Statistical theory of endothermic reactions

The rate of unimolecular decomposition of an excited molecule is calculated from the statistical theory based on conservation of the total energy and total momentum. The limits of applicability of the quasiequilibrium theory are examined. Distribution functions are calculated for the dissociation products from photon capture and electron impact.     

State selected ion-molecule reactions

A summary is given of recent state selected experimental data on charge transfer in the system [N₂+Ar]⁺. New results are reported on the reaction of Ar⁺(² P _J )+N₂, obtained at Orsay by threshold photoelectron-photoion coincidence techniques employing synchrotron radiation. Recent theoretical models dealing with [N₂+Ar]⁺ charge transfer are briefly discussed in regard to their capability to account for the most characteristic experimental observations.     

Gas-phase ion-molecule reactions in organophosphorus esters

Self-condensation ion-molecule reactions of trimethyl phosphite, triethyl phosphite, dimethyl phosphonate, trimethyl phosphate and 2, 2-dichlorovinyl dimethyl phosphate (dichlorvos) were investigated by ion trap mass spectrometry and Fourier transform ion cyclotron resonance mass spectrometry. Reaction paths for the main processes observed were elucidated by parent ion selection and for reaction times up to 500 ms. In parallel, high-resolution measurements were performed in order to determine the composition of the principal ions. Among the compounds under examination, trimethyl phosphite and triethyl phosphite mainly give [M + H](+) and [M + (RO)(2)P](+) (R = CH(3), C(2)H(5)) adduct ions, whereas trimethyl phosphate and dimethyl phosphonate display [2M + H](+) ions, as the only abundant products, formed by reaction of [M + H](+) and M. 2,2-Dichlorovinyl dimethyl phosphate mostly shows fragmentation processes. The reaction patterns of the compounds examined were related to their different structural features. Gas-phase basicities of the phosphoryl compounds were also determined or re-examined. Copyright 1999 John Wiley & Sons, Ltd.     

Intra-cluster ion-molecule reactions and photodissociation of molecular clusters

Zeitschrift für Physik D Atoms,Molecules and Clusters - Intra-cluster ion-molecule reactions in benzene clusters and benzene-water binary clusters were observed in Resonance-Enhanced...     

On the importance of ions and ion-molecule reactions to plasma-surface interface reactions

Ions are known to be key players in many plasma processes, including anisotropic etching, film deposition and surface modification. The relationship between plasma ions, film properties, and surface interactions of other plasma species is not, however, well known. Using our Imaging of Radicals Interacting with Surfaces (IRIS) technique, along with plasma ion mass spectrometry (PI-MS), and surface analysis data, we have measured the effects of ion bombardment on the surface interactions of SiF2 in SiF4 plasmas and of CF2 in C3F8 and C4F8 plasmas. SiF2 is a known product of F-atom etching of Si, and CF2 has also been cited as a product of fluorocarbon etching of Si. With both molecules, we measure surface generation when the surface is bombarded by all the plasma species. Removal of ions from the plasma molecular beam results in a net decrease in surface generation for both molecules at all powers. Results in both systems are compared with the gas-phase ion-molecule reaction data of Armentrout and coworkers. Preliminary guided-ion beam mass spectrometry results taken in the Armentrout laboratories for the Ar+ + C3F8 reaction system are also presented.     

Statistical theory of ion-molecular reactions with formation of three particles

Processes with the formation of three particles in the final channel have been investigated within the scope of the statistical theory of ion-molecular reactions proceeding through an intermediate complex. The probability of decomposition of the complex by a three-particle channel was expressed through the probability of its formation by this channel. A model of successive paired collisions was examined for probability of complex formation by three-particle channel. Threshold behavior of the reaction cross sections wasexamined in a simple model.In conclusion, the authors express their sincere gratitude to N. N. Tunitskii for his interest in the work and useful discussions.     

A theoretical method of researching ion-molecule reactions in solution

A method has been devised for researching the mechanisms of ion-molecule reactions in aqueous solutions, which involves deriving an approximate reaction path by means of a model Hamiltonian, in which the medium is described by a set of point Langevin dipoles. At the stationary points on the PES derived in that way, the solvation may be simulated by means of a more accurate scheme for the medium, namely by direct optimization of the potential energy for the solvate shell. The method has been tested on the reaction CO₂+OH^–HCO₃ ^–. The model Hamiltonian has been constructed by MINDO/3. Good agreement with experiment is obtained.Translated from Teoreticheskaya i Éxperimental'naya Khimiya, Vol. 23, No. 3, pp. 281–288, May–June, 1987.     

