Strong Vibronic Interactions Research Articles

Effects of vibronic interaction in cyano-bridged clusters containing Mn(III) and Mn(II) ions

The effects of vibronic interaction in the formation of the barrier for magnetization reversal in the cyanide-based binuclear Mn(III)–CN–Mn(II) clusters containing octahedrally coordinated Mn(III) and Mn(II) ions were examined. Along with the orbitally dependent superexchange between the Mn(II) and Mn(III) ions the model includes an axial component of the crystal field, spin–orbit coupling and vibronic interaction operating within the ground 3 T 1 ( t 2 4 ) term of the low-spin Mn(III) ion. The vibronic coupling parameters for the Mn(III) ion in the octahedral surrounding of six carbon ions have been evaluated in the framework of the exchange charge model of the crystal field accounting for the exchange and covalence effects. The dominating role of the tetragonal vibrations for the 3T 1 term of a single Mn(III) ion was demonstrated. The ability of the investigated complex to act as a Single-Molecule Magnet was analyzed as a function of both the temperature and the strength of vibronic coupling with the tetragonal vibrations. Very strong vibronic interaction strongly facilitates tunneling through the barrier, i.e., suppresses the barrier for magnetization reversal and prevents from long life time of magnetization.

Strong vibronic interactions and possible electron pairing in the photoinduced excited electronic states in nanosized molecules

Electron–phonon interactions in the photoinduced excited electronic states are investigated, and compared with those in the monoanionic and monocationic electronic states in acenes. The total electron–phonon coupling constants for the monoanionic ( l LUMO), monocationic ( l HOMO), and photoinduced excited electronic states ( l B 1 u ( HOMO → LUMO ) ) in acenes are estimated. The l B 1 u ( HOMO → LUMO ) values are much larger than the l LUMO and l HOMO values in acenes. The complete phase patterns difference between the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) in acenes, are the main reason why the C–C stretching modes around 1500 cm −1 afford much larger electron–phonon coupling constants in the photoinduced excited electronic states than in the monoanionic and monocationic electronic states in acenes, and the reason why the l B 1 u ( HOMO → LUMO ) values are much larger than the l LUMO and l HOMO values in acenes.

Combined experimental-theoretical study of the lower excited singlet states of paravinyl phenol, an analog of the paracoumaric acid chromophore

The low-lying excited singlet states of paravinyl phenol (pVP) are investigated experimentally and theoretically paying attention to their similarity to excited states of paracoumaric acid, the chromophore of the photoactive yellow protein (PYP). Resonance enhanced multiphoton ionization and laser induced fluorescence spectroscopic techniques are employed to obtain supersonically cooled, vibrationally resolved excitation and emission spectra related to the lowest (1)A'(V') excited state of pVP. Comprehensive analyses of the spectral structures are carried out by means of the equation-of-motion coupled cluster singles and doubles and time dependent density functional theory methods in combination with the linear vibronic coupling model and Franck-Condon calculations. The assignments of the spectral patterns are given, mostly in terms of excitations of totally symmetric modes. Weak activity of the non-totally-symmetric modes indicates low probability of photochemical processes in the Franck-Condon region of the (1)A'(V') state. The second (1)A'(V) and third (1)A" (Ryd) excited states of pVP are characterized with regard to their electronic structure, properties, and effects of geometry relaxations. The lengthening of the double bond relevant to the trans-cis isomerization of the PYP chromophore is found for the (1)A'(V) state. A possibility of photochemical processes and strong vibronic interactions in this state can be expected. The theoretical results for the (1)A"(Ryd) state predict that dissociation with respect to the O-H bond is possible.

