Raúl G. E. Morales

Synthesis and antiparasitic activity of 2-(Trifluoromethyl)benzimidazole derivatives

2-(Trifluoromethyl)benzimidazole derivatives substituted at the 1-, 5-, and 6-positions have been synthesized and in vitro tested against the protozoa Giardia lamblia, Entamnoeha histolytica. and the helminth Trichinella spiralis. Results indicate that all the compounds tested are more active as antiprotozoal agents than Albendazole and Metronidazole. One compound (20) was as active as Albendazole against T. spiralis. These compounds were also tested for their effect on tubulin polymerization and none inhibited tubulin polymerization.

ULTRAVIOLET ABSORPTION BANDS AND ELECTRONIC CHARGE TRANSFERS OF SALICYLIDENEANILINES IN SINGLET EXCITED STATES

Salicylideneaniline presents an electronic absorption band spectrum of complex patterns between 200 and 450 nm. However, we have successfully developed the electronic transition assignments of this molecular compound and its derivative species, substituted on the aniline ring of the salicylideneaniline structure. Thus, we have analyzed the substituent effect of the electron-acceptor groups such as ‒CN, ‒COCH3, and ‒NO2, as well as the electron-donor groups such as ‒CH3, ‒OCH3, and ‒N(CH3)2 on the electronic transition energies of the main absorption spectral region. From a theoretical point of view, we have characterized the orbital nature of the electronic transition energies and the net charge transfer in excited electronic states by means of molecular orbital theory calculations in the AM1 and ZINDO/S-CIS semiempirical frameworks. Our present study has permitted us to determine the three main electronic transitions localized under the broad absorption spectral band in the spectral region between 250 and 450 nm. Furthermore, we have experimentally characterized the effect of the substituents on the first three excited electronic states for every substituted salicylideneaniline compound.

Dipole moments of polyenic oligomeric systems. Part I. A one-dimensional molecular wire model

Ground-state dipolar moments of oligomeric compounds, containing electron-donor (D) and electron-acceptor (A) groups as two terminal units of the polyenic bridge (D-wire-A), can well be described by means of a one-dimensional molecular wire model, which considers a scattering process of electrons through the charge-transfer conduction channel. The dipole moment of the oligomers (μn) follows a non-linear dependence of the polyenic bridge length (L) according to μn = μ0 + μ∞(1 − e− γL) where μ0 is the dipole moment of the first compound of the series, without a polyenic unit (n = 0), μ∞ is a limit value for L ∞ and γ is the one-dimensional conduction constant of the π-molecular orbital channel of the molecular wire. This model can be extended to all those conjugated oligomers of the D-wire-A type where the electronic charge of the donor group can induce a soliton wave as far as through the polyenic bridge towards the acceptor group. Copyright

Molecular resistivities in organic polyenic wires: I. A one-dimensional photoconduction charge transfer model

Abstract In the present work we have developed a one-dimensional photoconduction charge transfer model, which allows us to determine the resistivities associated to π-conduction molecular channels of organic polyenic wires. In order to calculate the charge transmission coefficient through the conductor molecular wire, our model is based on the scattering process of electrons in metals according to Landauers one-dimensional conductor approach. This model can be applied to those D–bridge–A molecular systems, constituted by an electron-donor group (D), an electron-acceptor group (A) and a polyenic conductor molecular bridge, which present a photoinduced charge transfer absorption band. According to the present model, molecular resistivities and resistances in the charge transfer excited state of a series of polyenic compounds containing several kinds of electron-donor groups and CO, as an electron-acceptor group, have been calculated by means of the INDO/S-CI method, in the framework of ZINDO semiempirical molecular orbital theory. The calculations allow us to determine the linear and nonlinear contributions to the total molecular resistance of the π-conduction channel in the charge transfer excited state. Linear resistivities for a series of polyenic aldehydes substituted by an electron-donor group (D=–CH 3 , –OCH 3 , –NH 2 and –N[CH 3 ] 2 ) range between 18 and 42 μΩ cm and these results agree with the order of magnitude expected in classic conductors such as metals, nonmetals, and doped polyacetylenic compounds in the ground state.

INTERMOLECULAR PROTON TRANSFER PROCESS IN SULFUR TAUTOMERIC HETEROCYCLES. I. A QUANTUM CHEMICAL APPROACH

Abstract Molecular structures of two tautomeric aromatic heterocycles, 2-mercaptobenzoxazole and 2-mercaptobenzothiazole, were analyzed from a quantum chemical approach by means of AM1 semiempirical molecular orbital calculations. Thiones are the preferential tautomeric structures of these compounds in solid phase and solutions, and the AM1 profiles of potential wells agree the experimental evidences. In spite of the good description of the thione and thiol structures by the AM1 approach, the experimental tautomerism observed at room temperature can not be explained by means of an intramolecular protonic transfer mechanism. The potential barrier AM1 calculations of both tautomers range from 40 to 50 kcal/mol, approximately. Therefore, we have postulated a tautomeric process through a geometrical model of co-planar dimers in these heterocyclic compounds. These dimers are more stable than the respective monomers and this model permit to assume a simultaneous intermolecular proton transfer process between su...

