T. McAllister

Chemical kinetics of telluride pyrolysis

Abstract A reaction scheme is presented for the pyrolysis of alkyltellurides used in the growth by metalorganic chemical vapour deposition (MOCVD) of cadmium mercury telluride. Three possible mechanisms are accounted for: simple bond cleavage, complex elimination, and heterogeneous catalysis. The Arrhenius parameters for these reactions are chosen so that, in a computer simulation, the production of tellurium matches the experimentally observed production over a range of temperatures. In the case of dimethyltelluride, good agreement with experiment is obtained with the bond cleavage mechanism and an estimate of the bond dissociation energy D (CH 3 −TeCH 3 ) of about 270–280 kJ mol −1 . For the diethyl-, diisopropyl-, ditertiarybutyl- and diallyltellurides, the heterogeneous mechanism becomes progressively more important in determining the tellurium production rate, so that in diallyltelluride, which has the lowest onset temperature for pyrolysis, the heterogeneous mechanism dominates the others. In the case of ditertiarybutyl telluride, the elimination mechanism, although not dominant, plays a relatively more important role than it does in any of the other alkyl tellurides.

Graphite furnace mass spectrometry of cobalt

Abstract The atomisation of Co has been investigated in a pyrolytically coated graphite furnace operating in vacua with rapid scan mass spectrometric detection, and by thermochemical calculation of the equilibrium in the furnace during atomisation. The calculations show that the atomising reaction should be Co(s) → Co(g) for both nitrate and chloride samples at the observed appearance temperature for Co atoms. Neither CoO(g) nor Co2(g) were detectedby mass spectrometry during atomisation, but CoCl2(g) was detected from chloride samples during the ashing cycle. Appearance temperatures for Co may be estimated from semilog plots of the calculated equilibrium intensities of Co(g) with temperature, and show good agreement with experimentally determined values. Comparison of the equilibrium calculations with activation energies derived from Arrhenius plots by other workers indicates that the atomisation is thermodynamically controlled in the region of the appearance temperature.

Negative ions in the flame ionization detector and the occurrence of HCO4

Abstract Mass-analysis of the ions formed in an H 2 N 2 acetone diffusion flame similar to that of a flame ionization detector showed that the ions carrying the current fell into two groups, one at m/e = 60 and 61 (CO 3 − and HCO 3 − ) and the other at m/e = 77. The ion at m/e = 77 has been identified as HCO 4 − and clustering, free radical association or ion hydrate decomposition have been suggested as possible formation mechanisms. This confirms the theoretical conclusion that more than one type of ion is involved in carrying negative charge in the flame ionization detector. The occurrence of HCO 4 − and its possible formation by reactions of free radicals has implications for the atmospheric chemistry of Earth and Mars.

Ion-molecule reactions and proton affinities of methyl nitrite and nitromethane

Abstract At pressures of about 10 −3 Pa in an ion cyclotron resonance mass spectrometer equipped with a four-section cell, secondary ions of m/e = 45 and 76 are not formed in methyl nitrite, in contrast to their prominence in the isomer nitromethane. Secondary ions of m/e = 62 and 91 are formed in both isomers, but in methyl nitrite the parent ion of m/e = 61 reacts to give the secondary of m/e = 92 but not m/e = 62, whereas the converse holds in nitromethane. These observations may be explained by postulating that there is an exclusive structure for ions of m/e = 62 and 76 in each isomer: addition occurs on the central oxygen in methyl nitrite and on the terminal oxygens in nitromethane; in methyl nitrite, the parent ion is kinetically prohibited from attaining this structure in an ion-molecule collision. The primary ion of m/e = 60 reacts to give the secondary of m/e = 91 in both isomers: this is accepted as evidence that the structure of the ion of m/e = 60 is the same for both isomers. The proton affinity of methyl nitrite was found to be 787±9 kJ mol −1 , giving Δ H f (CH 3 ONOH + ) = 676 kJ mol −1 .

Ion-molecule reaction kinetics by ion cyclotron resonance mass spectrometry

Abstract The rate constants for the production of the secondary ions CO2H+, COH+, and N2H+ in gaseous mixtures of hydrogen with carbon dioxide, carbon monoxide and nitrogen have been measured by ion cyclotron resonance mass spectrometry. The results are similar to those of other techniques.

