Intra and intermolecular hydrogen bonding in formohydroxamic acid

The presence of hydrogen bonding interactions in several tautomeric forms of formohydroxamic acid (FHA) and 1:1 association among the tautomeric forms and water‐coordinated tautomeric forms of FHA is explored theoretically. Out of the seven equilibrium structures, four tautomeric forms have been selected for aggregation with single water molecule and dimer formation. Fifteen aggregates of FHA with H₂O have been optimized at MP2/AUG‐cc‐PVDZ level and analyzed for intramolecular and intermolecular H‐bond interactions. Twenty‐seven dimers of the four tautomeric forms have been obtained at MP2/6‐31+G* level. The stabilization energies associated with dimerization and adduct formation with water are the result of H‐bond interactions and range from very weak to medium. The atomic charges and NBO analysis indicate that the electrostatic and the charge transfer are the important components favoring H‐bond formation. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Analyte Detection: A Decade of Progress in the Development of Optical/Fluorescent Sensing Probes

The development of selective and sensitive chemical sensors capable of detecting metal ions, anions, neutral species, explosives and hazardous substances, selectively and sensitively has attracted considerable interest of various research groups. The presence of such analytes within the permissible limits is often beneficial, but the excess amounts may lead to lethal effects to both the environment as well as the living organisms. Owing to the toxicity of the heavy metal ions, toxic anions and nitro-aromatics which are main constituents of explosives, the timely detection of these materials is most desirable to ensure safety and security of the mankind. In this personal account, we present several classes of molecular sensors that were specifically designed in our lab during the past decade for detecting several species in solutions, solid state as well as biological media. Modulation of the optical properties in response to the presence of guest species, led to selective and sensitive detection protocols, and was supported by the theoretical studies wherever possible. We have also extended the application of some of these probes for the on-site detection of analytes by developing the paper strips, glass slides and even the wool and cotton fabrics loaded with probes. One such development represents detection of palladium in human urine and blood samples collected from clinical samples. Additionally, the sensing events in some cases have successfully been reproduced in the live cancer cells. Based on the ease and cost-effective synthesis of the molecular probes, we hope that this account shall provide significant information to researchers in understanding the structure dependent sensing capabilities of the molecular probes.     

One-pot synthetic approach to isolate a single isomer of bis(diethylenetriamine)cobalt(III) cation in bulk using benzoates and the isomeric effects of the cation on ion association with anions

Using a one-pot synthetic approach, a single isomer of bis(diethylenetriamine)cobalt(III) cation, [Co(dien)₂]³⁺ is obtained in bulk from the isomeric mixture (s-fac : u-fac : mer is 7 : 28 : 65) using sodium salts of benzoates (BBz-bromobenzoate, DNBz-dinitrobenzoate, MBz-methylbenzoate) in aqueous medium. Herein, we report the syntheses and characterization of three complexes of composition mer-[Co(dien)₂]Cl(p-BBz)₂·H₂O (1), s-fac-[Co(dien)₂](o,p-DNBz)₃·H₂O (2) and mer-[Co(dien)₂]Cl(p-MBz)₂·4H₂O (3) in the continuation of our earlier work, where benzoate (Bz), p-chlorobenzoate (CBz), p-nitrobenzoate (NBz) and p-aminobenzoate (ABz) were used. The isomeric identification of complex cation was initially made on the basis of spectroscopic characterization (UV–visible, IR and NMR). The binding properties of [Co(dien)₂]³⁺ with benzoates (p-BBz, o,p-DNBz, p-MBz, Bz, CBz, NBz or ABz) have been studied using standard UV–visible spectroscopic titrations in aqueous medium and comparison indicate ion association constants of s-fac > mer. The single-crystal X-ray diffraction structure analysis of 3 reveals the presence of discrete ions ([Co(dien)₂]³⁺, chloride, p-MBz) along with four lattice water molecules. The structure of 3, with formula [Co(dien)₂](p-MBz)₂Cl·4H₂O, consists of alternating layers made of benzoate ions and layers made of [Co(dien)₂]³⁺, chloride and water molecules. These layers result in the formation of their respective columns and intermolecular cohesion of p-MBz within the columns of [Co(dien)₂]³⁺ is achieved via electrostatic and H-bonding interactions.     

