Intracluster Reactions Research Articles

Intracluster Reactions of (CS2)n- and (OCS)n- Induced by Surface Impact

We investigated an intracluster reaction induced by impact of (CS 2 ) n - (n = 1-8) onto silicon and (La 0 . 7 Ce 0 . 3 )B 6 surfaces at collision energies of 100-2000 eV and 0-100 eV, respectively. Product anions scattered from the surfaces were mass analyzed by a tandem time-of-flight (TOF) mass spectrometer. In the impact onto the silicon surface, the intracluster reaction proceeds via (1) production of S - by dissociation of the core ion, CS 2 -, and (2) production of S m - (m = 2,3) by abstractions of S from surrounding CS 2 by S m - 1 -. In the impact onto a (La 0 . 7 Ce 0 . 3 )B 6 surface, essentially the same reaction mechanism operates except for the dimer reaction at collision energies less than 50 eV. The two isomers, CS 2 -(CS 2 ) and C 2 S 4 -, give S 2 -, and the latter dissociates further into C 2 S 2 -. A similar reaction mechanism is found to operate in the intracluster reaction of (OCS) n - .

Gas-phase ion chemistry of Glu/Met systems.

A combined chemical ionisation and tandem mass spectrometry (MS/MS) approach has been used for investigation of the gas-phase ion chemistry of systems containing the amino acids Glu and Met, and the dipeptides gamma-Glu-Met and Met-Glu. The metastable fragmentation of the protonated dimer, (Glu)2H(+), reveals an intracluster reaction leading to the elimination of the Glu residue. The main features of the ion-molecule reactions observed in the systems containing Glu and Glu + Met can be described in terms of sequential adduct formation. The results obtained for the thermal dehydration of Glu were used to rationalise the formation of the proton-bound structures (Glu-H2O...H(+)...(Glu-H2O) and (Glu-H2O)3-H(+). The adduct ions, [(Glu-H2O) + H + Glu](+) and [(Glu-H2O) + H + Met](+), and further association products were also observed. The results lead to a reconsideration of the structural aspects proposed earlier for these species in the sense that they suggest that the systems correspond to a mixture of isomeric covalent and proton-bound structures. The thermal effects on the decomposition of the neutral (gamma-Glu-Met) and its protonated form, (gamma-Glu-Met)H(+), at m/z 279 were investigated, and dramatic changes in the MI spectra of the m/z 279 ion with temperature were found. A mechanistic explanation for the observed evolution of higher mass ion peaks in the mass spectra is developed.

Intracluster Electron Transfer and Reactions in Alkali Metal−Methacrylate Clusters

Size-dependent stability and intracluster reactions have been investigated by photoionization mass spectrometry for alkali metal (M = Li, Na, and K)−methacrylate (CH2C(CH3)CO2R; RCH3 and C2H5) clus...

Intracluster electron transfer from a metal atom/cluster followed by anionic oligomerization of vinyl molecules

Intracluster electron transfer and oligomerization reaction were investigated by mass spectrometry of clusters of alkali metal atom (M) with acrylonitrile (AN; CH2=CHCN). In the photoionization mass spectra of M(AN)n, magic numbers were clearly observed at n = 3k (k = 1-4 for M = Na and K, k = 1 for M = Li). The results of photodissociation of neutral K(AN)n indicate that the n = 3 cluster has an anomalous stability relative to other sizes of clusters. The C=C bond in vinyl molecules is also found to be necessary to form the magic numbers by measuring the photoionization mass spectrum of K atom with propionitrile. These results strongly support the intracluster anionic oligomerization reaction initiated by electron transfer from the alkali atom. The quantum chemical calculations have revealed that the evaporation induced by excess energy generated by intracluster oligomerization is important to form the magic numbers in the present clusters.

Photodissociation of Mg +(NH 3) ion

Electronically excited states of Mg +(NH 3) are studied by photodissociation after mass selection. The dissociation spectrum shows relatively sharp vibronic transitions centered at about 28 000 and 36 000 cm −1 . These absorption bands are assigned to the 2 P– 2 S type transitions localized on the Mg + ion. In photodissociation, a photoinduced charge-transfer process to produce NH 3 + is observed in addition to evaporation and intracluster reaction processes to produce Mg + and MgNH 2 + ions, respectively. The mechanism for the production of these ions is discussed in terms of the predissociative and non-adiabatic interactions between the low-lying states.

