Simple Electrostatic Model Research Articles

Understanding Substituent Effects in Noncovalent Interactions Involving Aromatic Rings

Noncovalent interactions involving aromatic rings such as π-stacking, cation/π, and anion/π interactions are central to many areas of modern chemistry. Decades of experimental studies have provided key insights into the impact of substituents on these interactions, leading to the development of simple intuitive models. However, gas-phase computational studies have raised some doubts about the physical underpinnings of these widespread models. In this Account we review our recent efforts to unravel the origin of substituent effects in π-stacking and ion/π interactions through computational studies of model noncovalent dimers. First, however, we dispel the notion that so-called aromatic interactions depend on the aromaticity of the interacting rings by studying model π-stacked dimers in which the aromaticity of one of the monomers can be "switched off". Somewhat surprisingly, the results show that not only is aromaticity unnecessary for π-stacking interactions, but it actually hinders these interactions to some extent. Consequently, when thinking about π-stacking interactions, researchers should consider broader classes of planar molecules, not just aromatic systems. Conventional models maintain that substituent effects in π-stacking interactions result from changes in the aryl π-system. This view suggests that π-stacking interactions are maximized when one ring is substituted with electron-withdrawing groups and the other with electron donors. In contrast to these prevailing models, we have shown that substituent effects in π-stacking interactions can be described in terms of direct, local interactions between the substituents and the nearby vertex of the other arene. As a result, in polysubstituted π-stacked dimers the substituents operate independently unless they are in each other's local environment. This means that in π-stacked dimers in which one arene is substituted with electron donors and the other with electron acceptors the interactions will be enhanced only to the extent provided by each substituent on its own, unless the substituents on opposing rings are in close proximity. Overall, this local, direct interaction model predicts that substituent effects in π-stacking interactions will be additive and transferable and will also depend on the relative position of substituents on opposing rings. For cation/π and anion/π interactions, similar π-resonance-based models pervade the literature. Again, computational results indicate that substituent effects in model ion/π complexes can be described primarily in terms of direct interactions between the ion and the substituent. Changes in the aryl π-system do not significantly affect these interactions. We also present a simple electrostatic model that further demonstrates this effect and suggests that the dominant interaction for simple substituents is the interaction of the charged ion with the local dipole associated with the substituents. Finally, we discuss substituent effects in electrostatic potentials (ESPs), which are widely used in discussions of noncovalent interactions. In the past, widespread misconceptions have confused the relationship between changes in ESPs and local changes in the electron density. We have shown that computed ESP plots of diverse substituted arenes can be reproduced without altering the aryl π-density. This is because substituent-induced changes in the ESP above the center of aryl rings result primarily from through-space effects of substituents rather than through changes in the distribution of the π-electron density.

A theoretical study on structures, energetics, and spectra of Br–.nCO2 clusters: Towards bridging the gap between micro-domain and macro-domain

Structures, energetics, and spectra of Br(-).nCO(2) (n = 1-8) clusters are studied based on ab initio electronic structure theory. The geometry of each size of clusters is evaluated by employing second-order Moller-Plesset (MP2) perturbation theory. It is observed that the solvent CO(2) molecules approach the bromide moiety from one side in an asymmetric fashion except for the Br(-).8CO(2) cluster. Simple electrostatic model for charge-quadrupole interactions is valid for the Br(-).nCO(2) clusters. Reduced variational space based energy decomposition method shows that the electrostatic interaction is the major component and polarization and charge transfer energies are the other significant components of the total interaction energy. Both adiabatic and vertical electron detachment energies and solvation energies are calculated at MP2 level of theory. We have observed an excellent agreement between theory and experiment for the vertical detachment and solvation energies. Calculated quantities based on the analytical expression which connects the finite domain to macroscopic one are found to be very good in agreement with the available experimental results. The present study reveals a 2.6 eV increase in the detachment energy of bromide anion due to the solvation effect of CO(2), which is relatively small compared to that of the corresponding 4.7 eV increase in detachment energy in water.