Selective ion-molecule reactions of lactams with dimethyl ether ions

The ion-molecule reactions of dimethyl ether ions CH₃OCH₃ ⁺ and (CH₃OCH₃)H⁺, and four- to seven-membered ring lactams with methyl substituents in various positions were characterized by using a quadrupole ion trap mass spectrometer and a triple-quadrupole mass spectrometer. In both instruments, the lactams were protonated by dimethyl ether ions and formed various combinations of [M + 13] ⁺, [M + 15] ⁺, and [M + 45] ⁺ adduct ions, as well as unusual [M + 3] ⁺ and [M + 16] ⁺ adduct ions. An additional [M + 47] ⁺ adduct ion was formed in the conventional chemical ionization source of the triple-quadrupole mass spectrometer. The product ions were isolated and collisionally activated in the quadrupole ion trap to understand formation pathways, structures, and characteristic dissociation pathways. Sequential activation experiments were performed to elucidate fragment ion structures and stepwise dissociation sequences. Protonated lactams dissociate by loss of water, ammonia, or methylamine; ammonia and carbon monoxide; and water and ammonia or methylamine. The [M + 16] ⁺ products, which are identified as protonated lactone structures, are only formed by those lactams that do not have an N-methyl substituent. The ion-molecule reactions of dimethyl ether ions with lactams were compared with those of analogous amides and lactones.     

Statistical theory for the kinetics and dynamics of roaming reactions

We present a statistical theory for the effect of roaming pathways on product branching fractions in both unimolecular and bimolecular reactions. The analysis employs a separation into three distinct steps: (i) the formation of weakly interacting fragments in the long-range/van der Waals region of the potential via either partial decomposition (for unimolecular reactants) or partial association (for bimolecular reactants), (ii) the roaming step, which involves the reorientation of the fragments from one region of the long-range potential to another, and (iii) the abstraction, addition, and/or decomposition from the long-range region to yield final products. The branching between the roaming induced channel(s) and other channels is obtained from a steady-state kinetic analysis for the two (or more) intermediates in the long-range region of the potential. This statistical theory for the roaming-induced product branching is illustrated through explicit comparisons with reduced dimension trajectory simulations for the decompositions of H(2)CO, CH(3)CHO, CH(3)OOH, and CH(3)CCH. These calculations employ high-accuracy analytic potentials obtained from fits to wide-ranging CASPT2 ab initio electronic structure calculations. The transition-state fluxes for the statistical theory calculations are obtained from generalizations of the variable reaction coordinate transition state theory approach. In each instance, at low energy the statistical analysis accurately reproduces the branching obtained from the trajectory simulations. At higher energies, e.g., above 1 kcal/mol, increasingly large discrepancies arise, apparently due to a dynamical biasing toward continued decomposition of the incipient molecular fragments (for unimolecular reactions). Overall, the statistical theory based kinetic analysis is found to provide a useful framework for interpreting the factors that determine the significance of roaming pathways in varying chemical environments.     

In-source CID of guanosine: Gas phase ion-molecule reactions

In-source collision induced dissociation was applied to access second generation ions of protonated guanosine. The in-source gas-phase behavior of [BH2]+-NH3 (m/z 135, C5H3N4O+) was investigated. Adduct formation and reactions with available solvent molecules (H2O and CH3OH) were demonstrated. Several addition/elimination sequences were observed for this particular ion and solvent molecules. Dissociation pathways for the newly formed ions were developed using a QqTOF mass spectrometer, permitting the assignment of elemental compositions of all product ions produced. Reaction schemes were suggested arising from the ring-opened intermediate of the protonated base moiety [BH2]+, obtained from fragmentation of guanosine. The mass spectral data revealed that the in-source CH3OH-reaction product underwent more complex fragmentations than the comparable ion following reaction with H2O. A rearrangement and a parallel radical dissociation pathway were discerned. Apart from the mass spectrometric evidence, the fragmentation schemes are supported by density functional theory calculations, in which the reaction of the ring-opened protonated guanine intermediate with CH3OH and a number of subsequent fragmentations were elaborated. Additionally, an in-source transition from the ring-opened intermediate of protonated guanine to the ring-opened intermediate of protonated xanthine was suggested. For comparison, a low-energy collision induced dissociation study of xanthosine was performed. Its dissociation pathways agreed with our assumption.     

Microscopic cross section for ion-molecule reactions with polar molecules

Using Kapitsa's method of separating rapidly varying processes from slower translational motions, a microscopic capture cross section for gas phase ion-molecule reactions is derived (centrifugal barrier approximation), leading to rate coefficients lying between the values of Langevin's theory and those of the locked dipole approximation (LD).

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, , - ( ), , , , LD-.