Simulation of the photodetachment spectrum of the pyrrolide anion

The photodetachment spectrum of the pyrrolide anion, C 4H 4N −, has been measured recently [A.J. Gianola, T. Ichino, R.L. Hoenigman, S. Kato, V.M. Bierbaum, W.C. Lineberger, J. Phys. Chem. A 108 (2004), 10326]. The band associated with the 1 2 A 2 ground state of the pyrrolyl radical, C 4H 4N, can be identified in the spectrum by its resolved vibrational progression. In contrast, the second band, belonging to the 1 2 B 1 first excited state of pyrrolyl, is very weak, broad, and unresolved, which suggests the presence of strong vibronic interaction effects. We have performed a theoretical study of the spectrum in the framework of the linear vibronic coupling model, using ab initio calculated parameters. It is shown that a 1 2 B 1–1 2 A 2 conical intersection is responsible for the unresolved part of the spectrum. The potential-energy surfaces of the 1 2 A 2 and 1 2 B 1 states of pyrrolyl as a function of the a 1 and b 2 ground state normal coordinates of pyrrolide have been computed with the MRCI/aug-cc-pVDZ method. Only the b 2 modes can couple the involved electronic states in first order. Five totally symmetric modes ( a 1 symmetry) and four modes of b 2 symmetry have been identified as the vibronically most active tuning and coupling vibrations, respectively. Model Hamiltonians for the description of the dynamics in the coupled vibronic manifold of the 1 2 A 2 and 1 2 B 1 states, including different subsets of these nine modes, have been constructed. The simulated spectra predict a 1 2 A 2 band with sharp peaks and a very diffuse 1 2 B 1 band stretching from 2.6 to 3.3 eV with a maximum close to 3.0 eV. The calculated spectra are in good agreement with experiment. Reasons for the unexpectedly low intensity of the 1 2 B 1 band such as an extremely short lifetime of 1 2 B 1 vibronic levels or very different photodetachment cross sections for the 1 2 A 2 and 1 2 B 1 states are discussed.

Molecular Signatures in the Transport Properties of Molecular Wire Junctions: What Makes a Junction “Molecular”?

The simplest component of molecular electronics consists of a single-molecule transport junction: a molecule sandwiched between source and drain electrodes, with or without a third gate electrode. In this Concept article, we focus on how molecules control transport in metal-electrode molecular junctions, and where the molecular signatures are to be found. In the situation where the molecule is relatively short and the gap between injection energy and molecular eigenstates is large, transport occurs largely by elastic tunneling, stochastic switching is common, and the vibronic signature can be found using inelastic electron tunneling spectroscopy (IETS). As the energy gaps for injection become smaller, one begins to see stronger molecular signatures - these include Franck-Condon-like structures in the current/voltage characteristic and strong vibronic interactions, which can lead to hopping behavior at the polaron limit. Conformational changes induced by the strong electric field lead to another strong manifestation of the molecular nature of the junction. We overview some of this mechanistic landscape, focusing on significant effects of switching (both stochastic and controlled by the electric field) and of molecular vibronic coupling.

The Band 12 Issue in the Electron Momentum Spectra of Norbornane: A Comparison with Additional Green's Function Calculations and Ultraviolet Photoemission Measurements

In continuation of a recent study of the electronic structure of norbornane [J. Chem. Phys., 2004, 121, 10525] by means of electron momentum spectroscopy (EMS), we present Green's Function calculations of the ionization spectrum of this compound at the ADC(3) level using basis sets of varying quality, along with accurate evaluations at the CCSD(T) level of the vertical (26.5 eV) and adiabatic (22.1 eV) double ionization thresholds under C(2v) symmetry. The obtained results are compared with newly recorded ultraviolet photoemission spectra (UPS), up to binding energies of 40 eV. The theoretical predictions are entirely consistent with experiment and indicate that, in a vertical depiction of ionization, shake-up states at binding energies larger than approximately 26.5 eV tend to decay via emission of a second electron in the continuum. A band of s-type symmetry that has been previously seen at approximately 25 eV in the electron impact ionization spectra of norbornane is entirely missing in the UPS measurements and theoretical ADC(3) spectra. With regard to these results and to the time scales characterizing electron-electron interactions in EMS (10(-17) s) as compared with that (10(-13) s) of photon-electron interactions in UPS, and considering the p-type symmetry of the electron momentum distributions for the nearest 1b(1) and 1b(2) orbitals, this additional band can certainly not be due to adiabatic double ionization processes starting from the ground electronic state of norbornane, or to exceptionally strong vibronic coupling interactions between cationic states derived from ionization of the latter orbitals. It is therefore tentatively ascribed to autoionization processes via electronically excited and possibly dissociating states.

