Hyperaromatic stabilization of arenium ions: acid-catalyzed dehydration of 2-substituted 1,2-dihydro-1-naphthols

Rate constants for acid-catalyzed dehydration of cis-2-substituted 1,2-dihydro-naphthols are well correlated by the Taft relationship log k = -0.49 - 8.8σ(I), with minor negative deviations for OH and OMe. By contrast the trans substituents show a poor correlation with σ(I) and in most cases react more slowly than their cis isomers. The behavior is consistent with rate-determining formation of a 2-substituted carbocation (naphthalenium ion) intermediate that for cis reactants possesses a 2-C-H bond suitably oriented for hyperconjugation with the charge center. For the trans isomers the 2-substituent itself is oriented for hyperconjugation in the initially formed conformation of the cation. It is argued that k(cis)/k(trans) rate ratios for substituents (Me, 8.4; Bu(t), 12.7; Ph, 3.8; NH(3)(+), 160; OH, 440) reflect their hyperconjugating ability relative to hydrogen. Faster reactions of trans isomers are observed for substitutents known (RS, N(3)) or suspected (EtSO, EtSO(2)) of stabilizing the cation by a π or σ neighboring group effect. The good Taft correlation is taken to indicate that cis substuents are reacting normally, differentiated only by their inductive effects. The slower reactions of the trans isomers are the judged to be "abnormal". This is confirmed by comparing effects of cis and trans β-OH substituents on the reactivities of dihydro phenols, naphthols, and phenanthrols. Whereas k(H)/k(OH) for cis substituents varies by less than 8-fold and is consistent with the influence of an inductive effect of the OH group (k(H)/k(OH) ≈ 2000), k(H)/k(OH) for the trans substituents varies by 3 orders of magnitude, reflecting the additional influence of the lesser hyperconjugating ability of a C-OH bond compared to a C-H bond. The magnitude and variation of this difference is consistent with C-H hyperconjugation conferring aromatic character on the arenium ions.     

Spin transfer torque in non-collinear magnetic tunnel junctions exhibiting quasiparticle bands: a non-equilibrium Green’s function study

A non-equilibrium Green’s function formulation to study the spin transfer torque (STT) in non-collinear magnetic tunnel junctions (MTJs) exhibiting quasiparticle bands is developed. The formulation can be used to study the magnetoresistance and spin current too. The formulation is used to study the STT in model tunnel junctions exhibiting multiple layers and quasiparticle bands. The many body interaction that gives rise to quasiparticle bands is assumed to be a s ? f exchange interaction at the electrode regions of the MTJ. The quasiparticle bands are obtained using a many body procedure and the single particle band structure is obtained using the tight binding model. The bias dependence of the STT as well as the influence of band occupancy and s ? f exchange coupling strength on the STT are studied. We find from our studies that the band occupancy plays a significant role in deciding the STT and the s ? f interaction strength too influences the STT significantly. Anomalous behavior in both the parallel and perpendicular components of the STT is obtained from our studies. Our results obtained for certain values of the band occupation are found to show the trend observed from the experimental measurements of STT.     

Design and synthesis of oxaprozin-1,3,4-oxadiazole hybrids as potential anticancer and antibacterial agents

In the present study, we report design, synthesis and screening of new novel 5-substituted-2-mercapto-1,3,4-oxadiazole analogues appended to oxaprozin for their in vitro anticancer and antibacterial activity. The synthesised compounds were characterized using various spectroscopic techniques. Furthermore, the structure of 5b (2-(2-[4,5-diphenyloxazol-2-yl]ethyl)-5-(ethylthio)-1,3,4-oxadiazole) was unequivocally confirmed by X-ray analysis. Among the series 5c (2-(2-[4,5-diphenyloxazol-2-yl]ethyl)-5-(propylthio)-1,3,4-oxadiazole) showed most promising anticancer activity against A549 cancer cell line and all the reported analogues manifested satisfactory safety profiles against human normal cell line HEK293T. The products exhibited good antibacterial activity and among the tested 5j (2-(2-[4,5-diphenyloxazol-2-yl]ethyl)-5-([4-fluorobenzyl]thio)-1,3,4-oxadiazole) exhibited most potent.     

Poly(methyl methacrylate)-Assisted Exfoliation of Graphite and Its Use in Acrylonitrile-Butadiene-Styrene Composites

One of the applications of graphene in which its scalable production is of utmost importance is the development of polymer composites. Among the techniques used to produce graphene flakes, the liquid-phase exfoliation (LPE) of graphite stands out due to its versatility and scalability. However, solvents suitable for the LPE process are generally toxic and have a high boiling point, making the processing challenging. The use of low boiling point solvents could be convenient for the processing, due to the easiness of their removal. In this study, the use of poly(methyl methacrylate) (PMMA) as a stabilizing agent is proposed for the production of graphene flakes in a low boiling point solvent, that is, acetone. The graphene dispersions produced in the mixture acetone-PMMA have higher concentration, +175 %, and contain a higher percentage of few-layer graphene flakes (<5 layers), that is, +60 %, compared to the dispersions prepared in acetone. The as-produced graphene dispersions are used to develop graphene/acrylonitrile-butadiene-styrene composites. The mechanical properties of the pristine polymer are improved, that is, +22 % in the Young's modulus, by adding 0.01 wt. % of graphene flakes. Moreover, a decrease of ≈20 % in the oxygen permeability is obtained by using 0.1 wt. % of graphene flakes filler, compared to the unloaded matrix.     

Optimization of lignin recovery from sugarcane bagasse using ionic liquid aided pretreatment

Ionic liquid 1-ethyl-3-methylimidazolium acetate ([EMIM]oAc) was employed for the pretreatment of sugarcane bagasse (SCB) and extraction of lignin, a potentially valuable by-product of the biofuel industry. Response surface methodology based on central composite design was exploited and thereby an empirical model, exhibiting a coefficient of determination, R², of 0.9890, was established to optimize lignin recovery. In particular, a maximum lignin yield, equal to 90.1%, was calculated at the optimal pretreatment conditions, namely time: 120 min, temperature: 140 °C, and ionic liquid to bagasse ratio equal to 20:1 (wt/wt). The presence of guaiacyl and syringyl rings in lignin was confirmed by Fourier transform infrared spectroscopy (FTIR); whereas UV–Vis spectrophotometry showed that both p-coumaric acid and ferulic acid were contained in the lignin. Thermal analysis indicated a maximum decomposition rate of 2%/°C at 265 °C while Gel permeation chromatography analysis revealed that the molecular weight (M_w) of recovered lignin was equal to 1769 g/mol. Comparison of FTIR spectra of pretreated and untreated bagasse showed a negligible presence of lignin in the pretreated samples. Maximum delignification of bagasse after pretreatment was thus ensured. Thermal stability of the ionic liquid towards recyclability was proven by thermogravimetric analysis. The present study established adequate performance of neat and recycled ([EMIM]oAc) with regard to lignin recovery from SCB.     

Copper-catalyzed one-pot process to construct triazole-linked urea derivatives

A new one-pot synthetic methodology has been developed to construct the triazole-linked urea derivatives. In the same step, triazole forms from azide and alkyne via click reaction and urea forms from addition reaction of amines and isocyanates. The notable features of this work include wide substrate scope, atom economy, environmental friendliness, and easily accessible starting materials. The preliminary biological screening of synthesized compounds show promising cytotoxic activity in basal cell carcinoma (BCC) cell lines.