A novel study of charged carbon clusters using a tandem Van de Graaff accelerator

A conventional tandem Van de Graaff accelerator is used to produce charged carbon cluster beams. The unique capability of the method for studying highly charged clusters unaccessible to other methods of producing cluster beams is demonstrated.     

Fabrication of birefringent anisotropic model materials

Widely different techniques have been employed in the fabrication of transparent, birefringent, anisotropic model materials. Some of the techniques reported in the technical literature are briefly reviewed. A simple method of fabricating large plates of arbitrary layup is described.     

Adiabatic Photoreactions of Organic Molecules

An adiabatic photoreaction is a chemical process that occurs entirely on a single excited electronic energy surface. As a rule, most photoreactions of organic molecules start on an excited electronic surface but “jump” to a lower surface somewhere along the reaction coordinate. There are, however, exceptions to this general rule. For example, photoreactions involving small structural changes and minor alterations in covalent bonding (e.g., proton transfer and complex formation) are commonly found to occur adiabatically. The purpose of this review is to survey examples of more complicated adiabatic photoreactions such as fragmentation, electrocyclic rearrangements, and geometrical isomerizations. The concepts employed are presented in an introductory discussion.     

A class of zero-birefrngent polymers

A class of photoelastically insensitive materials consisting of a blend of rigid and flexible polyesters is described. The mechanical and optical response of these polymers is viscoelastic in character. For a wide range of the compositions, the birefringence changes from positive to negative under constant load or constant displacement. The influence of composition, time under load, and principal stress difference on the birefringence is studied. Isochromatic-fringe development in a diametrally loaded disk shows that a major region of the stressed body becomes optically insensitive after a reasonable period of time under load and remains insensitive for a time sufficiently large for associated photoelastic operations. A typical operating-time band is presented during which the optical response of a model characterized by the fringe order per unit thickness is reasonably small. It appears that this class of photoelastically insensitive materials can be employed to produce composite models with glass-fiber reinforcements. Compared to other zero-birefringent polymers, the present material has the advantage of easier processing (casting and curing), improved adhesion to glass fibers and closer matching of the refractive index with that of glass.     

Photophysical Studies on the Interaction of Acridinedione Dyes with Universal Protein Denaturant: Guanidine Hydrochloride

Photophysical studies of photoinduced electron transfer (PET) and non-PET based acridinedione dyes with guanidine hydrochloride (GuHCl) were carried out in water and methanol. Addition of GuHCl to photoinduced electron transfer (PET) based acridinedione dye (ADR 1) results in a fluorescence enhancement, whereas a non-PET based dye (ADR 2) shows no significant change in the fluorescence intensity and lifetime. Addition of GuHCl to ADR 1 dye in methanol results in single exponential decay behaviour, on the contrary a biexponential decay pattern was observed on the addition of GuHCl in water. Absorption and emission spectral studies of ADR 1 dye interaction with GuHCl reveals that the dye molecule is not in the protonated form in aqueous GuHCl solution, and the dye is confined to two distinguishable microenvironment in the aqueous phase. A large variation in the microenvironment around the dye molecule is created on the addition of GuHCl and this was ascertained by time-resolved area normalized emission spectroscopy (TRANES) and time-resolved emission spectroscopy (TRES). The dye molecule prefers to reside in the hydrophobic microenvironment, rather in the hydrophilic aqueous phase is well emphasized by time-resolved fluorescence lifetime studies. The mechanism of fluorescence enhancement of ADR 1 dye by GuHCl is attributed to the suppression of the PET process occuring through space.     

Asymmetric induction during electron transfer mediated photoreduction of carbonyl compounds: role of zeolites

Photochemistry of 17 aryl alkyl ketones included within cation exchanged zeolites has been examined. In solution five of the 17 ketones undergo intramolecular hydrogen abstraction reaction even in the presence of a chiral amine and the rest are photoreduced to the corresponding alcohol. Within zeolites all 17 ketones yielded in presence of a chiral amine, the corresponding alcohol as the major product. When a chiral amine was used as the coadsorbent within alkali ion exchanged zeolites, enantiomerically enriched alcohol was formed in all cases. The best chiral induction was obtained with phenyl cyclohexyl ketone (enantiomeric excess: 68%). 1H-13C Cross Polarization Magic Angle Spinning (CP-MAS) experiments, with a model ketone (perdeuterated acetophenone) and chiral amine (pseudoephedrine) included within MY zeolites, suggested that the cation brings the reactant and the chiral amine closer. The role of the cation in such a process is also revealed by the computation results. The results presented here highlight the importance of a supramolecular structure in forcing a closer interaction between a reactant and a chiral inductor that could be used to achieve asymmetric induction in photoproducts.     

Asymmetric induction during photocyclization of chiral and achiral alpha-oxoamides within achiral zeolites

The photochemistry of 31 alpha-oxoamides capable of undergoing gamma-hydrogen transfer has been examined within zeolites. These molecules, upon excitation, yield two products--a beta-lactam and oxazolidinone--in solution, both resulting from gamma-hydrogen transfer. While in benzene the major product is oxazolidinone, within an MY zeolite, the main product is a beta-lactam. In this investigation, we have focused our attention on asymmetric induction in the formation of the beta-lactam product. Two approaches--using a chiral inductor and chiral auxiliary--have been employed. While in solution, in the presence of chiral inductors, achiral alpha-oxoamides yield beta-lactams with zero enantioselectivity; within zeolites, an ee of up to 44% has been achieved. Alpha-oxoamides appended with a chiral auxiliary gave beta-lactams with less than 5% diastereoselectivity in solution while within zeolites, the same alpha-oxoamides gave the products with de's of up to 83%. Such a remarkable influence of zeolites is attributed to an alkali ion interaction with the reactant alpha-oxoamides and to the confined environment of the zeolite interior. At this stage, we have not been able to provide a model with predictive power and further work is needed to understand this valuable asymmetric induction strategy.     

Experimental signature of entrance channel effect in heavy mass region via evaporation residue cross section and spin distribution measurements

Evaporation residue (ER) cross sections and gamma multiplicity distributions have been measured for ¹⁶O + ¹⁸⁴W and ¹⁹F + ¹⁸¹Ta systems in the excitation energy range of 50–90 MeV, leading to the same compound nucleus ²⁰⁰Pb^∗. Comparison of experimental results of both the systems shows that ER cross sections and moments of gamma multiplicity distribution of ¹⁶O + ¹⁸⁴W system are significantly higher than those of ¹⁹F + ¹⁸¹Ta system at higher excitation energies. Present measurements directly shows the experimental signature of entrance channel effect even with the systems which are not very different with respect to their entrance channel mass asymmetry. It is further demonstrated that the reduction in the ER cross section and moments of spin distribution for ¹⁹F + ¹⁸¹Ta system is mainly due to the suppression of fusion of higher l values.