Rotational flexibility in NH4+-encapsulated 18-crown-6 studied by molecular dynamics and positron annihilation: where does the positron localize?

Conformational flexibility of the ammonium complex of macrocyclic 18-crown-6 at 100 K < or = T < or = 300 K is studied using a positron microprobe in conjunction with molecular dynamics simulations. The ammonium cation, encapsulated in a three-pointed hydrogen-bonding perching arrangement, undergoes unhindered facile rotation inside the crown cavity, and the ether backbone adopts itself to the dynamic cation conformation. Preferential localization of the positron in the vicinity of the encapsulated cation results in the positron sensing changes in the local electron density distribution arising from the unrestricted rotational motion of the cation as well as from the slow motion of the crown backbone and hence sensing only an average conformation of the complex.     

A rapid method for direct quantitative analysis on chromatographic layers

Summary A rapid method is described for direct quantitative determination on the chromatographic layer. The method is based on the measurement of the length of the chromatographic band in the direction of flow.     

Reductive nitrosylation of tetraoxometallates—XI. Molybdate hydroxylamine reaction in the presence of cyanide: Synthesis of mono- and dinitrosyl cyan

The new molybdenum cyanonitrosyl complexes, R₂[Mo(NO)(CN)₅]·2H₂O (R = Ph₄P and Bu₄N) and [Mo(NO)(CN)₃(L-L)]·H₂O [L-L =     

Fission track annealing and age determination of hornblende

Hornblende which occurs in igneous and metamorphic rocks is well suited for age determination by the fission track method. The tracks which are readily etched in hydrofluoric acid are annealed in 1 hr at 530° C. Extrapolation of the experimentally determined temperatures suggest that a temperature of 200° C for one m.y. will erase all the tracks present before the heating. Fission track ages of hornblende agree with the main orogenic metamorphic cycles.     

Low resistivity as-deposited ohmic contacts to 3C-SiC

The contact resistivities of Al and Ti ohmic contacts to n-type 3C-SiC were measured using the circular TLM method. The surface doping concentration under the contact was increased by ion-implantation of nitrogen into SiC. The contact resistivity was observed to decrease with increasing surface doping concentration for both Al and Ti contacts. The minimum value for the contact resistivities for Aland Ti contacts was 1.4x 10^-5and 1.5 x 10^-5 ω cm², respectively, at the surface doping concentration of 3 x 10²⁰ cm^-3 without any annealing of the contacts. These values are an order of magnitude lower than previously reported minimum values for as-deposited ohmic contacts on n-type 3C-SiC.     

Carbon-13 NMR spectroscopy of some pentacyclic triterpenoids

Carbon-13 NMR signal assignment of a number of polyoxygenated triterpenes of the olean-12-ene and urs-12-ene series, carried out by considering the changes in chemical shift produced by the change of oxygenation pattern(s), and using methyl oleanolate as a model, are reported.     

Design of medium energy beam transport line between the RFQ and the Linac in the radioactive ion beam facility at VECC,Kolkata

The design of a medium energy beam transport (MEBT) line comprising of a re-buncher and four quadrupoles, two upstream and the other two downstream of the re-buncher, has been presented. The design was done to ensure almost 100% transport of heavy-ion beams of about 99 keV/u energy from RFQ having a q/A not less than 1/14 through the re-buncher and then through IH Linac of about 0.6 m length in which beam would be accelerated to about 185 keV/u. The re-buncher has been designed to operate at 37.8 MHz, the resonating frequency of both the RFQ and the IH Linac. The entire beam line has been installed and recently O⁵⁺ beam from RFQ has been transported through the re-buncher and subsequently accelerated in the IH Linac successfully.     

Under what kind of parametric fluctuations is spatiotemporal regularity the most robust?

It was observed that the spatiotemporal chaos in lattices of coupled chaotic maps was suppressed to a spatiotemporal fixed point when some fractions of the regular coupling connections were replaced by random links. Here we investigate the effects of different kinds of parametric fluctuations on the robustness of this spatiotemporal fixed point regime. In particular we study the spatiotemporal dynamics of the network with noisy interaction parameters, namely fluctuating fraction of random links and fluctuating coupling strengths. We consider three types of fluctuations: (i) noisy in time, but homogeneous in space; (ii) noisy in space, but fixed in time; (iii) noisy in both space and time. We find that the effect of different kinds of parametric noise on the dynamics is quite distinct: quenched spatial fluctuations are the most detrimental to spatiotemporal regularity; spatiotemporal fluctuations yield phenomena similar to that observed when parameters are held constant at the mean value, and interestingly, spatiotemporal regularity is most robust under spatially uniform temporal fluctuations, which in fact yields a larger fixed point range than that obtained under constant mean-value parameters.