Structural,electronic properties and intramolecular hydrogen bonding of substituted 2-[(E)-imino methyl] benzenethiol in ground and first excited state by quantum chemical methods

Quantum chemical calculations of geometric structure, the intramolecular hydrogen bond, harmonic vibrational frequencies, NMR spin–spin coupling constants, and physical properties such as chemical potential and chemical hardness of the 2-(E)-imino methyl benzenethiol and its nineteen derivatives were carried out using density functional theory (DFT/B3LYP/6-311++G**) method in the gas phase and the water solution. Furthermore, the topological properties of the electron density distributions for S–H···N intramolecular hydrogen bond have been analyzed in terms of the Bader’s theory of atoms in molecules (AIM). Natural bond orbital (NBO) analysis also performed for better understanding the nature of intramolecular interactions, the results of analysis by quantum theory of AIM and NBO method fairly supported the DFT results. Besides, MEP was performed by the DFT method. On the other hand, the aromaticity of the formed ring has been measured using several well-established indices of aromaticity such as nucleus-independent chemical shift, harmonic oscillator models of the aromaticity, para-delocalization index, average two-center indices, and aromatic fluctuation index. Also, the excited-state properties of intramolecular hydrogen bonding in these systems have been investigated theoretically using the time-dependent DFT method.     

Periodic solutions for Hamiltonian systems under strong constraining forces

We consider periodic solutions of Hamiltonian systems in Euclidean spaces whose motion is constrained to a submanifold M. We prove that under some nondegeneracy assumptions, periodic solutions persist when the constraint is replaced by a strong restoring potential.     

Stable standing waves of nonlinear Klein-Gordon equations

In this paper we give sufficient conditions for the stability of the standing waves of least energy for nonlinear Klein-Gordon equations.     

Design of novel antibiotics that bind to the ribosomal acyltransfer site

The structure of neamine bound to the A site of the bacterial ribosomal RNA was used in the design of novel aminoglycosides. The design took into account stereo and electronic contributions to interactions between RNA and aminoglycosides, as well as a random search of 273 000 compounds from the Cambridge structural database and the National Cancer Institute 3-D database that would fit in the ribosomal aminoglycoside-binding pocket. A total of seven compounds were designed and subsequently synthesized, with the expectation that they would bind to the A-site RNA. Indeed, all synthetic compounds were found to bind to the target RNA comparably to the parent antibiotic neamine, with dissociation constants in the lower micromolar range. The synthetic compounds were evaluated for antibacterial activity against a set of important pathogenic bacteria. These designer antibiotics showed considerably enhanced antibacterial activities against these pathogens, including organisms that hyperexpressed resistance enzymes to aminoglycosides. Furthermore, analyses of four of the synthetic compounds with two important purified resistance enzymes for aminoglycosides indicated that the compounds were very poor substrates; hence the activity of these synthetic antibiotics does not appear to be compromised by the existing resistance mechanisms, as supported by both in vivo and in vitro experiments. The design principles disclosed herein hold the promise of the generation of a large series of designer antibiotics uncompromised by the existing mechanisms of resistance.     

Preconcentration of trace amounts of beryllium in water samples on octadecyl silica cartridges modified by quinalizarine and its determination with atomic absorption spectrometry

A simple method has been developed for rapid and selective extraction, preconcentration and determination of trace amounts of beryllium. The extraction is carried out by octadecyl silica cartridge, modified with quinalizarine as a chelating agent. The effect of different parameters, such as sample matrix, flowrates of sample solution and eluent, pH, type and least amount of eluent for elution of Be(+2) ions from cartridge, breakthrough volume and limit of detection, were evaluated. Also, the effects of various cationic and anionic interferences on percent recovery of Be(+2) were studied. Be(+2) was extracted from solution at pH 6-6.6 and was eluted from modified cartridge with 5 ml of 0.5 M HNO(3). Extraction efficiencies >99% were obtained by elution of the cartridges with minimal amount of eluent. A preconcentration factor of 200 and a detection limit of 200 ng per 1000 ml were obtained. The method was applied to the recovery and determination of Be(+2) in different water and alloy samples.     

Determination of explosives in environmental water samples by solid-phase microextraction-liquid chromatography

When explosives are present in natural aqueous media, their concentration is usually limited to trace levels. A preconcentration step able to remove matrix interferences and to enhance sensitivity is therefore necessary. In the present study, we evaluated solid-phase microextraction (SPME) technique for the recovery of nine explosives from aqueous samples using high-performance liquid chromatography with ultraviolet detection (HPLC-UV). Several parameters, including adsorption and desorption time, coating type, rate of stirring, salt addition, and pH, were optimized to obtain reproducible data with good accuracy. Carbowax coating was the only adsorbent found capable of adsorbing all explosives including nitramines. Method detection limits (MDL) were found to range from 1 to 10 microg/L, depending on the analyte. SPME/HPLC-UV coupling was then applied to the analysis of natural ocean and groundwater samples and compared to conventional solid-phase extraction (SPE/HPLC-UV). Excellent agreement was observed between both techniques, but with an analysis time around five times shorter, SPME/HPLC-UV was considered to be applicable for quantitative analysis of explosives.     

Ab initio and DFT studies on the mechanism of the 1,3-dipolar cycloaddition reaction between nitrone and sulfonylethene chloride

The molecular mechanism for the 1,3-dipolar cycloaddition of nitrone with sulfonylethene chlorides has been studied using ab initio and DFT methods at the HF, MP2 and B3LYP levels together with the 6-31G* basis set. Relative rates, stereo and regioselectivity, have been analysed and discussed. For this cycloaddition four reactive channels associated with the formation of two pairs of diastereoisomeric regioisomers have been characterized. Analysis of the geometries of the corresponding transition structures shows that the cycloaddition takes place along a concerted but asynchronous mechanism. Activation energies as asynchronicity are dependent on the computation level. Thus, while HF calculations gave large barriers, MP2 calculations tend to underestimate them. DFT calculations gave reasonable values. These 1,3-dipolar cycloadditions present an endo stereoselectivity while the meta regioselectivity depends on the computational level. Thus, while HF and DFT calculations predict meta path, in agreement with the experimental results, MP2 calculation predict ortho regioselectivity. The frontier molecular orbitals analysis shows that the reaction is controlled by the (HOMO_dipole–LUMO_{dipolarophile}) interaction in agreement with the charge transfer analysis carried out at the transition structures. Inclusion of diffuse functions at the B3LYP/6-31+G* level increases the energy barriers about 4 kcal/mol, giving a similar endo/meta selectivity. Solvent effects have been taken into account, by means of self-consistent reaction field.