Low-lying isomers of the B9(-) boron cluster: the planar molecular wheel versus three-dimensional structures

The B(9)(-) cluster was found previously to be an unprecedented molecular wheel containing an octacoordinate planar boron with D(8h) symmetry in a combined photoelectron spectroscopy (PES) and theoretical study [H. J. Zhai et al., Angew. Chem., Int. Ed. 42, 6004 (2003)]. However, the PES spectra of B(9)(-) exhibit minor features that cannot be explained by the global minimum D(8h) structure, suggesting possible contributions from low-lying isomers at finite temperatures. Here we present Car-Parrinello molecular dynamics with simulated annealing simulations to fully explore the potential energy surface of B(9)(-) and search for low-lying isomers that may account for the minor PES features. We performed density functional theory (DFT) calculations with different exchange-correlation functionals and ab initio calculations at various levels of theory with different basis sets. Two three-dimensional low-lying isomers were found, both of C(s) symmetry, 6.29 (C(s)-2) and 10.23 (C(s)-1) kcal/mol higher in energy than the D(8h) structure at the highest CCSD(T) level of theory. Calculated detachment transitions from the C(s)-2 isomer are in excellent agreement with the minor features observed in the PES spectra of B(9)(-). The B(9)(-) cluster proves to be a challenge for most DFT methods and the calculated relative energies strongly depend on the exchange-correlation functionals, providing an excellent example for evaluating the accuracies of various DFT methods.     

Ab initio calculation of bowl, cage, and ring isomers of C20 and C20-

High-level ab initio calculations have been carried out to reexamine relative stability of bowl, cage, and ring isomers of C(20) and C(20)(-). The total electronic energies of the three isomers show different energy orderings, strongly depending on the hybrid functionals selected. It is found that among three popular hybrid density-functional (DF) methods B3LYP, B3PW91, PBE1PBE, and a new hybrid-meta-DF method TPSSKCIS, only the PBE1PBE method (with cc-pVTZ basis set) gives qualitatively correct energy ordering as that predicted from ab initio CCSD(T)/cc-pVDZ [CCSD(T)-coupled-cluster method including singles, doubles, and noniterative perturbative triples; cc-pVDZ-correlation consistent polarized valence double zeta] as well as from MP4(SDQ)/cc-pVTZ [MP4-fourth-order Moller-Plesset; cc-pVTZ-correlation consistent polarized valence triple zeta] calculations. Both CCSD(T) and MP4 calculations indicate that the bowl is most likely the global minimum of neutral C(20) isomers, followed by the fullerene cage and ring. For the anionic counterparts, the PBE1PBE calculation also agrees with MP4/cc-pVTZ calculation, both predicting that the bowl is still the lowest-energy structure of C(20)(-) at T=0 K, followed by the ring and the cage. In contrast, both B3LYP/cc-pVTZ and B3PW91/cc-pVTZ calculations predict that the ring is the lowest-energy structure of C(20)(-). Apparently, this good reliability in predicting the energy ordering renders the hybrid PBE method a leading choice for predicting relative stability among large-sized carbon clusters and other carbon nanostructures (e.g., finite-size carbon nanotubes, nano-onions, or nanohorns). The relative stabilities derived from total energy with Gibbs free-energy corrections demonstrate a changing ordering in which ring becomes more favorable for both C(20) and C(20)(-) at high temperatures. Finally, photoelectron spectra (PES) for the anionic C(20)(-) isomers have been computed. With binding energies up to 7 eV, the simulated PES show ample spectral features to distinguish the three competitive C(20)(-) isomers.     

Infrared predissociation spectroscopy of M+ (C6H6)(1-4)(H2O)(1-2)Ar(0-1) cluster ions, M = Li, Na

Infrared predissociation (IRPD) spectra of Li(+)(C(6)H(6))(1-4)(H(2)O)(1-2)Ar(0-1) and Na(+)(C(6)H(6))(2-4)(H(2)O)(1-2)Ar(1) are presented along with ab initio calculations. The results indicate that the global minimum energy structure for Li(+)(C(6)H(6))(2)(H(2)O)(2) has each water forming a π-hydrogen bond with the same benzene molecule. This bonding motif is preserved in Li(+)(C(6)H(6))(3-4)(H(2)O)(2)Ar(0-1) with the additional benzene ligands binding to the available free OH groups. Argon tagging allows high-energy Li(+)(C(6)H(6))(2-4)(H(2)O)(2)Ar isomers containing water-water hydrogen bonds to be trapped and detected. The monohydrated, Li(+) containing clusters contain benzene-water interactions with varying strength as indicated by shifts in OH stretching frequencies. The IRPD spectra of M(+)(C(6)H(6))(1-4)(H(2)O)(1-2)Ar are very different for lithium-bearing versus sodium-bearing cluster ions emphasizing the important role of ion size in determining the most favorable balance of competing noncovalent interactions.     