Multistate vibronic interactions and nonadiabatic wave packet dynamics in the second photoelectron band of chlorine dioxide

Abstract We report theoretical investigations on the second photoelectron band of chlorine dioxide molecule by ab initio quantum dynamical methods. This band exhibits a highly complex structure and represents a composite portrait of five excited energetically close-lying electronic states of ClO2+. Much of this complexity is likely to be arising due to strong vibronic interactions among these electronic states – which we address and examine herein. The near equilibrium MRCI potential energy surfaces (PESs) of these five cationic states reported by Peterson and Werner [J. Chem. Phys. 99 (1993) 302] for the C2v configuration, are extended for the Cs geometry assuming a harmonic vibration along the asymmetric stretching mode. The strength of the vibronic coupling parameters of the Hamiltonian are calculated by ab initio CASSCF-MRCI method and conical intersections of the PESs are established. The diabatic Hamiltonian matrix is constructed within a linear vibronic coupling scheme and the resulting PESs are employed in the nuclear dynamical simulations, carried out with the aid of a time-dependent wave packet approach. Companion calculations are performed for transitions to the uncoupled electronic states in order to reveal explicitly the impact of the nonadiabatic coupling on the photoelectron dynamics. The theoretical findings are in good accord with the experimental observations. The femtosecond nonradiative decay dynamics of ClO2+ excited electronic states mediated by conical intersections is also examined and discussed.

Rotational analysis of jet-cooled phenylpentamethyldisilane: vibronic interaction with the intramolecular charge-transfer state

In order to determine an equilibrium structure of phenylpentamethyldisilane in the S 0 and S 1 states, high-resolution laser-induced fluorescence spectra were measured. An analysis of rotational envelopes of the 0 0 0 and 6b 0 1 bands was carried out. As a result, the most stable form was found to be a configuration in which the disilanyl group lies in a plane perpendicular to the phenyl ring, and it was found that a conjugative interaction between the σ Si–Si and π orbitals becomes strong in the S 1 state. A strong vibronic interaction, which gives a substantial transition intensity to the “forbidden” 6b 0 1 band, is discussed. The structure of the 6b 1 level in the S 1 state indicates that a σ Si–Siπ * type charge-transfer state exhibits an appreciable contribution to the vibronic interaction. In ( p- yanophenyl)ethynylpentamethyldisilane, it was also found that the similar strong vibronic interactions involved in their spectral feature.

Silver halidemicro- and nanocrystals embedded in an alkali halide matrix:suppression of the Jahn-Teller effect in nanoparticles

Self-organized microcrystals and nanocrystals (quantumdots) of AgCl and AgBr embedded in KCl and KBr crystallinematrices and retaining the orientation of the host latticewere studied by optically detected magnetic resonance. It wasunambiguously shown that self-organized microcrystalline silverhalides can be grown inside alkali halide crystals with theproperties of bulk crystals since the optically detectedmagnetic resonance spectra of the embedded microcrystals werepractically the same or close to those in bulk AgCl and AgBrand could be used as a `fingerprint' for AgCl and AgBr. ForAgCl nanocrystals in a KCl matrix the anisotropy of theg-factor both for isolated self-trapped holes and forself-trapped holes forming self-trapped excitons was found tobe substantially reduced compared with those of bulk AgClcrystals. This implies a considerable suppression of theJahn-Teller (JT) effect in nanoparticles. A rather general mechanismof the suppression of the JT effect in nanocrystalsis proposed, taking into account the additional deformationfield appearing because of the strong vibronic interaction atthe interface. It was concluded that the distribution ofexchange interactions for electron-hole pairs and triplet excitons in the KBr:AgBr system is due to a distribution of AgBrcrystal sizes. The holes seem to be self-trapped in the AgBrbecause of the dynamical JT effect. The exchange splittingincreases for distant electron-hole pairs with a decrease ofAgBr size. The spectra with exchange splitting larger thanthat in bulk AgBr (1.9 cm-1) seem to belong to AgBr nanocrystals. In contrast to AgCl the wavelength of theluminescence in AgBr micro- and nanocrystals embedded in a KBrmatrix decreases with the decrease of AgBr crystal size.

