Theoretical search for small linear doubly charged anions

Super-electrophiles of tri- and tetra-anions stabilized by selected terminal groups and their role in binding noble gas atoms

Stabilization of multiply-charged atomic clusters in the gas phase has been a topic of great interest not only because of their potential applications as weakly-coordinating anions, but also for their ability to promote unusual reactions and serve as building blocks of materials. Recent experiments have shown that, after removing one terminal ligand from the closo-dodecacyano-borate, B12(CN)122-, the cluster can strongly bind an argon atom at room temperature. Bearing this in mind, here, we have developed more than a dozen highly stable tri- and tetra-anions using density functional theory (DFT) calculations with hybrid functional (B3LYP) and semi-empirical dispersion corrections. The interactions between the clusters and noble gas atoms, including Ne, Ar and Kr, are studied. The resulting super-electrophilic sites embedded in these charged clusters can bind noble gas atoms with binding energies up to 0.7 eV. This study enriches the database of highly-charged clusters and provides a viable design rule for super-electrophiles that can strongly bind noble gas atoms.

Record-high stability and compactness of multiply-charged clusters aided by selected terminal groups

Multiply-charged clusters with compact sizes that are stable in the gas phase are important due to their potential applications as weakly-coordinating ions and building blocks of bulk materials.

Ground and excited states of the diatomic dianion Cl2(2-)

The QCISD(T)/aug-cc-pVQZ and CIS/aug-cc-pVQZ calculations have been carried out to obtain potential energy curves (PECs) of the Cl2(2-) diatomic dianion in order to address possibility of its formation in the merged beam fragmentation of Cl2(-) questioned based on the observation of the Cl(-)+Cl+e(-) channel. Results show that two of the excited states, namely A(1)Σg and a(3)Σg are metastable with PECs having wells deep enough to suite several bound states, with minima located at Re=2.8280 Å and Re=2.5972 Å, and Coulomb barriers of 1648.288 and 1403.835 cm(-1) heights located at 4.0320 and 3.6130 Å, respectively. Transition probabilities and tunneling predissociation lifetimes corresponding to these metastable states are also calculated and analyzed. Ground state X(1)Σg and excited states B(1)Σu, C(1)Πg and D(1)Πu calculated for this dianion are all repulsive. Calculated Franck-Condon factors suggest that Cl2(2-) can be produced in its excited states via an electron impact process initiating from the ground states of Cl2 and Cl2(-) .

Small gas-phase dianions of Zn3O42−, Zn4O52−, CuZn2O42−, Si2GeO62−, Ti2O52−and Ti3O72−

We have searched for new species of small oxygen-containing gas-phase dianions produced in a secondary ion mass spectrometer by Cs+ ion bombardment of solid samples with simultaneous exposure of their surfaces to O2 gas. The targets were a pure zinc metal foil, a copper-contaminated zinc-based coin, two silicon-germanium samples (Si(1-x)Ge(x)(with x= 6.5% or 27%)) and a piece of titanium metal. The novel dianions Zn3O(4)(2-), Zn4O(5)(2-), CuZn2O(4)(2-), Si2GeO(6)(2-), Ti2O(5)(2-) and Ti3O(7)(2-) have been observed at half-integer m/z values in the negative ion mass spectra. The heptamer dianions Zn3O(4)(2-) and Ti2O(5)(2-) have been unambiguously identified by their isotopic abundances. Their flight times through the mass spectrometer are approximately 20 micros and approximately 17 micros, respectively. The geometrical structures of the two heptamer dianions Ti2O(5)(2-), and Zn3O(4)(2-) are investigated using ab initio methods, and the identified isomers are compared to those of the novel Ge2O(5)(2-) and the known Si2O(5)(2-) and Be3O(4)(2-) dianions.

New stable multiply charged negative atomic ions in linearly polarized superintense laser fields

Singly charged negative atomic ions exist in the gas phase and are of fundamental importance in atomic and molecular physics. However, theoretical calculations and experimental results clearly exclude the existence of any stable doubly-negatively-charged atomic ion in the gas phase, only one electron can be added to a free atom in the gas phase. In this report, using the high-frequency Floquet theory, we predict that in a linear superintense laser field one can stabilize multiply charged negative atomic ions in the gas phase. We present self-consistent field calculations for the linear superintense laser fields needed to bind extra one and two electrons to form He-, He2-, and Li2-, with detachment energies dependent on the laser intensity and maximal values of 1.2, 0.12, and 0.13 eV, respectively. The fields and frequencies needed for binding extra electrons are within experimental reach. This method of stabilization is general and can be used to predict stability of larger multiply charged negative atomic ions.