Ion—molecule reactions and proton affinities of methyl thio- and isothiocyanate

Abstract The ion—molecule reactions of the isomers CH 3 SCN and CH 3 NCS have been investigated by ion cyclotron resonance mass spectrometry at pressures up to 2 · 10 −3 Pa. For both compounds the most important features were the reactions of parent ions ( m/e = 73) and fragment ions ( m/e = 45–47) to produce ions of m/e = 74, and the reactions of fragment ions to produce secondary ions of m/e = 73. The reactions producing ions of m/e = 74 in CH 3 SCN were about 3.5 times faster than those in CH 3 NCS. This is attributed to the different action of excited states of the ions m/e = 45, 46 and 73 in CH 3 NCS compared to CH 3 SCN and to alternative reactions of the ions m/e = 45 and 46, producing m/e = 72 ions, in CH 3 NCS, rather than to differences in collision rate constants. The remainder of the reactions were production of ions m/e = 44–46 by the doubly charged CHNCS 2+ in CH 3 NCS; ions m/e = 61, 90, 95, 102 and 129 in CH 3 SCN; and ions m/e = 59 in both isomers. In the course of this work the proton affinities of the isomers were found to be identical within the range 766–795 kJ mol −1 and the appearance potentials of the fragment ions from CH 3 SCN were found to be similar to those from CH 3 NCS, with the exception of the ions CH 2 SCN + (12.7 eV) and CH 2 NCS + (12.0 eV).

Ion—molecule reactions in mixtures of CS2 with H2 and CH4

CS2H+ and CSH+ ions are the most significant secondary ions found in mixtures of CS2 with H2 and CH4 at pressures of about 10−3 Nm−2 in an ion cyclotron resonance mass spectrometer. The CS2+ primary ions are unreactive and CS2H+ ions are formed by reactions of H2+ and CH4+ only, the rate constants of which are very much less than the theoretical values, due perhaps to the interference of charge transfer reactions. Double resonance experiments of CS2/CH4 and CS2/C2H6 mixtures indicate that the limits for the heat of formation of CS2H+ are 1037 kJ mole−1 < ΔHf(CS2H+) < 1124 kJ mole−1. CSH+ ions are formed by reactions of CS+ only. Small amounts of CH3S+ and CH3CS+ are formed from CH3+. In CS2 alone, CS2+ ions with kinetic energies of at least 1.7 and 2.6 eV form C2S2+ and CS3+ ions, respectively.

Atomisation and chemical ionisation of Si, Ge and Sn in fuel-rich flames

Abstract The relationship between atomisation and ionisation for Si, Ge and Sn in fuel-rich C 2 H 2 and H 2 flames has been studied by means of flame ionisation mass spectroscopy, thermochemical calculation of burnt gas equilibrium composition, and computer simulation of chemical ionisation kinetics. The mass spectra obtained from C 2 H 2 /Ar/O 2 flames are similar to those from H 2 diffusion flames: Sn yields Sn + and SnOH + , Ge and Si yield GeOH + and SIO + . These similarities are in contrast to the substantial differences in calculated atomisation found between the C 2 H 2 and H 2 flames. The discrepancies between atomisation and ionisation are reconciled by a chemical ionisation mechanism in which the ions SiH + , GeH + and SnH + are important intermediates. The ratios of atomic ions to protonated monoxides, M + :MOH + are determined by the thermochemistry for the reaction, MOH + + H ⇌ M + + H 2 O.

High-resolution ion cyclotron resonance spectroscopy of mixtures of CH4 with N2O and CO2

Abstract By means of an ion cyclotron resonance (ICR) mass spectrometer operating at high mass resolution, the secondary ion signal at m/e = 29 in a N 2 O/CH 4 mixture has been shown to be due to CHO + as well as N 2 H + and C 2 H s + . High resolution has also shown that the secondary ion CH 4 N + is being produced at m/e = 30. The reactions producing CHO + in a CO 2 /CH 4 mixture have also been determined at high resolution. In this mixture, a secondary ion of composition C 2 H 3 O + was detected at low resolution. This ion also occurs in CO/CH 4 mixtures. Precursors of this ion were determined by double resonance experiments and found to be CO 2 + , CH 4 + and CO + .

Protonated combustion products of benzene, toluene and cyclohexane in the flame ionization detector

Abstract Mass spectra of H2/Ar diffusion flames with benzene additive show that ions of m/z 79, 81, 91 and 95 occur within the flame. Deuteration experiments indicate that these ions have the formulae C6H7+, C6H9+, C7H7O+ and C6H7O+. They also appear in the mass spectra of flames with toluene and cyclohexane additives, but in the ICR mass spectra of H2O/C6H6 mixtures only C6H7+ was found. C6H7+, C6H9+ and C6H7O+ are more prominent early in the flames and their presence may be explained by assuming proton transfer from H3O+ or HCO+ to the unburnt additive or to the initial products of combustion, i.e. benzene, C6H8 and C6H6O (probably phenol), from hydrogen and hydroxyl radical addition and abstraction reactions. C7H7+ is more prominent in the middle of the flame, where C7H6 is presumably formed by reactions of hydrocarbon radicals.