Harpoon Mechanism Research Articles

Organic Molecules Mimic Alkali Metals Enabling Spontaneous Harpoon Reactions with Halogens.

The harpoon mechanism has been a milestone in molecular reaction dynamics. Until now, the entity from which electron harpooning occurs has been either alkali metal atoms or non-metallic analogs in their excited states. In this work, we demonstrate that a common organic molecule, octamethylcalix[4] pyrrole (omC4P), behaves just like alkali metal atoms, enabling the formation of charge-separated ionic bonding complexes with halogens omC4P+ ⋅ X- (X=F-I, SCN) via the harpoon mechanism. Their electronic structures and chemical bonding were determined by cryogenic photoelectron spectroscopy of the corresponding anions and confirmed by theoretical analyses. The omC4P+ ⋅ X- could be visualized to form from the reactants omC4P+X via electron harpooning from omC4P to X at a distance defined by the energy difference between the ionization potential of omC4P and electron affinity of X.

Theoretical Study of the Photochemical Mechanisms of the Electronic Quenching of NO(A2Σ+) with CH4, CH3OH, and CO2.

The electronic quenching of NO(A2Σ+) with molecular partners occurs through complex non-adiabatic dynamics that occurs on multiple coupled potential energy surfaces. Moreover, the propensity for NO(A2Σ+) electronic quenching depends heavily on the strength and nature of the intermolecular interactions between NO(A2Σ+) and the molecular partner. In this paper, we explore the electronic quenching mechanisms of three systems: NO(A2Σ+) + CH4, NO(A2Σ+) + CH3OH, and NO(A2Σ+) + CO2. Using EOM-EA-CCSD calculations, we rationalize the very low electronic quenching cross-section of NO(A2Σ+) + CH4 as well as the outcomes observed in previous NO + CH4 photodissociation studies. Our analysis of NO(A2Σ+) + CH3OH suggests that it will undergo facile electronic quenching mediated by reducing the intermolecular distance and significantly stretching the O-H bond of CH3OH. For NO(A2Σ+) + CO2, intermolecular attractions lead to a series of low-energy ON-OCO conformations in which the CO2 is significantly bent. For both the NO(A2Σ+) + CH3OH and NO(A2Σ+) + CO2 systems, we see evidence of the harpoon mechanism and low-energy conical intersections between NO(A2Σ+) + M and NO(X2Π) + M. Overall, this work provides the first detailed theoretical study on the NO(A2Σ+) + M potential energy surface of each of these systems and will inform future velocity map imaging experiments.

An "inverse" harpoon mechanism.

Electron-transfer reactions are ubiquitous in chemistry and biology. The electrons’ quantum nature allows their transfer across long distances. For example, in the well-known harpoon mechanism, electron transfer results in Coulombic attraction between initially neutral reactants, leading to a marked increase in the reaction rate. Here, we present a different mechanism in which electron transfer from a neutral reactant to a multiply charged cation results in strong repulsion that encodes the electron-transfer distance in the kinetic energy release. Three-dimensional coincidence imaging allows to identify such “inverse” harpoon products, predicted by nonadiabatic molecular dynamics simulations to occur between H2 and HCOH2+ following double ionization of isolated methanol molecules. These dynamics are experimentally initiated by single-photon double ionization with ultrafast extreme ultraviolet pulses, produced by high-order harmonic generation. A detailed comparison of measured and simulated data indicates that while the relative probability of long-range electron-transfer events is correctly predicted, theory overestimates the electron-transfer distance.

Energy Transfer in the 2 u (1 D 2) Ion-Pair State of I2 by Inelastic Collisions with Noble Gas Atoms.