Small gas-phase dianions produced by sputtering and gas flooding

We have extended our previous experiment [Schauer et al., Phys. Rev. Lett. 65, 625 (1990)] where we had produced small gas-phase dianion clusters of C(n) (2-)(n > or =7) by means of sputtering a graphite surface by Cs(+) ion bombardment. Our detection sensitivity for small C(n) (2-) could now be increased by a factor of about 50 for odd n. Nevertheless, a search for the elusive pentamer dianion of C(5) (2-) was not successful. As an upper limit, the sputtered flux of C(5) (2-) must be at least a factor of 5000 lower than that of C(7) (2-), provided that the lifetime of C(5) (2-) is sufficiently long to allow its detection by mass spectrometry. When oxygen gas (flooding with either O(2) or with N(2)O) was supplied to the Cs(+)-bombarded graphite surface, small dianions of OC(n) (2-)(5< or =n < or =14) and O(2)C(7) (2-) were observed in addition to C(n) (2-)(n > or =7). Similarly, Cs(+) sputtering of graphite with simultaneous SF(6) gas flooding produced SC(n) (2-)(6< or =n< or =18). Mixed nitrogen-carbon or fluorine-carbon dianion clusters could not be observed by these means. Attempts to detect mixed metal-fluoride dianions for SF(6) gas flooding of various Cs(+)-bombarded metal surfaces were successful for the case of Zr, where metastable ZrF(6) (2-) was observed. Cs(+) bombardment of a silicon carbide (SiC) wafer produced SiC(n) (2-) (n=6,8,10). When oxygen gas was supplied to the Cs(+)-bombarded SiC surface, small dianions of SiOC(n) (2-) (n=4,6,8) and of SiO(2)C(n) (2-) (n=4,6) as well as a heavier unidentified dianion (at mz=98.5) were observed. For toluene (C(7)H(8)) vapor flooding of a Cs(+)-bombarded graphite surface, several hydrocarbon dianion clusters of C(n)H(m) (2-)(n> or =7) were produced in addition to C(n) (2-)(n> or =7), while smaller C(n)H(m) (2-) with n< or =6 could not be observed. BeC(n) (2-) (n=4,6,8,10), Be(2)C(6) (2-), as well as BeC(8)H(m) (2-) (with m=2 and/or m=1) were observed for toluene vapor flooding of a Cs(+)-bombarded beryllium metal foil. The metastable pentamer (9)Be(12)C(4) (2-) at mz=28.5 was the smallest and lightest dianion molecule that we could detect. The small dianion clusters of SC(n) (2-), OC(n) (2-), BeC(n) (2-), and SiO(m)C(n) (2-) (m=0,1,2) have different abundance patterns. A resemblance exists between the abundance patterns of BeC(n) (2-) and SiC(n) (2-), even though calculated molecular structures of BeC(6) (2-) and SiC(6) (2-) are different. The abundance pattern of SC(n) (2-) is fairly similar to that of C(n) (2-).

Photodetachment spectroscopy of PtBr42−: Probing the Coulomb barrier of a doubly charged anion

We probe the repulsive Coulomb barrier of the doubly charged anion PtBr(4) (2-) by photodetachment spectroscopy. The results are discussed in terms of models for the photoemission process, the excitation spectrum of PtBr(4) (2-), and calculations of the energy-dependent tunneling probability for various model potentials.

First-principles T-matrix calculations of double-ionization energy spectra of atoms and molecules

Strong electron correlation plays an important role in the determination of double ionization energy, which is required for removing or adding two electrons, particularly in small-sized systems. Starting from the state-of-the-art GW approximation, we evaluate the particle-particle ladder diagrams up to the infinite order by solving the Bethe-Salpeter equation of the T-matrix theory to calculate the double-ionization energy spectra of atoms and molecules (Be, Mg, Ca, Ne, Ar, Kr, CO, C(2)H(2), Li(2), Na(2), and K(2)) from first principles. The ladder diagrams up to the infinite order are significant to calculations of double-ionization energy spectra. The present results are in good agreement with available experimental data as well as the previous calculations using, e.g., the configuration-interaction method.

Open Shell Dianions Likely To Be Stable in the Gas Phase with Respect to Autoionization

We address the challenge set by Dreuw and Cederbaum [Dreuw, A.; Cederbaum, L. S. Chem. Rev. 2002, 102, 181-200] to develop a set of "small" open shell stable dianions. We offer two families of such species, based on a central diradical center with attached anionic sites. Both families achieve dianion stabilization through alternating zones of positive and negative charge. First, quasi-linear systems such as NB(C2)n-Q-(C2)nBN become diradical dianions stable to autoionization in two cases:  (a) for Q a divalent (carbene) carbon when n is two or greater and (b) for Q a C4 ring diradical when n is one or greater. Second, carbenes with certain anionic aromatic substituents can be stable with respect to autoionization. π-Acid substituted carbenes (A2Q) have triplet ground states generally. If A is cyclopentadienyl anion stabilized by cyano substitution, the resulting triplet dianion is stable with respect to autoionization. In bisphenyl carbenes the triplet is relatively stabilized when ortho substituents force the system toward D2d symmetry. The dianion of bis(orthochlorophenyl) carbene produced by para-substitution with BN retains the triplet preference and is stable with respect to autoionization. These results obtained first by density functional calculations in a small basis, B3LYP/6-31G(d), are confirmed and extended by ROMP2 and CCSD calculations in a more flexible basis, 6-31+G(d). DFT has a systematic bias against systems with excess electrons, which is paradoxically a virtue in the screening of candidate dianions since the DFT calculation does not make false predictions of stability.

