Susan T. Graul

Gas-phase acidities of carboxylic acids and alcohols from collision-induced dissociation of dimer cluster ions

Abstract The gas-phase acidities (Δ G acid ) of fifteen carboxylic acids and six alcohols have been determined in a flowing afterglow—triple quadrupole instrument from a correlation of the relative yields of anions A − and B − produced by collisionally activated dissociation of the mixed dimer (AHB) − . The acidity of 2,3-butanedione has also been determined, using thermal energy proton transfer bracketing reactions.

A flowing afterglow-triple quadrupole study of the mechanisms and intermediates in the gas-phase reactions of CH3OH+2 with CH3OH

Abstract The detailed mechanism of formation of protonated dimethyl ether in ionized gaseous methanol has been investigated with a flowing afterglow-triple quadrupole instrument. The structures and unimolecular decomposition reactions of isomeric cluster ions related to those observed in ionized methanol have been examined as models for the proposed bimolecular reaction intermediates. Results from isotope labeling experiments provide evidence for backside nucleophilic attack in the reaction between CH 3 OH + 2 and CH 3 OH, producing (CH 3 ) 2 OH + and H 2 O. Collision-induced dissociation of the protonated methanol dimer and trimer results in both desolvation and formation of protonated dimethyl ether, the latter presumably via rearrangement of the collisionally activated proton-bound cluster ion to the backside displacement precursor. Both CH 3 OH) 2 H + and (CH 3 OH) 3 H + undergo inefficient but exothermic bimolecular displacement reactions with CH 3 OH, producing (CH 3 ) 2 OH + (CH 3 OH) and CH 3 ) 2 OH + (CH 3 OH) 2 , respectively.

Collisional activation of intramolecular nucleophilic displacement reactions: the formation of acetolactone from dissociation of α-haloacetate negative ions

Abstract The energetics and products of low energy collision-induced dissociation (CID) of a number of α-substituted acetate ions and β-substituted alkoxide ions have been investigated. Analysis of the appearance energies of the fragment ions produced yields limiting values for the endothermicities of the dissociation processes. These limits provide information about the neutral product and mechanism of dissociation. Several of these ions appear to undergo cyclization accompanied by intramolecular nucleophilic displacement in the course of dissociation. For example, the appearance energies measured for the production of X − by CID of XCH 2 CO − 2 (X = Cl, Br, I) indicate that the neutral C 2 H 2 O 2 product is the cyclic species acetolactone ( 1 ). Similarly, production of X − by dissociation of XCH 2 CH 2 O − (X = F, Cl, CH 3 O, CH 2 CHO) is accompanied by formation of the cyclic C 2 H 4 O species ethylene oxide ( 5 ).

Energy-resolved collision-induced dissociation of proton-bound cluster ions as a structural probe: the acetonitrile—water system

Abstract The energy-resolved collision-induced dissociation of proton-bound clusters of acetonitrile and water has been examined. Comparison are made with the fragmentations of mixed methanol—water and mixed dimethyl ether-water clusters. The cluster fragmentations display collision-energy dependences that can be related to the cluster ion structures. Our experimental results confirm predictions of a core H3O+ ion in (CH3CN)n(H2O)mH+ clusters with n + m = 3, 4, and further implicate a core H3O+ ion in the analogous dimethyl ether-water clusters. In contrast, a methanol molecule constitutes the protonated core in methanol-water clusters. Evidence for the production of pure water cluster ions by collision-induced dissociation of mixed acetonitrile—water cluster ions is also presented.

The dynamics of photodissociation of the gas phase N2O·H2O)+ cluster ion

Abstract The photodissociation dynamics of the positive cluster ion of N 2 O and H 2 O have been examined for the wavelength range 657–458 nm (1.89–2.71 eV). The major products at all wavelengths are N 2 O + H 2 O, which are formed by excitation to a repulsive upper surface followed by rapid dissociation. The next most abundant products are H 2 O + N 2 O, which are formed predominantly by rapid dissociation from a repulsive surface. At the longest wavelength (657 nm), there is evidence for a second mechanism for production of H 2 O + + N 2 O, possibly involving a bound excited state. A minor N 2 OH + + OH channel is observed at all wavelengths, and arises by vibrational predissociation in the ground state after photoexcitation to a bound excited state. Molecular orbital calculations indicate that at least two isomers of the cluster ion exist on the ground state potential energy surface. One isomer is best represented as N 2 O + · H 2 O and is a logical precursor for the N 2 O + /H 2 O products and the major portion of the H 2 O + /N 2 O products that is formed by a repulsive dissociation mechanism. The second isomer is N 2 OH + ·OH, probably the precursor for the N 2 OH + /OH products and perhaps a minor portion of the H 2 O + /N 2 O products observed from excitation at 657 nm. Phase space modeling of the N 2 OH + kinetic energy release distribution suggests a binding energy of about 1.2 eV for the N 2 OH + ·OH cluster. The relative binding energy from ab initio calculations suggests that the N 2 O + /H 2 O cluster is bound by about 0.5 eV. Mechanisms for formation of the observed photofragments are proposed.

