Molecular Models in Chemistry Education at University and Upper Secondary School - Structure of Amides

Molecular models derived from results of quantum-chemical calculations present an important category of didactic instruments in chemistry education in upper secondary school and, particularly, at university. These models can be used especially as tools for supporting the students’ understanding by visual learning, which can adequately address complexity of many chemical topics, incorporate appropriate didactic principles, as well as utilize the benefits brought up by the actual information technology. The proposed molecular models are non-trivial examples of didactic application of computational chemistry techniques in illustration of electron interactions in amidic group, namely the interaction of the free electron pair on the nitrogen atom with the carbonyl group and also the interaction of atoms in the amide group with other surrounding atoms in the molecule. By these molecular models it is possible to explain acid-base properties of amides applying knowledge of electron density distribution in the molecules and the resulting electrostatic potential. Presentation of the structure and properties of the amides within education is important also for the reason that amidic functions are involved in many important natural substances (e.g. proteins, peptides, nucleic acids or alkaloids), synthetic macromolecular substances (e.g. Silon) or pharmaceutical preparations (e.g. paracetamol). Molecular models then serve to support better understanding of the structure of these substances and, in relation to it, their properties.

Comparison the Effectiveness of Chitosanin Blue Crab Shells with Ciprofloxacin to the Development of Eschericia coli

Background: Escherichia coliis a normal intestinal flora that have contributions on the normal functioning of human digestion, but it will turn into a pathogen when it is outside the intestinal tract. E. coli including family Enterobacteriaceae, facultative anaerobic. Characteristics of E. coli is a bacteria gram-negative rod-shaped, motile with flagella or nonmotile, fermentation of glucose, and producing gas. Chitosan has antimicrobial effect against bacteria. Amine moieties of chitosancontent positively charged which is very reactive, and have the ability to bind with the cell walls of bacteria are negative charged. Amine cluster also has a free electron pair, so it was able to attract minerals Ca2+ in the cell walls of bacteria.Objective: to know the comparative effectiveness of chitosanon the shell of a small crab attaching with ciprofloxacin antibiotics against the development of the bacteria E. coli.Methods: this research uses experimental methods. Techniques used in measuring antimicrobial activity is the diffusion disc method.Results: the results showed that the average diameter of the inhibitory zones of chitosanagainst E. coli in a row with a concentration of 4%, 5%, 6%, 7%, and 8% is 14.22 mm, 11.94 mm, 9.18 mm, 11.19 mm, and 8.76 mm. Whereas the average diameter of the inhibitory zones of ciprofloxacin is 42.31 mm.Conclusion: Chitosanon small crab attaching shells with a concentration of the most effective in inhibiting the development of the bacteria E. coli is chitosanwith concentrations of 4% with the average drag zone amounting to 14,22 mm.International Journal of Human and Health Sciences Vol. 02 No. 04 October’18. Page : 217-219

Theoretical Study of the Coordination of Semicarbazone and Its Methylated Derivatives

Semicarbazone is a molecule with a group R1R2C=N-NR3-C(=O)-NR4R5.The oxygen atom has two free electron pairs; each nitrogen atom has one free electron pair. These free electron pairs are potential sites of coordination. The simplest molecule in this series is the semicarbazone which the formula is H2C=N-NH-C(=O)NH2. By replacing the oxygen atom by a sulfur atom is obtained a thiosemicarbazone. Some semicarbazones, such as nitrofurazone, and some thiosemicarbazones are known to have many properties: antiviral, antibacterial, antitrypanosomal, anticonvulsant, antitumor, anticancer. They are usually mediated by an association with copper or iron.Indeed transition metal complexes with given chemical structures are useful alternatives in the treatment of certain diseases since coordination of active ingredients deeply modifies both the physiological properties of metals and ligands in the meaning of overall improvement of these properties.The present work focuses on quantum study of the complexation of semicarbazone and its methylated derivatives. The purpose of this study is to determine the most favorable coordination site of each of these ligands. It was found that the oxygen atom appears more favorable to the coordination of semicarbazones.Complexes of these ligands with the Zn (II) were modeled. The calculations were made by the method DFT / B3LYP with the orbital basis 6-31G (d, p).

Removal of triazine-based herbicides on specific polymeric sorbent: batch studies

The triazine-based herbicides removal from aqueous solution on specific polymeric adsorbent was studied. Poly(divinylbenzene) modified with maleic anhydride in Diels-Alder reaction was selected for the sorption experiments because molecular structures of triazine derived herbicides exhibit complementarity to the arrangement of functional groups in the polymer. The presence of carboxyl groups in adsorbent structure resulted in specific directional interactions, such as hydrogen bonds, which can intensify adsorption ability towards triazines. In the case of both atrazine and terbuthylazine the effect is more intensive, whereas in sorption of simazine and propazine the non-specific interactions have higher importance than hydrogen bonds. Specific interactions in investigated systems are between the hydrogen atom of the amino group of triazine and the carbonyl oxygen atom of the carboxyl group of the modified poly(divinylbenzene) (O…H–N). Only in the case of terbuthylazine the creation of hydrogen bonds between hydroxyl hydrogen atom of carboxylic group and nitrogen atom containing free electron pair from triazine (O–H…N) was observed. The sorption of simazine, atrazine and propazine does not depend on pH in the acidic region, whereas in the case of terbuthylazine an increase in sorption efficiency is observed while pH decreases.