Probing hydrogen bond network formation in anion–water clusters through high energy collision experiments

In high-energy collisions (50 keV in the laboratory system) between anion–water clusters [X−⋅(H2O)n, X−=Cl−, CN−, O2−, NO2−, NO3−; n=1–6] and argon, H3O+ is formed with an abundance that is determined by the cluster size—the larger the cluster, the more H3O+. The mechanism for H3O+ formation is believed to be a nearly vertical ionization process (two-electron stripping) followed by an exothermic intracluster hydrogen transfer reaction between an ionized water and a neutral water. The abundance of H3O+ can be used as a probe to determine how extended the water hydrogen bond network is in the initial anion–water cluster and to distinguish between surface solvation (water network preserved) and internal solvation (water network broken). In this work, it is demonstrated that in the hydration of hexacyanoplatinate(IV) dianion complexes, surface solvation is important despite the large number of available water binding sites; however, the competition between a cyanide-bound water and a “naked” cyanide ligand for a water molecule favors the cyanide-bound water because of the splitting of the excess charges between six ligands (between −1/3 and −1 charge at each ligand on average). We also investigate anion–methanol clusters in which the hydrogen bond network is less extended with the result of a less abundant oxonium ion compared to the hydronium ion from similar size water clusters.

Femtosecond time-resolved electron spectroscopy of the intracluster reaction in Ba⋯FCH [formula omitted

Applying femtosecond pump–probe spectroscopy combined with the coincidence detection of photoelectrons and photoions, we have analyzed the energetics of the photoinitiated intracluster reaction in Ba⋯FCH 3 on the femtosecond time scale. The rate-determining process of internal conversion from the initially excited à state of the complex to the next lower electronic à ′ state was directly observed within a few 100 fs after the initiation of the reaction by the 130 fs laser pulse at 618 nm. Thus, the theoretical analysis of the reaction which is based on the calculation of the corresponding potential energy surfaces was confirmed.

Gas-Phase Oligomerization of Propene Initiated by Benzene Radical Cation

Initiation of the radical cation polymerization of propene has been observed following the selective ionization of benzene in the gas phase by resonance two-photon ionization−high-pressure mass spectrometry (R2PI−HPMS) and by selected ion flow tube (SIFT) techniques. In this system, the aromatic initiator (C6H6) has an ionization potential (IP) between those of the reactant's monomer (C3H6) and its covalent dimer (C6H12), i.e, IP(C3H6) > IP(C6H6) > IP(C6H12). Therefore, direct charge transfer from C6H6•+ to C3H6 is not observed due to the large endothermicity of 0.48 eV, and only the adduct C6H6•+(C3H6) is formed. However, coupled reactions of charge transfer with covalent condensation are observed according to the overall process C6H6•+ + 2C3H6 → C6H12•+ + C6H6, which results in the formation of a hexene product ion, C6H12•+. The formation of this ion can make the overall process of charge transfer and covalent condensation significantly exothermic. At higher concentrations of propene, the reaction products are the propene oligomers (C3H6)n•+ with n = 2−7 and the adduct series C6H6•+(C3H6)n with n ≤ 6. The significance of the coupled reactions is that the overall process leads exclusively to the formation of the condensation product (C3H6)n•+ and avoids other competitive channels in the ion/molecule reactions of propene. Gas-phase nominal second-order rate coefficients for the overall reaction into both channels are in the range of (1−3) × 10-12 cm3 s-1. The rate coefficients into both channels, especially for the formation of the C6H12•+ dimer, have large negative temperature dependencies. Consistent with the gas-phase results, the intracluster reactions of C6H6•+ produced selectively by R2PI of mixed benzene/propene clusters also do not form the monomer ion C3H6•+ but form higher propene clusters (C3H6)n•+ that contain at least the C6H12•+ hexene ion. The similarity of the reaction mechanisms in the gas phase and in preformed clusters suggests that the mechanism may also apply in the condensed phase in common aromatic solvents such as benzene and toluene.

Intracluster anionic oligomerization of acrylic ester molecules initiated by electron transfer from an alkali metal atom.