Guided‐ion beam measurements of X++NO (X=Ar, N2) reactions

Cross section and product ion time‐of‐flight measurements are presented for the X++NO (X=Ar, N2) collision systems over a collision energy range of 0.1–20 eV (c.m.). The experiments are carried out in an octopole guided‐ion beam apparatus. Charge transfer is the main channel observed and dissociative charge‐transfer products are observed for collision energies equal to or greater than their respective thermodynamic thresholds. A weak channel leading to ArN+ and/or ArO+ is observed in the X=Ar system. The charge‐transfer cross sections of the two collision systems are similar in magnitude and have approximately an E−1/2 dependence at near‐thermal collision energies. The near‐thermal cross sections are significantly less than the Langevin–Gioumousis–Stevens predictions for a reaction mediated by ion–induced dipole interactions and complex formation. The time‐of‐flight distributions of the Ar+, N+2+NO charge‐transfer products are characteristic of near‐resonant charge‐transfer processes. At all collision ene...

Guided-ion beam measurements of the Kr+ + NO charge-transfer reaction

Abstract Guided-ion beam measurements of Kr + + NO charge-transfer reaction cross sections are presented over a collision energy range of 0.08 to 16 eV (c.m.). At energies below 2 eV, the charge-transfer cross section is approximately two orders of magnitude smaller than the Langevin capture cross section and decreases with collision energy displaying an E −0.4 T dependence. The cross section reaches a minimum at about 3 eV, increases with collision energy until about 8 eV, and then decreases abruptly at higher energies. N + and O + are observed at and above their respective thermodynamic thresholds. The results are discussed with respect to energy resonance and Franck-Condon models.

Selected ion flow tube studies of the atomic oxygen radical cation reactions with ethylene and other alkenes

Abstract The chemical ionization reaction of the atomic oxygen radical cation with ethylene have been investigated extensively at 300 K in 0.5 Torr of helium in a selected ion flow tube (SIFT). To help understand the ethylene data, five additional terminal alkenes (propene, isobutene, isoprene, styrene, and vinylidene chloride) were also examined. Considerable care was taken to account for the extremely high reactivity of O·+ (i.e. correction for reaction with trace levels of impurity in the helium) and the possibility of generation of electronically excited reactant ions. Ethylene was found to react on 93% of encounters, and to give 26% of its parent radical cation, 18% of the vinyl cation, 47% of acetylene radical cation, and 9% of protonated carbon monoxide. A mechanistic proposal for how these ions arise is presented. Extension of the mechanistic proposal for ethylene accounts for the reactivity observed for the other alkenes as well. Correlation of the yield of charge transfer product from each alkene with ionization energy (IE) suggests that primary event in the reaction is charge transfer. One exception is styrene: It has the lowest IE of the alkenes examined but the highest yield of molecular radical cation, leading to the suggestion that styrene may yield electronically excited ion products.

COMPETITIVE ELIMINATION OF METHANE AND ETHYLENE FROM TRIMETHYLSILYL-SUBSTITUTED SILYLENIUM IONS

Abstract Metastable dissociations have been studied for trimethylsilyldimethylsilylenium ion, (CH 3 ) 3 Si–Si + (CH 3 ) 2 , and four bridged analogs, (CH 3 ) 3 Si–X–Si + (CH 3 ) 2 (X = CH 2 , O, NH, CC). Several dissociation pathways are observed, with branching ratios that vary significantly with the bridging group. This article focuses on two competing pathways: elimination of methane and of ethylene. The energetics of ethylene elimination from (CH 3 ) 3 Si–Si + (CH 3 ) 2 have been characterized by molecular orbital and density functional theory, and the kinetic energy distributions of the products of the dissociation have been modeled with statistical phase space theory, yielding good agreement with experiment. Methane elimination occurs across the two silicon centers, and is accompanied by a large release of kinetic energy, suggestive of a concerted reaction and a considerable reverse activation barrier in the exit channel. Electronic structure calculations combined with statistical phase space modeling of the dissociation kinetics suggest that the product from methane elimination is a disilacyclobutyl cation. A mechanism for this elimination reaction is proposed. A comparison of the kinetic energy release distributions observed for methane elimination from the bridged ions suggests that an analogous mechanism is involved for the entire series.