Potential energy surface of O2-(H2O) and factors controlling water-to-O2- binding motifs

The potential energy surface (PES) of O(2)(-)(H(2)O) is investigated by varying the interoxygen distance of O(2)(-) via ab initio calculations with a large basis set. Although two stationary points, C(s) and C(2v) conformers, are found along the interoxygen-distance coordinate, only the C(s) conformer is identified as a minimum-energy species. We find a critical distance, r(c), separating these two conformers in the PES. The C(s) conformer prevails at interoxygen distances of O(2)(-) that are less than r(c), while the C(2v) conformer dominates at the distances larger than r(c). The structural features of these two conformers are also discussed. Although the water deformation energy is shown to be the stabilization source responsible for the prevalence of the C(s) cluster conformer at the interoxygen distances of O(2)(-) less than r(c), the ionic hydrogen bonding is the major driving force for transformation of the water binding motif from C(s) to C(2v) when the interoxygen distance of O(2)(-) increases.     

Low-lying isomers and finite temperature behavior of (H2O)6 -

(H2O)(6) (-) appears as a "magic" number water cluster in (H2O)(n) (-) mass spectra. The structure of the (H2O)(6) (-) isomer dominating the experimental population has been established only recently [N. I. Hammer et al., J. Phys. Chem. A 109, 7896 (2005)], and the most noteworthy characteristic of this isomer is the localization of the excess electron in the vicinity of a double-acceptor monomer. In the present work, we use a quantum Drude model to characterize the low-energy isomers and the finite temperature properties of (H2O)(6) (-). Comparison with ab initio calculations shows that the use of a water model employing distributed polarizabilities and distributed repulsive sites is necessary to correctly reproduce the energy ordering of the low-lying isomers. Both the simulations and the ab initio calculations predict that there are several isomers of (H2O)(6) (-) significantly lower in energy than the experimentally observed species, suggesting that the experimental distribution is far from equilibrium.     

First principles study on the solvation and structure of C2O4(2-)(H2O)n, n = 6-12

The structures and energies of hydrated oxalate clusters, C2O4(2-)(H2O)n, n = 6-12, are obtained by density functional theory (DFT) calculations and compared to SO4(2-)(H2O)n. Although the evolution of the cluster structure with size is similar to that of SO4(2-)(H2O)n, there are a number of important and distinctive futures in C2O4(2-)(H2O)n, including the separation of the two charges due to the C-C bond in C2O4(2-), the lower symmetry around C2O4(2-), and the torsion along the C-C bond, that affect both the structure and the solvation energy. The solvation dynamics for the isomers of C2O4(2-)(H2O)12 are also examined by DFT based ab initio molecular dynamics.     

All-boron analogues of aromatic hydrocarbons: B17- and B18-

We have investigated the structural and electronic properties of the B(17)(-) and B(18)(-) clusters using photoelectron spectroscopy (PES) and ab initio calculations. The adiabatic electron detachment energies of B(17)(-) and B(18)(-) are measured to be 4.23 ± 0.02 and 3.53 ± 0.05 eV, respectively. Calculated electron detachment energies are compared with experimental data, confirming the presence of one planar C(2v) ((1)A(1)) isomer for B(17)(-) and two nearly isoenergetic quasi-planar C(3v) ((2)A(1)) and C(s) ((2)A') isomers for B(18)(-). The stability and planarity/quasi-planarity of B(17)(-) and B(18)(-) are ascribed to σ- and π-aromaticity. Chemical bonding analyses reveal that the nature of π-bonding in B(17)(-) and B(18)(-) is similar to that in the recently elucidated B(16)(2-) and B(19)(-) clusters, respectively. The planar B(17)(-) cluster can be considered as an all-boron analogue of naphthalene, whereas the π-bonding in the quasi-planar B(18)(-) is reminiscent of that in coronene.     

