Victor Vargas

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.

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.

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.

Bridge effect in charge transfer absorption bands of para-substituted β-nitrostyrenes

Charge transfer electronic transitions in a series of para-substituted β-nitro-styrenes are analyzed in order to characterize the π-conduction channel between the electron-donor substituents and the NO2 electron-acceptor group, particularly the bridge effect of the olefinic molecular structure.

Cooperative Association of p-Alkylbenzene Sulfonates Sodium Salts to Poly-2-(dimethylamino) Ethylmethacrylate-N-alkyl Quaternized at the Water/Chloroform Interface

The association of p-alkyl benzene sulfonate sodium salts with several poly-2-(dimethylamino) ethylmethacrylate-N-alkyl quaternized bromides at the water/chloroform interface was studied. High association percentages were found which increase with both the size of the sulfonate molecule and the length of the polyelectrolyte side chain: octyl, decyl, dodecyl and tetradecyl. The results fit well to the Hills equation for the association of anionic surfactant to polyelectrolytes. Benzene sulfonate shows an anticooperative behavior for the association, whereas 4-methyl benzene sulfonate and 4-ethylbenzene sulfonate are increasingly cooperative for the association. From the association constants, the standard free energies of transfer from water to the interfacial polyelectrolytes were determined. Their values were a linear function of the number of carbon atoms of the aliphatic residue of the benzene sulfonate molecule ranging from −15.7 kJ/mol of CH2 groups for the octyl polyelectrolyte to −18.9 kJ/mol of CH2 groups for the tetradecyl derivative. These values are similar to those reported for the association of some of these sulfonates molecules with ammonium type cationic micelles. The results for the incremental free energy of transfer by mol of methylene group from water to micelles are in the same order of magnitude as those reported for p-alkyl phenoxides and p-alkylbenzoates to hexadecyltrimethyl ammonium bromide micelles. The results shows that amphipathic counterions as small as 4-ethylbenzenesulfonate are enough to induce a cooperative effect in their association to cationic polyelectrolytes.

Luminiscent Properties of Benzoxazolinones

Abstract The UV absorption and fluorescence spectral bands of 2-benzoxazolinone and 6-methoxy-2-benzoxazolinone have been recorded in solution at room temperature. The phosphorescence spectral bands, their Quantum yields and mean life times, have been measured in solution of ethanol at 77 K. Whilst internal conversion does not compete with fluorescence in these compounds, an efficient intersystem crossing in the triplet state occurs in a nonradiative pathway to ground state with a low phosphorescence quantum yield.

Association of 4-n-Alkylbenzene Sulfonates with Hydrophobically Modified Poly (Diallyldimethylammonium) Chlorides

The association of poly (diallyldimethylammonium) chloride (PDDAC) hydrophobically modified with octyl, decyl, and dodecyl chlorides, with 4-n-alkylbenzene sulfonates, was studied in aqueous solution by ultrafiltration. Experimental data agree well with the Hills equation for the association process. The association degree increases with the size of the sulfonate anion and decreases as the polyelectrolyte side chains length increases. This kind of polyelectrolyte is able to form hydrophobic microdomains lowering the availability of association sites due to a hypercoiling process. The association process was, in all cases, cooperative, having a maximum with the octyl derivative. The association degrees were significantly higher than those found with PDDAC itself. The standard free-energy contribution to the association process was between −19 and −20 kJ/mol of benzene sulfonate. These values are comparable with those recently found for a family of hydrophobic cationic polyelectrolytes at the water/chloroform interface. Therefore, in terms of the association process, it does not matter if the polyelectrolyte is adsorbed at a liquid/liquid interface or if it is soluble in a pure phase. The contribution of the methylene groups was very small, ranging from −0.20 to −0.50 kJ/mol. This reveals that the main interaction governing the association is the aromatic ring–ammonium group interaction.

BRIDGE EFFECT OF THE C˭N BOND AND LONG DISTANCE ELECTRONIC EFFECTS OF ELECTRON-DONOR (D) SUBSTITUENTS ON N-(4-D-BENZYLIDENE)-4-NITROANILINES AND N-(4-NITROBENZYLIDENE)-4-D-ANILINE

ABSTRACT By means of 13C-NMR spectroscopy and ab initio molecular orbital theory calculations we have analyzed the bridge effect of the ˭N bond and the long distance electronic effect of the electron-donor substituents (D: -NO2, -Cl, -H, -CH3, -OCH3, and -N(CH3)2) on N-(4-D-benzylidene)-4-nitroanilines (DCNA) and N-(4-nitrobenzylidene)-4-D-ani-line (DNCA), in the ground electronic state. From the 13C-NMR spectral assignment signals and our molecular orbital calculations on a Gaussian HF/6-31G* basis set, we have found a linear functional dependence of the chemical shifts on the electronic charge of the C1, C4, 1′ and C4′ centers. Furthermore, we have determined the effect of the nitrogen centres on the molecular bridge by means of the chemical shifts of the carbon centres, the theoretical charge densities and the dipolar moments. From an electronic point of view, our results permit determine in a quantitative way the local charge accumulation capacity on the ˭N bond induced by the electron-donor substituent, as well as, determine the bridge effect on the dipolar moment.

INTERMOLECULAR ASSOCIATION OF AMPHIPATHIC POLYELECTROLYTES IN AQUEOUS SOLUTIONS

The intermolecular association of amphipathic polyelectrolytes is studied using fluorescence and conductivity methods. This type of interaction is evidenced by the decrease in the average distances between charges as the polyelectrolyte side chain increases in length. This modify the dissociation degree and consequently, the linear charge density parameter. These distances were calculated with the Manning counterion condensation theory for the conductivity of polyelectrolyte solutions. The determination of the ratio I1/I3 of the fluorescence bands of pyrene with polymer content reveals the formation of hydrophobic microdomains at very low concentrations, smaller than the required concentration to produce a significant change in the average distances between charges. These distances and also the I1/I3 ratio vary with polymer concentration and their values were dependent on the size of the side chain in the polyelectrolyte. Finally, the experimental behavior of the solution viscosity and electrical conductivity of polyelectrolytes, which increase drastically with dilution, can be explained as a continuous change in the average distances between charges which produce conformational changes.