We investigated the energy transfer in the 2u (1D2) ion-pair state of I2 by collision with noble gas atoms, Ar, Kr, and Xe, using an optical–optical double resonance/fluorescence detection technique. By analyzing the temporal profiles of the emission from the laser-excited 2u (1D2) state at various noble gas pressures, the quenching rate constants were determined to be (4.55 ± 0.42) × 10–10, (4.23 ± 0.11) × 10–10, and (6.83 ± 0.16) × 10–10 cm3 molecule–1 s–1 for quenching by Ar, Kr, and Xe, respectively. The 2g (1D2) ion-pair state, lying in the vicinity of the 2u (1D2) state, was identified as a destination state by collision with Ar and Kr. Collision with Xe provided a new reactive pathway forming the excimer XeI(B). The rate constants were determined to be = (9.61 ± 0.63) × 10–11 cm3 molecule–1 s–1 and = (4.87 ± 0.34) × 10–11 cm3 molecule–1 s–1 for the formation of the 2g (1D2) state by collision with Ar and Kr, respectively, and = (6.55 ± 0.19) × 10–11 cm3 molecule–1 s–1 for the formation of XeI(B). The collisional cross sections calculated from the quenching rate constants were considerably larger than the molecular size, owing to the harpoon mechanism.

Stereodynamic Control of Collision-Induced Nonadiabatic Dynamics of NO (A2Σ+) with H2, N2, and CO: Intermolecular Interactions Drive Collision Outcomes.

Intermolecular interactions, stereodynamics, and coupled potential energy surfaces (PESs) all play a significant role in determining the outcomes of molecular collisions. A detailed knowledge of such processes is often essential for a proper interpretation of spectroscopic observations. For example, nitric oxide (NO), an important radical in combustion and atmospheric chemistry, is commonly quantified using laser-induced fluorescence on the A2Σ+ ← X2Π transition band. However, the electronic quenching of NO (A2Σ+) with other molecular species provides alternative nonradiative pathways that compete with fluorescence. While the cross sections and rate constants of NO (A2Σ+) electronic quenching have been experimentally measured for a number of important molecular collision partners, the underlying photochemical mechanisms responsible for the electronic quenching are not well understood. In this paper, we describe the development of high-quality PESs that provide new physical insights into the intermolecular interactions and conical intersections that facilitate the branching between the electronic quenching and scattering of NO (A2Σ+) with H2, N2, and CO. The PESs are calculated at the EOM-EA-CCSD/d-aug-cc-pVTZ//EOM-EA-CCSD/aug-cc-pVDZ level of theory, an approach that ensures a balanced treatment of the valence and Rydberg electronic states and an accurate description of the open-shell character of NO. Our PESs show that H2 is incapable of electronically quenching NO (A2Σ+) at low collision energies; instead, the two molecules will likely undergo scattering. The PESs of NO (A2Σ+) with N2 and CO are highly anisotropic and demonstrate evidence of electron transfer from NO (A2Σ+) into the lowest unoccupied molecular orbital of the collision partner, that is, the harpoon mechanism. In the case of ON + CO, the PES becomes strongly attractive at longer intermolecular distances and funnels population to a conical intersection between NO (A2Σ+) + CO and NO (X2Π) + CO. In contrast, for ON + N2, the conical intersection is preceded by an ∼0.40 eV barrier. Overall, our work shines new light into the impact of coupled PESs on the nonadiabatic dynamics of open-shell systems.

Radiative lifetimes and self-quenching rate constants of the ion-pair states of halogen molecules

A series of charge-transfer excited states of halogen molecules is known as ion-pair states. The radiative lifetime and self-quenching rate constant for a certain excited state are among the most important physical properties representing the concerned ion-pair state. Thus, the determination of these parameters is indispensable for discussing the radiative and nonradiative dynamics in the excited manifolds under collision conditions. In the present study, the optical–optical double resonance spectroscopy technique was used to measure the fluorescence lifetimes and self-quenching rate constants of the ion-pair states of halogen molecules. The fluorescence lifetimes and self-quenching constants of five ion-pair states for Cl2 as well as one ion-pair state for Br2 were newly determined. For I2, fluorescence lifetimes of five ion-pair states and self-quenching rate constants of thirteen ion-pair states were evaluated for the first time. The lifetimes and self-quenching constants of all 18 ion-pair states of I2 so far identified spectroscopically were determined. The collision-free lifetimes of the ion-pair states range from several nanoseconds to several tens of nanoseconds, except for thef′0g+(1D2) state, which are shorter than those of lower-valence electronic states. All ion-pair states of halogen molecules have very large self-quenching cross-sections, which can be explained by the harpoon mechanism.