Synthesis,characterization and single crystal x-ray analysis of a complex of iron(II) bis(2,4-dimethylphenyl)dithiophosphate with 4-ethylpyridine

Complex of iron(II) bis(2,4-dimethylphenyl)dithiophosphate with 4-ethylpyridine [{(2,4- (CH₃)₂C₆H₃O)2PS2}2Fe{NC₅H₄(C₂H₅)-4}₂] is synthesized and characterized by elemental analysis, magnetic moment, IR spectroscopy and single crystal X-ray analysis. Complex crystallizes in the monoclinic sp. gr. P2₁/n, Z = 2. Crystal structure consists of mononuclear units with Fe(II) ion chelated by four S atoms of the two diphenyldithiophosphate ligands in bidentate manner. N atoms from two 4-ethylpyridine ligands are axially coordinated to the Fe(II) atom leading to an octahedral geometry.     

Behavior of acetyl modified amino acids in reverse micelles: a non-invasive and physiochemical approach

The well-characterized, monodisperse nature of reverse micelles formed by sodium bis-(2-ethylhexyl)sulfosuccinate/water/isooctane and their usefulness in assimilating compounds of varied interests have been exploited to investigate the effect of acetyl modified amino acids (MAA) viz., N-acetyl-L-glycine (NAG), N-acetyl-L-aspartic acid (NAA) and N-acetyl-L-cysteine (NAC), on the water pool and physiochemical properties. Non-invasive techniques such as FTIR and UV-vis absorption spectroscopy have been employed to analyze the interactions of MAA with core water and the AOT headgroup. The micropolarities on both sides of AOT interface have further been investigated by UV-vis absorption probes, methyl orange (MO) and methylene blue (MB). The dynamics of water and temperature induced percolation process have also been studied. The MAA molecules have been found to assist the process with the increase in water content where as a contrary behavior has been observed with the increase in temperature. Conductivity results have been further rationalized in terms of scaling equations, which delineate the dynamic nature of the percolation process. The results have also been analyzed in the light of activation energy of the percolation process and thermodynamics of droplet clustering.     

Synthesis of alkyl 6‐methyl‐4‐(2‐trifluoromethylphenyl)‐1,2,3,4‐tetrahydro‐2H‐pyrimidine‐2‐one‐5‐carboxylates possessing a N‐3 nitro substituent to determine calcium channel modulation structure‐activity relationships

The Bigenelli acid catalyzed condensation of 2‐trifluoromethylbenzaldehyde ( 1 ), urea ( 2 ) and an alkyl acetoacetate ( 3 ) afforded the respective alkyl (Me, Et, i‐Pr, i‐Bu) 6‐methyl‐4‐(2‐trifluoromethylphenyl)‐1,2,3,4‐tetrahydro‐2H‐pyrimidine‐2‐one‐5‐carboxylate ( 4‐7 ). Subsequent N³‐nitration of the alkyl esters ( 4‐7 ) using Cu(NO₃)₂ 3H₂O and Ac₂O furnished the target alkyl 6‐methyl‐3‐nitro‐4‐(2‐trifluoromethylphenyl)‐1,2,3,4‐tetrahydro‐2H‐pyrimidine‐2‐one‐5‐carboxylates ( 8‐11 ). The N³‐nitro compounds ( 8‐11 ) were less potent calcium channel antagonists (IC₅₀ values in the 1.9 × 10^?7 to 3.9 × 10^?6 M range) on guinea pig ileal longitudinal smooth muscle than the reference drug nifedipine (Adalat®, IC₅₀ = 1.4 × 10^?8 M). In vitro calcium channel modulation studies on guinea pig left atrium (GPLA) showed that the methyl and ethyl esters ( 8‐9 ) induced a weak‐to‐modest positive inotropic (agonist) effect, and that the inactive isopropyl ( 10 ) and isobutyl ( 11 ) esters did not alter the cardiac contractile force of GPLA.     