Laser induced dispersed fluorescence spectroscopy of 107 vibronic levels of NO2 ranging from 12 000 to 17 600 cm−1

We report the laser induced dispersed fluorescence spectra (LIDFS) of NO2 recorded when exciting 107 vibronic levels lying between 11 961 and 17 627 cm−1. These levels result from the vibronic interactions between the highly excited vibrational levels of the electronic ground state, X̃ 2A1, and the isoenergetic levels of the electronic excited state, à 2B2. One observes that the intensity patterns of almost half of the LIDFS can be interpreted in terms of Franck–Condon factors (FCFs) between a single bright parent level belonging to à 2B2 and the low-lying vibrational levels of X̃ 2A1. This means that the vibronic interactions occurring between the vibrational levels of X̃ 2A1 and à 2B2 are not strong enough to systematically mix together several bright parent states of à 2B2. We have also found that vibronic eigenstates scattering over several hundreds of cm−1 can be assigned to the same bright parent state. However, these features, which are characteristic of individual vibronic levels, must be faced with the statistical properties of the NO2 visible spectrum, well-known to exemplify molecular quantum chaos, at least above 16 500 cm−1 [J. Chem. Phys. 103, 1732 (1995)]. Complementarily, we briefly present results of the breakdown of vibrational and rotational selection rules, as observed in LIDFS. This phenomenon reflects the onset of strong vibronic and rovibronic interactions that progressively destroy the above mentioned one-bright-parent-state model and leads, at higher energies, to a regime of rovibronic chaos.

Suppression of the local Jahn-Teller effect in nanostructures: Self-trapped holes and excitons in AgCl nanocrystals

A strong decrease in the g-factor anisotropy was revealed by optically detected magnetic resonance for self-trapped Jahn-Teller holes (both isolated and forming self-trapped excitons) in AgCl nanocrystals embedded into the KCl crystal lattice. This is evidence for considerable suppression of the Jahn-Teller effect in nanoobjects. The suggested mechanism of suppression of the Jahn-Teller effect in nanocrystals is associated with an additional deformation field arising in nanocrystals owing to a strong vibronic interaction at the interface.

The radical cation of syn-tricyclooctadiene and its rearrangement products

The syn dimer of cyclobutadiene (tricyclo[4.2.0.0(2.5)]octa-3,7-diene, TOD) is subjected to ionization under different conditions and the resulting species are probed by optical and ESR spectroscopy. By means of quantum chemical modelling of the potential energy surfaces and the optical spectra, it is possible to assign the different products that arise spontaneously after ionization or after subsequent warming or illumination of the samples. Based on these findings, we propose a mechanistic scheme which involves a partitioning of the incipient radical cation of TOD between two electronic states. These two states engage in (near) activation-less decay to the more stable valence isomers, cyclooctatetraene (COT*+) and a bis-cyclobutenylium radical cation BCB*+. The latter product undergoes further rearrangement, first to tetracyclo[4.2.0.0(2,4).0(3,5]oct-7-ene (TCO*+) and eventually to bicyclo[4.2.0]octa-2,4,7-triene (BOT*+) which can also be generated photochemically from BCB*+ or TCO*+. The surprising departure of syn-TOD*+ from the least-motion reaction path leading to BOT*+ can be traced to strong vibronic interactions (second-order Jahn-Teller effects) which prevail in both possible ground states of syn-TOD*+. Such effects seem to be more important in determining the intramolecular reactivity of radical cations than orbital or state symmetry rules.

High-resolution laser spectroscopy of NO2 just above the X̃ 2A1–Ã 2B2 conical intersection: Transitions of K−=0 stacks

The visible absorption spectrum of NO2 is very dense and irregular, and shows signs of a chaotic frequency and intensity distribution in the higher energy region. The complexity of the spectrum is related to a conical intersection of the potential energy surfaces of the two lowest electronic states. Above the conical intersection strong vibronic interactions lead to hybrid eigenstates, which can be viewed as mixtures of low vibrational levels of the electronically excited state and high vibrational levels of the electronic ground state. As a contribution to the elucidation of the nature of the vibronic bands of NO2 we have measured high-resolution spectra of a number of vibronic bands in the region between 10 000 and 14 000 cm−1 by exciting a supersonically cooled beam of NO2 molecules with a narrow-band Ti:Sapphire ring laser. The energy absorbed by the molecules was detected by a bolometer, and in some cases, laser-induced fluorescence was detected. The hyperfine structure is dominated by the Fermi-contact interaction and the magnitude of this interaction is a direct measure of the (electronic) composition of the hybrid eigenstates. In the present paper we have restricted our analysis to transitions of K−=0 stacks. The fine- and hyperfine structure of each rotational transition can be analyzed by using an effective Hamiltonian approach. The very good agreement that is found between the calculated transition strengths and the measured line intensities is evidence that in the spectral region studied, rovibronic interactions play a minor role. The composition of the hybrid eigenstates is compared with ab initio calculations reported in the literature, leading to the conclusion that measurements of the hyperfine structure are a helpful tool in characterizing vibronic bands.