Al(6)(2-) - fusion of two aromatic Al(3)(-) units. A combined photoelectron spectroscopy and ab initio study of M(+)[Al(6)(2-)] (M = Li,Na, K,Cu, and Au)

Photoelectron spectroscopy is combined with ab initio calculations to elucidate the structure and chemical bonding of a series of MAl(6)(-) (M = Li, Na, K, Cu, and Au) bimetallic clusters. Well-resolved photoelectron spectra were obtained for MAl(6)(-) (M = Li, Na, Cu, and Au) at several photon energies. The ab initio calculations showed that all of the MAl(6)(-) clusters can be viewed as an M(+) cation interacting with an Al(6)(2-) dianion. Al(6)(2-) was found to possess an O(h) ground-state structure, and all of the MAl(6)(-) clusters possess a C(3v) ground-state structure derived from the O(h) Al(6)(2-). Careful comparison between the photoelectron spectral features and the ab initio one-electron detachment energies allows us to establish firmly the C(3v)ground-state structures for the MAl(6)(-) clusters. A detailed molecular orbital (MO) analysis is conducted for Al(6)(2-) and compared with Al(3)(-). It was shown that Al(6)(2-) can be considered as the fusion of two Al(3)(-) units. We further found that the preferred occupation of those MOs derived from the sums of the empty 2e' MOs of Al(3)(-), rather than those derived from the differences between the occupied 2a(1)' and 2a(2)' ' MOs of Al(3)(-), provides the key bonding interactions for the fusion of the two Al(3)(-) into Al(6)(2-). Because there are only four bonding MOs (one pi and three sigma MOs), an analysis of resonance structures was performed for the O(h)Al(6)(2-). It is shown that every face of the Al(6)(2-) octahedron still possesses both pi- and sigma-aromaticity, analogous to Al(3)(-), and that in fact Al(6)(2-) can be viewed to possess three-dimensional pi- and sigma-aromaticity with a large resonance stabilization.     

Vacuum ultraviolet photoionization of C3

Photoionization efficiency (PIE) curves for C(3) molecules produced by laser ablation are measured from 11.0 to 13.5 eV with tunable vacuum ultraviolet undulator radiation. A step in the PIE curve versus photon energy, obtained with N(2) as the carrier gas, supports the conclusion of very effective cooling of C(3) to its linear (1)Sigma(g)(+) ground state. The second step observed in the PIE curve versus photon energy could be the first experimental evidence of the C(3)(+)((2)Sigma(g)(+)) excited state. The experimental results, complemented by ab initio calculations, suggest a state-to-state vertical ionization energy of 11.70 +/- 0.05 eV between the C(3)(X(1)Sigma(g)(+)) and the C(3)(+)(X(2)Sigma(u)(+)) states. An ionization energy of 11.61 +/- 0.07 eV between the neutral and ionic ground states of C(3) is deduced using the data together with our calculations. Accurate ab initio calculations are performed for both linear and bent geometries on the lowest doublet electronic states of C(3)(+) using Configuration Interaction (CI) approaches and large basis sets. These calculations confirm that C(3)(+) is bent in its electronic ground state, which is separated by a small potential barrier from the (2)Sigma(u)(+) minimum. The gradual increase at the onset of the PIE curve suggests a geometry change between the ground neutral and cationic states. The energies between several doublet states of the ion are theoretically determined to be 0.81, 1.49, and 1.98 eV between the (2)Sigma(u)(+) and the (2)Sigma(g)(+),( 2)Pi(u), (2)Pi(g) excited states of C(3)(+), respectively.     

An isotopic-independent highly accurate potential energy surface for CO2 isotopologues and an initial (12)C(16)O2 infrared line list