     

Modified statistical theory of energy transfer in ion-molecule collisions

The modified statistical theory developed previously for potentials appropriate to interactions in neutral-neutral collisions, is now extended to more strongly attractive potentials involved in ion-neutral collisions. The model system is the collisional deactivation of C₅H₉⁺ by a variety of both polar and non-polar neutral molecules. A 12 - 6 - 4 potential is used for ion interaction with non-polar neutrals, and a 12 - 6 - 4 - 2 potential, as modified by Su and Bowers to take into account the rotational energy of the neutral, for interaction with polar neutrals. Calculated is (ΔE), the average energy lost by the ion in a collision, and compared with experiment. For C₅H₉⁺-CH₄ collisions, the calculated (ΔE) agrees with experiment within 5%. Predictions of the theory, namely that (ΔE) should increase with excitation energy and should decrease with the size of the excited reactant, are found to be in fair agreement with the somewhat ambiguous experimental evidence.     

Secondary ion-molecule reactions in matrix-assisted laser desorption/ionization

Ion-molecule charge- and proton-transfer reactions in the desorption plume are considered for the case of matrix-assisted laser desorption/ionization (MALDI) with ultraviolet laser excitation, and it is proposed that they are major determinants of the observed mass spectrum. Specific MALDI phenomena which are discussed include the dominance of singly charged ions and analyte-matrix or analyte-analyte signal suppression. Should any be formed, highly charged products can be reduced by reaction with neutral matrix, yet singly charged ions cannot generally be neutralized in the same manner. Ion suppression effects can also be explained by similar reactions, which in some cases involve interconversion of dissimilar ion types. The plume is proposed often to be more under thermodynamic rather than kinetic control owing to these secondary reactions. UV/MALDI mass spectra should therefore be largely predictable, given sufficient thermodynamic information, and appropriate experimental conditions of sufficient analyte and plume density. Copyright 2000 John Wiley & Sons, Ltd.     

Gas-phase ion-molecule reactions of small nitroalkanes and their deprotonated anions

Gas-phase reactions of nitromethane (1), nitroethane (2), 2-nitropropane (3), 2-methyl-2-nitropropane (4) and nitrocyclopropane (5) were studied at 300 K using the flowing afterglow technique. These nitroalkanes react with gas-phase bases HO(-), CH(3)O(-) and HOO(-) very rapidly with rate coefficients of (2.5-4.3) x 10(-9) cm(3) s(-1) and reaction efficiencies of 60-100%, for example, k = 3.2 x 10(-9) cm(3) s(-1) (86%) for 5 reacting with hydroperoxide anion. Proton transfer (PT) is the only reaction observed for 1 while elimination (E2) is the exclusive pathway for 4 yielding isobutene and NO(2)(-). Both PT and E2 reactions are observed for 2, 3 and 5, the former being the major pathway. Deprotonated anions of 1, 2, 3 and 5 were subjected to reactivity studies with CH(3)I, CO(2), CS(2) and SO(2). Nucleophilic substitution (S(N)2) reaction occurs with CH(3)I while characteristic products CS(2)O(-) and SO(3)(-) are formed from CS(2) and SO(2), respectively, along with competing adduct formation. The SN(2) rate is greater, whereas the reactivities with the triatomic reagents are smaller for deprotonated nitrocyclopropane than for the other acyclic anions. These observations strongly suggest that the reactions of nitroalkane [M - H](-) anions occur through initial attack from the terminal oxygen; the nitrocyclopropane carbanion is more strained and, thus, less stabilized by resonance [R(2)C(-) - NO2 <--> R(2)=NO(2)(-)] resulting in the greater basicity/nucleophilicy and the less negative charge on the oxygen site.     

Fragmentation of organic anions induced by exothermic addition reactions

The addition reactions of a series of carbanions with CO₂, COS and CS₂ have been studied in a flowing afterglow apparatus. Carbon dioxide simply forms the adduct, but the more highly exothermic additions to COS and CS₂ lead both to addition and addition followed by fragmentation. A number of novel fragmentation pathways have been observed for additions to diazo anions and to anions derived by proton abstraction from allenes. In addition to these reactions, highly basic anions are observed to undergo sulfur atom transfer reactions with CS₂.     

Gas-phase ion-molecule reactions of the CH3O+ cation

The methoxy cation, CH₃0⁺, formed by collision-induced charge reversal of methoxr anions with a kinetic energy of 8 keY, has been differentiated from the isomenric CH₂OH⁺ ion by performing low kinetic energy ion-molecule reactions In the radiofrequency-only quadrupole of a reverse-geometry double-focusing quadrupole hybrid mass spectrometer. The methoxy cation reacts with CH₃SH, CH₃?CH=CH₂, (CH₃)₂O, and CH₃CH₂Cl by electron transfer, whereas the CH₂OH⁺ ion reacts by proton transfer with these substrates     

Measuring H-bonding in supramolecular complexes by gas phase ion-molecule reactions

H/D and guest-exchange ion-molecule reactions have been used as a new tool to elucidate the operation of multiple hydrogen bonding in gas-phase complexes formed between phosphonate cavitands and ethyl-substituted ammonium ions.