Francesco Lelj

Conformational behavior of gaseous glycine by a density functional approach

Density functional calculations with large basis sets have been performed for nine conformers of neutral glycine. The results obtained by standard, even gradient corrected functionals are significantly different from the best post‐Hartree–Fock computations and the available experimental data. Inclusion of some Hartree–Fock exchange significantly improves matters, thus providing a promising protocol for the study of noncovalent interactions in biomolecules.

Capsule formation, carboxylate exchange, and DFT exploration of cadmium cluster metallocavitands: highly dynamic supramolecules.

A family of molecular heptacadmium carboxylate clusters templated inside [3 + 3] Schiff base macrocycles has been isolated and studied by variable temperature solution and solid-state NMR spectroscopy, single-crystal X-ray diffraction (SCXRD), and density functional theory (DFT) calculations. These metallocavitand cluster complexes adopt bowl-shaped structures, induced by metal coordination, giving rise to interesting host-guest and supramolecular phenomena. Specifically, dimerization of these metallocavitands yields capsules with vacant coordination and hydrogen-bonding sites accessible to encapsulated guests. Strong host-guest interactions explain the exceptionally high packing coefficient (0.80) observed for encapsulated N,N-dimethylformamide (DMF). The guest-accessible hydrogen-bonding sites arise from an unusual mu(3)-OH ligand bridging three cadmium ions. Thermodynamic and kinetic studies show that dimerization is an entropy-driven process with a highly associative mechanism. In DMF the exchange rate of peripheral cluster supporting carboxylate ligands is intrinsically linked to the rate of dimerization and these two seemingly different events have a common rate-determining step. Investigation of guest dynamics with solid-state (2)H NMR spectroscopy revealed 3-fold rotation of an encapsulated DMF molecule. These studies provide a solid understanding of the host-guest and dynamic properties of a new family of metallocavitands and may help in designing new supramolecular catalysts and materials.

A hybrid density functional study of the first‐row transition‐metal monocarbonyls

The results of a systematic density functional study, carried out using a self‐consistent hybrid approach including exact exchange contribution, on the all monocarbonyls of first‐row transition metals, are reported. Geometries, harmonic wave numbers, and binding energies, obtained using both standard generalized gradient corrected and hybrid functionals, are compared with previous published theoretical data and the available experimental findings. It is shown that hybrid functionals give results close to highly correlated post Hartree–Fock approaches and which are sensibly different from those obtained by standard local, even gradient corrected, methods. A nice agreement has been also found between theoretical and experimental binding energies. A natural bond orbital analysis confirms the role of the π interaction in the metal‐carbonyl bond and gives an explanation to the preference for bent structures found in chromium and copper monocarbonyls.

General trends in the molecular physics of azabiphenyls

Some phenylpyridines and bipyridines have been studied by UV photoelectron spectroscopy and quantum-chemical methods. This study, together with some new computations on the previously reported bipyrimidines, has allowed a comprehensive analysis of general trends along the whole series of biphenyl-like molecules. Several physico-chemical characteristics have been considered in this respect, including geometric structure, conformational behaviour, relative stabilities, dipole moments and ordering of ionic states. The results obtained have been analysed in terms of conjugative and steric interactions by means of fragment analysis. On these grounds simple thumb-rules have been suggested, which may prove useful for a better understanding of polyphenyl-like molecules.

Optical non-linear properties of the [MXY] neutral mixed-ligand dithiolenes (M=Ni, Pd, Pt; X=R2timdt, dmit, mnt; Y=R2timdt, dmit, mnt; X≠Y). The role of coordinated metal, substituents and of high lying excited states

Abstract The second-order response of the [MXY] (M= Ni, Pd, Pt; X=R2timdt (monoanion of 1,3-dialkyl imidazolidine-2,4,5-trithione), dmit (2-thioxo-1,3-dithiole-4,5-dithiolate), mnt (maleonitriledithiolate); Y=R2timdt, dmit, mnt; X≠Y) neutral mixed-ligand dithiolenes has been investigated within the TD-DFT approach. Scalar relativistic contributions have been considered on both optimized geometry and hyperpolarizability using the zero-order regular approximation (ZORA). All β tensor components have been calculated off-resonance at 4.48, 3.50 and 2.50 μm. A small HOMO–LUMO energy gap and the presence of a heavy metal make the [Pt(H2timdt)(dmit)] response the largest one among the analyzed ones. In particular, the static βvec value is −40.3(×10−30 esu).

