Department of Chemistry
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Item Magnetic resonance lineshapes in powdered and amorphous systems(IAS, 1986-04) Sivasubramanian, S.C.A survey is made of the theory and applications ofEPR andNMR absorption lineshapes observed in powdered and amorphous materials. The ‘spin Hamiltonian’ and ‘resonance condition’ formalisms are reviewed, andepr andnmr lineshapes are discussed which typify the singularity characteristics in powdered materials. In the amorphous or ‘glassy’ state, the measurable spin resonance parameters often have to be viewed as being ‘randomly distributed’ according to a probability density function. Several recent probability-theory based approaches to lineshape computation for modelling the amorphous state are discussed.Item X-ray diffraction, electron paramagnetic resonance, and electron spin echo modulation studies of the lead oxide-lead chloride-cupric chloride ternary glass system(ACS, 1991-08-01) Sivasubramanian, S.C.X-ray diffraction, EPR, and ESEM studies are reported for a novel ternary glass 43PbO-56PbCl2-1 CuCl2. The X-ray PDF data lead to a structural model in which octahedral building units of Cu04Cl2 are predominant. EPR spectra of the glass at X and Q bands have been fitted, by line shape simulation, to the distributed spin Hamiltonian parameters g^rnean) = 2.34, = 0.03, gx(mean) = 2.06, ± = 0.008, p = 0.95, /4,(mean) = 131.1 X 10"4 cm"1, and /4x(mean) = 5.8 X 10"4 cm"1. These data suggest a distribution of bonding geometries for Cu2+ in the glass structure, with the above-mentioned elongated octahedral Cu04Cl2 dominating. ESEM results suggest that Cu2+ may be surrounded by four Pb2+ in the second coordination shell at a distance of 3.8 ÁItem X-ray diffraction, electron paramagnetic resonance, and electron spin echo modulation studies of the lead oxide-lead chloride-cupric chloride ternary glass system(ACS, 1991-08-01) Sivasubramanian, S.C.X-ray diffraction, EPR, and ESEM studies are reported for a novel ternary glass 43PbO-56PbCl2-1 CuCl2. The X-ray PDF data lead to a structural model in which octahedral building units of Cu04Cl2 are predominant. EPR spectra of the glass at X and Q bands have been fitted, by line shape simulation, to the distributed spin Hamiltonian parameters g^rnean) = 2.34, = 0.03, gx(mean) = 2.06, ± = 0.008, p = 0.95, /4,(mean) = 131.1 X 10"4 cm"1, and /4x(mean) = 5.8 X 10"4 cm"1. These data suggest a distribution of bonding geometries for Cu2+ in the glass structure, with the above-mentioned elongated octahedral Cu04Cl2 dominating. ESEM results suggest that Cu2+ may be surrounded by four Pb2+ in the second coordination shell at a distance of 3.8 Á.Item Synthesis and characterization of two stable paramagnetic octahedral chromium(IV) complexes with dianionic tridentate SNO donor ligands and of a chromium(III) complex with a ONO donor ligand(Elsiever, 2008-04-01) Sivasubramanian, S.C.Two novel paramagnetic octahedral chromium(IV) complexes with dianionic tridentate SNO donor ligands containing extended π-system have been synthesized while only a paramagnetic octahedral chromium(III) complex is obtained when a related dianionic tridentate ONO donor ligand is used under similar conditions. These bischelate complexes [Cr(abtsal)2] (1) (abtsalH2 is the Schiff base of o-aminobenzenethiol and salicylaldehyde), [Cr(4-PhTSCsal)2] · H2O (2) (4-PhTSCsalH2 is the Schiff base of 4-phenylthiosemicarbazide and salicylaldehyde) and K[Cr(sap)2] · H2O (3) (sapH2 is the tridentate Schiff base of salicylaldehyde and o-aminophenol) are characterized by elemental analyses, magnetic moment measurements, IR, UV–Vis and EPR spectroscopic studies. Compound 3 has been structurally characterized by X-ray crystallography. Measured room temperature (RT) magnetic moment values are 2.98 BM for 1 and 2.83 BM for 2 indicating a d2 system with a triplet ground state in both the cases. On the other hand, the magnetic moment value for 3 is found to be 3.74 BM at RT and is consistent with the presence of three unpaired electrons for a d3 Cr(III) ion. The magnetic moment values rule out the large spin–orbit coupling which is substantiated by the presence of RT EPR signals. Compounds 1 and 2 exhibit very similar powder EPR spectra at RT and LNT, which show the allowed transition ΔMs = ±1 (g = 2.004 for both 1 and 2) as well as the “forbidden” half-field transition (ΔMs = ±2) at g = 4.105 for 1 and g = 4.318 for 2, respectively. The X-band LNT frozen glass EPR spectrum of 1 in DMF shows the presence of zero-field split rhombic symmetry character, and results in the parameters g ≅ 2.0, D = 740 G, and E = 260 G. It suggests that the intensity of ΔMs = ±2 forbidden transition is large due to the large D value. The X-band frozen glass EPR spectrum of compound 3 in DMF is found to be very similar to that reported for trans-[Cr(py)4F2]+ in DMF–H2O–MeOH glass. The large difference (∼700 mV) in the reduction potential for the two octahedral complexes 1 (−1.40 V) and 3 (−0.70 V) is attributed to the difference in their metal ion oxidation states.Item Dioxygen binding and activation by a highly reactive Cr(II) compound containing S,N-donors derived from o-aminothiophenol(Taylor & Francis, 2012-08-07) Sivasubramanian, S.C.We report the synthesis, characterization, and reactivity of a Cr(II) complex, [Cr(H2O)(LISQ)2] (1) [(LISQ)1− is o-iminothionebenzosemiquinonate(1−) π-radical], that is highly stable in solid state in the presence of air but undergoes spontaneous change in solution, both in the presence and absence of air. Physicochemical studies in solution show that a superoxo-CrIII species, [Cr(O2)(OH)(LISQ)2]− is generated initially in DMF solution of 1 in the presence of air owing to its immediate deprotonation followed by O2 binding to the deprotonated species. The formation of this superoxo-CrIII species is prominent and gradual in the presence of CH3OH, a scavenger of CrO2+ species. This Cr(O2)2+ species in turn is converted to another highly reactive O=Cr(IV) intermediate [O=Cr(OH)(LISQ)2]− which undergoes disproportionation producing an unstable O=Cr(V) species, [O=Cr(OH)(LISQ)2] and a stable Cr(III) compound, [Cr(OH)(DMF)(LISQ)2] (2). The rate of this disproportionation is enhanced in the presence of MnCl2, [N(n-Bu)4]PF6 and KSCN. The generated O=Cr(IV) species interacts with DNA with complete cleavage. The O=Cr(V) species slowly disappears from solution as revealed from EPR studies.Item A paramagnetic octahedral trans-dihydroxy chromium(IV) complex with dianionic tetradentate Schiff base salophen and crystal structure of its trans-diisothiocyanato analog(Taylor & Francis, 2012-09-06) Sivasubramanian, S.C.A paramagnetic octahedral trans-dihydroxychromium(IV) complex, [Cr(OH)2(salophen)] (1) (H2salophen = N,N′-bis(salicylidene)-1,2-phenylenediamine), has been synthesized and characterized by elemental analysis, magnetic moment measurement, IR, UV-Vis, and EPR spectroscopic studies. Measured room temperature (RT) magnetic moment value is 2.79 BM for 1, indicating a d2 (S = 1) system with a triplet ground state. Compound 1 exhibits powder EPR spectra at RT and LNT, which show the allowed transition ΔM s = ±1 (g = 2.0038) as well as the “forbidden” half-field transition (ΔM s = ±2) at g = 4.2080. Two successive reduction waves are observed in the cyclic voltammogram of 1 in CH3CN at −0.84 and −1.63 V (vs. Ag/AgCl), respectively. Compound 1 readily reacts with Mn2+ ion, a Cr(IV)–specific reductant and also undergoes –OH substitution reactions in solution with NCS− and imidazole. The trans-diisothiocyanato analog, [Cr(NCS)2(salophen)] (2), with μ eff = 2.80 BM has been structurally characterized by X-ray crystallography and found to contain two N-bonded axial thiocyanato ligands with slightly different axial Cr–N bond lengths (N(3)–Cr(1), 2.032(2); N(4)–Cr(1), 2.015(2) Å). Compound 2 and the corresponding Cr(III) compound K[Cr(NCS)2(salophen)] · H2O (3) show significant difference in their electronic structures as revealed from their electronic spectra.Item Imidazolium ionic liquid-tagged palladium complex: an efficient catalyst for the Heck and Suzuki reactions in aqueous media(RSC, 2014) Kumar, Anil; Sivasubramanian, S.C.; Khungar, BhartiAn air stable, water soluble, and efficient ionic liquid-tagged Schiff base palladium complex was prepared. The synthesized complex was well characterized by NMR, mass spectrometry, FT-IR, UV-visible spectroscopy and powder X-ray diffraction. The complex was used as a catalyst for the Suzuki and Heck cross-coupling reactions in water. Good to excellent yields were achieved using a modest amount of the catalyst. In addition, the catalyst can be easily reused and recycled for six steps without much loss in activity, exhibiting an example of sustainable and green methodology