The role of near resonance electronic energy transfer on the collisional quenching of NO (A2Σ+) by C6H6 and C6F6 at low temperature

We present measurements of the collisional fluorescence quenching cross section of NO (A2Σ+) by benzene and hexafluorobenzene at a series of temperatures using a custom built slow flow cell and a pulsed de Laval nozzle test cell. The measurements show a deviation from the common harpoon mechanism of collisional fluorescence quenching, specifically the large quenching cross section of 147 Å2 at 300 K measured for benzene which is not predicted by the harpoon mechanism due to the low electron affinity of benzene (−1.5 eV). We speculate that the quenching of NO (A2Σ+) by benzene and hexafluorobenzene occurs through near resonance electronic energy transfer from the NO A-state to the A-state of the quencher via a pseudo-emission process. The predicted T−1/3 dependence of the cross section for this model is consistent with the experimental observations.

Reactivity of Metal Clusters.

We summarize here the research advances on the reactivity of metal clusters. After a simple introduction of apparatuses used for gas-phase cluster reactions, we focus on the reactivity of metal clusters with various polar and nonpolar molecules in the gas phase and illustrate how elementary reactions of metal clusters proceed one-step at a time under a combination of geometric and electronic reorganization. The topics discussed in this study include chemical adsorption, addition reaction, cleavage of chemical bonds, etching effect, spin effect, the harpoon mechanism, and the complementary active sites (CAS) mechanism, among others. Insights into the reactivity of metal clusters not only facilitate a better understanding of the fundamentals in condensed-phase chemistry but also provide a way to dissect the stability and reactivity of monolayer-protected clusters synthesized via wet chemistry.

Collision induced state-to-state energy transfer dynamics between the 2u ((1)D2) and 2g ((1)D2) ion-pair states of I2.

We report the first observation of collision induced state-to-state energy transfer from the 2u ((1)D2) (v2u = 3-7) ion-pair state of I2 using a perturbation facilitated optical-optical double resonance technique through the c (1)Πg∼ B (3)Π(0) hyperfine mixed double-faced valence state as the intermediate state. The excitation of the 2u ((1)D2) state yielded the weak UV fluorescence from the wide range of vibrational levels in the nearby 2g ((1)D2) state. The vibrational distribution in the 2g ((1)D2) state derived by the Franck-Condon simulation of the UV fluorescence showed that the population in the 2u ((1)D2) state transfers mostly to the 2g ((1)D2) vibronic levels which are located energetically above the laser-prepared level. The radiative lifetimes and the self-quenching rate constants were determined to be 21.3 ± 0.1 and 44.6 ± 0.8 ns, and (1.30 ± 0.01) × 10(-9) and (2.26 ± 0.17) × 10(-9) cm(3) molecule(-1) s(-1) for the 2u ((1)D2) (v2u = 3) and 2g ((1)D2) (v2g = 5) states, respectively. The rate constant for the 2u ((1)D2) - 2g ((1)D2) collision induced state-to-state energy transfer was also evaluated to be (1.89 ± 0.01), (3.07 ± 0.07), and (3.77 ± 0.05) × 10(-10) cm(3) molecule(-1) s(-1) for the v2u = 3, 5, and 7 levels, respectively. The very large self-quenching cross sections for the ion-pair states of I2 could be explained by the harpoon mechanism.

Rebinding dynamics of NO to microperoxidase-8 probed by time-resolved vibrational spectroscopy.

