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1.
K. Harikrishna Reddy Dr. Dandamudi Usharani Prof. John F. Nixon Prof. Eluvathingal D. Jemmis 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(33):9142-9152
The potential energy surfaces of both neutral and dianionic SnC2P2R2 (R=H, tBu) ring systems have been explored at the B3PW91/LANL2DZ (Sn) and 6‐311+G* (other atoms) level. In the neutral isomers the global minimum is a nido structure in which a 1,2‐diphosphocyclobutadiene ring (1,2‐DPCB) is capped by the Sn. Interestingly, the structure established by X‐ray diffraction analysis, for R=tBu, is a 1,3‐DPCB ring capped by Sn and it is 2.4 kcal mol?1 higher in energy than the 1,2‐DPCB ring isomer. This is possibly related to the kinetic stability of the 1,3‐DPCB ring, which might originate from the synthetic precursor ZrCp2tBu2C2P2. In the case of the dianionic isomers we observe only a 6π‐electron aromatic structure as the global minimum, similarly to the cases of our previously reported results with other types of heterodiphospholes. 1 , 4 , 19 The existence of large numbers of cluster‐type isomers in neutral and 6π‐planar structures in the dianions SnC2P2R22? (R=H, tBu) is due to 3D aromaticity in neutral clusters and to 2D π aromaticity of the dianionic rings. Relative energies of positional isomers mainly depend on: 1) the valency and coordination number of the Sn centre, 2) individual bond strengths, and 3) the steric effect of tBu groups. A comparison of neutral stannadiphospholes with other structurally related C5H5+ analogues indicates that Sn might be a better isolobal analogue to P+ than to BH or CH+. The variation in global minima in these C5H5+ analogues is due to characteristic features such as 1) the different valencies of C, B, P and Sn, 2) the electron deficiency of B, 3) weaker pπ–pπ bonding by P and Sn atoms, and 4) the tendency of electropositive elements to donate electrons to nido clusters. Unlike the C5H5+ systems, all C5H5? analogues have 6π‐planar aromatic structures as global minima. The differences in the relative ordering of the positional isomers and ligating properties are significant and depend on 1) the nature of the π orbitals involved, and 2) effective overlap of orbitals. 相似文献
2.
We have developed methodology for the synthesis of aryl/alkyl cyanamides from amines in one-pot four steps reaction using cheap, readily available and air stable copper source as catalyst under mild reaction conditions. We have also studied the application of cyanamides. In this connection, we could construct aryl tetrazolamine from cyanamides using click reaction 相似文献
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4.
G. M. Brahmanandhan J. Malathi D. Khanna S. Selvasekarapandian N. Nidhya R. Usharani M. T. Jose V. Meenakshisundaram 《Journal of Radioanalytical and Nuclear Chemistry》2007,274(2):373-377
Samples of natural and manufactured building materials used by the people of Gobichettipalayam town have been analyzed for
226Ra, 232Th and 40K using gamma-ray spectrometry. Radium equivalent activity of the materials has been measured using the formula given by OECD
and the geometric mean value of sand, clay and cements are found to be 53.53 Bq·kg−1, 89.09 Bq·kg−1 and 72.25 Bq·kg−1, respectively. The radium equivalent activities obtained in the building materials are all well below the acceptable limit.
The indoor gamma-dose has been measured using thermoluminescence dosimeters and it was found in the range of 1051.2–3946.0
μGy/year. The annual effective indoor gamma radiation dose to the people of Gobichettipalayam town has been found to be 0.8
mSv/y. 相似文献
5.
Nagarajan Usharani Gladstone Christopher Jayakumar Swarna Vinodh Kanth Jonnalagadda Raghava Rao Bangaru Chandrasekaran 《高分子科学杂志,A辑:纯化学与应用化学》2013,50(8):666-673
The present study explains the molecular level interaction of valeraldehyde with collagen. Valeraldehyde is a monoaldehyde, which involves crosslinking with protein through covalent linkages. The role of valeraldehyde as a crosslinking agent for collagen stabilization was studied. Molecular modeling approaches was used to understand the interaction of collagen like peptide with valeraldehyde, which mimic the aldehyde tanning processes involved in protein stabilization. Crosslinking efficiency of valeraldehyde was found to increase with an increase in concentration due to the higher availability of aldehydic groups involved in crosslinking with collagen. Valeraldehyde interacted collagen membrane showed an increase in thermal stability by 25°C at pH 8. In the presence of valeraldehyde, collagen fibrils nucleation center was shifted from a lower to a higher range. Shift in the nucleation center was observed in the reduction of gelling time. Water accessibility in valeraldehyde interacted collagen membrane was reduced due to a higher crosslinking rate in the collagen. Modified collagen membrane by valeraldehyde at incubation of about 96 h showed higher resistance to collagenolytic activity of 81%. The amino groups reacting appear to be involved in crosslinking with valeraldehyde. Several interaction sites were identified and the docking energy obtained was ?5.539 kcal/mol. The participation of the aldehyde group with amino groups in collagen was observed, which plays a dominant role in the stabilization of peptide by valeraldehyde. It was found that complexes exhibit covalent bonding, hydrogen bonding and electrostatic interaction in the process of stabilization. 相似文献
6.
