Ab initio quantum-chemical computations of the electronic states in HgBr2 and IBr: Molecules of interest on the Earth's atmosphere.

The electronic states of atmospheric relevant molecules IBr and HgBr2 are reported, within the UV-Vis spectrum range (170nm≤λphoton≤600 nm) by means of the complete-active-space self-consistent field/multi-state complete-active-space second-order perturbation theory/spin-orbit restricted-active-space state-interaction (CASSCF/MS-CASPT2/SO-RASSI) quantum-chemical approach and atomic-natural-orbital relativistic-correlation-consistent (ANO-RCC) basis sets. Several analyses of the methodology were carried out in order to reach converged results and therefore to establish a highly accurate level of theory. Good agreement is found with the experimental data with errors not higher than around 0.1 eV. The presented analyses shall allow upcoming studies aimed to accurately determine the absorption cross sections of interhalogen molecules and compounds with Hg that are relevant to better comprehend the photochemical processes taking place in the atmosphere.

Acidities of closo-1-COOH-1,7-C2B10H11 and amino acids based on icosahedral carbaboranes.

Carborane clusters are not found in Nature and are exclusively man-made. In this work we study, both experimentally and computationally, the gas-phase acidity (measured GA = 1325 kJ·mol(-1), computed GA = 1321 kJ·mol(-1)) and liquid-phase acidity (measured pKa = 2.00, computed pKa = 1.88) of the carborane acid closo-1-COOH-1,7-C2B10H11. The experimental gas-phase acidity was determined with electrospray tandem mass spectrometry (ESI/MS), by using the extended Cooks kinetic method (EKM). Given the similar spatial requirements of the title icosahedral cage and benzene and the known importance of aminoacids as a whole, such a study is extended, within an acid-base context, to corresponding ortho, meta, and para amino acids derived from icosahedral carborane cages, 1-COOH-n-NH2-1, n-R with {R = C2B10H10, n = 2, 7, 12}, and from benzene {R = C6H4, n = 2, 3, 4}. A remarkable difference is found between the proportion of neutral versus zwitterion structures in water for glycine and the carborane derived amino acids.

Tuning the photophysical properties of anti-B18H22: efficient intersystem crossing between excited singlet and triplet states in new 4,4'-(HS)2-anti-B18H20.

The tuning of the photophysical properties of the highly fluorescent boron hydride cluster anti-B18H22 (1), by straightforward chemical substitution to produce 4,4'-(HS)2-anti-B18H20 (2), facilitates intersystem crossing from excited singlet states to a triplet manifold. This subsequently enhances O2((1)Δg) singlet oxygen production from a quantum yield of ΦΔ âˆ¼ 0.008 in 1 to 0.59 in 2. This paper describes the synthesis and full structural characterization of the new compound 4,4'-(HS)2-anti-B18H20 (2) and uses UV-vis spectroscopy coupled with density functional theory (DFT) and ab initio computational studies to delineate and explain its photophysical properties.

Electronic structure and geometries of o-carborane derived cyclic structures [{µ-1,2-(C2B10H10)(n)M(n)}Ag(m)]z-, M = {Au, Hg}, n = {3, 4}, m = {0, 1, 2}, z = {n-m, -m}.

To date, crystal structures of the cyclic mercury complexes [µ-1,2-(C(2)B(10)H(10))(n)Hg(n)], with n = {3, 4}, have been found. However, the same structures substituting Hg by Au, which implies a charge z = {-3, -4} for n = {3, 4} respectively, have not been found. In this work, we present geometrical and electronic structure properties of the title structures [µ-1,2-(C(2)B(10)H(10))(n)M(n)](z-), M = {Au, Hg}, n = {3, 4}, z = {n, 0}, and study the stability of structures with one and two added Ag(+) ions.

Distinct photophysics of the isomers of B18H22 explained.

