Dénes Szieberth

A benchmark theoretical study of the electronic ground state and of the singlet-triplet split of benzene and linear acenes.

A benchmark theoretical study of the electronic ground state and of the vertical and adiabatic singlet-triplet (ST) excitation energies of benzene (n=1) and n-acenes (C(4n+2)H(2n+4)) ranging from naphthalene (n=2) to heptacene (n=7) 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, 5) and single-reference methods [HF, MP2, MP3, MP4SDQ, CCSD, CCSD(T)] 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 [CCSD(T)] in the limit of an asymptotically complete basis set (cc-pVinfinityZ), the S(0)-T(1) vertical excitation energies of benzene (n=1) and n-acenes (n=2-7) 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 CCSD(T)/cc-pVinfinityZ 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 approximately 4 kcal/mol (approximately 0.17 eV) in the limit of an infinitely large polyacene.

Hydrolysis of Imidazole-2-ylidenes

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.

Dibenzophosphapentaphenes: Exploiting P Chemistry for Gap Fine-Tuning and Coordination-Driven Assembly of Planar Polycyclic Aromatic Hydrocarbons

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.

Significant Cation Effects in Carbon Dioxide–Ionic Liquid Systems

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.

Diastereoselective synthesis of 1,2,3,6-tetrahydrophosphinine 1-oxides with an exocyclic P-function by a Michael type addition

The anions generated from diphenylphosphine oxide or dialkyl phosphites add easily at the α,β-double-bond of 1,2-dihydrophosphinine oxides 1 to afford a single diastereomer of 3-substituted tetrahydrophosphinine oxides 2–4 existing in a twist-boat conformation.

Pyridyl-Functionalised 3H-1,2,3,4-Triazaphospholes: Synthesis, Coordination Chemistry and Photophysical Properties of Low-Coordinate Phosphorus Compounds

Novel conjugated, pyridyl-functionalised triazaphospholes with either tBu or SiMe3 substituents at the 5-position of the N3 PC heterocycle have been prepared by a [3+2] cycloaddition reaction and compared with structurally related, triazole-based systems. Photoexcitation of the 2-pyridyl-substituted triazaphosphole gives rise to a significant fluorescence emission with a quantum yield of up to 12 %. In contrast, the all-nitrogen triazole analogue shows no emission at all. DFT calculations indicate that the 2-pyridyl substituted systems have a more rigid and planar structure than their 3- and 4-pyridyl isomers. Time-dependent (TD) DFT calculations show that only the 2-pyridyl-substituted triazaphosphole exhibits similar planar geometry, with matching conformational arrangements in the lowest energy excited state and the ground state; this helps to explain the enhanced emission intensity. The chelating P,N-hybrid ligand forms a Re(I) complex of the type [(N^N)Re(CO)3 Br] through the coordination of nitrogen atom N(2) to the metal centre rather than through the phosphorus donor. Both structural and spectroscopic data indicate substantial π-accepting character of the triazaphosphole, which is again in contrast to that of the all-nitrogen-containing triazoles. The synthesis and photophysical properties of a new class of phosphorus-containing extended π systems are described.

FROM 1,3-DIPHOSPHETANE-2,4-DIYLS TO CYCLIC ANIONS AND CATIONS

The interest in diradicals has grown by the increasing number of stable diradicals which have been synthesized during the last few years. The diphosphacyclobutane-diyles have a diradicaloid molecular structure and they were the first diradicals to be prepared in the gramme scale. The combination of stability and high yield synthesis creates the opportunity both to change the substitution pattern or to prepare different valence isomers. Current experimental and computational studies reveal that electron transfer reactions of maintain the cyclic system. In the contribution, the oxidation reaction of 1 to the radical cation 2 and the cyclic diphosphaallyl cation 3 will be reported. Additionally, the remarkable mechanism of the reaction of 1 at first to the radical anion 4 but particularly to the diphosphacyclobutadiene dianion 5 under reductive conditions will be presented in detail.

The Photoelectron Spectrum and Conformation of Phenylphosphine and Phenylarsine

He(I) and He(II) photoelectron spectra of phenylphosphine and phenylarsine have been investigated and assigned. The rotational barrier of the phosphino group has been investigated at the MP2/6-31G(d,p)//MP2/6-31G(d,p) and HF/6-31G(d,p)//HF/6-31G(d,p) levels of theory, and that of the arsino group at the HF/6-31G(d,p)//6-31G(d,p) levels of theory. The rotational barrier of the two molecules is nearly the same. The energy difference between the two possible conformers of the molecules is low (1.5 kJ/mol at the MP2/6-31G(d,p) level of theory), allowing nearly free rotation about the P-C bond. The photoelectron spectrum cannot be interpreted by considering the most stable rotamer, but all possible conformers should be taken into account. The present interpretation is consistent with the smallnp- π interaction concluded from other investigations. The rotational barrier ofo-phosphinophenol is significantly larger than for phenylphosphine, and the photoelectron spectrum of this compound can be interpreted by considering a single conformer, and no appreciable interaction between the π-system of the ring and the phosphorus lone pair.

Synthetic, structural and theoretical studies on the new 2,3-dihydro-1,2,4-thia-, selena- and tellura-diphospholes, P2EC2But2(H)Me, (E=S, Se, Te) and their [M(CO)5] complexes (M=Cr, Mo, W)

Abstract Methylation of the 1,2,4-thia-, selena- and tellura-diphospholes P 2 EC 2 Bu t 2 (E=S, Se, Te) with MeLi, followed by protonation with HCl, affords the new 2,3-dihydro-1,2,4-thia-, selena- and tellura-diphospholes. Further treatment with [M(CO) 5 (THF)] (M=Cr, Mo, W) yields the corresponding η 1 -metal pentacarbonyl complexes, of which [M(CO) 5 {η 1 -P 2 EC 2 Bu t 2 (H)Me}], (E=S, M=W; E=Se, M=Cr, M=Mo, M=W) have been structurally characterised by single crystal X-ray diffraction studies. Theoretical calculations carried out on the different isomers of the 2,3-dihydro-1,2,4-thia-, selena- and tellura-diphospholes, and the 2,3-dihydro-1 H -1,2,4-triphosphole are also presented and discussed.

Structure and other molecular properties of actinide trichlorides AnCl3 (An = Th-Cm).

The ground-state molecular properties of the trichlorides of light actinides (An = Th-Cm) have been predicted by state-of-the-art quantum chemical calculations. The ground electronic states have been determined by multireference calculations at the CASPT2 level including both scalar and spin-orbit relativistic effects. These studies supported the expected single-configuration character of ThCl3 and CmCl3 with their well-defined 6dσ/7s hybrid and 5f(7) configurations, respectively. In contrast, the intermediate actinides (PaCl3-AmCl3) with partly filled 5f shells have numerous very low-lying excited states and consequently a mixed character of the spin-orbit ground states. Apart from the planar ThCl3 the ground-state molecular geometries proved to be pyramidal with C(3v) symmetry. The gradually decreasing An-Cl bond distances reveal the actinide contraction known for the atomic and ionic radii of these actinide atoms. Other ground-state molecular properties as vibrational frequencies and natural charges have been obtained by density functional theory calculations using the B3LYP exchange-correlation functional in conjunction with small-core relativistic energy-consistent pseudopotentials for the actinides.