The C4H6•+ Potential Energy Surface. 1. The Ring-Opening Reaction of Cyclobutene Radical Cation and Related Rearrangements

Ab initio MO and density functional calculations indicate that the ring opening of the cyclobutene radical cation (CB•+) follows two competitive pathways, whose energy barriers differ by less than 1 kcal/mol at the highest level of theory employed, RCCSD(T)/cc-pVTZ//UQCISD/6-31G*. The first corresponds to a conrotatory rearrangement to the cis-butadiene radical cation (cis-BD•+). The second one leads to trans-BD•+ via a very flat potential energy plateau which comprises cyclopropylcarbinyl-type structures of the type proposed some time ago by Bauld, but the controtatory stereochemistry is preserved also along this process. State correlation diagrams indicate that the rearrangement leading to trans-BD•+ may occur adiabatically along a C2 reaction coordinate. Despite this, the transition state has no symmetry. This seemingly “unnecessary” loss of symmetry is traced back to the proximity of the 2A and 2B surfaces in the vicinity of the C2 stationary points, where the two states encounter a strong vibronic interaction which leads to breaking of the C2 symmetry. These vibronic interactions are also responsible for the general flattening of the potential energy surface in this area.

Faraday Laser Magnetic Resonance Spectroscopy of Vibrationally Excited C2D

We studied the gas phase spectrum of the deuterated ethynyl radical C2D in the region between 3196 and 3243 cm−1using a Faraday LMR spectrometer in combination with a CO overtone laser. The C2D radicals were generated in a dc glow discharge containing helium, deuterium, and acetylene. We observed a hot band between two vibronic2Π states with an origin at 3225 cm−1. The lower level is assigned to the first excited bending level of the electronicXground state. The upper level corresponds to the first excited electronic stateAat 3513 cm−1, which was observed previously [J. Mol. Struct.190,41–60 (1988)]. This region is subject to strong vibronic interaction, caused by mixing of the electronicXground state with theAstate at 3513 cm−1. From the analysis of the spectra we could determine the orbitalgfactor of the upper level, which gave important information about the mixing ratios. In addition we were able to derive a precise term value for the first excited bending level of the electronicXground state. The experimentally derived molecular parameters are compared with theoretically calculated values, obtained byab initiocalculations.

Mixed Valent Nickel and Manganese Oxide Ceramics—Model Systems with Superconducting Properties?

Various oxidic solids of the perovskite and K 2NiF 4 type with Mn(III) and low-spin Ni(III) have been synthesized with clear structural and spectroscopic evidence for strong vibronic Jahn-Teller interactions. In the latter case clustering seems to induce a change to the high-spin state. In mixed valent Mn(III)/Mu(IV) compounds the Jahn-Teller effect is suppressed by band-broadening effects above a critical Mn(IV) concentration. Mixed valence Ni(III)/Ni(IV) ceramics could also be prepared, in which the positive hole is presumably located predominantly on the oxygen ligands, however. Superconducting properties are not observed down to 4 K. The possible significance of a strongly Jahn-Teller unstable E ground state, which occurs in the cases of Cu 2+, Mn 3+, and low-spin Ni 3+ in octahedral coordination, for the appearance of superconductivity is discussed.

Jet cooled NO2 intra cavity laser absorption spectroscopy (ICLAS) between 11200 and 16150 cm−1