The photophysics of the two isomers of octadecaborane(22), anti- and syn-B(18)H(22), have been studied by UV-vis spectroscopic techniques and theoretical computational methods. In air-saturated hexane, anti-B(18)H(22) shows fluorescence with a high quantum yield, Φ(F) = 0.97, and singlet oxygen O(2)((1)Δ(g)) production (Φ(Δ) ∼ 0.008). Conversely, isomer syn-B(18)H(22) shows no measurable fluorescence, instead displaying much faster, picosecond nonradiative decay of excited singlet states. Computed potential energy hypersurfaces (PEHs) for both isomers rationalize these data, pointing to a deep S(1) minimum for anti-B(18)H(22) and a conical intersection (CI) between its S(0) and S(1) states that lies 0.51 eV higher in energy. Such an energy barrier to nonradiative relaxation is not present in the PEH of syn-B(18)H(22), and the system therefore has sufficient initial energy on excitation to reach the (S(0)/S(1)) CI and to then decay to the ground state without fluorescence. The computational analysis of the geometries at stationary points along the PEH of both isomers shows that the determining factor for the dissimilar photophysics of anti- and syn-B(18)H(22) may be due to the significant differences in the geometrical rearrangements at their respective conical intersections. Thus, the syn isomer shows one very large, B-B elongation of 1.2 Å from 1.8 Å in the ground state to 3.0 Å at the CI, whereas the anti isomer shows smaller elongations (below 1 Å) in several B-B connectivities at its (S(0)/S(1))(CI). The absorbed energy in S(1) for the anti-B(18)H(22) is therefore redistributed vibrationally into several regions of the molecule rather than almost completely into a single vibrational mode as in the case for the syn isomer. The consequent prolonged S(1) lifetime for the anti isomer allows for relaxation via fluorescence.

A distance-dependent parameterization of the extended Hubbard model for conjugated and aromatic hydrocarbons derived from stretched ethene.

The Hubbard model, which is widely used in physics but is mostly unfamiliar to chemists, provides an attractive yet simple model for chemistry beyond the self consistent field molecular orbital approximation. The Hubbard model adds an effective electron-electron repulsion when two electrons occupy the same atomic orbital to the familiar Hückel Hamiltonian. Thus it breaks the degeneracy between excited singlet and triplet states and allows an explicit treatment of electron correlation. We show how to evaluate the parameters of the model from high-level ab initio calculations on two-atom fragments and then to transfer the parameters to large molecules and polymers where accurate ab initio calculations are difficult or impossible. The recently developed MS-RASPT2 method is used to generate accurate potential energy curves for ethene as a function of carbon-carbon bond length, which are used to parameterize the model for conjugated hydrocarbons. Test applications to several conjugated/aromatic molecules show that even though the model is very simple, it is capable of reasonably accurate predictions for bond lengths, and predicts molecular excitation energies in reasonable agreement with those from the MS-RASPT2 method.

Can (pi)6 + (pi)4 = 10? Exploring cycloaddition routes to highly unsaturated 10-membered rings.

