Browsing by Author "K. Ravi Kumar"

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Complexes of 3d metal ions of the Schiff-base, 4-hydroxy-N′-(4′-octadecyloxybenzylidene)benzohydrazide: Synthesis, spectral studies, crystal structure and magnetic interactions
(Elsevier Ltd, 2009) Angad Kumar Singh; Sanyucta Kumari; K. Ravi Kumar; B. Sridhar; Miroslaw Wrzecion; Jerzy Mroziński; T.R. Rao
A novel hydrazide-based mesogenic (nematic) Schiff-base, 4-hydroxy-N′-(4′-octadecyloxybenzylidene)benzohydrazide, (Hhodbbh) (abbreviated as HL3), was prepared and its structure studied by elemental analyses, mass, NMR and IR spectra and single crystal XRD (triclinic space group Pover(1, -) with Z = 2) techniques. The Schiff-base behaves as a non-deprotonated bi-dentate species in its metal complexes of the general formula, [M(HL3)2](OAc)2 [M = Mn, Co, Ni, Cu and Zn]. The IR and NMR spectral data imply bonding of HL3 species through amidic oxygen and azomethine nitrogen atoms. The MnII and CoII complexes show gradual decrease in the χMT product with lowering of temperature and negative Weiss constants characteristic of anti-ferromagnetic interaction. The MnII complex shows magnetic hysteresis loops at 2.0 K. © 2009 Elsevier Ltd. All rights reserved.
Coordination of a mesogenic Schiff-base with MnII, CoII, NiII, CuII and ZnII: Synthesis, spectral studies and crystal structures
(Elsevier Ltd, 2008) Angad Kumar Singh; Sanyucta Kumari; T.N. Guru Row; Jai Prakash; K. Ravi Kumar; B. Sridhar; T.R. Rao
A novel mesogenic (nematic) Schiff-base, N,N′-di-4-(4′-pentyloxybenzoate)salicylidene diaminoethane, H2dpbsde (abbreviated as H2L5) was synthesized and its structure studied. The Schiff-base crystallizes in the non-centrosymmetric space group Pna21 with Z = 4, and the mesogenic isomorphous nickel and copper complexes, [NiL5]2 and [CuL5], in the centrosymmetric space group P21/c with Z = 4. The (L5)2- species coordinates to the metal ions through two phenolate oxygens and two azomethine nitrogens. Both the [NiL5]2 and [CuL5] complexes involve cis-MN2O2 planes; the former complex has a low-spin distorted square-pyramidal geometry with a Ni-Ni bonding of 3.337 Å and the latter, a square-planar geometry. © 2008 Elsevier Ltd. All rights reserved.
Crystal structure and ligational aspects of the mesogenic Schiff base, N,N′-di-(4-hexadecyloxysalicylidene)diaminoethane, with some rare earth metal ions
(Elsevier S.A., 2009) Sanyucta Kumari; Angad Kumar Singh; K. Ravi Kumar; B. Sridhar; T.R. Rao
A mesogenic Schiff base, N,N′-di-(4-hexadecyloxysalicylidene)diaminoethane, H2dhdsde (abbreviated as H2L1) that exhibit smectic-C (SmC) mesophase, was synthesized and its structure studied by elemental analyses, mass, NMR & IR spectra and single crystal XRD (triclinic space group P over(1, ̄) with Z = 1) techniques. Bi-dentate bonding of the Schiff base in the mesogenic LaIII complex was implied on the basis of IR & NMR spectral data. As per the spectral studies of the complexes, the Zwitterionic species of the ligand (H2L1) coordinates to LnIII ion through two phenolate oxygens rendering the overall geometry around the metal ion to distorted square antiprism (Ln = La, Pr, Nd, Sm, Eu) and monocapped octahedron (Ln = Gd, Tb, Dy, Ho). © 2009 Elsevier B.V. All rights reserved.
Ligational aspects of the hydrazide-based Schiff-base, N-(4″-pyridylcarboxamido)-4-(4′-hexyloxybenzoato)salicylaldimine towards some 3d metal ions and crystal structures of the Schiff-base and Zn(II) complex
(Elsevier Ltd, 2008) Angad Kumar Singh; Sanyucta Kumari; K. Ravi Kumar; B. Sridhar; T.R. Rao
A novel hydrazide-based mesogenic (nematic) Schiff-base, N-(4″-pyridylcarboxamido)-4-(4′-hexyloxybenzoato)salicylaldimine, Hpychbsal (abbreviated as H2L) was prepared and its structure studied by elemental analyses, mass, NMR and IR spectra and single crystal XRD (triclinic space group P over(1, ̄) with Z = 4) techniques. The Schiff-base behaves as a uninegative bi/tridentate species in its non-mesogenic complexes of the general formula, [M(HL)2] [M = Mn, Co, Ni, Cu and Zn] and as a dinegative tridentate species with Zn(II) in the distorted square-pyramidal complex, [ZnL · 2py], for which the crystal structure was solved (triclinic space group P over(1, ̄) with Z = 2). The IR and NMR spectral data imply bonding of HL- and L2- species through phenolate oxygen and/or amidic oxygen and azomethine nitrogen atoms. © 2008 Elsevier Ltd. All rights reserved.