Triple-Bond Covalent Radii

A system of additive covalent radii is proposed for sigma(2) pi(4) triple bonds involving elements from Be to E 112 (eka-mercury). Borderline cases with weak multiple bonding are included. Only the elements in Group 1, the elements Zn-Hg in Group 12 and Ne in Group 18 are then totally excluded. Gaps are left at late actinides and some lanthanides. The standard deviation for the 324 included data points is 3.2 pm.

The Repulsive Coulomb Barrier along a Dissociation Path of the Be Dianion

We present ab initio calculations of the repulsive Coulomb barrier for several geometrically stable isomers of the BeC(2-)(4) dianion. We describe how the deformation of certain isomers can account for the experimental Coulomb explosion images of the dianion. For the most stable linear isomer, C(-)(2)BeC(-)(2), we examined the electron tunneling process along the dissociation path to obtain C(-)(2) plus BeC(-)(2). We found the crossing point for autodetachment to be R(c)(dis)= 3.25 A. R(dis) is the bond length between C(-)(2) and BeC(-)(2); at this point, the electron tunneling energy is equal to the maximum of the repulsive Coulomb barrier. In the framework of the Wenzel-Kramer-Brioullin theory, the electron-loss lifetime of the metastable C(-)(2)BeC(-)(2) dianion at the equilibrium geometry, R(dis) = 1.64 A, was estimated to be about 5 ms. This lower limit is in agreement with the experimental results in which the BeC(2-)(4) dianion has a lifetime much longer than 5 micros.

Gas-phase stability of derivatives of the closo-hexaborate dianion B(6)H(6)(2-)

B(6)H(6)(2-) does not represent a stable gas-phase dianion, but emits spontaneously one of its excess electrons in the gas phase. In this work we address the question whether small stable gas-phase dianions can be constructed from the parent B(6)H(6)(2-) dianion by substitution of the hydrogens with appropriate ligands. Various hexa-, tetra-, and disubstituted derivatives B(6)L(6)(2-), B(6)H(2)L(4)(2-), and B(6)H(4)L(2)(2-) (L = F, Cl, CN, NC, or BO) are investigated with ab initio methods in detail. Four stable hexasubstituted B(6)L(6)(2-) (L = Cl, CN, NC, or BO) and three stable B(6)H(2)L(4)(2-) (L = CN, NC, or BO) gas-phase dianions could be identified and predicted to be observable in the gas phase. The trends in the electron-detachment energies depending on various ligands are discussed and understood in the underlying electrostatic pattern and the electronegativities of the involved elements.

Effect of Protonation on Peroxo-Copper Bonding: Spectroscopic and Electronic Structure Study of [Cu(2)((UN-O-)(OOH)](2+)

Spectroscopic studies of a &mgr;-1,1-hydroperoxo-bridged copper dimer are combined with SCF-Xalpha-SW molecular orbital calculations to describe the vibrational and electronic structure of the hydroperoxo-copper complex and compare it to that of previously studied peroxo-copper species. Four vibrational modes of the Cu(2)OOH unit in the resonance Raman and infrared spectra are assigned on the basis of isotope shifts: nu(O-O) = 892 cm(-)(1), nu(as)(Cu-O) = 506 cm(-)(1), nu(s)(Cu-O) = 322 cm(-)(1), and nu(O-H) = 3495 cm(-)(1). The 892 cm(-)(1) O-O stretch of the &mgr;-1,1-hydroperoxo-bridged copper dimer is 89 cm(-)(1) higher than that of the unprotonated complex. Resonance Raman profiles of the 892 cm(-)(1) O-O stretch are used to assign an electronic absorption band at 25 200 cm(-)(1) (epsilon = 6700 M(-)(1) cm(-)(1)) to a hydroperoxide pi-to-Cu charge transfer (CT) transition. This band is approximately 5000 cm(-)(1) higher in energy than the corresponding transition in the unprotonated complex. The pi-to-Cu CT transition intensity defines the degree of hydroperoxide-to-copper charge donation, which is lower than in the unprotonated complex due to the increased electronegativity of the peroxide with protonation. The lower Cu-O covalency of this hydroperoxo-copper complex shows that the high O-O stretching frequency is not due to increased pi-to-Cu charge donation but rather reflects the direct effect of protonation on intra-peroxide bonding. Density functional calculations are used to describe changes in intra-peroxide and Cu-O bonding upon protonation of the peroxo-copper complex and to relate these changes to changes in reactivity.