An isotopic-independent, highly accurate potential energy surface (PES) has been determined for CO(2) by refining a purely ab initio PES with selected, purely experimentally determined rovibrational energy levels. The purely ab initio PES is denoted Ames-0, while the refined PES is denoted Ames-1. Detailed tests are performed to demonstrate the spectroscopic accuracy of the Ames-1 PES. It is shown that Ames-1 yields σ(rms) (root-mean-squares error) = 0.0156 cm(-1) for 6873 J = 0-117 (12)C(16)O(2) experimental energy levels, even though less than 500 (12)C(16)O(2) energy levels were included in the refinement procedure. It is also demonstrated that, without any additional refinement, Ames-1 yields very good agreement for isotopologues. Specifically, for the (12)C(16)O(2) and (13)C(16)O(2) isotopologues, spectroscopic constants G(v) computed from Ames-1 are within ±0.01 and 0.02 cm(-1) of reliable experimentally derived values, while for the (16)O(12)C(18)O, (16)O(12)C(17)O, (16)O(13)C(18)O, (16)O(13)C(17)O, (12)C(18)O(2), (17)O(12)C(18)O, (12)C(17)O(2), (13)C(18)O(2), (13)C(17)O(2), (17)O(13)C(18)O, and (14)C(16)O(2) isotopologues, the differences are between ±0.10 and 0.15 cm(-1). To our knowledge, this is the first time a polyatomic PES has been refined using such high J values, and this has led to new challenges in the refinement procedure. An initial high quality, purely ab initio dipole moment surface (DMS) is constructed and used to generate a 296 K line list. For most bands, experimental IR intensities are well reproduced for (12)C(16)O(2) using Ames-1 and the DMS. For more than 80% of the bands, the experimental intensities are reproduced with σ(rms)(ΔI) < 20% or σ(rms)(ΔI∕δ(obs)) < 5. A few exceptions are analyzed and discussed. Directions for future improvements are discussed, though it is concluded that the current Ames-1 and the DMS should be useful in analyzing and assigning high-resolution laboratory or astronomical spectra.     

B2(BO)2(2-)-diboronyl diborene: a linear molecule with a triple boron-boron bond

We have produced and investigated an unique boron oxide cluster, B4O2(-), using photoelectron spectroscopy and ab initio calculations. Relatively simple and highly vibrationally resolved PES spectra were obtained at two photon energies (355 and 193 nm). The electron affinity of neutral B4O2 was measured to be 3.160 +/- 0.015 eV. Two excited states were observed for B4O2 at excitation energies of 0.48 and 0.83 eV above the ground state. Three vibrational modes were resolved in the 355 nm spectrum for the ground state of B4O2 with frequencies of 350 +/- 40, 1530 +/- 30, and 2040 +/- 30 cm(-1). Ab initio calculations showed that neutral B4O2 (D(infinity h), 3sigma(g)-) and anionic B4O2(-) (D(infinity h), 2pi(u)) both possess highly stable linear structures (O[triple bond]B-B=B-B[triple bond]O), which can be viewed as a B2 dimer bonded to two terminal boronyl groups. The lowest nonlinear structures are at least 1.5 eV higher in energy. The calculated electron detachment energies from the linear B4O2- and the vibrational frequencies agree well with the experimental results. The three observed vibrational modes are due to the B-B, B=B, and B[triple bond]O symmetric stretching vibrations, respectively, in the linear B2(BO)2. Chemical bonding analyses revealed that the HOMO of B2(BO)2, which is half-filled, is a bonding pi orbital in the central B2 unit. Thus, adding two electrons to B2(BO)2 leads to a B[triple bond]B triple bond in [O[triple bond]B-B[triple bond]B-B[triple bond]O]2-. Possibilities for stabilizing B2(BO)2(2-) in the form of B2(BO)2Li2 are considered computationally and compared with other valent isoelectronic, triple bonded species, B2H2Li2, B2H2(2-), and C2H2. The high stability of B2(BO)2(2-) suggests that it may exist as a viable building block in the condensed phase.     

Lanthanide-transition-metal carbonyl complexes: new [Co4(CO)11]2- clusters and lanthanide(II) isocarbonyl polymeric arrays