Linkage of water vapor distribution in the lower stratosphere to organized Asian summer monsoon convection
(Springer Science and Business Media Deutschland GmbH, 2021) Bhupendra Bahadur Singh; Raghavan Krishnan; D.C. Ayantika; Ramesh K. Vellore; T.P. Sabin; K. Ravi Kumar; Simone Brunamonti; Sreeharsha Hanumanthu; Teresa Jorge; Peter Oelsner; Sunil Sonbawne; Manish Naja; Suvarna Fadnavis; Thomas Peter; Manoj K. Srivastava
Accumulation of water vapor in the upper troposphere/lower stratosphere (UT/LS) over the Asian continent is a recognized feature during the boreal summer monsoon. While there has been a debate on the role of monsoon convective intensities on the UT/LS water vapor accumulations, there are ambiguities with regard to the effects of organized monsoon convection on the spatial distribution of water vapor. We provide insights into this aspect using high precision balloon measurements of water vapor from a high-elevation site Nainital (29.4° N, 79.5° E), India, located in the Himalayan foothills and satellite retrievals of water vapor from the Microwave Limb Sounder (MLS). We also use precipitation estimates from the Tropical Rainfall Measuring Mission (TRMM) satellite (i.e., merged product 3B42 and precipitation radar 3A25 estimates of rain rate and rain type viz convective/stratiform), reanalysis circulation data, as well as numerical model simulations. We first evaluate the MLS estimates of water vapor mixing ratios with in situ high precision hygrometer balloon observations over Nainital. It is seen from our analyses of the MLS data that the LS water vapor distribution is closely linked to the organization of the South Asian monsoon convection and its influence on the UT/LS circulation. This link between LS water vapor distribution and organized monsoon convection is also captured in the in situ observations on 3 August 2016. It is evidenced that periods of organized summer monsoon convective activity over the Indian subcontinent and Bay of Bengal promote divergence of water vapor flux in the UT/LS; additionally the Tibetan anticyclonic circulation causes widespread distribution of the UT/LS water vapor. In addition to the effects of Asian monsoon convection, we also note that global climate drivers such as El Niño-Southern Oscillation (ENSO), Brewer–Dobson circulation (BDC), and Quasi-Biennial Oscillation (QBO) can contribute to nearly 38% of the UT/LS water vapor variability over the Asian monsoon region. The main result of our study indicates that widespread spatial distribution and accumulation of water vapor in the LS (about 80% of total accumulation between May and August months) tend to co-occur with organized monsoon convection, intensified divergence of water vapor flux in the UT/LS and intensified Tibetan anticyclone. On the other hand, the circulation response and LS water vapor distribution to pre-monsoon localized deep convection tend to have a limited spatial scale confined to Southeast Asia. Results from model experiments suggest that the UT/LS circulation pattern to organized monsoon convection has resemblance to stationary Rossby waves forced by organized latent heating, with the westward extending response larger by about 15° longitudes as compared to that of the pre-monsoon localized deep convection. © 2021, The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.
Polymorphic signature of the anti-inflammatory activity of 2,2′-{[1,2-Phenylenebis(methylene)]bis(sulfanediyl)}bis(4, 6-dimethylnicotinonitrile)
(2014) Rashmi Dubey; Praveen Singh; Ajeet K. Singh; Manoj K. Yadav; D. Swati; Manjula Vinayak; Carmen Puerta; Pedro Valerga; K. Ravi Kumar; B. Sridhar; Ashish K. Tewari
Weak noncovalent interactions are the basic forces in crystal engineering. Polymorphism in flexible molecules is very common, leading to the development of the crystals of same organic compounds with different medicinal and material properties. Crystallization of 2,2′-{[1,2-phenylenebis(methylene)] bis(sulfanediyl)}bis(4,6-dimethylnicotinonitrile) by evaporation at room temperature from ethyl acetate and hexane and from methanol and ethyl acetate gave stable polymorphs 4a and 4b, respectively, while in acetic acid, it gave metastable polymorph 4c. The polymorphic behavior of the compound has been visualized through single-crystal X-ray and Hirshfeld analysis. These polymorphs are tested for anti-inflammatory activity via the complete Freund's adjuvant-induced rat paw model, and compounds have exhibited moderate activities. Studies of docking in the catalytic site of cyclooxygenase-2 were used to identify potential anti-inflammatory lead compounds. These results suggest that the supramolecular aggregate structure, which is formed in solution, influences the solid state structure and the biological activity obtained upon crystallization. © 2014 American Chemical Society.
Synthesis and spectral studies of the mesogenic Schiff-base, N,N′-di-(4′-pentyloxybenzoate) salicylidene-1,8-diamino-3,6-dioxaoctane and crystal structure of the Zn(II) complex
(Elsevier Ltd, 2008) Angad Kumar Singh; Sanyucta Kumari; K. Ravi Kumar; B. Sridhar; T.R. Rao
A novel Schiff-base, N,N′-di-(4′-pentyloxybenzoate)salicylidene-1,8-diamino-3,6-dioxaoctane (H2L4) with the mesogenic phase, nematic droplets, was prepared and its structure studied by elemental analyses and mass, NMR and IR spectra. The di-negative tetra-dentate bonding of the Schiff-base in the non-mesogenic complex, [ZnL4], as implied on the basis of IR and NMR spectral data, was confirmed in the crystal structure of the monomeric tetrahedral Zn(II) complex. As per the crystal structure of the complex, the dinegative species of the ligand, L42 -, coordinates to the Zn(II) ion through two phenolate oxygens and two azomethine nitrogens, rendering the overall geometry around Zn(II) to distorted tetrahedron. © 2007 Elsevier Ltd. All rights reserved.