Maurizio Cignitti

UV spectroscopic study and conformational analysis of domperidone

Abstract The UV absorption spectra of domperidone, 2-(3 H )-benzimidazolone and 5-Cl-2-(3 H )-benzimidazolone in CH 3 CN have been studied both in the absence and in the presence of hard acids. A theoretical analysis of the electronic transitions of 2-(3 H )-benzimidazolone has also been performed by means of quantum-mechanical methods. Molecular mechanics (force field) calculations have been carried out to characterize the conformational space of domperidone. The results show a large number of conformers lying with 3 kcal mol −1 with respect to the lowest energy structure.

A STUDY OF THE DONOR PROPERTIES OF THE CARBONYL GROUP OF PYRIDONES

Abstract The donor properties of the CO group of pyridones have been examined by studying the IR spectra of their complexes with mercury-(II)-chloride and with hydroquinone. The complexes with mercury chloride are formed by a weak O⋯Hg donor-acceptor bond, while the others have the character of strongly H-bonded salts of hydroxypyridinium cations. The dipole moment of N-methyl-3-pyridone, not reported so far, has also been measured, (7·2 D). These results, and the agreement between the theoretical and the experimental dipole moments, support the prediction that the donor properties of the CO group are very similar for the three isomers.

Isomeric methoxypyridine-1-oxides and 1-methoxypyridones electronic spectra and structure

Abstract The electronic spectra of the isomeric forms of hydroxypyridine-1-oxides and 1-hydroxypyridones were calculated by the CNDO/CI method. Transition energies, intensities and assignments were compared with the u.v. spectra recorded for the first time on all the methyl-substituted isomers under similar conditions in non-aqueous solvents. Ground state properties of the same compounds were calculated after a full geometry optimization using the CNDO/2 and MINDO/3 approximations.

Tautomeric forms of oxy- and oxo-derivatives of 1-3-5-triazine. Part II. The ultraviolet absortion of 2-4-6-trimethoxy-l-3-5-triazine and 2-4-6-trioxo-1-3-5-trimethyl-hexahydro-triazine

Abstract The spectra of the two tautomeric isomers V and VI have been studied in the range 200–350 mμ, using n-hexane, dioxane, acetonitrile and water as solvents. Both molecules possess a shoulder at about 45,000 cm −1 which is assigned to a n → π* transition. The recorded spectra give evidence for a strong absorption band slightly above 50,000 cm −l , which should be assigned to a π → π* transition. These results are regarded as an indication that the lowest energy levels of the two tautomeric forms have a rather similar pattern, the main difference being in their transition moments, and, hence, in the intensity of the corresponding bands.

Protonation of acetamidoxime: An ab initio molecular orbital study

Abstract Stabilities of acetamidoxime isomers, conformers and tautomers were investigated by ab initio calculations using different basis sets (up to 6–31G*). Protonation energies evaluated for the most stable tautomers indicate that the best protonation site of the amino—oxime structures is the oxime nitrogen whereas the imino—nitrogen is the preferred site for the other tautomeric forms.

494—Apparent equilibrium constants for self-stacking: Calorimetric study of 5′AMP in aqueous solutions

Abstract Data available in the literature show that the numerical values of the apparent equilibrium constants, K st , of self-association of nucleotides and nucleobases differ by up to an order of magnitude, for the same compound. In addition to the known influence of the salts present in water solution it would also appear that the experimental technique used may influence the numerical values of K st . With regard to this last point, we report here a study of the self-association of 5′AMP using microcalorimetry. The thermodynamic parameter values obtained confirm their dependence on the experimental techniques utilized.

390 - Quantitative study of donor-acceptor interactions with biological relevance. I. C-T complexes between substituted pyridinium ions and phenolic structures

Abstract The apparent association constants and enthalpies of various C–T complexes between substituted pyridinium ions and phenolic structures have been determined by U.V. absorption spectroscopy. All the complexes are 1:1 with apparent association constants in the range 0.2–1.0 M−1. The ΔHo of the reactions are of the order of 10 kJ mol−1. As expected the presence of the OH groups in the donor molecule is the main structural feature influencing the interaction; while their relative position (o, m, p) is irrelevant in this respect.

The relative stabilities of isomeric HCN dimers

Abstract The relative ground-state energies of various HCN dimers, structures supposed to be key intermediates in the prebiotic synthesis of proteins, have been evaluated within the INDO semi-empirical molecular orbital procedure. The influence of a complete optimization of the geometrical parameters has been evaluated and compared with previous calculations.

Charge-transfer interactions in some 2-, 3-, 4-substituted N-methylpyridinium iodides and tri-iodides

Abstract The charge transfer (CT) interactions involving the substituted pyridinium ion are of a remarkable biological interest. In particular the pyridinium ions having substituents in position 3 of the heterocyclic ring constitute models for the study of similar, more complex structures. In this paper some spectral characteristics are reported which show such CT interactions between N-methyl-2-, -3-, -4-metoxypyridinium ions and iodide ions, and between N-methyl-2-, -3-, -4-tiomethylpyridinium ions and iodide and tri-iodide ions. The pyridinium ions substituted in position 3 turned out to be the strongest electron-acceptors. The 1H N.M.R. investigations concerning the N+-CH2 chemical shift confirmed that : I. a substituent of the -OCH3 and -SCH3 type in the pyridine ring causes a rise in the negative charge on the nitrogen in relation to the position in the ring according to the sequence 2 > 4 > 3 ; 2. the substitution of the -O-CH3 group with the -S-CH3 group seems to modify the nitrogen charge-density only when the substitution takes place in position 2.

Conformational properties of o-alkoxy-benzamides in different solvents

Abstract Electronic absorption/emission, 1 H NMR spectroscopy and quantum mechanical calculations have been used to characterize the ground-state conformation of o-methoxy-and o-hexyloxy-benzamides in different solvents. Experimental evidence supports the assumption that a conformation having an intramolecular H-bond between one amide hydrogen and the o-alkoxy oxygen is significantly populated even in aqueous solutions.