This paper uses DFT and G3(MP2) calculations to examine whether unbridged 10-membered rings can be made by (pi)6 + (pi)4 cycloadditions to (Z)- and (E)-hexatrienes, hexa-1,5-dien-3-ynes, (Z)-hexa-1,3-dien-5-ynes, hexa-1,2,3,5-tetraenes, and (Z)-hexa-3-ene-1,5-diynes. Cycloadditions to four 4pi reactants, buta-1,3-diene, butenyne, butatriene, and butadiyne, are explored. Thirty different basic cycloadditions are identified, and all are shown to be exothermic according to G3(MP2) calculations; strain energies in the products are comparable with that of cyclodecane itself, despite the presence of trans-alkene, alkyne, allene, cumulene, and s-trans diene moieties. The major obstacles to the isolation of 6 + 4 cycloaddition products are competing (pi)4 + (pi)2 cycloadditions and, especially, rapid Cope rearrangement of the products, but, in many cases, the judicious introduction of substituents can overcome these problems so that practical syntheses should be possible. Reactions between (E)-hexa-1,3,5-triene and s-trans-buta-1,3-diene are shown to have substantially lower activation energies than those involving (Z)-hexa-1,3,5-triene reacting with either s-cis- or s-trans-buta-1,3-diene. Conformationally locked derivatives of s-cis,s-cis (E)-hexa-1,3,5-trienes can lead to derivatives of (Z,Z,E)-cyclodeca-1,3,7-triene that are stable to Cope rearrangement, and reactions should proceed at close to ambient temperatures with suitable activating groups. We predict that it should be possible to prepare suitably substituted derivatives of at least 11 more highly unsaturated ring systems: (5Z,7Z)-cyclodeca-1,2,5,7-tetraene, (1Z,3Z)-cyclodeca-1,3-dien-7-yne, (2Z,7E)-cyclodeca-1,2,3,7-tetraene, (Z)-cyclodeca-1,2,3-trien-7-yne, (4Z,8E)-cyclodeca-1,2,4,8-tetraene, (Z)-cyclodeca-1,2,4,5,7-pentaene, (Z)-cyclodeca-1,2,4-trien-8-yne, (1Z,7E)-cyclodeca-1,7-dien-3-yne, (R,S,E)-cyclodeca-1,2,4,5,8-pentaene, cyclodeca-1,2,4,5,8,9-hexaene, and (R,S)-cyclodeca-1,2,4,5-tetraen-8-yne. In three other cases, we predict that cycloaddition will be followed by unusual and intriguing rearrangements. Cycloadditions can be accelerated by the presence of electron-withdrawing groups in either the 6pi or 4pi reactants. Transannular cyclizations of some products may lead to interesting stereocontrolled routes to 6,6- and/or 5,7-bicyclic structures.

On the electronic structure and stability of icosahedral r-X2Z10H12 and Z12H(12)(2-) clusters; r = {ortho, meta, para}, X = {C, Si}, Z = {B, Al}.

We report on the electronic structure of the 12-vertex icosahedral clusters r-X(2)Z(10)H(12) and Z(12)H(12)(2-), where X = {C, Si} and Z = {B, Al}. The least stable cluster--with the lowest HOMO-LUMO gap (E(g))--corresponds to the ortho-X(2)Z(10)H(12) isomers for all values of X = {C, Si} and Z = {B, Al}. The well-known energetic order E(para) < E(meta) < E(ortho) for r-carboranes is also valid for all compounds except r-C(2)Al(10)H(12). Substitution of two atoms of carbon or silicon into the icosahedral cage B(12)H(12)(2-) enhances considerably the stability of the system as analyzed from E(g) gaps, as opposite to Al(12)H(12)(2-), where similar gaps are found upon double carbon or silicon substitution regardless of the positions in the cage. In order to highlight similarities and differences in the title clusters, topological analysis of the electron density was performed, together with analysis of the deviation from polyhedron icosahedral form with (i) volumes, skewness and kurtosis calculations; and (ii) continuous shape measures.

Solution and computed structure of O-lithium N,N-diisopropyl-P,P-diphenylphosphinic amide. Unprecedented Li-O-Li-O self-assembly of an aryllithium.

The structural characterization of an ortho-lithiated diphenylphosphinic amide is described for the first time. Multinuclear magnetic resonance ((1)H, (7)Li, (13)C, (31)P) studies as a function of temperature and concentration employing 1D and 2D methods showed that the anion exists as a mixture of one monomer and two diastereomeric dimers. In the dimers the chiral monomer units are assembled in a like and unlike manner through oxygen-lithium bonds, leading to fluxional ladder structures. This self-assembling mode leads to the formation of Li(2)O(2) four-membered rings, a structural motif unprecedented in aryllithium compounds. DFT computations of representative model compounds of ortho-lithiated phosphinic amide monomer and Li(2)C(2) and Li(2)O(2) dimers with different degrees of solvation by THF molecules showed that Li(2)O(2) dimers are thermodynamically favored with respect to the alternative Li(2)C(2) structures by 4.3 kcal mol(-1) in solvent-free species and by 2.3 kcal mol(-1) when each lithium atom is coordinated to one THF molecule. Topological analysis of the electron density distribution revealed that the Li(2)O(2) four-membered ring is characterized by four carbon-lithium bond paths and one oxygen-oxygen bond path. The latter divides the Li-O-Li-O ring into two Li-O-Li three-sided rings, giving rise to two ring critical points. On the contrary, the bond path network in the Li(2)C(2) core includes a catastrophe point, suggesting that this molecular system can be envisaged as an intermediate in the formation of Li(2)O(2) dimers. The computed (13)C chemical shifts of the C-Li carbons support the existence of monomeric and dimeric species containing only one C-Li bond and are consistent with the existence of tricoordinated lithium atoms in all species in solution.