Femtosecond vibrational spectroscopy was used to probe the rebinding kinetics of NO to microperoxidase-8 (Mp), an ideal model system for the active site of ligand-binding heme proteins, including myoglobin and hemoglobin, after the photodeligation of MpNO in glycerol/water (G/W) solutions at 294 K. The geminate rebinding (GR) of NO to Mp in viscous solutions was highly efficient and ultrafast and negligibly dependent on the solution viscosity, which was adjusted by changing the glycerol content from 65% to 90% by volume in G/W mixtures. The kinetics of the GR of NO to Mp in viscous solutions was well represented by an exponential function with a time constant of ca. 11 ps. Although the kinetic traces of the GR of NO to Mp in solutions with three different viscosities (18, 81, and 252 cP) almost overlap, they show a slight difference early in the decay process. The kinetic traces were also described by the diffusion-controlled reaction theory with a Coulomb potential. Since the ligand is deligated in a neutral form, an ionic pair of NO(-) and Mp(+) may be produced before forming the Mp-NO bond by an electron transfer from Mp to NO as the deligated NO is sufficiently near to the Fe atom of Mp. The strong reactivity between NO and ferrous heme may arise from the Coulomb interaction between the reacting pair, which is consistent with the harpooning mechanism for NO binding to heme.

Bonding description of the Harpoon mechanism

The lowest lying states of LiH have been widely used to develop and calibrate many different methods in quantum mechanics. In this paper, we show that the electron-transfer processes occurring in these two states are a difficult test for chemical bonding descriptors and can be used to assess new bonding descriptors on its ability to recognise the harpoon mechanism. To this aim, we study the bond formation mechanism in a series of diatomic molecules. In all studied electron reorganisation mechanisms, the maximal electron-transfer variation point along the bond formation path occurs when about half electron has been transferred from one atom to another. If the process takes place through a harpoon mechanism, this point of the reaction path coincides with the avoided crossing. The electron sharing indices and one-dimensional plots of the electron localisation function and the Laplacian of the electron density along the molecular axis can be used to monitor the bond formation in diatomics and provide a distinction between the harpoon mechanism and a regular electron reorganisation process.

Special and general superatoms.

Bridging the gap between atoms and macroscopic matter, clusters continue to be a subject of increasing research interest. Among the realm of cluster investigations, an exciting development is the realization that chosen stable clusters can mimic the chemical behavior of an atom or a group of the periodic table of elements. This major finding known as a superatom concept was originated experimentally from the study of aluminum cluster reactivity conducted in 1989 by noting a dramatic size dependence of the reactivity where cluster anions containing a certain number of Al atoms were unreactive toward oxygen while the other species were etched away. This observation was well interpreted by shell closings on the basis of the jellium model, and the related concept (originally termed "unified atom") spawned a wide range of pioneering studies in the 1990s pertaining to the understanding of factors governing the properties of clusters. Under the inspiration of a superatom concept, advances in cluster science in finding stable species not only shed light on magic clusters (i.e., superatomic noble gas) but also enlightened the exploration of stable clusters to mimic the chemical behavior of atoms leading to the discovery of superhalogens, alkaline-earth metals, superalkalis, etc. Among them, certain clusters could enable isovalent isomorphism of precious metals, indicating application potential for inexpensive superatoms for industrial catalysis, while a few superalkalis were found to validate the interesting "harpoon mechanism" involved in the superatomic cluster reactivity; recently also found were the magnetic superatoms of which the cluster-assembled materials could be used in spin electronics. Up to now, extensive studies in cluster science have allowed the stability of superatomic clusters to be understood within a few models, including the jellium model, also aromaticity and Wade-Mingos rules depending on the geometry and metallicity of the cluster. However, the scope of application of the jellium model and modification of the theory to account for nonspherical symmetry and nonmetal-doped metal clusters are still illusive to be further developed. It is still worth mentioning that a superatom concept has also been introduced in ligand-stabilized metal clusters which could also follow the major shell-closing electron count for a spherical, square-well potential. By proposing a new concept named as special and general superatoms, herein we try to summarize all these investigations in series, expecting to provide an overview of this field with a primary focus on the joint undertakings which have given rise to the superatom concept. To be specific, for special superatoms, we limit to clusters under a strict jellium model and simply classify them into groups based on their valence electron counts. While for general superatoms we emphasize on nonmetal-doped metal clusters and ligand-stabilized metal clusters, as well as a few isovalent cluster systems. Hopefully this summary of special and general superatoms benefits the further development of cluster-related theory, and lights up the prospect of using them as building blocks of new materials with tailored properties, such as inexpensive isovalent systems for industrial catalysis, semiconductive superatoms for transistors, and magnetic superatoms for spin electronics.