Active Site Hydrophobic Residues Impact Hydrogen Tunneling Differently in a Thermophilic Alcohol Dehydrogenase at Optimal versus Nonoptimal Temperatures

A growing body of data suggests that protein motion plays an important role in enzyme catalysis. Two highly conserved hydrophobic active site residues in the cofactor-binding pocket of ht-ADH (Leu176 and V260) have been mutated to a series of hydrophobic side chains of smaller size, as well as one deletion mutant, L176Δ. Mutations decrease k(cat) and increase K(M)(NAD(+)). Most of the observed decreases in effects on k(cat) at pH 7.0 are due to an upward shift in the optimal pH for catalysis; a simple electrostatic model is invoked that relates the change in pK(a) to the distance between the positively charged nicotinamide ring and bound substrate. Structural modeling of the L176Δ and V260A variants indicates the development of a cavity behind the nicotinamide ring without any significant perturbation of the secondary structure of the enzyme relative to that of the wild type. Primary kinetic isotope effects (KIEs) are modestly increased for all mutants. Above the dynamical transition at 30 °C for ht-ADH [Kohen, A., et al. (1999) Nature 399, 496], the temperature dependence of the KIE is seen to increase with a decrease in side chain volume at positions 176 and 260. Additionally, the relative trends in the temperature dependence of the KIE above and below 30 °C appear to be reversed for the cofactor-binding pocket mutants in relation to wild-type protein. The aggregate results are interpreted in the context of a full tunneling model of enzymatic hydride transfer that incorporates both protein conformational sampling (preorganization) and active site optimization of tunneling (reorganization). The reduced temperature dependence of the KIE in the mutants below 30 °C indicates that at low temperatures, the enzyme adopts conformations refractory to donor-acceptor distance sampling.

Isomer-Selected Photoelectron Spectroscopy of Isolated DNA Oligonucleotides: Phosphate and Nucleobase Deprotonation at High Negative Charge States

Fractionation according to ion mobility and mass-to-charge ratio has been used to select individual isomers of deprotonated DNA oligonucleotide multianions for subsequent isomer-resolved photoelectron spectroscopy (PES) in the gas phase. Isomer-resolved PE spectra have been recorded for tetranucleotides, pentanucleotides, and hexanucleotides. These were studied primarily in their highest accessible negative charge states (3-, 4-, and 5-, respectively), as provided by electrospraying from room temperature solutions. In particular, the PE spectra obtained for pentanucleotide tetraanions show evidence for two coexisting classes of gas-phase isomeric structures. We suggest that these two classes comprise: (i) species with excess electrons localized exclusively at deprotonated phosphate backbone sites and (ii) species with at least one deprotonated base (in addition to several deprotonated phosphates). By permuting the sequence of bases in various [A(5-x)T(x)](4-) and [GT(4)](4-) pentanucleotides, we have established that the second type of isomer is most likely to occur if the deprotonated base is located at the first or last position in the sequence. We have used a combination of molecular mechanics and semiempirical calculations together with a simple electrostatic model to explore the photodetachment mechanism underlying our photoelectron spectra. Comparison of predicted to measured photoelectron spectra suggests that a significant fraction of the detected electrons originates from the DNA bases (both deprotonated and neutral).

Kelvin probe force gradient microscopy of charge dissipation in nano thin dielectric layers