Organometallic gold complexes of carborane. Theoretical comparative analysis of ortho, meta, and para derivatives and luminescence studies.

The synthesis and X-ray analysis of complexes [(micro-1,12-C2B10H10){Au(PPh3)}2] and [(micro-1,2-C2B10H10){Au(PMe3)}2] have provided the experimental data needed to analyse two points. The first point is the use of these data to carry out a computational study with the aim of comparing the electronic structures and relative stabilities of the organometallic isomers [(micro-1,n-C2B10H10){Au(PR3)}2] (n=2, 7, 12; R=Ph, Me) with those of the parent carborane clusters ortho-, meta- and para-carborane and the influence of the monophosphine substituents. The second point is focused in the influence of the steric demand of the monophosphine in the presence or not of aurophilic interactions in the ortho derivatives [(micro-1,2-C2B10H10){Au(PR3)}2] (R=Me, Ph). The photoluminescent behaviour of both the carboranes and the organometallic complexes is presented.

What Electronic Structures and Geometries of Carborane Mono- and ortho-, meta-, and para-Diradicals are Preferred?

Structures, relative stabilities, singlet-triplet gaps, and the ground-state character of mono- and diradicals derived from the three icosahedral carborane cage isomers have been computed by unrestricted broken-symmetry DFT and by CASPT2 methods. Whereas the bond dissociation energies (BDE) leading to the carborane monoradicals are close to the benzene BDE, the most stable carborane radicals are derived from dissociations of hydrogens farthest away from the carbon atoms. All the monomeric carborane diradicals are determined to have singlet ground states. However, the energetic accessibility of triplet states in some of the species offers the potential of building diradical multidimensional carborane network architectures with interesting magnetic properties.

What Is the Limit of Atom Encapsulation for Icosahedral Carboranes?

The stability of endohedral carboranes X@{1,n-C2B10H12} (X = Li(+), Be(2+); n = 2, 7, 12) and X@{CB11H12(-)} (X = Li(+), Be(2+)) is studied using electronic structure calculations with the B3LYP/6-311+G(d,p) model. Our calculations suggest that all endohedral compounds are local energy minima; for the exohedral complexes X···cage, the global energy minimum always corresponds to the X atom above a triangular face of the icosahedron. In the latter the X atom is furthest apart from the carbon atoms of the cage. As opposite to exohedral {Be(2+)···cage} complexes, no global energy minima were found for exohedral complexes {Li(+)···cage} whereby a carbon atom is present in the triangular face of the icosahedron below the Li(+) cation.

A computational study of the lowest singlet and triplet states of neutral and dianionic 1,2-substituted icosahedral and octahedral o-carboranes.

This work introduces a calibrated B3LYP/6-31G(d) study on the electronic structure of singlet and triplet neutral species of 1,2-substituted icosahedral 1,2-R(2)-1,2-C(2)B(10)H(10) and octahedral 1,2-R(2)-1,2-C(2)B(4)H(4) molecules with R = {H, OH, SH, NH(2), PH(2), CH(3), SiH(3)} and their respective dianions formed by proton removal on each R group. A variety of small adiabatic singlet-triplet gaps DeltaE(ST) are obtained from these systems ranging from 2.93 eV (R = NH(2))

Strikingly long C...C distances in 1,2-disubstituted ortho-carboranes and their dianions.