Stabilities and intracluster reactions have been investigated by photoionization mass spectrometry for clusters composed of an alkali metal atom (M; Na and K) and acrylic ester molecules, CH(2)=CHCO(2)R, such as methyl acrylate (MA; R = CH(3)) and ethyl acrylate (EA; R = C(2)H(5)). The following two features are commonly observed in the photoionization mass spectra of M(CH(2)=CHCO(2)R)(n): (1) The ion with n = 3 is clearly observed as a magic number. (2) Fragmented cluster ions with the loss of ROH, [M(CH(2)=CHCO(2)R)(n) - ROH] are detected only for n = 3. These features are both explained by an intracluster oligomerization reaction initiated by electron transfer from the metal atoms. The magic number trimer is concluded to have the stable structure of cyclohexane derivatives as a result of oligomerization. The fragmentation reaction is explained by Dieckmann cyclization after anionic oligomerization to produce another isomer of the trimer. The intracluster electron transfer is also supported by theoretical calculation for Na(MA) based on density functional theory.

Mechanism of the laser initiated ultrafast intracluster reaction in Ba···FCH3 and Ba···FCD3

The dynamics of the intracluster reaction Ba···FCH3 → BaF + CH3 in the van der Waals complex of barium and monofluoromethane (CH3F or CD3F) initiated by resonant excitation to its electronic à state has been analyzed on the femtosecond timescale. To elucidate the mechanism we have compared the results of pump–probe experiments for the two complexes Ba···FCH3 and Ba···FCD3, both excited by 120 fs laser pulses at the resonant wavelength of 618 nm. The measured decay times of the parent ion signals are τIC = 270 ± 30 fs for Ba···FCH3 and τIC = 430 ± 20 fs for Ba···FCD3. The detailed analysis is based on the ab initio calculated potential energy surfaces of the parent complex. It is found that the reaction proceeds ia the next lower electronic Ã′ state of the complex which is populated after the initial à state excitation by internal conversion with the time constant τIC. The subsequent BaF formation out of the intermediate Ã′ state is restricted to very short times of about 50 fs for both parent clusters due to the competing dissociation of the complex (Ba···FCH3 → Ba + CH3F). Thus, the effective formation times for the BaF reaction products exceed only slightly the obtained time constants τIC for internal conversion.

The Transition State: A Theoretical Approach Edited by Takayuki Fueno (Osaka University and Aichi Institute of Technology). Kodansha: Tokyo and Gordon Breach Publishers: Amsterdam. 1999. xvi + 328 pp. $120.00. ISBN 90-5699-216-3

The transition state is the critical configuration of a reaction system situated at the highest point of the most favorable reaction path on the potential-energy surface, its characteristics governing the dynamic behavior of reacting systems decisively. This text presents an accurate survey of current theoretical investigations of chemical reactions, with a focus on the nature of the transition state. Its scope ranges from general basic theories associated with the transition states, to their computer-assisted applications, through to a number of reactions in a state-of-the-art fashion. It covers various types of gas-phase elementary reactions, as well as some specific types of chemical processes taking place in the liquid phase. Also investigated is the recently developing transition state spectroscopy. Chapter 1. Introduction; Chapter 2. Determination of Transition State Structures on Potential Energy Surfaces; Chapter 3. Molecular Symmetry and Transition State; Chapter 4. Transition State Theoretical Calculations of the Canonical Rate Constants for Bimolecular Reactions; Chapter 5. Development of the Microcanonical Statistical Rate Theory for Unimolecular Reactions; Chapter 6. Intracluster Reaction Dynamics of Ar{sub 4}{sup +}; Chapter 7. Transition State for Chemical Reactions in Solution; Chapter 8. Structures and Reactions of Compounds Containing Heavier Main Group Elements; Chapter 9. Transition States in Organometallicmore » Reactions; Chapter 10. Chemical Reaction Dynamics and Potential Ridge: Beyond the Transition State; Chapter 11. Differential Geometry in Chemical Reaction Dynamics; Chapter 12. Toward Transition State Spectroscopy: Experimental Approaches Using Weakly Bonded Clusters; Chapter 13. Relativistic Effects on Transition State Structures and Properties: Transition State Spectroscopy of IHI and BrHI; Chapter 14. A Time-Dependent Theoretical Approach to Transition State Spectroscopy; Chapter 15. Scattering Theory for Photodetachment and Molecular Dissociation as a Direct Probe of Transition State.« less

Intracluster reactions of singly ionised magnesium atoms with dimethyl ether

Clusters of magnesium and dimethyl ether (CH 3OCH 3) have been prepared in a supersonic expansion pick-up source. Solvated ions Mg +(dimethyl ether) n have been observed for n=2 and higher. Surprisingly, the single-photon ionisation process is very inefficient to form these clusters. Reaction products have been found to arise from an ion-molecule reaction where Mg + (3s 2 S) attacks the methyl oxygen bond in dimethyl ether. This reaction is made possible by addition of solvent molecules in the cluster, and the products are found with a maximum population when solvated by four molecules. This process, already observed in collisions of similar clusters and magnesium ions, is manifest here as an intracluster reaction.