Application of Kelvin probe force gradient microscopy (KPFGM) to visualize the local charge dissipation in thin dielectric layers is considered. By this method, the local charge behavior in nano thin SiO2, Si3N4, and LaScO3 dielectric layers has been studied. Local charging of the layers has been performed at the point contact with a conductive probe. KPFGM potential images reveal variations of the surface potential in the locally charged areas, which makes it possible to detect the injected charge and to study its behavior. Special experiments on the SiO2 layers with embedded Si-nanocrystals, when lateral spreading of injected charge had been suppressed, permitted to demonstrate high (better than 20 nm) lateral resolution of KPFGM observations. A simple electrostatic model has been developed to estimate the total amount of injected charge. The obtained estimations made it possible to control charge retention in the dielectric layer and possible leaks into the substrate. The studied dielectric layers demonstrate a broadening of the charged area with time t, proportionally to t1/2, what indicates the domination of the diffusion mechanism in charge lateral spreading on the large time scale. These observations permitted to determine the diffusion coefficients, mobilities, and diffusion activation energies for charges in the studied dielectric layers. To obtain the correct information on the injected charge behavior, the parasitic charge dissipation through the surface film of adsorbed water should be reduced to a negligible level. It was achieved by working in moderate vacuum conditions with an additional sample heating.

Impact of film thickness on threshold voltage of polysilicon TFTs

The threshold voltage is a key parameter in the silicon MOSFETS design and operation. In this paper, we study the factors that contribute to the changes of threshold voltage of thin-film LPCVD polysilicon transistors when varying the thickness of the active layer. The results show that the threshold voltage depends strongly on the film thickness. For high thicknesses, the threshold voltage is shown to be determined by the trapped holes at grain boundaries. The variation of this parameter with film thickness can be attributed to inter-granular trap states density variation in the film. For low thicknesses, a simple electrostatic model of the study structure, associated with a numerical method of solving 2D-Poisson's equations, shows that the changes of threshold voltage of polysilicon TFT depends on grain-boundary properties and charge-coupling between the front and back gates. Based on this consideration, the usual threshold voltage expression is modified. The results so obtained are compared with the available experimental data, which show a satisfactory match thus justifying the validity of the proposed relation.

Universal Electrostatic Origin of Cation Ordering in A2BO4 Spinel Oxides

The crystal structures of A(2)BO(4) spinel oxides are classified as either normal or inverse, representing different distributions of the A and B cations over the tetrahedrally and octahedrally coordinated cation sites. These structures undergo characteristic structural changes as a function of temperature: (i) the nominally disordered inverse structure orders crystallographically at low T, and (ii) at finite temperatures, both inverse and normal develop characteristic distributions of cations associated with order-disorder structural changes. We show here that all of these universal features emerge naturally from a simple point-ion electrostatic (PIE) model with a single adjustable parameter. Monte Carlo simulations of the PIE Hamiltonian provide quantitative order-disorder characteristic temperatures. We show that, with the help of the PIE model, the magnitude of the temperatures can be inferred from the nominal charges of the atomic species in the spinel. Indeed, we show that characteristic order-disorder temperatures in 3-2 spinels (nominal charges Z(A) = 3 and Z(B) = 2) are approximately an order of magnitude lower than in 2-4 spinels, thus explaining why typical 3-2 samples exhibit much larger degrees of disorder than those belonging to the 2-4 class.

An Electrostatic Charge Partitioning Model for the Dissociation of Protein Complexes in the Gas Phase

Electrosprayed multi-protein complexes can be dissociated by collisional activation in the gas phase. Typically, these processes follow a mechanism whereby a single subunit gets ejected with a disproportionately high amount of charge relative to its mass. This asymmetric behavior suggests that the departing subunit undergoes some degree of unfolding prior to being separated from the residual complex. These structural changes occur concomitantly with charge (proton) transfer towards the subunit that is being unraveled. Charge accumulation takes place up to the point where the subunit loses physical contact with the residual complex. This work develops a simple electrostatic model for studying the relationship between conformational changes and charge enrichment during collisional activation. Folded subunits are described as spheres that carry continuum surface charge. The unfolded chain is envisioned as random coil bead string. Simulations are guided by the principle that the system will adopt the charge configuration with the lowest potential energy for any backbone conformation. A finite-difference gradient algorithm is used to determine the charge on each subunit throughout the dissociation process. Both dimeric and tetrameric protein complexes are investigated. The model reproduces the occurrence of asymmetric charge partitioning for dissociation events that are preceded by subunit unfolding. Quantitative comparisons of experimental MS/MS data with model predictions yield estimates of the structural changes that occur during collisional activation. Our findings suggest that subunit separation can occur over a wide range of scission point structures that correspond to different degrees of unfolding.