Two types of Ln(II)-Co(4) isocarbonyl polymeric arrays, [(Et(2)O)(3)(-)(x)()(THF)(x)()Ln[Co(4)(CO)(11)]]( infinity ) (1-3; x = 0, 1) and [(THF)(5)Eu[Co(4)(CO)(11)]]( infinity ) (4), were prepared and structurally characterized. Transmetalation involving Ln(0) and Hg[Co(CO)(4)](2) in Et(2)O yields [(Et(2)O)(3)Ln[Co(4)(CO)(11)]]( infinity ) (1, Ln = Yb; 2, Ln = Eu). Dissolution of the solvent-separated ion pairs [Ln(THF)(x)()][Co(CO)(4)](2) (Ln = Yb, x = 6; Ln = Eu) in Et(2)O affords [(Et(2)O)(2)(THF)Yb[Co(4)(CO)(11)]]( infinity ) (3) and [(THF)(5)Eu[Co(4)(CO)(11)]]( infinity ) (4). In these reactions, oxidation and condensation of the [Co(CO)(4)](-) anions result in formation of the new tetrahedral cluster [Co(4)(CO)(11)](2)(-). The two types of Ln(II)-Co(4) compounds contain different isomers of [Co(4)(CO)(11)](2)(-), and, consequently, the structures of the infinite isocarbonyl networks are distinct. The cluster in [(Et(2)O)(3)(-)(x)()(THF)(x)()Ln[Co(4)(CO)(11)]]( infinity ) (1-3) possesses pseudo C(3)(v)() symmetry (an apical Co, three basal Co atoms; one face-bridging, three edge-bridging, seven terminal carbonyls) and connects to Ln(II) centers through eta(2),micro(4)- and eta(2),micro(3)-carbonyls to generate a 2-D puckered sheet. In contrast, [(THF)(5)Eu[Co(4)(CO)(11)]]( infinity ) (4) incorporates a C(2)(v)() symmetric cluster (two unique Co environments; two face-bridging, one edge-bridging, eight terminal carbonyls), and isocarbonyl linkages (eta(2),micro(4)-carbonyls) to Eu(II) atoms create a 1-D zigzag chain. Complexes 1-4 contain the first reported eta(2),micro(4)-CO bridges between a Ln and a transition-metal carbonyl cluster. Infrared spectroscopic studies revealed that the isocarbonyl associations to Ln(II) persist in solution. The solution structure and dynamic behavior of the [Co(4)(CO)(11)](2)(-) cluster in 1 was investigated by variable-temperature (59)Co and (13)C NMR spectroscopies.     

Small gas-phase dianions of Zn3O(4)(2-), Zn4O(5)(2-), CuZn2O(4)(2-), Si2GeO(6)(2-), Ti2O(5)(2-) and Ti3O(7)(2-)

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.     

Electronic structure and chemical bonding in MO(n)- and MO(n) clusters (M = Mo, W; n = 3-5): a photoelectron spectroscopy and ab initio study

Photoelectron spectroscopy (PES) and ab initio calculations are combined to investigate the electronic structure of MO(n)- clusters (M = W, Mo; n = 3-5). Similar PES spectra were observed between the W and Mo species. A large energy gap between the first and second PES bands was observed for MO3- and correlated with a stable closed-shell MO3 neutral cluster. The electron binding energies of MO4- increase significantly relative to those of MO3-, and there is also an abrupt spectral pattern change between MO3- and MO4-. Both MO4- and MO5- give PES features with extremely high electron binding energies (>5.0 eV) due to oxygen-2p-based orbitals. The experimental results are compared with extensive density functional and ab initio [CCSD(T)] calculations, which were performed to elucidate the electronic and structural evolution for the tungsten oxide clusters. WO3 is found to be a closed-shell, nonplanar molecule with C3v symmetry. WO4 is shown to have a triplet ground state (3A2) with D2d symmetry, whereas WO5 is found to be an unusual charge-transfer complex, (O2-)WO3+. WO4 and WO5 are shown to possess W-O* and O2-* radical characters, respectively.     

The Dinitramide Anion, N(NO(2))(2)(-)

The infrared and Raman spectra of the NH(4)(+), K(+), and Cs(+) salts of N(NO(2))(2)(-) in the solid state and in solution have been measured and are assigned with the help of ab initio calculations at the HF/6-31G and MP2/6-31+G levels of theory. In agreement with the variations observed in the crystal structures, the vibrational spectra of the N(NO(2))(2)(-) anion are also strongly influenced by the counterions and the physical state. Whereas the ab initio calculations for the free N(NO(2))(2)(-) ion indicate a minimum energy structure of C(2) symmetry, Raman polarization measurements on solutions of the N(NO(2))(2)(-) anion suggest point group C(1) (i.e., no symmetry). This is attributed to the very small (<3 kcal/mol) N-NO(2) rotational barrier in N(NO(2))(2)(-) which allows for easy deformation.     

Accurate ab initio intermolecular potential energy surface for the quintet state of the O2(3Sigma(g)-)-O2(3Sigma(g)-) dimer