Cluster reaction of [Ag8]−/[Cu8]− with chlorine: Evidence for the harpoon mechanism?

To examine the question whether the harpoon mechanism can account for the reactive behavior of microscopic charged systems, we have investigated the reactivity of coinage metal clusters in gas phase. Our studies reveal that the reactivity between [Cu8]−/[Ag8]− and chlorine gas is consistent with the harpoon mechanism. An increased reactive cross section is noted through our theoretical estimation based on two methods, ascribed to a long-range transfer of valence electrons from the [Cu8]−/[Ag8]− cluster to chlorine. Insights into this reactivity will be of interest to other researchers working on obtaining a better understanding of the reaction mechanisms of such superatomic species.

Multidimensional steric effect for the XeF* (B, C) formation in the oriented Xe* (3P2, MJ= 2) + oriented NF3reaction

Steric effect for the XeF* (B, C) formations in the oriented Xe* ((3)P(2), M(J) = 2) + oriented NF(3) reaction has been observed as a function of the mutual configuration between the molecular orientation and the atomic orientation in the collision frame. Molecular steric opacity function has been determined as a function of the atomic orbital alignment (L(Z)') in the collision frame. The larger reactivity at the side with the smaller reactivity at the molecular axis direction is observed for the XeF* (B, C) channels at each atomic orbital alignment. A good correlation between the shape of the molecular steric opacity function and the molecular geometry of NF(3) is recognized. The L(Z)' selectivity in the molecular steric opacity function is different between the XeF* (B, C) channels; in the sideways direction, the XeF* (B) channel is favorable at L(Z)' = 0, while the XeF* (C) channel is favorable at |L(Z)'| = 1. In contrast, at the molecular axis direction, the XeF* (B) channel is favorable at |L(Z)'| = 1, while the XeF* (C) channel is favorable at L(Z)' = 0. We propose the collision-induced harpoon mechanism for the XeF* (B, C) formation in the Xe* ((3)P(2)) + NF(3) reaction.

An ab initio study of the potential energy surfaces for the collision between a Cs atom and an I2 molecule

For the Cs + I2 collision system, a systematic theoretical study is first reported using the ab initio method. Three of eight possible channels are considered. The nonadiabatic coupling between the covalent state and the ionic one is calculated from different angles, especially the T-shape collision. The complete ion-pair formation potential energy surfaces of the T-shape collision in two electronic states (ionic 2B2 state and covalent 2A1 state) and the reactive surface of the linear collision are constructed at the QCISD(T)/SDD level. The main features of potential energy surfaces, such as the minimum energy reaction path, the crossing radius (Rc), and energy minimum geometries, are analyzed. The cross section of this titled system is calculated based on the harpoon mechanism and compared with the available experimental data and those obtained for the M + I2 (M = Li, Na) systems.Key words: ab initio two-state potential energy surfaces, nonadiabatic coupling, ion-pair formation, cross section.

Orientation dependence of the reaction of K + CH 3CN

Calculations are reported at the MP2/6-311++G(2df,2pd) level of theory on the reaction of K atoms with CH 3CN, with different orientations of the CH 3CN molecule with respect to the incoming K atom. Approach along a minimum energy pathway does not involve a simple harpoon mechanism. Stretching of the C–C bond of CH 3CN facilitates electron transfer. Electron transfer occurs on a repulsive surface for CN end approach and on an attractive surface for methyl end approach. For approach from the methyl end of the molecule, the barrier to the formation of CH 2CN − is lower than that for the formation of CH 3CN − or CN − as the K atom approaches closer to the CH 3CN molecule.

