LEP constraints on grand unified theories

Recent developments on grand unified theories (GUTs) in the context of the LEP measurements of the coupling constants will be reviewed. The three coupling constants at the electroweak scale have been measured at LEP quite precisely. One can allow these couplings to evolve with energy following the renormalization group equations for the various groups and find out whether all the coupling constants meet at any energy. It was pointed out that the minimalSU (5) grand unified theory fails to satisfy this test. However, various extensions of the theory are still allowed. These extensions include (i) supersymmetricSU (5) GUT, with some arbitrariness in the susy breaking scale arising from the threshold corrections, (ii) non-susySU (5) GUTs with additional fermions as well as Higgs multiplets, which has masses of the order of TeV, and (iii) non-renormalizable effect of gravity with a fine tuned relation among the coupling constants at the unification energy. The LEP results also constrain GUTs with an intermediate symmetry breaking scale. By adjusting the intermediate symmetry breaking scale, one usually can have unification, but these theories get constrained. For example, the left-right symmetric theories coming from GUTs can be broken only at energies higher than about ～ 10¹⁰ GeV. This implies that if right handed gauge bosons are found at energies lower than this scale, then that will rule out the possibility of grand unification. Another recent interesting development on the subject, namely, low energy unification, will be discussed in this context. All the coupling constants are unified at energies of the order of ～ 10⁸ GeV when they are embedded in anSU (15) GUT, with some particular symmetry breaking pattern. But even in this case the results of the intermediate symmetry breaking scale remain unchanged.     

Study of energy transfer from 7-amino coumarin donors to the rhodamine 6G acceptor in lecithin vesicles and sodium taurocholate-lecithin mixed aggregates

The energy transfer using 7-amino coumarin dyes as the donor and rhodamine 590 (Rh6G) as the acceptor was investigated in lecithin vesicles and sodium taurocholate (NaTC)-lecithin mixed aggregates using steady-state and time-resolved fluorescence spectroscopy. All energy transfer parameters were calculated. The coumarin 153-Rh6G pair is the most efficient donor-acceptor pair as reflected by the value of k(ET). With addition of NaTC in lecithin, in the case of the coumarin 153-Rh6G pair, the energy transfer rate or efficiency does not change very much, whereas in the case of the coumarin 151-Rh6G pair, the energy transfer rate decreases 2-fold upon going from lecithin vesicles to NaTC-lecithin mixed aggregates where the molar ratio is 2.5. It is mainly due to the deeper location of coumarin 153 in the lipid bilayer or in mixed aggregates. Rotational relaxation data also support this idea.     

Metal binding characteristics of a laterally nonsymmetric aza cryptand upon functionalization with a pi-acceptor group

The laterally nonsymmetric aza cryptand synthesized by condensing tris(2-aminoethyl)amine (tren) with tris[[2-(3-(oxomethyl)phenyl)oxy]ethyl]amine readily forms mononuclear inclusion complexes with both transition and main-group metal ions. In these complexes, the metal ion occupies the tren-end of the cavity making bonds with the three secondary amino and the bridgehead N atoms. When a strong pi-acceptor group such as 2,4-dinitrobenzene is attached to one of the secondary amines, the binding property of the cryptand changes drastically. When perchlorate or tetrafluoroborate salts of Ni(II), Cu(II), Zn(II), or Cd(II) are used, the metal ion enters the cavity which can be monitored by the hypsochromic shift of the intramolecular charge-transfer transition from the donor amino N atom to the acceptor dinitrobenzene. However, in the presence of coordinating ions such as Cl(-), N(3)(-), and SCN(-), the metal ion comes out of the cavity and binds the cryptand outside the cavity at a site away from the dinitrobenzene moiety. Four such complexes are characterized by X-ray crystallography. Thus, a metal ion can translocate between inside and outside of the cryptand cavity depending upon the nature of the counter anion.     

The crystal structure of a novel copper(II) complex with asymmetric ligand derived from l-histidine

The crystal structure of a novel copper(II) complex with a potentially hexadentate Schiff base ligand derived from l-histidine has been solved by a single-crystal X-ray diffraction method at pH 7.4. The copper(II) ion is coordinated asymmetrically by a tetradentate ligand with the amino and imidazole imido nitrogen atoms on one side versus imino nitrogen and carboxylate oxygen atoms on the other side in a distorted square-planar geometry. The formation of an infinite chain through carboxylate coordination is observed. The novel ligand is obtained by the reaction between the l-histidine molecules coordinated to copper(II) and 4-hydroxy-4-methylpentan-2-one formed by aldol condensation of acetone. This complex provides insights into a possible structural arrangement between copper(II) and l-histidine which is physiologically important and abundantly present in biological systems.     

Further studies on the postpolymerization of methyl methacrylate

A search has been made for electroinitiators of postpolymerization. A number of hydroxycarboxylic acids and some of their sodium salts have been found to be quite efficient. Factors controlling the rate of polymerization and yield have been described. Surprisingly, citric acid, though not an electroinitiator of polymerization, is a good initiator of postpolymerization. Other active species that deserve mention are tartaric acid and its salts. A free radical mechanism involving formation of oligomeric initiator fragments, growth by attachment of imbibed monomer, and termination by chain transfer has been proposed to explain the observed facts.     

An octahedral nickel(II) complex of a hexadentate N<Subscript>2</Subscript>O<Subscript>2</Subscript>S<Subscript>2</Subscript> thioether ligand: synthesis,characterization, X-ray and electronic structure

A hexadentate dibasic thioether N₂O₂S₂ donor ligand (H ₂ L) and its octahedral nickel(II) complex, [Ni(L)] have been synthesized and characterized by physicochemical and spectroscopic techniques. The structures of both H ₂ L and its nickel complex were confirmed by single-crystal X-ray diffraction studies. The cyclic voltammogram of the complex shows a quasi-reversible Ni(II)/Ni(III) oxidation couple (E _1/2 = 0.88 V) along with a ligand-based reduction (E _1/2 = ?0.83 V). The electronic structures and electrochemical properties have been interpreted with the help of DFT calculations. The electronic transitions as calculated by TDDFT/CPCM method are used to assign the UV–Vis absorption bands.     

Critical behaviour of a polar-polar binary system from optical birefringence measurements

High resolution measurements of optical birefringence (Δn) have been carried out to study the critical behaviour at the nematic-isotropic (N–I) and smectic A-nematic (SmA–N) phase transitions in a binary system composed of nonyloxycyanobiphenyl (9OCB) and heptylcyanobiphenyl (7CB). From a fitting procedure compatible with the weakly first order nature of the N–I transition, the order parameter critical exponent β at the N–I transition has been found to be in well agreement with the tricritical hypothesis. The pretransitional behaviour at the SmA–N transition has been investigated with the aid of a differential parameter extracted from Δn data and the yielded critical exponent α has been found to be non-universal in nature. A weak birefringence have been found to develop on approaching the N–I transition temperature, which is tentatively attributed to surface induced ordering. In this case, a treatment identical to that for the SmA–N transition yields a tricritical nature for the N–I transition in the investigated mixtures. Such an outcome has also appeared to be in well agreement with those obtained from the direct analysis of the Δn data (i.e. in parameterisation of β).