Reduced partition function ratios for isotopes of hydrated alkali and alkaline earth ions calculated by a simple electrostatic model

We describe a simple electrostatic model of hydrated ions [M(H2O)n]+ (n = 3–18, M = Li, Mg, Ca, K) which enables to calculate ion vibration frequency of the ground state. In this model the considered ion with a reduced mass vibrates in quasispherical well formed by the ion-dipole attractive potential and repulsive valence potential, these simplifications allowed to solve one dimensional Schrödinger equation, whilst the calculated ground state was considered as one of triply degenerated state of the three dimensional motion of the ion vs. hydration shell. The reduced partition function ratios were calculated from the vibration frequencies using Urey's (1947) harmonic approximation formula. The results obtained in this way are in good agreement with those obtained by much more laborious ab initio molecular orbital methods, like SCF Hartree-Fock, DFT, MP2, etc. Moreover, we were able to extend calculations to hydrated Li and K ions surrounded with two shells of water molecules. These results are the first estimations of the upper limit of isotope fractionation in water solutions, which are 99.3‰ for Li and only 2.5‰ for K isotopes.

Probing electrostatic interactions and structural changes in highly charged protein polyanions by conformer-selective photoelectron spectroscopy

We have recorded the first conformer-selective photoelectron spectra of a protein polyanion in the gas-phase. Bovine cytochrome c protein was studied in 8 different negative charge states ranging from 5- to 12-. Electron binding energies were extracted for all charge states and used as a direct probe of intramolecular Coulomb repulsion. Comparison of experimental results with simulations shows that the experimental outcome can be reproduced with a simple electrostatic model. Energetics are consistent with a structural transition from a folded to an unfolded conformational state of the protein as the number of charges increases. Furthermore, the additional ion-mobility data show that the onset of unfolding can be assigned to charge state 6- where three conformers can be distinguished.

Influence of the ferroelectric polarization on the electronic structure of BaTiO3 thin films

AbstractMicron scale ferroelectric (FE) domains have been written into a 20‐nm‐thick epitaxial thin film of BaTiO3 (001) [BTO(001)] on a Nb‐doped SrTiO3 substrate using a piezoforce microscope (PFM). The domain‐dependent electronic structure has been studied using fully energy‐filtered photoemission electron microscopy (PEEM) and synchrotron radiation. Shifts, induced by FE polarization, of up to 300 meV were observed in the work function of the sample. The surface is Ba‐O terminated. Polarization‐induced distortion of the electronic structure was observed in the valence band and on the Ba 3d, Ti 2p and O 1s core levels of BTO. Polarization‐dependent surface adsorption was observed. A simple electrostatic model based on net surface charge is not sufficient to explain the observed modifications in the electronic levels. Copyright © 2010 John Wiley & Sons, Ltd.

