Vincent G. Anicich

Absolute ion-molecule rate constants from drift cell ion cyclotron resonance spectroscopy☆

Abstract The various exact methods of calculating power absorption and its relationship to ion-molecule rate constants in an ion cyclotron resonance spectrometer are summarized. An approximate method that does not require computer analysis is developed and related to other approximate methods. A number of rate constants of model second order reactions are measured and rate constants obtained from the various exact analyses and the approximate method. The approximate method gives rate constants within 5% of the general exact method of Comisarow for most cases. The utility of the graphical approximate method in determining reaction order and mechanism is demonstrated by including pseudo second order and third order reactions. The systems studied include the parent ion reactions in methane, ethylene, vinyl fluoride, vinyl chloride, trifluoroethylene, tetrafluoroethylene and benzene.

ICR Mass spectra of fluoroalkenes: II. Ion-Molecule reactions in the 1,1-difluoroethylene system

Abstract The sole reaction of the molecular ion of 1,1-difluoroethylene has been shown to be the formation of collisionally-stabilized dimer ion, C 4 H 4 F 4 ±, in a third-order process, the rate constant of which is measured to be 1.1±0.2X10 −24 cm 6 molecule −2 sec −1 . Relative rate constants have been obtained for ten reactions of fragment ions with the parent molecule and problems associated with the thermochemistry of these reactions have been discussed.

Ion chemistry of fluoroethylene parent ions thermal energy reactions in mixtures of fluoroethylenes

Abstract The total rate constant for parent ion loss has been measured in all possible mixtures of fluoroethylene molecules. These include C 2 H 4 , C 2 H 3 F, CH 2 CF 2 , cis-CHFCHF, trans-CHFCHF, CHF 3 , and C 2 F 4 . These total ion loss rate constants were separated into their charge exchange and condensation components. The condensation products are principally C 2 (H, F) 4 + ions resulting from methylene switching reactions or C 3 (H, F) 5 + ions resulting from C(H, F) 3 elimination. All condensation products result from highly specific mechanisms. Discussions of the details of possible mechanisms are given. Isotopic labelling studies utilising C 2 D 4 , cis- and trans-CDFCDF and CDF 3 are also investigated and mechanistic conclusions presented. Ion cyclotron resonance spectroscopy is utilised.

ICR mass spectra of fluoroalkenes: III. Thermal ion-molecule reactions in C2HF3 and C2F4 by ion cyclotron resonance

Abstract The ion—molecule reactions of both parent and fragment ions in C2HF3 and C2F4 have been observed. Absolute rate constants have been measured for the parent ion reactions in both systems and in the mixed system. In each case, the only condensation reaction involves elimination of the ·CF3 radical although other reaction channels are thermodynamically allowed. The rates of the CF3 elimination reactions vary from 0.3 to 5.6 X 10−11 cm3 molecule−1 sec−1. Substantial decreases in the rates are observed at low pressures. This effect is attributed to internal energy in the reactant ions. The reversible charge transfer reaction was also observed in the mixed system. The equilibrium constant for this reaction has been measured yielding ΔGD = 0.023 ± 0.001 eV as the free energy of the charge exchange (C2F4⨥ → C2HF3⨥). This value agrees well with previously determined differences in ionization potentials. Thermodynamic implications are discussed for the fragment ion reactions.

Ion chemistry of fluoroethylene parent ions: cis- and trans-1,2-difluoroethylene

Abstract The ion-molecule reactions of the parent ions in cis-CHFCHF and trans-CHFCHF have been observed using ion cyclotron resonance spectroscopy. The only observed reaction in both cases was CHFCHF + + CHFCHF → C 3 H 3 F 2 + + CHF 2 with rate constants k (cis) = 6.3 × 10 −12 and k (trans) = 3 × 10 −12 cm 3 molecule −1 sec −1 . The relative ionization potentials of cis- and trans-CHFCHF were measured by determining the forward and reverse rate constants for the reaction cis-CHFCHF + + trans-CHFCHF ⇄ cis-CHFCH + trans-CHFCHF + and by measuring the relative ionic concentrations after 40–50 collisions in the ICR spectrometer. A value of the equilibrium constant K 3 ± 0.4, was obtained in both instances yielding Δ IP = 0.030 ± 0.003 eV or 0.70 ± 0.08 kcal/mole. By comparing this value with the known heat of isomerisation between cis and trans-CHFCHF, the difference in the heats of formation of the ions is Δ H o f (cis-trans) = 0.07 eV or 1.6 kcal/mole.

Association reactions at low pressure. V. The CH+3/HCN system. A final word?

The reaction of the methyl cation with hydrogen cyanide is revisited. We have confidence that we have resolved a long standing apparent contradiction of experimental results. A literature history is presented along with one new experiment and a re‐examination of an old experiment. In this present work it is shown that all of the previous studies had made consistent observations. Yet, each of the previous studies failed to observe all of the information present. The methyl cation does react with HCN by radiative association, a fact which had been in doubt. The product ions formed in the two‐body and three‐body processes react differently with HCN. The collisionally stabilized association product formed by a three‐body mechanism, does not react with HCN and is readily detected in the experiments. The radiatively stabilized association product, formed by a slow two‐body reaction, is not detected because it reacts with HCN by a fast proton transfer reaction forming the protonated HCN ion. Previous studies eit...

Reactions of the parent ions in vinyl chloride and vinyl fluoride as studied by ion cyclotron resonance

Abstract The absolute thermal energy rate constants for the reactions of the parent ions in C 2 H 3 Cl and C 2 H 3 F have been measured for the first time. The rate constants measured were 9.6 × 10 −11 and 2.1 × 10 −10 cm 3 molecule −1 sec −1 respectively. Relative rate constants have also been measured that agree with previously reported ion cyclotron resonance (ICR) values. The quantitative kinetic energy dependence of the above rate constants has been measured from thermal energies to above 6 eV. Previously published single source medium pressure work and tandem mass spectral results indicate that the reactions have two pathways; one second order and the other one third order (under their operating conditions) and the product distributions of the second and third order pathways were measurably different. Correlation has been made between the present work and the previously published work that indicates that the medium pressure source and tandem instruments create ions with a substantially larger amount of excess internal and/or kinetic energy than does ICR. A unified picture for the parent ion condensation reactions in C 2 H 3 Cl and C 2 H 3 F emerges when the different ion energetics are considered. Exact solutions of two possible kinetic schemes are presented and compared with published and present data.