A model study for the breaking of N 2 from CN x within DFT

A hypothetical CN 2 structure was investigated as a model to study the release of N 2 from the octahedral hole of 3D carbon based ultra hard compounds, which is the most important drawback in the attempts to synthesize ultra hard compounds like C 3N 4 and C 11N 4. Full structure relaxations using DFT methods led to a structure at the energy minimum showing a significantly enlarged NN distance of 1.34 Å compared to the molecular N 2 (1.09 Å). While for small volume changes a high hardness for CN 2 of 405 GPa is calculated, we found that enlargements of the cell constant lead to the release of N 2 that could be followed calculating the ELF and the charge transfer within the AIM theory. The whole procedure simulates an inverted “harpoon mechanism”.

Formation of Electronically Excited Fragments Resulting from Collisions of He(23S) Atoms with CCl4 and Related Chloromethanes

Abstract The photoemissions from fragments produced by collisions of He(23S) atoms with CHCl3, CCl2F2, CCl3F, CCl4, and CBrCl3 have been studied in the collision energy range of 120–210 meV using a crossed beam. The Cl2(D′–A′) band is predominant for CCl4 and CBrCl3, while the A–X and B–X bands of CF are intense for CCl2F2 and CCl3F. The sum of the emission cross sections (σem) for several systems of neutral fragments in the 190–600 nm region was estimated to be in the (0.3–1.2) × 10−20 m2 range for these chloromethanes at a collision energy of 150 meV. The dependences of σem’s for the Cl2(D′–A′) band from CHCl3 and CCl4 on the collision energy are either nearly zero or slightly negative. The sum of σem’s for Cl2 and other ion-pair bands reveals a fairly good correlation with the electron affinity of the targets. These results support the harpoon mechanism for the formation of Cl2(D′) from the He(23S) + chloromethanes reactions.

Reaction pathway, energy barrier, and rotational state distribution for Li (2 2PJ)+H2→LiH (X 1Σ+)+H

By using a pump-probe technique, we have observed the nascent rotational population distribution of LiH (v=0) in the Li (2 2PJ) with a H2 reaction, which is endothermic by 1680 cm−1. The LiH (v=0) distribution yields a single rotational temperature at ∼770 K, but the population in the v=1 level is not detectable. According to the potential energy surface (PES) calculations, the insertion mechanism in (near) C2v collision geometry is favored. The Li (2 2PJ)–H2 collision is initially along the 2A′ surface in the entrance channel and then diabatically couples to the ground 1A′ surface, from which the products are formed. From the temperature dependence measurement, the activation energy is evaluated to be 1280±46 cm−1, indicating that the energy required for the occurrence of the reaction is approximately the endothermicity. As Li is excited to higher states (3 2S or 3 2P), we cannot detect any LiH product. From a theoretical point of view, the 4A′ surface, correlating with the Li 3 2S state, may feasibly couple to a repulsive 3A′ surface, from which the collision complex will rapidly break apart into Li (2 2PJ) and H2. The probability for further surface hopping to the 2A′ or 1A′ surfaces is negligible, since the 3A′ and 2A′ surfaces are too far separated to allow for an efficient coupling. The Li (3 2P) state is expected to behave similarly. The observation also provides indirect evidence that the harpoon mechanism is not applicable to this system.

Excited state reactions of metals in clusters: pluridimensional harpoon and solvation effects.

Excited state reactions of metals produce electronically excited products efficiently, as revealed by studies performed both in the gas phase and in free Van der Waals complexes. The reaction mechanism is assigned to an excited state charge transfer from the metal to the molecular reactant (i.e. a harpoon mechanism). The present work uses the well established cluster isolated chemical reaction (CICR) technique and addresses these processes when the metal ... molecule Van der Waals pair is deposited at the surface of a large argon cluster. Such work is aimed at investigating the effect of the cluster substrate on the preparation and dynamics of the reaction. We have revisited the pluridimensional character of the harpoon reaction in these systems. More specifically, we studied the reaction of excited calcium with HBr near the calcium resonance line at 422.7 nm, forming CaBr in the A and B states. As in previous Van der Waals experiments, we could explore the dynamics of the reaction by recording action spectra. These spectra exhibit noticeable differences from those observed for unsupported Ca...HBr complexes. In particular the bending movement of the Ca...HBr complex which gives access to the transition state of the reaction is partly hindered by the presence of the argon cluster.