Dissociative photoionisation of acetylene-ethane van der Waals clusters

Intracluster ion–molecule reactions have been studied by dissociative photoionization of mixed acetylene–ethane van der Waals clusters in a supersonic molecular beam. The studies were carried out by recording threshold photoelectron–photoion coincidence mass spectra of the clusters at different photon energies between 10.8 and 11.3 eV. Intracluster reactions have been observed in mixed dimer, trimer, pentamer, and hexamer clusters. In the case of the mixed dimer ion [(C 2H 2)(C 2H 6)] +, two fragment ions result from intramolecular reactions: C 3H 5 + (CH 3 loss), and C 2H 4 + (C 2H 4 loss). This indicates that the dimer ion isomerizes into the 2-butene ion prior to dissociation, as is the case for ethylene dimer ions. Mixed trimer ions also dissociate by CH 3 loss. No intracluster hydride transfer prior to dissociation is observed, although this reaction channel is important in bimolecular C 2H 2 + + C 2H 6 collisions. Two very specific intracluster reactions leading to C 2H 4 loss were observed for pentamer and hexamer mixed cluster ions. A model involving evaporation and intracluster reactions leading to CH 3 and C 2H 4 loss is proposed to explain this surprising result.

Exciplex formation reaction in benzaldehyde-1,4-dimethoxybenzene clusters

It was demonstrated that the intracluster exciplex formation reaction occurs by the laser excitation of benzaldehyde (BA) in BA/1,4-dimethoxybenzene (DMB) clusters. The exciplex formation was inferred from spectral broadening, the red-shifted emission spectrum from BA monomer fluorescence and the linear laser power dependence of the emission intensity. The exciplex emission appears in the region from 400 to 600 nm and its lifetime was estimated to be less than 20 ns which is remarkably shorter than the fluorescence lifetime of the BA monomer (>1 μs). Exciplex formation dynamics in the clusters is discussed on the basis of the fluorescence excitation spectrum of the exciplex and the fluorescence lifetime.

Multiphoton lonization and Ab lnitio Calculation of C<sub>4</sub>H<sub>5</sub>N-(NH<sub>3</sub>)<sub>n</sub> Hydrogen-bond Clusters

Multiphoton ionization of binary clusters (C4H5N) -NH3 at 355 and 532 nm have been investigated using a TOF mass spectrometer, The experimenal results showed that normal cluster ions and protonated cluster ions were produced at both laser wavelengths. The protonated products came from the intracluster proton transfer reaction. The existence of NH3 in the clusters increased the ionization cross sections of the clusters, The higher ionization efficiency at 355 nm results from the (2 + 1) resonance multiphoton ionization of the pyrrole molecule. A peculiar low abundance of (C4H5N)- (NH3)(2)(+) was observed, ab initio calculations indicated that when ammonia number n greater than or equal to 3, two hydrogen bonds between ammnia chain and pyrrole ring can be formed, and the cluster become more stable. The channel ratio for the formation of protonated and unprotonated products increases with the increase of n, in agreement with the trend of proton affinity of (C4H5N) - (NH3)(n),

Electron attachment to nitric oxide clusters and electron impact ionization of carbon monoxide clusters

The formation of both positively and negatively charged ions after interaction of electrons with different cluster beams is investigated, by the use of highly monochomatized electron beams. In the case of the electron attachment to neutral NO clusters, the formation of the monomer ion NO− could be observed for the first time. The only explanation for the creation of such an ion is an intracluster reaction. In the case of carbon monoxide clusters, the appearance energies were determined with high accuracy. From these data, we derived the binding energy of the cluster dinier. Our results are in good agreement with photoionization studies.

