A benchmark theoretical study of the electronic ground state and of the vertical and adiabatic singlet-triplet ͑ST͒ excitation energies of benzene ͑n =1͒ and n-acenes ͑C 4n+2 H 2n+4 ͒ ranging from naphthalene ͑n =2͒ to heptacene ͑n =7͒ is presented, on the ground of single-and multireference calculations based on restricted or unrestricted zero-order wave functions. High-level and large scale treatments of electronic correlation in the ground state are found to be necessary for compensating giant but unphysical symmetry-breaking effects in unrestricted single-reference treatments. The composition of multiconfigurational wave functions, the topologies of natural orbitals in symmetry-unrestricted CASSCF calculations, the T1 diagnostics of coupled cluster theory, and further energy-based criteria demonstrate that all investigated systems exhibit a 1 A g singlet closed-shell electronic ground state. Singlet-triplet ͑S 0 -T 1 ͒ energy gaps can therefore be very accurately determined by applying the principles of a focal point analysis onto the results of a series of single-point and symmetry-restricted calculations employing correlation consistent cc-pVXZ basis sets ͑X=D, T, Q, 5͒ and single-reference methods ͓HF, MP2, MP3, MP4SDQ, CCSD, CCSD͑T͔͒ of improving quality. According to our best estimates, which amount to a dual extrapolation of energy differences to the level of coupled cluster theory including single, double, and perturbative estimates of connected triple excitations ͓CCSD͑T͔͒ in the limit of an asymptotically complete basis set ͑cc-pVϱZ͒, the S 0 -T 1 vertical excitation energies of benzene ͑n =1͒ and n-acenes ͑n =2-7͒ amount to 100.79, 76.28, 56.97, 40.69, 31.51, 22.96, and 18.16 kcal/mol, respectively. Values of 87.02, 62.87, 46.22, 32.23, 24.19, 16.79, and 12.56 kcal/mol are correspondingly obtained at the CCSD͑T͒ / cc-pVϱZ level for the S 0 -T 1 adiabatic excitation energies, upon including B3LYP/cc-PVTZ corrections for zero-point vibrational energies. In line with the absence of Peierls distortions, extrapolations of results indicate a vanishingly small S 0 -T 1 energy gap of 0 to ϳ4 kcal/ mol ͑ϳ0.17 eV͒ in the limit of an infinitely large polyacene.

A synthetic route to planar P-modified polycylic aromatic hydrocarbons (PAHs) is described. The presence of a reactive σ(3),λ(3)-P moiety within the sp(2)-carbon scaffold allows the preparation of a new family of PAHs displaying tunable optical and redox properties. Their frontier molecular orbitals (MOs) are derived from the corresponding phosphole MOs and show extended conjugation with the entire π framework. The coordination ability of the P center allows the coordination-driven assembly of two molecular PAHs onto a Au(I) ion.

The direct reaction of an imidazole-2-ylidene in a predominantly aqueous environment [about 0.1 M solution in a H(2)O (>60%)/THF solvent system] was investigated for the first time. The reaction yielded a stable solution of the corresponding imidazolium-hydroxide of pH 13, which is in agreement with results from an ab initio molecular dynamics simulation. In contrast, hydrolysis of the carbene in a mainly aprotic environment (>80% THF) gives a hydrogen-bridged carbene-water complex which could be detected by NMR and IR spectroscopies for the first time. This complex converts slowly to two isomeric ring opened products and is at higher water concentration in dynamic equilibrium with the imidazolium hydroxide. A computational mechanistic study of the carbene hydrolysis with a gradually increasing number of water molecules revealed that the imidazolium-hydroxide structure can only be optimized with three or more water molecules as reactants, and with the increasing number of water molecules its stability is increasing with respect to the carbene-water complex. In agreement with the experimental results, these findings point out that solvent stabilization and basicity of the hydroxide ion plays a crucial role in the reaction. With increasing number of water molecules the barriers connecting the reaction intermediates are getting smaller, and the ring opened hydrolysis products can be derived from imidazolium-hydroxide type intermediates. Computational studies on the hydrolysis of a nonaromatic imidazolidine-2-ylidene analogue clearly indicated the analogous ring-opened product to be by 10-12 kcal/mol more stable than the appropriate ion pair and the carbene-water complex, in agreement with the known aromatic stabilization of imidazol-2-ylidenes. Accordingly, these molecules hydrolyze with exclusive formation of the ring-opened product.

Carbon dioxide–ionic liquid systems are of great current interest, and significant efforts have been made lately to understand the intermolecular interactions in these systems. In general, all the experimental and theoretical studies have concluded so far that the main solute–solvent interaction takes effect through the anion, and the cation has no, or only a secondary role in solvation. In this theoretical approach it is shown that this view is unfounded, and evidence is provided that, similarly to the benzene–CO2 system, dispersion interactions are present between the solute and the cation. Therefore, this defines a novel site for tailoring solvents to tune CO2 solubility.

Titanium oxide ultrathin films were grown on Ag(100) by evaporation of titanium in an O(2) atmosphere. The growth of the oxide films was monitored by means of XPS. The Ti2p XPS spectra indicate the formation of films with a TiO(2) stoichiometry in the whole range of coverages studied here. The STM results show that titania films appear to grow as islands of uniform thickness up to the completion of the first layer. Up to the formation of one complete monolayer, a (5 × 1) LEED pattern is observed. This pattern can be interpreted as a coincidence mesh and the lattice parameters of the oxide layer are very close to those of TiO(2) films with a lepidocrocite-like structure. However, the STM images show a long-range coincidence between the periodicity of the oxide film and that of the substrate along the short side of the oxide unit cell revealing that this lattice parameter is not exactly equal to that of the substrate. Above the monolayer coverage, additional spots become visible in the LEED pattern which can be interpreted in terms of the unit cell of rutile (110). The structural determination was carried out for the monolayer of titania by means of XPD and LEED intensity analysis. The results of these investigations demonstrate that the titania monolayer has a lepidocrocite-like structure. The DFT calculations carried out for the titania monolayer show the higher stability of the lepidocrocite structure with respect to other structures derived from crystallographic planes of bulk TiO(2) phases. Moreover, these calculations allow us to determine the oxide-substrate interface energy as well as to clarify the effect of the strain on the stability of the oxide layer.