Playing with PtII and ZnII Coordination to Obtain Luminescent Metallomesogens

Blue–green luminescent terpyridine-containing Pt^II and Zn^II complexes are reported. Equipped with lipophilic gallate units, which act as monodentate ancillary coordinating ligands and/or as anions, they display low-temperature mesomorphic properties (lamello-columnar and hexagonal mesophases for Pt^II and Zn^II complexes, respectively). The mesomorphic properties were investigated by polarised optical microscopy, differential scanning calorimetry, thermogravimetric analysis and X-ray scattering of bulk materials and oriented thin films. The model of self-assembly into the lamello-columnar phase of the Pt^II complex has been described in detail. The optical properties of the complexes were investigated in the liquid and condensed liquid crystalline states, highlighting the delicate balance between the role of the metal in determining the type of excited state responsible for the emission, and the role of the ancillary ligand in driving intermolecular interactions for proper mesophase formation.     

Influence of polarization functions on molecular electrostatic potentials

We study the effects of d-polarization functions, centered on the heavy atoms, on the SCF molecular electrostatic potentials calculated for some molecules. The positions and energies of the minima found are very sensitive to the inclusion of polarization functions in the basis set used. The variations depend on the heavy atom involved and on the possible anisotropy of its charge distribution. These variations are particularly important for second-row atoms.     

Stable doubly charged positive molecular ions formed by direct attachment of alpha particles to HCN and HNC

The existence of [HeNCH](2+) and [HeCNH](2+) is examined by reliable high-level ab initio methods. It is shown that both species are metastable with respect to the proton loss process, but the activation barriers are high enough to accommodate several resonant states. Their estimated lifetimes are practically infinite for v<3, implying that the attachment of alpha particles to HNC and HCN yields kinetically stable dications. This could be an important factor in the cyanide chemistry in interstellar and comet environments.     

Ternary Complexes Stabilized by Chalcogen and Alkaline-Earth Bonds: Crucial Role of Cooperativity and Secondary Noncovalent Interactions

High-level G4 calculations show that the strength of chalcogen interactions is enhanced dramatically if chalcogen compounds simultaneously form alkaline-earth bonds. This phenomenon is studied by exploring binary YX₂⋅⋅⋅N-Base complexes and two types of ternary MCl₂⋅⋅⋅YX₂⋅⋅⋅N-Base, YX₂⋅⋅⋅N-Base⋅⋅⋅MCl₂ complexes, in which YX₂ is a chalcogen compound (Y=S, Se; X=F, Cl), the N-Bases are sp, sp², and sp³ bases (NCH, HN=CH₂, NH₃), and MCl₂ are alkaline-earth BeCl₂ or MgCl₂ derivatives. Starting from the chalcogen-bonded complexes YX₂⋅⋅⋅NH₃ and YX₂⋅⋅⋅HN=CH₂, the binding site of a new incoming alkaline-earth bond is found, surprisingly, to depend on the nature of the halogen atom attached to the chalcogen. For the YF₂ binary complexes the association site is the F atom of the YF₂ subunit, whereas for YCl₂ it is the N atom of the nitrogen base. Regarding YX₂⋅⋅⋅NCH complexes, N is the most favorable site for an alkaline-earth interaction in ternary complexes, regardless of which YX₂ derivative is used. The explanation relies on the interplay of all the noncovalent interactions involved: the strong cooperativity between chalcogen and alkaline-earth bonds, and the appearance of secondary noncovalent interactions in the form of hydrogen bonds.     

Accelerating charge transfer in a triphenylamine-subphthalocyanine donor-acceptor system

We have designed, synthesized and probed a dodecafluoro-subphthalocyaninato boron(III) unit, which bears a triphenylamine moiety in its axial position, as a novel electron donor-acceptor system.     

A theoretical study of hydration effects on the prototropic tautomerism of selenouracils

The prototropic tautomerism of 2-, 4-selenouracil and 2,4-diselenouracil has been studied using density functional theory (DFT) methods, at the B3LYP/6-311 + G(3df,2p)//B3LYP/6-31G(d,p) level. The relative stability order of selenouracil tautomers does not resemble that of uracil tautomers, but it is similar to that of thiouracils, even though the energy gaps between the different tautomers of selenouracils are smaller than for thiouracils. The tautomerism activation barriers are high enough as to conclude that only the oxo-selenone or the diselenone structures should be found in the gas phase. The specific interaction with one water molecule reduces these barriers by a half, but still the oxo-selenone form is always the most stable tautomer. The addition of a second water molecule has a relatively small effect, as well as bulk effects, evaluated by means of a continuum-polarized model. For isolated 2- and 4-selenouracils, the more favorable tautomerization process corresponds to a hydrogen transfer towards the selenium atom, the activation barriers for transfer towards the oxygen atom being much higher. This situation changes when specific and bulk effects are included, and the latter process becomes the more favorable one. For 2,4-diselenouracil the more favorable tautomerization, in the gas phase, corresponds to the H shift from N1 to the Se atom at C2, while solvation effects favor the transfer from N3 to the Se atom at C4.     

Revealing Unexpected Mechanisms for Nucleophilic Attack on S?S and Se?Se Bridges

The reactivity of disulfide and diselenide derivatives towards F^? and CN^? nucleophiles has been investigated by means of B3PW91/6‐311+G(2df,p) calculations. This theoretical survey shows that these processes, in contrast with the generally accepted view of disulfide and diselenide linkages, do not always lead to S? S or Se? Se bond cleavage. In fact, S? S or Se? Se bond fission is the most favorable process only when the substituents attached to the S or the Se atoms are not very electronegative. Highly electronegative substituents (X) strongly favor S? X bond fission. This significant difference in the observed reactivity patterns is directly related to the change in the nature of the LUMO orbital of the disulfide or diselenide derivative as the electronegativity of the substituents increases. For weakly electronegative substituents, the LUMO is a σ‐type S? S (or Se? Se) antibonding orbital, but as the electronegativity of the substituents increases the π‐type S? X antibonding orbital stabilizes and becomes the LUMO. The observed reactivity also changes with the nature of the nucleophile and with the S or Se atom that undergoes the nucleophilic attack in asymmetric disulfides and diselenides. The activation strain model provides interesting insights into these processes. There are significant similarities between the reactivity of disulfides and diselenides, although some dissimilarities are also observed, usually related to the different interaction energies between the fragments produced in the fragmentation process.     

Structure and charge distribution of some alkynoyl cations

We have performedab initio calculations to determine the structure and charge distribution of some alkynoyl cations and their parent alkynoyl fluorides. We have used Mulliken population analysis and a new technique developed by Yáñez, Stewart and Pople. Our results indicate that the mesomeric form O⁺C–CC–R is one of the most important contributors to the structure of these cations, in agreement with experimental conclusions. We have also found that the participation of mesomeric form O=C=C=C⁺ -R is not negligible and increases with -substitution. In the 3-phenylpropynoyl cation substantial delocalization of charge into the phenyl group occurs. Calculations from YSP population analysis are in good agreement with experimental evidence.