Structural versatility of peptides from Cα,α-dialkylated glycines: a conformational energy calculation and X-ray diffraction study of homopeptides from 1-aminocyclopentane-1-carboxylic acid☆

Abstract Coformational energy calculations on the 1-aminocyclopentane-1-carboxylic acid monopeptide Ac-Acc 5 -NHMe indicate that this C α,α -dialkylated, cyclic α-amino acid residue is conformationally restricted and that its minimum energy conformation falls in the α/3 10 -helical region. The results of the theoretical analysis are in agreement with the crystal-state structural tendency of p BrBz(Acc 5 ) 4 O t Bu·2 MeOH , p BrBz(Acc 5 ) 5 O t Bu· MeOH , and Z(Acc 5 ) 6 O t Bu, determined by X-ray diffraction and also described in this work (formation of 3 10 -helices). The implications for the use of Acc 5 residues in designing conformationally constrained analogues of bioactive peptides are briefly discussed.

Absorption Spectra of the Potential Photodynamic Therapy Photosensitizers Texaphyrins Complexes: A Theoretical Analysis †

A systematic study of a class of divalent transition-metal texaphyrin complexes (M-Tex(+), M = Mn, Fe, Co, Ni, Cu, Zn), recently proposed as active photosensitizers in photodynamic therapy (PDT), was undertaken for the ground and excited electronic states. Geometry optimizations were performed by using the PBE0 exchange-correlation functional coupled with the 6-31G(d) basis set, while electronic excitations energies were evaluated by means of time-dependent density functional response theory (TD-DFT) at the PBE0/6-31+G(d) // PBE0/6-31G(d) level of theory. Solvent effects on excitation energies were taken into account in two ways:  by considering solvent molecules explicitly coordinated to the metal center and as bulk effects, within the conductor-like polarizable continuum model (C-PCM). The influence of the metal cation on the so-called Q-band, localized in the near-red visible region of the spectrum, was carefully examined since it plays a basic role in the drug design of new photodynamic therapy photosensitizers. The differences between experimental and computed excitation energies were found to be within 0.3 eV.

Chemisorption of atomic and molecular oxygen on the (100) surface of silicon; a theoretical study

Abstract The chemisorption of atomic and molecular oxygen on the (100) surface of silicon has been studied by the MNDO method in the framework of a cluster approach. The results show that both atomic and molecular chemisorption are exothermic and occur preferentially at bridge positions. For the atomic process an energy minimum for the open site has been found below the surface, between the first and second layer. Dissociative chemisorption is anyway preferred by a thermodynamic point of view, but involves an activation energy. As a consequence molecular chemisorption could also be observed.

On the chemisorption of water on the (100) surface of silicon

Abstract The chemisorption of water on the (100) surface of silicon has been studied by the MNDO method in the framework of the cluster approach. Molecular chemisorption occurs preferentially at bridge positions, whereas the dissociative process leads essentially to OH (at bridge position) and H (at on-top position) chemisorbed species. The dissociative chemisorption represents the best process from a thermodynamic point of view, but involves an activation energy. As a consequence molecular chemisorption could also be observed. The formation of dihydric phases has been also investigated but is energetically unfavourable.

STABILITY AND STRUCTURE OF FORMAMIDE AND UREA DIMERS IN AQUEOUS-SOLUTION - A THEORETICAL-STUDY

The microscopic characteristics of concentrated aqueous solutions of formamide and urea have been investigated through energy minimization of clusters consisting of one or two solute molecules surrounded by up to 19 water molecules. The computations performed for single solute molecules correctly reproduce the pattern of solent molecules in the first hydration shell found by molecular-dynamics simulations and lead to reasonable solvation enthalpies. The computations performed for two solute molecules indicate that direct CONH bridges are not to be expected in aqueous solution. Solute molecules are, instead, linked by water chains, leading to compact structures which are also compatible with the local tetrahedral environment of the solvent molecules.