Spectroscopic studies of mass selected clusters of Sr+ solvated by H2O and D2O

In this paper we present the results of spectroscopic studies on mass selected Sr+(H2O)n, n=1–4 and Sr+(D2O)n, n=1–6 clusters. Mass spectra of nascent clusters formed in our laser vaporization source show that hydrated metal ion species are predominant for n⩽4. Clusters larger than this size are more abundant in the hydrogen loss form SrOH+(H2O)n−1. The cluster size at which the product switching occurs is slightly larger (n=5) in the deuterated species. Photodissociation of all clusters results in both ligand loss and H/D atom loss occurring via an intracluster reaction. The monomer and dimer cluster species exhibit distinct absorption bands attributable to electronic excitation of the 5s valence electron of Sr+. Metal–ligand stretching frequencies are extracted from Franck–Condon progressions in the excited state. Spectroscopic parameters agree well with ab initio calculations [Bauschlicher et al., J. Chem. Phys. 96, 4453 (1992), and Sodupe et al., Chem. Phys. Lett. 212, 624 (1993)]. Analysis of the product branching ratios allows us to conclude that, when below the threshold for excited state dissociation, rapid internal conversion followed by an intracluster reaction preferentially occurs on the ground state surface, while evaporation occurs primarily in the excited state. We find evidence in the spectral profiles of clusters with n⩾3 for a consistent “substructure” characterized by a pπ state lacking σ-type interactions with the ligand molecules.

Multiphoton ionization and fragmentation processes of methyl iodide clusters at 266 and 355 nm

The multiphoton ionization (MPI) and dissociation dynamics of methyl iodide clusters at 266 and 355 nm are studied using a reflectron time-of-flight (RTOF) mass spectrometer. The excitation wavelengths were chosen to access the repulsive valence A state and the Rydberg C state. MPI mass spectrum at 266 nm consists mostly of fragment ions, CH 3 +, I +, and I 2 +. This is consistent with a ladder-switching mechanism in which rapid one-photon dissociation occurs from the repulsive potential energy surface followed by multiphoton ionization of neutral photofragments. The observation of considerable amount of parent ion and methyl-rich fragments (CH 3I) n CH 3 + ( n = 1–9) at 355 nm excitation is indicative of the ionic-ladder process in which the parent ions are produced directly by absorbing three photons and then dissociates through an additional one-photon absorption to produce fragment ions. These processes are further supported by the laser power dependence of product ions. The results are interpreted in terms of the electronic state diagram of the methyl iodide and effects resulting from intracluster chemical reactions.

Three-Body Dissociation Dynamics of Excited States of O3(D2O)

In these experiments, the dynamics of the three-body dissociative photodetachment of O3-(D2O) at 258 nm are directly probed for the first time. Photodetachment of a negatively charged precursor, with coincident energy analysis of the photoelectron, allows production of energy-selected excited-state O3(D2O) complexes. By measurement of the laboratory velocities and recoil angles of the O + O2 + D2O products simultaneously, a kinematically complete description of the three-body dissociation dynamics of O3(D2O) is obtained. The results show that clustering of D2O to O3- stabilizes the system by 0.75 ± 0.09 eV. Photodetachment to the triplet states of O3 in the complex results in three-body dissociation, with no clear evidence observed for quenching or intracluster reaction in the complex. The molecular-frame differential cross section (MF-DCS), showing how the three products scatter in the molecular frame, is presented and discussed in light of DFT calculations of possible equilibrium geometries of the parent O3-(D2O) anion.

The reaction of O(1D) with H2O, D2O monomers and clusters and the intracomplex reaction in N2O–X2O (X=H,D) photo-initiated at 193 and 212.8 nm

The reaction of O(1D) with water and water clusters was re-examined. We monitored the nascent product state distributions in the reaction photo-initiated by the dissociation of N2O at 193 and 212.8 nm, and the corresponding photo-initiated intracluster reaction. The study at two different dissociation wavelengths and the use of D2O allowed us to obtain direct information on the effect of initial collision kinetic energy on the energy distribution in the product. Based on the new results obtained we conclude that the reaction of O(1D) with water occurs through abstraction mechanism with a relatively short lived collision complex. In the case of the intracluster reaction, we have indication that more internal energy is deposited in the N2 moiety, compared to the dissociation of an isolated N2O. In addition the results indicate that the reaction between the oxygen atom and the water in the complex involved the formation of a short lived collision complex, with a lifetime of probably only few rotations of OH.