Estimation of cut-off wavelength of rare earth doped single-mode fibers

A new empirical relation is proposed describing spectral variation of mode-field radius (MFR) as inferred from measurements in the far-field of the fiber. It is shown that using this relation, it is possible to estimate the cut-off wavelength (λ_c) of the fiber. The proposed technique is successfully tested through measurements made on two standard step index single-mode fibers, as well as on an erbium doped fiber (EDF) having λ_c falling within its strong absorption band around 980 nm. This empirical formula is more accurate than the widely used Marcuse's formula to describe spectral dependence of MFR determined through measurements made in the fiber's far-field. The proposed technique is especially suited for estimation of λ_c of doped fibers in which λ_c falls within an absorption band.     

Wei Hua’s four-parameter potential: Comments and computation of molecular constants αe and εeXe

The value of adjustable parameterC in the four-parameter potentialU(r) =D _e [(1 - exp[-b(r -r _e)])/(1 -C exp[-b(r -r _e)])]² has been expressed in terms of molecular parameters and its significance has been brought out. The potential so constructed, withC derived from the molecular parameters, has been applied to ten electronic states in addition to the states studied by Wei Hua. Average mean deviation for these 25 states has been found to be 3.47 as compared to 6.93, 6.95 and 9.72 obtained from Levine, Varshni and Morse potentials, respectively. Also Dunham’s method has been used to express rotation-vibration interaction constant (α_e) and anharmonicity constant (ω_ex_e) in terms ofC and other molecular constants. These relations have been employed to determine these quantities for 37 electronic states. For α_e, the average mean deviation is 7.2% compared to 19.7% for Lippincott’s potential which is known to be the best to predict these values. Average mean deviation for (ω_ex_e) turns out to be 17.4% which is almost the same as found from Lippincott’s potential function.     

Transfer-Free Graphene Growth on Dielectric Substrates: A Review of the Growth Mechanism

Ever since the more-than-decade-old discovery of application of mechanical exfoliation to obtain graphene, this 2-dimensional material was known for its soaring promise in various applications, owing to its excellent properties. Graphene, most popularly grown on metallic substrates by chemical vapour deposition, needs to be transferred onto dielectric substrates for multiple optical and electronic applications. During such complex and expensive transfer steps, defects are introduced into graphene, which deteriorates the quality and thus, properties of graphene. An alternative approach to surmount these problems is the elimination of the transfer process and to directly grow graphene on dielectric substrates, for future electronic and optical applications. This review presents a comprehensive and an up-to-date account of the development of synthesis methods, challenges and future directions for transfer-free graphene growth on dielectric substrates. Special emphasis is given on the fundamentals of growth mechanisms of various transfer-free graphene synthesis processes on dielectric materials.     

Encapsulating Au‐Fe3O4 Nanodots into AIE‐active Supramolecular Assemblies: Ambient Visible‐light Harvesting “Dip‐Strip” Photocatalyst for C−C/C−N Bond Formation Reactions

The present study demonstrates the development of a supramolecular porous ensemble consisting of hetero‐oligophenylene derivative 6 and Au‐Fe₃O₄ nanodots. Supramolecular assemblies of AIE‐active hetero‐oligophenylene derivative 6 served as reactors for the generation of Au‐Fe₃O₄ nanodots. The as prepared supramolecular ensemble functioned as an efficient recyclable photocatalytic system for C(sp²)?H bond activation of anilines for the construction of quinoline carboxylates. Interestingly, the “dip catalyst” prepared by depositing PTh‐co‐PANI‐6: Au‐Fe₃O₄ nanodots on a filter paper served as a recyclable strip (up to 10 cycles) for C?C/C?N bond formation reaction.