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Chemical Science Colloquium     

 

December 11, 2015

Prof. Parimal K. Bharadwaj, IIT Kanpur

A Chemist's journey through supramolecular chemistry of cryptands

Chemistry of macrobicyclic cryptands are useful in chemical, biological as well as materials research. Laterally non-symmetric cryptands constitute an important class of molecules which can be suitably modified to carry out different exciting studies. This lecture will deal with such exciting studies of cryptands and will empahsize its role for future resarch in supramolecular chemistry.

January 08, 2016

Prof. D.D.Sarma, SSCU, IISc.

Solid Solution, Vegard’s Law, Chemical Pressure and all that

It is well known that lattice parameters of most solid solutions can be interpolated almost linearly with the composition between those of the two end-members. This, confirmed within a reasonable degree of accuracy by a huge body of structural investigations over many decades using the x-ray diffraction technique, is the celebrated Vegard's Law. In fact, many properties, such as the bandgap of a semiconductor solid solution, are also known to show a similar linear behaviour with composition between the end-members. These observations form the basis of the vast literature aimed at tuning of properties by making solid solutions. There is a closely associated concept of chemical pressure achieved by the substitution of typically a cation in a solid with another homovalent ion of dissimilar size defining a solid solution. This technique not only allows to exert a positive pressure by doping a smaller cation akin to the physical pressure, but also makes it feasible to explore of the negative pressure regime inaccessible otherwise with the help of a larger sized dopant. While these concepts are universally accepted and much used in designing new compounds with tailor made properties, a closer, microscopic inspection reveals many surprises not fully appreciated in the past. Based on some of our recent studies1,2, I shall illustrate the limits of these concepts and provide a rationale for their apparent success in spite of several conceptual difficulties.

1. Soham Mukherjee et al., Microscopic description of the evolution of the local structure and an evaluation of the chemical pressure concept in a solid solution, Phys. Rev. B 89, 224105 (2014).
2. Soham Mukherjee et al., Unpublished results.

February 10, 2016
Prof. Toshiaki Enoki, Tokyo Institute of Technology, Japan

Unconventional Electronic and Magnetic Properties of Nanographene

Graphene, which is formed by fusing an infinite number of benzene rings in 2D manner, is described in terms of massless Dirac fermion in relativistic quantum mechanics. When a graphene sheet is cut into nanofragments, a variety of open-edged graphene nanostructures having different geometries and sizes are created. Here their electronic structures can be understood on the basis of the effect of the edges (zigzag and armchair edges), which work as the boundary condition to the Dirac fermion. Meanwhile the nanofragments are considered as nano-sized polycyclic aromatic hydrocarbon molecules and are extrapolated to anthrathene, naphthalene, and finally benzene as the size decreases. Accordingly the properties of the graphene nanostructures can be understood also in terms of phenomenological Clar’s aromatic sextet rule in chemistry. In particular the chemistry approach becomes intuitive more in understanding their properties and predictable for investigating functionalities for the nanostructures as various functional groups terminating the edge carbon atoms play an important role in their functionalities. Molecular science approach to graphene nanostructures are presented on the basis of investigations with STM/STS, AFM, X-ray absorption spectra, electron transport and magnetic measurements, and DFT calculations. According to our findings, zigzag-edged nanostructures have active nonbonding states in the vicinity of the edge region, which are the origin of electronic, magnetic and chemical activities. In contrast, electron wave interference gives rise to a standing wave, resulting in thermodynamic stability of armchair-edged nanostructures. In addition, chemical species, which terminate edge carbon atoms, give another modification of the electronic structures. The interplay of edge geometry and chemistry is found to be the source of divergent varieties in the electronic, magnetic and chemical properties of graphene nanostructures.
References
1. T. Enoki, Phys. Scr. T146, 014008 (2012) (Nobel Symposium on Graphene and Quantum Matter).
2. S. Fujii and T. Enoki, Angew. Chem. Int. Ed. 51, 7236 (2012).
3. T. Enoki and T. Ando, Physics and Chemistry of Graphene, Pan Stanford Publishing, 2013.
4. S. Fujii and T. Enoki, Acc. Chem. Res. 46, 220 (2013).
5. S. Fujii, M. Ziatdinov, M. Ohtsuka, K. Kusakabe, M. Kiguchi and T. Enoki, Faraday Discussions, 173, 173 (2014).

February 16, 2016
Prof. Amy C. Rosenzweig,Depts. of Molecular Biosciences and of Chemistry Northwestern University, IL.

Biological methane oxidation

Methane gas is underutilized as a feedstock for production of liquid fuels due to low conversion efficiencies and high capital costs. Increasing natural gas reserves combined with an ongoing price spread between natural gas and gasoline have led to renewed interest in bioconversion of methane. In nature, methanotrophic bacteria activate methane with high selectivity under mild conditions using methane monooxygenases (MMOs). However, the use of MMOs in bioconversion processes is hindered by low efficiency, low carbon yields, and suboptimal kinetics. Improving these properties requires detailed understanding of the MMO enzyme systems, which has not been achieved for the primary MMO in nature, particulate MMO (pMMO). pMMO comprises three subunits, PmoA, PmoB, and PmoC, arranged in a trimeric complex. Despite extensive research and the availability of multiple crystal structures, the nature of the pMMO active site remains controversial and the chemical mechanism has not been elucidated. Mounting evidence indicates that methane is oxidized at a dinuclear copper center in the PmoB subunit, but several key questions remain unresolved. In addition, neither the roles of additional metal centers observed by crystallography nor the function of the transmembrane domains has been established. Progress toward answering these fundamental questions will be reported.

March 18, 2016
Uday Maitra, Indian Institute of Science

Playing ‘Games’ with Bile Acids!

Bile acids are naturally occurring degradation products of cholesterol, which are present in the bile as their taurine and glycine conjugates. They perform a number of important biological functions such as the prevention of cholesterol gallstone formation, emulsification of fats for subsequent digestion, etc.1

During the past two decades our group has developed new molecular and supramolecular chemistry with bile acids and their derivatives. We have used modified bile acids as chiral templates and chiral auxiliaries, as molecular tweezers, and as organo- and hydrogelators. More recently, we have used a variety of bile acid derived metallogels for designing nanostructured and photoluminescent materials with interesting properties. Some of these results will be highlighted in this lecture.2

1 S. Mukhopadhyay, U. Maitra, Curr. Sci., 2004, 87, 1666-1683.
2 For a recent review of our work see: V.S. Sajisha, U. Maitra, Chimia, 2013, 67, 44-50.

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