Arcangelo Millefiori

Tautomerism and intramolecular hydrogen bonding in β-thioxoketones. An ab initio molecular orbital study on monothiomalondialdehyde

The energetics of tautomerism and intramolecular hydrogen bonding of monothiomalondialdehyde have been studied by fully optimizing with the 3–21G basis set the molecular geometry of several conformers followed by single-point calculations at a higher level of theory. At MP2/6-31G** level the z-enol tautomer is predicted to be 2.09 kJ mol–1 more stable than the z-enethiol tautomer, in agreement with experimental results, while the inclusion of zero-point vibrational energy pushes the equilibrium considerably towards the z-enethiol structure. The OHS hydrogen bond is ca. five times stronger than the SHO bond and appears to be a decisive factor in the stabilization of the enolic structure. The complete set of Δ(SCF6-31G**) ionization potentials (valence and core) is reported and the results are briefly discussed by reference to the effects of the hydrogen bonding on the relative stabilities of the tautomers in the ionized states.

Geometries and energies of small Gen(n= 2–6) clusters: an ab initio molecular orbital study

The molecular geometries and relative stabilities of various electronic configurations of Gen clusters (n= 2–6) have been investigated by means of ab initio calculations by employing an effective core potential for core electrons and a double zeta plus polarization functions basis set for valence electrons. The calculations have produced the following optimal configurations: Ge2(D∞h, 3Σ–g), Ge3(equilateral triangle, D3h3A′2), Ge4(planar rhombus D2h1Ag), Ge5(oblate distorted trigonal bipyramid, D3h1A′1), Ge6(edge-capped trigonal bipyramid, C2v1A1).Binding energies have been evaluated by single-point CISD(Q) calculations at the SCF optimal ground-state geometry of the clusters. The results are in qualitative agreement with experimental data. The comparison of Gen and Sin properties shows close similarity between Ge and Si.

Calculations on π—π* transitions on benzophenone, Phenylpyridyl ketones and dipyridyl ketones

Abstract The “Molecules in Molecules” (MIM) method, after a suitable choice of inerent parameters, is used to interpretate the electronic spectrum of benzophenone (C 2 symmetry) and of analogous phenylpyridyl and dipyridyl ketones. Theoretical results are compared with u.v. experimental findings.

Spectroscopic and electrochemical properties of para-methoxyazobenzene derivatives

Ultraviolet photoelectron spectroscopy, gas-phase absorption spectroscopy, polarographic half-wave reduction potentials and INDO/S-CI molecular-orbital calculations have been employed to investigate the electronic structure of some p-methoxyazobenzene derivatives. The photoelectron bands in the low-energy region of the spectrum have been assigned. The ag(n–) and the au(π1) ionizations display almost the same sensibility towards the substituent. The observed trends are qualitatively predicted by calculation. The electronic transitions are interpreted with the aid of INDO/S-CI calculations. Use is also made of the approach of Haselbach and Schmelzer approach. Reversible half-wave reduction potentials are used as a measure of the solution electron affinity of the molecule. Good linear relations are found between E1/2 and ELUMO and the Hammett σ constant. Correlations between electrochemical and spectroscopic data provide evidence that variations in the experimental transition energies along the series, with the exception of the nitro-derivatives, originate from changes in the (–J+ 2K) and ΔGsolv terms.

Electronic structure of NS compounds. A CNDO study of transition metal thionitrosyl complexes Ni(S2N2H)2 and Pd(S2N2H)2

Abstract Conformational preferences, valence molecular orbitals and electronic charge distribution in thionitrosyl complexes Ni(S 2 N 2 H) 2 and Pd(S 2 N 2 H) 2 have been studied by means of CNDO calculations. Both complexes are predicted to exist in the cis conformation, in agreemetn with the experimental data. Valence molecular orbital energies and compositions show that covalent interactions between transition metal and ligand are important for the metal-ligand bonding. The population analysis gives evidence of inequivalent nitrogen and sulfur atoms and of rather polar SN bonds in the planar ring. The charge distribution is tentatively correlated with the available experimental XPS binding energies.

Tautomerism in arylazonaphthols by dipole moment analysis

Abstract The azo-hydrazone equilibrium in some arylazonaphthols has been studied by dipole moment measurements and HMO calculations. Results show that in 1,4-derivatives comparable amounts of both forms are present. The 1,2-derivatives appear to be essentially azo-compounds, but an overestimate of azo proportion cannot be excluded.

Dipole moments of pyridyl ketones

In a previous work the electronic absorption spectra of phenyl-pyridyl and dipyridy1 ketones were studied [l J . Results of calculations performed by the MIM method gave a satisfactory interpretation of the spectra, but were not able to give valuable information concerning the preferred conformation of the molecules. A contribution presented to this problem is the dipole moment measurements and calculations. Experimental dipole moments were obtained by Guggenheim’s method [Z] . Calculated dipole moments for all possible isomers were obtained by the vector composition of the group moments and mesomeric-n-moments through the fragments of the molecule. Mesomeric moments were calculated from the ground state wave functions computed by the MIM method utilizing the procedure and parametrization previously described [l] . As group moments we took the pyridine moment (2.21 D) and the formaldehyde moment (2.30 D). Since the difference in rr-electron energy between the various isomers with respect to the most stable one results negligible, we assume that the isomeric distribution in pyridyl ketones is determined mainly by the dipolar interaction energy, which was evaluated by the equation

Electronic structure of the 1,5-dithia-2,4,6,8-tetrazocine ring. Ab initio and CNDO-CI study

Calculs sur la dithia-1,5 tetrazocine-2,4,6,8 et son derive diamino-3,7, avec plusieurs ensembles de base (STO-3G, STO-3G+5d, 4-31G, 4-31G+5d) pour les calculs ab initio: les stabilites varient avec les ensembles de base. Analyse de population de Mulliken; transitions electroniques du derive diamino

Gas phase ultraviolet photoelectron spectra of hydroxyanthraquinones

Abstract The gas phase u.v. photoelectron spectra of anthraquinone, 1,4-OH-1,8-OH-2,6-OH-1-NH 2 -and 1-NH 2 -4-OH-anthraquinones have been measured using HeI radiation. Semiempirical HAM/3 calculations have been employed to help in the band assignment. The lowest ionization potential of all the studied compounds is of π-type, except of anthraquinone where the n -O(CO) ionization is the lowest one. The results show that the intramolecular OH ⋯ O bonding stabilizes the n -orbital by ca 0.4 eV, and that the IP n+ −IP n− energy difference should not exceed 0.5 eV, very close to that found in para -benzoquinone. No hydrogen bonding effects of the NH 2 group are apparent.

Ab initio molecular orbital study of the geometric and electronic structure of dimethylsulfurdiimine

Abstract The (E,Z) and (E,E) forms of dimethylsulfurdiimine were fully optimized by ab initio STO-3G* calculations. The (E,Z) conformation is predicted to be more stable than the (E,E) conformation by ca. 41 kJ/mol, in agreement with experimental data. The sulfur 3d orbitals play a crucial role in describing the structural characteristics of the molecule. 4-31G and equivalent 31G core-less calculations have been employed to obtain atomic charge densities, bond overlap populations and molecular orbital energies. A good agreement was obtained between the results of all-electron and pseudo-potential calculations.