Effects of The Nanoplasma Electrons on Coulomb Explosion of Xenon Clusters

The traditional description of Coulomb explosion (CE) of multicharged clusters driven by ultraintense, near-infrared, femtosecond laser fields (peak intensities of 1015−4 × 1016) W cm−2 at pulse lengths of 25−230 fs) under the initial conditions of complete and vertical outer ionization is transcended in the presence of a nanoplasma within the cluster. Unique features of the energetics of CE of many-electron elemental Xen (n = 55−6099) clusters manifest the interrelations between electron dynamics of outer ionization and nuclear CE dynamics. Molecular dynamics simulations of high-energy electrons and ions focused on the cluster size and laser parameter domains, where the contribution of electron impact inner ionization is important, the population of the persistent nanoplasma is considerable, and the effects of the nanoplasma on CE nuclear dynamics and energetics are significant. Relations were established for the cluster size and the laser parameter dependence of CE energetic attributes, that is, the average and maximal kinetic energies and the energy distribution, which mark the breakdown of the initial conditions of the complete cluster vertical ionization and of the simple electrostatic models for this situation. Incomplete vertical outer ionization, which retains a persistent nanoplasma, is exhibited for short pulses and low intensities. The CE energetics is then described in terms of the lychee model, where the persistent nanoplasma is initially confined within a neutral sphere in the center of the cluster while ions from an exterior shell undergo CE. This model accounts for the cluster size dependence of the average and maximal CE energies, for their initial site dependence, and for the narrow kinetic energy distribution at low energies. Complete or incomplete nonvertical ionization, when the time scales for electron outer ionization dynamics and nuclear CE dynamics are inseparable, is exhibited for long pulses. For complete nonvertical ionization, we relied on an empirical analysis of the cluster size scaling of the average and maximal CE energies, which can be reasonably well described in terms of the simple electrostatic model for complete cluster vertical outer ionization. For incomplete nonvertical outer ionization, CE kinetic energy distribution for long pulses is broad with a pronounced structure, which is due to a distribution of {Xeq+} ions with different charges. The gross features of the initial site dependence of CE energies and the broad distribution of the kinetic energies for incomplete nonvertical outer ionization can be rationalized in terms of the lychee model, which is strictly applicable for incomplete vertical ionization.

First-principles study of competing ferroelectric and antiferroelectric instabilities inBaTiO3/BaOsuperlattices

We report a first-principles study of (BaTiO${_3}$)$_m$/(BaO)$_n$ superlattices for a wide range of periodicities $m/n$. We show that such a system develops a polar zone-center instability for sufficiently large \textit{m}/\textit{n} ratio, which can be understood, at least qualitatively, from a simple electrostatic model and should lead to a ferroelectric ground-state. However, the analysis of the phonon dispersion curves also points out the appearance of stronger antiferroelectric instabilities at the zone boundaries around $m=4$, before the critical ratio for ferroelectricity is reached and which still dominate beyond it. The dominant character of the anti-ferroelectric instability is explained from the depolarizing field which hardens the ferroelectric mode. This analysis allows us to predict that, (BaTiO${_3}$)$_m$/(BaO)$_n$ superlattices should present an antiferroelectric ground state for $m$ larger than 4, which should smoothly evolve to a multidomain structure for increasing $m$ values and only become ferroelectric for large $m$.

Enhanced ionization of theH2molecule driven by intense ultrashort laser pulses

We report correlated two-electron ab initio calculations for the hydrogen molecule H{sub 2} in interaction with intense ultrashort laser pulses, via a solution of the full three-dimensional time-dependent Schroedinger equation. Our results for ionization and excitation probabilities (at 800 and 400 nm) as a function of internuclear distance R show strong evidence of enhanced ionization, in both single and double ionization, as well as enhanced excitation, in single and double excitation, as the internuclear distance R increases from the equilibrium value R{sub e}. The enhancement of all these molecular processes exhibits a maximum at a critical distance R{sub c}, which can be predicted from simple electrostatic and recollision models.

Experimental Determination of the Excited-State Polarizability and Dipole Moment in a Thin Organic Semiconductor Film

We report the evolution of the electronic structure of excited (sub)monolayer films of vanadyl naphthalocyanine (VONc) at the interface with highly oriented pyrolytic graphite (HOPG). Using two-photon photoemission spectroscopy, an unoccupied state corresponding to the lowest unoccupied molecular orbital of VONc is observed. The energy of this state shows a significant dependence on coverage, interpreted in the context of the electrostatic environment at the interface. On the basis of a simple electrostatic model, we were able to determine the excited-state polarizability and dipole moment of VONc at the interface with HOPG. The results suggest that local electric fields may have a major influence on interfacial energy level alignment in the excited-state manifold, with direct consequences for interfacial charge-transfer dynamics.