Structure and hydrogen bonding of the hydrated selenite and selenate ions in aqueous solution

The selenite ion has an asymmetric hydration sphere with loosely electrostatically bound water molecules outside the free electron pair.

Influence of protonation of peripheral substituents on photophysical and photochemical properties of tetrapyrazinoporphyrazines

Octasubstituted zinc tetrapyrazinoporphyrazines with four N,N-dimethylaminophenyls and four phenyl or pyridin-3-yl substituents were synthesized and fully characterized. Their fluorescence quantum yields in DMF or pyridine were very low, almost undetectable, as a consequence of ultrafast intramolecular charge transfer. Titration of their DMF solutions with sulfuric acid led to increase of the fluorescence quantum yields by two orders of magnitude when the full protonation of peripheral substituents was achieved. Intramolecular charge transfer is no longer a favorable way of excited-state relaxation at full protonation of N,N-dimethylaminophenyl substituents because of loss of donor centers (free electron pair on its nitrogen). Similarly, singlet oxygen quantum yields also increased by two orders of magnitude when sulfuric acid was added to tetrapyrazinoporphyrazine solutions in DMF. Protonation at azomethine nitrogens of tetrapyrazinoporphyrazine macrocycle was observed at higher acid concentrations and it led to considerable decrease of fluorescence quantum yields. Octaphenyl zinc tetrapyrazinoporphyrazine and octa(pyridin-3-yl) zinc tetrapyrazinoporphyrazine were used as controls without intramolecular charge transfer. Their fluorescence and singlet oxygen quantum yields were high in DMF and decreased at higher concentrations of sulfuric acid due to protonation of azomethine nitrogens. The results suggest that the photophysical and photochemical properties of studied compounds may be controlled by changes of pH of medium.

Electronic effects on enol acidity and keto-enol equilibrium constants for ring-substituted 2-tetralones

Equilibrium constants for the ionization of a variety of phenyl-substituted 2-tetralones (pKaK), for the ionization of their enols (pKaE), and for keto-enol tautomerization (pKE) were determined. Hammett plots of pKaK and pKaE vs. σ- are linear with slopes (-ρ) of -1.66 ± 0.06 and -0.90 ± 0.03, respectively, except for deviations of the points corresponding to 6-nitro-2-tetralone (1b) and its enol. We have previously attributed the negative deviation of 1b from the correlation for the acidities of the ketones obtained with the more limited set of data to the lack of a free electron pair on C-1 of the free tetralone (Nevy et al.). The negative deviation of the point for 1b from the correlation for the acidities of the enols suggests that charge transfer from the hydroxyl group of the enol to the nitro group is less important than it is for phenols. This study represents the first systematic study of electronic effects on equilibria among ketone, enol, and enolate in aqueous solution. Key words: enol, acidity, equilibrium, substituent, conjugation.

1H and 13C NMR spectra of 2,2’-disubstitutedtrans-azobenzenes suitable for preparation of metallized azo dyes

The 1H and 13C NMR spectra have been measured of six trans-azobenzenes substituted at 2 and 2’ positions with substituents favourable for complex formation with a metal (OH, NH2, NHCOCH3, COOH). From the standpoint of NMR such substituted trans-azobenzenes are present in solution in a rapid equilibrium following from rotation around the bond between C-1 of phenyl group and N atom of azo linkage. The predominant form has the substituent in the syn-position with respect to the free electron pair of the nearer azo nitrogen atom. The equilibrium is affected by dipolar aprotic solvents (such as hexadeuteriodimethyl sulfoxide) by decreasing the presence of the predominant form by 1 to 11%.

13C, 15N, and 19F NMR spectra of 2-phenylhydrazonopropanedinitriles and methyl 2-phenyl hydrazonocyanoacetates

13C, 15N, and 19F NMR spectra of the azo coupling products obtained from 4-X-benzenediazonium salts (X = H, F, NO2) and propanedinitrile (I) or methyl cyanoacetate (II) and of their 15N isotopomers have been measured. The 13C chemical shifts of cyano groups have been assigned unambiguously on the basis of the 2J(15Nβ13C) coupling constants, and mutual comparison of the 15N chemical shifts in compounds I and II enabled also the assignment of the15N chemical shifts of these groups. The 13C chemical shifts of cyano groups in cis position with respect to free electron pair at Nβ nitrogen atom are shifted downfield, whereas δ(15N) of the same groups exhibit upfield shifts as compared with the cyano in trans position. The azo coupling products of benzenediazonium salts and methyl cyanoacetate contain predominantly the E isomer even after long-term standing in hexadeuteriodimethyl sulphoxide.