S.S. Kubakaddi K.R. Usharani 《Physica E: Low-dimensional Systems and Nanostructures》2005,25(4):497-503
We calculate the diffusion thermopower for a degenerate two-dimensional electron gas in real lattice-mismatched semiconductor quantum wells (QWs) at low temperatures. We consider explicitly two scattering mechanisms: (i) the surface roughness-induced piezoelectric effect, a new important scattering source, arising due to a large fluctuating density of roughness-induced piezoelectric charges and (ii) the surface roughness. The scattering parameter p of energy dependence of the momentum relaxation time and the diffusion thermopower Sd, of each of the mechanisms separately and also when both the mechanisms are combined, are calculated as a function of electron concentration and well width. The diffusion thermopower, as a function of electron concentration, due to piezoelectric field shows a change in sign for lower concentrations. Interestingly, the diffusion thermopower, due to this mechanism, as a function of well width also shows a change in sign and it is dominant for larger well widths. The numerical calculations are presented for In0.2Ga0.8As/GaAs and AlN/GaN QWs. The piezoelectric mechanism is expected to be very important in systems with large piezoelectric constant and lattice mismatch. 相似文献
7.
Sumana Ghosh Dandamudi Usharani Susmita De Eluvathingal D. Jemmis Santanu Bhattacharya 《化学:亚洲杂志》2008,3(11):1949-1961
Distamycin‐based tetrapeptide ( 1 ) was covalently tethered to both ends of the central dihydroxyazobenzene moiety at either the 2,2′ or 4,4′ positions. This afforded two isomeric, distamycin–azobenzene–distamycin systems, 2 (para) and 3 (ortho), both of them being photoisomerizable. Illumination of these conjugates in solution at approximately 360 nm induced photoisomerization and the time course of the process was followed by UV/Vis and 1H NMR spectroscopy. The kinetics of the thermal reversion at various temperatures of cis to trans isomers of the conjugates obtained after photoillumination were also examined. This afforded the respective thermal‐activation parameters. Both the molecular architecture and the location of the substituent around the core azobenzene determined the rate and activation‐energy barrier for the cis‐to‐trans back‐isomerization of these conjugates in solution. Duplex–DNA binding of the conjugates and the changes in DNA‐binding efficiency upon photoisomerization was also examined by CD spectroscopy, thermal denaturation studies, and a Hoechst displacement assay. The conjugate 2 showed higher DNA‐binding affinity and a greater change in the DNA‐binding efficiency upon photoisomerization compared with its 2,2′‐disubstituted counterpart. The experimental findings were substantiated by using molecular‐docking studies involving each conjugate with a model duplex d[(GC(AT)10CG)]2 DNA molecule. 相似文献
8.
R Mas-Ballesté AR McDonald D Reed D Usharani P Schyman P Milko S Shaik L Que 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(37):11747-11760
The intramolecular gas-phase reactivity of four oxoiron(IV) complexes supported by tetradentate N(4) ligands (L) has been studied by means of tandem mass spectrometry measurements in which the gas-phase ions [Fe(IV) (O)(L)(OTf)](+) (OTf=trifluoromethanesulfonate) and [Fe(IV) (O)(L)](2+) were isolated and then allowed to fragment by collision-induced decay (CID). CID fragmentation of cations derived from oxoiron(IV) complexes of 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane (tmc) and N,N'-bis(2-pyridylmethyl)-1,5-diazacyclooctane (L(8) Py(2) ) afforded the same predominant products irrespective of whether they were hexacoordinate or pentacoordinate. These products resulted from the loss of water by dehydrogenation of ethylene or propylene linkers on the tetradentate ligand. In contrast, CID fragmentation of ions derived from oxoiron(IV) complexes of linear tetradentate ligands N,N'-bis(2-pyridylmethyl)-1,2-diaminoethane (bpmen) and N,N'-bis(2-pyridylmethyl)-1,3-diaminopropane (bpmpn) showed predominant oxidative N-dealkylation for the hexacoordinate [Fe(IV) (O)(L)(OTf)](+) cations and predominant dehydrogenation of the diaminoethane/propane backbone for the pentacoordinate [Fe(IV) (O)(L)](2+) cations. DFT calculations on [Fe(IV) (O)(bpmen)] ions showed that the experimentally observed preference for oxidative N-dealkylation versus dehydrogenation of the diaminoethane linker for the hexa- and pentacoordinate ions, respectively, is dictated by the proximity of the target C?H bond to the oxoiron(IV) moiety and the reactive spin state. Therefore, there must be a difference in ligand topology between the two ions. More importantly, despite the constraints on the geometries of the TS that prohibit the usual upright σ trajectory and prevent optimal σ(CH) -σ*?z?2 overlap, all the reactions still proceed preferentially on the quintet (S=2) state surface, which increases the number of exchange interactions in the d block of iron and leads thereby to exchange enhanced reactivity (EER). As such, EER is responsible for the dominance of the S=2 reactions for both hexa- and pentacoordinate complexes. 相似文献
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This theoretical work addresses the mechanistic switch between hydroxylase (alcohol formation) and desaturase (olefin formation) activities during alkane oxidation by two non-heme high-valent oxoiron reagents, the enzyme taurine:α-ketoglutarase dioxygenase (TauD) and the synthetic shape-selective catalyst (TpOBzFe), toward cyclohexadiene, cyclohexane, cyclopentane, and ethane. As we show, the desaturase/hydroxylase steps obey unique orbital selection rules, and the mechanistic switch is determined by intrinsic reactivity factors that depend on the ligand-sphere flexibility of the oxoiron species, the substrate, and the spin states of the reaction pathways. Testable predictions are outlined. 相似文献