Simplified electrostatic model for the thermodynamic excess potentials of binary strong electrolyte solutions with size-dissimilar ions

Modern theories of electrolyte solutions are physically accurate but difficult to apply for real-life systems; a need therefore exists to theoretically derive simplified and practically useful mathematical expressions for thermodynamic excess functions. This can be done by incorporating ion-size dissimilarity into the classical Debye-Hückel model [Physik Z. 24, 185 (1923)], under conditions at which non-electrostatic contributions are negligible. If the contact distance between the central (β) ion and a cloud (α) ion is a for counter-ions and b for co-ions, two basic cases exist, b < a and b > a. In both, a ‘smaller-ion shell’ (SiS) at the edge of the ionic cloud, bordered by the spherical surfaces of radius b and a, admits only the smaller α ions [Thomlinson and Outhwaite, Mol. Phys. 47, 1113 (1982)]. In the b < a case, the SiS contributes an ionic repulsion effect and the overall extra-electrostatic potential energy, Ψ b < a (κ) − κ, reciprocal screening length–exhibits a minimum. For b > a, the SiS contributes an ‘extra ionic attraction’ and the overall extra-electrostatic energy, Ψ b > a (κ) declines monotonically with increasing κ. The entire Ψ contribution, Ψ±, is a linear combination of the Ψs of the two counter β ions. The effectiveness of Ψ± is demonstrated for real-life electrolyte systems, based on experimental mean ionic activity coefficients and their concentration dependencies. Fitting theory with experiment generates ion-size parameters that represent realistic interionic collision distances in solution, unlike parallel parameters based on other simplified theories.

Dynamic response of tubular dielectric elastomer transducers

In this paper, a numerical model for the dynamic response of tubular dielectric elastomer transducers is presented and validated with experimental results for the first time. Dielectric elastomers (DE) are soft polymer based smart materials that can be potentially employed in applications such as actuation, sensing and energy harvesting ( Kornbluh, 2004; Carpi et al., 2005; Waki et al., 2008). In our previous work, the quasi-static response of tubular DE transducers was studied ( Goulbourne et al., 2007; Son and Goulbourne, 2009). Here, a numerical model is developed to predict the dynamic response of tubular DE transducers. Inertia effects are included in our previous static model which yields a system of partial differential equations. The results of the dynamic response of the tubular DE transducers are obtained by numerically solving the simplified partial different equations using a finite difference scheme. The capacitance change induced by the dynamic deformation of the tubular DE is also calculated by a simple electrostatic model, illustrating dynamic passive sensing. Several tubular DE transducer samples (VHB 4905 and silicone) were fabricated and an experimental setup was developed to investigate the dynamic response by measuring capacitance and radial deformation. In the sensing experiments, a sweep of dynamic pressure profiles (0–5 Hz) are applied. It is observed that silicone transducers have a larger dynamic sensing range. In the actuation experiments, the deformation of the silicone actuator is monitored while a voltage signal (4.5 kV) is applied from 0 to 30 Hz. The silicone actuator shows a good actuation response. The comparison between numerical and experimental results for the DE transducers shows an overall error of 3%.

Integration of Nanowire Devices in Out-of-Plane Geometry

We report the fabrication of arrays of single and multiple out-of-plane nanowire devices on a single substrate, an important step for the fabrication of novel three-dimensional devices and the integration of individually addressable nanowires onto current Si planar technology platforms. Vertical nanowire device fabrication can greatly increase device densities; however integrating such devices into arrays with registry to the substrate requires precise control over the number and position of the nanowires. Here we report the directed assembly of gold nanoparticle seeds into patterned arrays for the growth of nanowires using chemical recognition and electrophoretic methods. Chemical recognition provides highly reproducible control of the position and number of nanoparticles per pattern element and is shown to be in good agreement with a simple electrostatic model. Individually addressed out-of-plane, vapor-liquid-solid grown Ge nanowires with single and multiple nanowires per element are fabricated and electrically characterized.