We have combined the high sensitivity of the ICLAS technique with the rotational cooling effect of a slit jet expansion in order to observe and to understand the visible and near infrared NO2 spectrum. By this way, an equivalent absorption pathlength of several kilometers through rotationally cooled molecules has been achieved. Due to the vibronic interaction between the two lowest electronic states, X̃2A1 and à 2B2, this spectrum is vibronically dense and complex. Moreover, the dense room temperature rotational structure is perturbed by additional rovibronic interactions. In contrast, the rotational analysis of our jet cooled spectrum is straightforward. The NO2 absorption spectrum is vanishing to the IR but, owing to the high sensitivity of the ICLAS technique, we have been able to record the NO2 spectrum down to 11200 cm−1 with a new Ti:sapphire ICLAS spectrometer. As a result 249 2B2 vibronic bands have been observed (175 cold bands and 74 hot bands) in the 11200–16150 cm−1 energy range. Due to the cooling effect of the slit jet we have reduced the rotational temperature down to about 12 K and at this temperature the K = 0 subbands are dominant. Consequently, we have analysed only the K = 0 manifold for N ⩽ 7 of each vibronic band. The dynamical range of the band intensities is about one thousand. Due to the strong vibronic interaction between the X̃ 2A1 and à 2B2 electronic states, we observed not only the a1 vibrational levels of the à 2B2 state but also the b2 vibrational levels of the X̃ 2A1 state interacting with the previous ones. By comparison with the calculated density of states, we conclude that we have observed about 65% of the total number of 2B2 vibronic levels located in the studied range. However, there are more missing levels in the IR because of the weakness of the spectrum in this range. The correlation properties of this set of vibronic levels have been analysed calculating the power spectrum of the absorption stick spectrum which displays periodic motions: the dominant period, at 714 ± 20 cm−1, corresponds to the bending motion of the à 2B2 state. The other observed periods remain unassigned. In contrast the next neighbor spacing distribution (NNSD) shows a strong level repulsion, i.e. a manifestation of quantum chaos. These two observations, apparently contradictory, can be rationalized as follows: the short time dynamics, for t < 10−12 s, is “regular” while for longer times the dynamics becomes “chaotic”. We suggest that this behavior may be observed directly with a pump and probe fs laser experiment.

The geometric phase in two electronic level systems

The effects of the geometric phase on vibronic states associated with the lower potential surface of two electronic level Hamiltonians are examined. We obtain the general formula for the gauge potential arising from the vibronic interaction. It is shown that this gauge potential is split into a topological part and a magnetic part, where the topological part gives rise to the phase factor of +1 or −1 when it is integrated along a closed trajectory in the nuclear coordinate space, and the magnetic part gives rise to a contribution depending on the local character of the trajectory and exists only in systems without time-reversal symmetry. For particular examples, we consider the E⊗e and E⊗(b1+b2) Jahn–Teller systems with strong vibronic interactions. It is demonstrated that the ground states have vibronic standing wave states whose nuclear probability density distributions are localized in one of the equivalent minima on the lower potential surface. We also consider Zeeman splittings of degenerate vibronic states, where the degeneracy arises from time-reversal symmetry.

Difference-frequency laser spectroscopy of the 3ν2+ν3 band of C2H

A spectrum of C2H 2Π–2Σ+ transition centered at 2928 cm−1 has been measured in the gas phase using a high resolution difference-frequency infrared laser spectrometer. The C2H molecules are produced in a hollow cathode discharge through a mixture of C2H2 (∼20 mTorr) and H2 (∼500 mTorr). The discharge amplitude modulation technique is used to enhance sensitivity and selectivity. About 125 lines are rotationally analyzed and accurate molecular constants are determined through a least-squares fit. The band is assigned to the 3ν2+ν3 combination band. The assignment is consistent with the observed 2Π–2Σ+ symmetry and is in good agreement with the recent theoretical calculation of the vibrational frequency and the spin–orbit coupling constant. The relatively large spin–orbit coupling constant of the (0 3 1) state is indicative of strong vibronic interaction between the à and X̃ states. A small perturbation appears near the end of observed Q-branch series, which is probably caused by a Σ vibronic state. New assignments of the progressions (0 v2 0) and (0 v2 1) toward higher v2 are proposed based on the existing gas phase and rare gas matrix data with the help of recent ab initio theoretical predictions.

Magnetic moments of multielectron mixed-valency dimer clusters in a dynamic intermediate vibronic coupling approach

The vibronic aspect of the Jahn-Teller effect is considered for multielectron mixed-valency dimer clusters. A basic intermediate-coupling set is used in the numerical diagonalization of the vibronic Hamiltonian. The temperature dependence of the vibronic magnetic moment is derived for symmetrical and distorted mixed-valency dimer clusters. Strong vibronic interaction can suppress double exchange and lead to an antiferromagnetic ground state for the cluster. Distortions that eliminate the inversion center localize the surplus electron and suppress the ferromagnetic effect caused by double exchange.