A new potential energy surface (PES) for the quintet state of rigid O(2)((3)Sigma(g)(-)) + O(2)((3)Sigma(g)(-)) has been obtained using restricted coupled-cluster theory with singles, doubles, and perturbative triple excitations [RCCSD(T)]. A large number of relative orientations of the monomers (65) and intermolecular distances (17) have been considered. A spherical harmonic expansion of the interaction potential has been built from the ab initio data. It involves 29 terms, as a consequence of the large anisotropy of the interaction. The spherically averaged term agrees quite well with the one obtained from analysis of total integral cross sections. The absolute minimum of the PES corresponds to the crossed (D(2d)) structure (X shape) with an intermolecular distance of 6.224 bohrs and a well depth of 16.27 meV. Interestingly, the PES presents another (local) minimum close in energy (15.66 meV) at 6.50 bohrs and within a planar skewed geometry (S shape). We find that the origin of this second structure is due to the orientational dependence of the spin-exchange interactions which break the spin degeneracy and leads to three distinct intermolecular PESs with singlet, triplet, and quintet multiplicities. The lowest vibrational bound states of the O(2)-O(2) dimer have been obtained and it is found that they reflect the above mentioned topological features of the PES: The first allowed bound state for the (16)O isotope has an X structure but the next state is just 0.12 meV higher in energy and exhibits an S shape.     

Microsolvation of the dicyanamide anion: [N(CN)(2)(-)](H(2)O)n (n = 0-12)

Photoelectron spectroscopy is combined with ab initio calculations to study the microsolvation of the dicyanamide anion, N(CN)(2)(-). Photoelectron spectra of [N(CN)(2)(-)](H2O)n (n = 0-12) have been measured at room temperature and also at low temperature for n = 0-4. Vibrationally resolved photoelectron spectra are obtained for N(CN)(2)(-), allowing the electron affinity of the N(CN)2 radical to be determined accurately as 4.135 +/- 0.010 eV. The electron binding energies and the spectral width of the hydrated clusters are observed to increase with the number of water molecules. The first five waters are observed to provide significant stabilization to the solute, whereas the stabilization becomes weaker for n > 5. The spectral width, which carries information about the solvent reorganization upon electron detachment in [N(CN)(2)(-)](H2O)n, levels off for n > 6. Theoretical calculations reveal several close-lying isomers for n = 1 and 2 due to the fact that the N(CN)(2)(-) anion possesses three almost equivalent hydration sites. In all the hydrated clusters, the most stable structures consist of a water cluster solvating one end of the N(CN)(2)(-) anion.     

The repulsive Coulomb barrier along a dissociation path of the BeC(2-)(4) 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.     

The singlet electronic ground state isomers of dialuminum monoxide: AlOAl, AlAlO, and the transition state connecting them

The singlet electronic ground state isomers, X (1)Sigma(g) (+) (AlOAl D(infinityh)) and X (1)Sigma(+) (AlAlO C(infinitynu)), of dialuminum monoxide have been systematically investigated using ab initio electronic structure theory. The equilibrium structures and physical properties for the two molecules have been predicted employing self-consistent field (SCF) configuration interaction with single and double excitations (CISD), multireference CISD (MRCISD), coupled cluster with single and double excitations (CCSD), CCSD with perturbative triples [CCSD(T)], CCSD with iterative partial triple excitations (CCSDT-3 and CC3), and full triples (CCSDT) coupled cluster methods. Four correlation consistent polarized valence (cc-pVXZ) type basis sets were used. The AlAlO system is rather challenging theoretically. The two isomers are confirmed to have linear structures at all levels of theory. The symmetric isomer AlOAl is predicted to lie 81.9 kcal mol(-1) below the asymmetric isomer AlAlO at the cc-pV(Q+d)Z CCSD(T) level of theory. The predicted harmonic vibrational frequencies for the X (1)Sigma(g) (+) AlOAl molecule, omega(1)=517 cm(-1), omega(2)=95 cm(-1), and omega(3)=1014 cm(-1), are in good agreement with experimental values. The harmonic vibrational frequencies for the X (1)Sigma(+) AlAlO structure, omega(1)=1042 cm(-1), omega(2)=73 cm(-1), and omega(3)=253 cm(-1), presently have no experimental values with which to be compared. With the same methods the barrier heights for the isomerization AlOAl-->AlAlO and AlAlO-->AlOAl reactions were predicted to be 84.3 and 2.4 kcal mol(-1), respectively. The dissociation energies D(0) for AlOAl (X (1)Sigma(g) (+)) and AlAlO (X (1)Sigma(+))-->AlO (X (2)Sigma(+))+Al ((2)P) were determined to be 130.8 and 48.9 kcal mol(-1), respectively. Thus, both symmetric AlOAl (X (1)Sigma(g) (+)) and asymmetric AlAlO (X (1)Sigma(+)) isomers are expected to be thermodynamically stable with respect to the dissociation into AlO (X (2)Sigma(+)) + Al ((2)P) and kinetically stable for the isomerization reaction (AlAlO-->AlOAl) at sufficiently low temperatures.