Bridge Effect in Charge Transfer Absorption Bands. Para-substituted Benzylideneacetones

Abstract The electronic absorption charge transfer bands in a series of para - substituted benzalketones are analyzed in order to stablish the role of the electron-donor substkuent as well as the electronic properties of the molecular structure of the π-conduction channel. Absorption bands assignment of the π-π∗ electronic transitions in the near ultraviolet spectral region is carry out from an experimental and theoretical point of view. The photo-induced charge transfer spectral bands in these aromatic compounds follow the same spectral pattern than the para-substituted benzaldehydes and acetophenones and the electronic transition takes place in the π,π∗(1La) excited state. However, our semiempirical M.O. calculations show that this charge transfer process involve the electron-acceptor carbonyl group and the olefinic bond bridge as a second electron-acceptor group.

A Spectroscopic Model for the Study of Preferential in the Ground and Excited Electronic States.

Abstract The preferential solvation of a solute molecule in mixed solvents is analysed in terms of the spectral solvent shifts. A spectroscopic model is developed in order to know the preferential solvation degree in the ground electronic state as well as in the first excited electronic state by measuring the absorption and fluorescence spectra respec tively. Triphenylene was found to be a good model probe molecule in n-butanol/carbon tetrachloride mixtures for both electronic states. Furthermore Tryphenylene in its ground electronic state was studied in chloroform/methanol mixtures.

Indirect Determination of Electronic Transition Frequencies of Coronene, Triphenylene and 1,2,5,6-Dibenzanthracene in the Vapor Phase

Abstract Frequencies of absorption band maxima for the first two electronic singlet transitions of coronene, triphenylene and 1,2,5,6-dibenzanthracene in vapor phase were calculated indirectly from solution spectral data. The calculations were based on two different models: the first one was a linear correlation between the solution absorption frequencies in nonpolar solvents and the solvent refractive indices. The accuracy of both methods was checked with anthracene and some of its derivatives: both methods gave a good agreement with the experimental values.

1π→1π* Ultraviolet Absorption Bands and Electronic Charge Transfers in Singlet Excited States of Sulfur Aromatic Heterocycles

Abstract Indoline‐2‐thione (BC), benzimidazole‐2‐thione (BN), benzoxazole‐2‐thione (BO), and benzothiazole‐2‐thione (BS) define an interesting series of aromatic compounds containing a NCS synthonic unit in a heterocyclic ring of five centers, substituted by atomic centers of the type C, N, O, or S, where the main electronic absorption bands are localized in the spectral range of ultraviolet A or B. The first two singlet electronic transitions of this series, 1S0→1S1(n,π*) and 1S0 → 1S2(π,π*), determine the main spectroscopic characteristic of these compounds in order to be used as potential photochemical actinometers of solar ultraviolet radiation. Furthermore, the second electronic transition, localized in the 270–360 nm ultraviolet spectral range, presents a hipsochromic spectral shift as function of the electronic nature of the heteroatomic centers in the heterocyclic ring. In order to determine a spectroscopic assignment of the main absorption bands in aqueous solution and analyze the effect of the substituent on the electronic charge distributions in the ground and the first two singlet excited electronic states, we have used a semiempirical molecular orbital calculation in the INDO/S‐CIS approach. On the other hand, we have carried out a molecular orbital calculation in the AM1 framework, in order to determine the energetic stability of the thiones with respect to the thiol compounds.

Long distance electronic effects of para-substituted β-nitrostyrenes by 13C-NMR spectroscopy

Abstract By means of 13C-NMR spectroscopy and AM1 molecular orbital calculations of para-β-nitrostyrenes, we have found a characteristic long distance charge transfer pattern, where the olefinic bridge (CHCH) and the aromatic ring (Ph) carbon centres are perturbed according to the donor-nature of the para-substituent groups. After a complete spectral assignment of the 13C-NMR signals, the chemical shifts (δ) of the C1, C3 and Cβ centres show a linear functional dependence with the charge densities (qAM1), while in the same molecular series C2 and Cα are practically constants. On the other hand, an analysis of the electron-donor substituent effect at the para-position of the aromatic carbonyl compounds on the C4 centre, has permitted us to find a good correlation between the C4 chemical shift and the electronegativity of this vicinal centre.