion source of an analytical mass spectrometer was investigated as a function of oven temperature. Evidence was obtained for the existence in the vapor of both monomer and dimer. Partial heats of sublimation to monomer and dimer at 0°K were found to be 44.9±0.7 and 45.5±2 kcal respectively. The heat of dissociation of (LiI)2 into gaseous monomers was found to be 44.4±2.2 kcal. The partial heat of sublimation of monomer was used with supplementary thermochemical data to obtain values for the lattice energy and the heat of dissociation of the diatomic molecule.Appearance potentials for the major ions in the spectrum where determined. Upper limits for the energy of dissociation of the gaseous diatomic molecule were derived from the appearance potentials of Li+ and I+ which were slightly higher than the thermochemical value.

Cross Sections and Intramolecular Isotope Effects in AB—HD Ion—Molecule Reactions

Isotopic ion—molecule reactions producing ABH+ and ABD+ ions (AB=N2, CO, O2, and CO2) when AB—HD mixtures are subjected to electron bombardment in the mass‐spectrometer ion source have been studied as a function of reactant ion velocity. Good agreement between the experimental and theoretical cross sections for secondary ion formation is obtained when both AB+—HD and HD+—AB interactions are taken as possible reaction paths. Ionization‐efficiency curves of the secondary ion products furnish supporting evidence for the assumed reaction mechanisms. Identification of reactant ions by ionization‐efficiency curves is not sufficient to establish the thermochemistry of the ion—molecule reaction; distributions of vibronic states produced by electron impact are also required. The vibrational population of excited reactant ions formed in the electron impact process has been estimated from the squares of the overlap integrals of respective ground and excited ionized states. Data taken from published photoionization e...

Study of the H+ and OH− Hydrated Ions by the CNDO/2 Method

The ions obtained by hydration of H+ and OH−, e.g., H3O+, H5O2, H7O3, H9O4+, H3O2−, H5O3−, and H7O4− have been studied theoretically using the CNDO/2 method. The results indicate chain structures are generally preferred albiet weakly over cyclic structures by the total energy criterion and in the case of positive ions over proton centered structures. Binding energies are found to be in reasonable agreement with experimental data. Charge distributions and individual bond energy effects have been examined in detail and are generally consistent with the notion that protons serve as bridging centers for delocalization which remain strongly positive throughout both series to shield the fragment oxygens from each other.

Hydrogen cluster ion equilibria

The equilibria H3++H2=H5+ and H5++H2=H7+ have been studied using both hydrogen and deuterium in a cooled corona discharge ion source fitted with a very small ion exit aperture. Enthalpies of reaction are in relatively good agreement with results obtained in earlier studies by Hiraoka and Kebarle using a pulse electron impact ion source. Absolute values of equilibrium constants and entropies of reaction are in somewhat poorer agreement. The enthalpy determined for the reaction yielding H5+ was found to be in slightly better agreement with the recent theoretical study of Yamaguchi, Gaw, and Schaeffer than results obtained in earlier examinations. A discussion of potential errors in the experimental determination of these equilibria is presented.