Simple electrostatic model applicable to biomolecular recognition.

An exact, analytic solution for a simple electrostatic model applicable to biomolecular recognition is presented. In the model, a layer of high-dielectric constant material (representative of the solvent, water), whose thickness may vary separates two regions of low-dielectric constant material (representative of proteins, DNA, RNA, or similar materials), in each of which is embedded a point charge. For identical charges, the presence of the screening layer always lowers the energy compared to the case of point charges in an infinite medium of low-dielectric constant. Somewhat surprisingly, the presence of a sufficiently thick screening layer also lowers the energy compared to the case of point charges in an infinite medium of high-dielectric constant. For charges of opposite sign, the screening layer always lowers the energy compared to the case of point charges in an infinite medium of either high or low dielectric constant. The behavior of the energy leads to a substantially increased repulsive force between charges of the same sign. The attractive force between charges of opposite signs is weaker than in an infinite medium of low dielectric constant material but stronger than in an infinite medium of high dielectric constant material. The presence of this behavior, which we name asymmetric screening, in the simple system presented here confirms the generality of the behavior that was established in a more complicated system of an arbitrary number of charged dielectric spheres in an infinite solvent.

Dissociation of dicarboxylate and disulfonate dianions

Collision-induced dissociation (CID), along with infrared multiple photon dissociation/detachment (IRMPD) techniques, is utilized to study a series of doubly substituted aromatic dianions containing sulfonate and carboxylate functionalities (1,2- and 1,3-benzenedisulfonate, 1,5-naphthalenedisulfonate, 2,6-naphthalenedisulfonate, 4-sulfobenzoate, 2,6-naphthalenedicarboxylate, and terephthalate dianions). The molecules were chosen because of the electronegativity of the CO(2) and SO(3) moieties along with their varied spatial separation in order to investigate the effect of the repulsive Coulomb barrier (RCB) on the dianions' respective dissociation pathways. Density functional theory calculations of the structures, electron detachment and dissociation energies, as well as vibrational frequencies are performed. Calculated infrared active vibrational frequencies are largely in agreement with the IRMPD spectra which provide support for interpretations based upon computed energies. Calculated and experimental results show that fragmentation dominates over electron detachment as the lowest energy dissociation pathway for these systems and the nature of this dissociation is dictated by properties of the substituent group. CID and IRMPD of dianions with two sulfonate groups (SO(3)(-)) resulted in a single dissociation channel leading to observation of SO(3)(-) and its anion conjugate pair, whereas the carboxylate (CO(2)(-)) containing dianions dissociated via loss of one or both CO(2) molecules and an electron. The SO(3)(-) collisional dissociation exhibited a clear energetic threshold toward ionic fragmentation with an isomeric dependence that is in reasonable agreement with a simple electrostatic model of the RCB, as well as with published reports on electron photodetachment. The loss of one or both CO(2) units and an electron from CID of the carboxylate dianions appeared with no threshold (dissociation occurs with no collision gas), implying these dianions to be metastable toward the dissociation pathway. However, calculations show these ions to be energetically stable toward dissociation as well as electron detachment. More importantly, in the case of the 2,6-naphthalenedicarboxylate dianion, experiments performed at the FELIX Fourier-transform ion cyclotron resonance facility and the ELISA electrostatic storage ring, where ions are collisionally cooled prior to analysis, showed this ion to be stable (tau>1.5 s). We conclude that the carboxylate (CO(2)(-)) containing dianions formed in the present CID experiment are electronically stable but vibrationally metastable due to internal energy imparted in the harsh electrospray conditions. The delocalized nature of the excess electrons associated with the carboxylate containing dianions may lead to circumvention of the RCB by dissociating via neutral fragmentation followed by (or accompanied by) electron detachment.