Neutral and especially dianionic 6- and 12-vertex closo ortho-carboranes (o-carboranes) 1,2-R2-1,2-C2BnHn (R = H, CH3, NH2, OH, F, SiH3, PH2, SH, Cl, as well as e-, CH2-, NH-, O-, SiH2-, PH-, and S- exhibit extremely large variations (over 1 A!) of the cage CC distances, from 1.626 to 2.638 A, at the B3LYP/6-31G//B3LYP/6-31G DFT level. These CC "bond lengths," among the longest ever reported, generally are greater in the icosahedral than those in the corresponding octahedral systems and depend strongly on the substituents. While 1,2-(NH2)2-1,2-C2B10H10 has the longest Cc...Cc separation in neutral species (1.860 A), Cc...Cc distances can be much larger in the corresponding dianions. These range from 1.823 A (R- = e-) to 2.638 A (R- = CH2-) for 1,2-(R-)2-1,2-C2B10H10 and from 1.626 A (R- = SiH2-) to 3.099 A (R- = NH-) for 1,2-(R-)2-1,2-C2B4H4. Remarkably, there is no abrupt discontinuity over the entire range of CC lengths. Consequently, the relationship between the gradual changes in the distances and the nature of the bonding was analyzed by means of the form of the Kohn-Sham orbitals, the Wiberg Cc...Cc bond indices, and Bader AIM method. Cluster carboranes, and possibly other heteroboranes, thus appear to offer unique opportunities for modulating Cc...Cc distances.

On the low-lying excited states of sym-triazine-based herbicides.

We report a joint computational and luminescence study on the low-lying excited states of sym-triazines, namely, 1,3,5-triazine (1) and the ubiquitous herbicides atrazine [6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine (2)] and ametryn [6-methylthio-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine (3)]. Geometrical structures, energetics, and transition and state properties of I and 2 were computed at the TD-DFT, CASSCF, and CASPT2 levels of theory. The fluorescence and phosphorescence emission spectra, lifetimes, and fluorescence quantum yields were measured for the three compounds, and from these, the energies of the lowest excited states and their corresponding radiative rates were determined. The predictions from CASPT2 calculations are in good agreement with the experimental results obtained from the luminescence studies and allow the interpretation of different absorption and emission features.

MALDI-Fourier transform mass spectrometric and theoretical studies of donor-acceptor and donor-bridge-acceptor fullerenes.

Fourier-transform ion cyclotron resonance (FTICR) mass spectrometric studies have been performed on donor-acceptor and donor-bridge acceptor fullerene-based systems. Matrix-assisted laser desorption/ionization (MALDI) was used for ion production; both the positive and negative ion modes were utilized. In addition, collision-induced dissociation (CID) experiments were carried out to study the movement of the charge (electron or hole) upon fragmentation. The experiments are complemented by ab initio theoretical calculations yielding both molecular orbital energies and electron density distributions. It was found that the theoretical electron density map predicted the experimentally observed fragmentation correctly in every case. Both the calculations and the MS experiments may be useful in studying these and related donor-acceptor systems in view of their use for charge separation and eventually, solar energy production.

Ab initio predictions of ferroelectric ternary fluorides with the LiNbO3 structure.

First-principles periodic density-functional theory calculations suggest ternary fluorides LiMgF3, NaCaF3 and LiNiF3 should adopt the ferroelectric LiNbO3 structure at low temperatures; LiMgF3 and LiNiF3 are predicted to have negative enthalpies of formation from the binary fluorides.

Possible nonactivated conductivity of the low-dimensional ternary nitride Ca(2)GeN(2).

The electronic structure of the recently reported ternary nitride Ca(2)GeN(2) has been studied by means of first-principles density functional calculations. The relatively short nonbonded Ge...Ge contacts along the direction approximately perpendicular to the plane of the bent GeN(2) units confer a large dispersion to the energy bands based on the antibonding pi-type level of GeN(2). This feature as well as the smaller but non-negligible interactions along the perpendicular directions suggests that this material, despite being built from discrete units, may exhibit a metallic behavior. This study suggests that the nature of the alkaline-earth atom has a crucial influence on the electrical conductivity of this type of phase.