Studies on pyridinium hexafluorostannate and its derivatives

Pyridinium hexafluorostannate, (C5H5NH)2SnF6, has been prepared by the reaction of stannous chloride or tin metal with pyridinium poly(hydrogen fluoride), PPHF, and identified by chemical analysis, IR and NMR (H-1, F-19, C-13). Making use of (C5H5NH)2SnF6 as a precursor, the following important hexafluorostannate salts have been synthesized in high yields at room temperature by ionic exchange: M2SnF6 (M = NH4, Na, K, Rb, Cs) and BaSnF6. These salts have been characterised by chemical analysis and infrared spectroscopy. Indexed powder X-ray diffraction data for Na2SnF6, Rb2SnF6 and Cs2SnF6 have been reported.

Synthesis of amino thiols and isocysteines via regioselective ring opening of sulfamidates with tetrathiomolybdate

Herein we present a simple and highly efficient method for the synthesis of beta and gamma-amino thiols via regioselective ring opening of sulfamidates with tetrathiomolybdate 1. The generality of this methodology has been shown by synthesizing carbohydrate derived beta-amino thiol. The scope and versatility of this methodology has been demonstrated by synthesizing biologically important unnatural amino acids like isocysteines in optically pure form. (C) 2011 Elsevier Ltd. All rights reserved.

Raman Spectroscopy of the Charge Transfer Complex (TTF)x C60 Br8

We report Raman studies on powder samples of the charge transfer complex (TTF)(x)C60Br8 at room temperature. The phonons show considerable softening with respect to the frequencies observed in the Raman spectrum of solid C60Br8. The strongest mode at 1464 cm(-1) in C60Br8 is red shifted to a doublet with peaks at 1414 and 1421 cm(-1), implying an average phonon softening Delta omega of -47 cm(-1). A comparison with the phonon softening of the corresponding A(g)(2) mode in alkali-doped C-60 (Delta omega similar to -36 cm(-1) for A(6)C(60), A = K, Rb or Cs) suggests that 8 electrons are transferred per C60Br8 molecule in the charge transfer complex. The mode at 503 cm(-1) in C60Br8 is shifted upwards, similar to that in A(6)C(60) compounds.

Pyridinidm Hexafluorotitanate and its Derivatives

Pyridinium hexafluorotitanate, (C5H5NH)(2)TiF6, has been prepared by the reaction of titanium metal with pyridinium poly(hydrogen fluoride), PPHF, at room temperature. Making use of (C5H5NH)(2)TiF6 as a precursor, ammonium and alkali metal hexafluorotitanates, M(2)TiF(6) (M = NH4, Na, K, Rb and Cs) have been synthesized by metathesis. These hexafluorotitanates have been characterized by chemical analyses, infrared and NMR (H-1 and F-19) spectroscopy and powder X-ray diffraction methods. Indexed powder X-ray diffraction data for Rb2TiF6 and Cs2TiF6 have been reported.

Modulation of lifetimes and diastereomeric discrimination in triplet-excited substituted butane-1,4-diones through intramolecular charge-transfer quenching

Triplet lifetimes have been determined for the diastereomers of a broad set of butane-l,4-dione derivatives (1-3). A remarkable dependence of lifetimes on conformational preferences is revealed in that the lifetimes are shorter for the meso diastereomers of 1-3 than those for the racemic ones. The intramolecular beta-phenyl quenching is promoted in the case of meso diastereomers by virtue of the gauche relationship between the excited carbonyl group and the beta-aryl ring, while a distal arrangement in the lowest energy conformation (H-anti) in racemic diastereomers prevents such a deactivation. The involvement of charge transfer in the intramolecular beta-phenyl quenching is suggested by the correlation of the triplet lifetimes of the meso diastereomers of compounds 2 with the nature of the substituent on the beta-phenyl rings. In the case of racemic diastereomers, beta-methoxy substitution on the beta-phenyl ring (2-OCH3, 3-OCH3) also led to a decrease of the triplet lifetimes when compared to those of the nonsubstituted compounds (2-H, 3-H). This shortening is accounted for by the deactivation of a small proportion of the excited molecules through beta-phenyl quenching. In addition to the above factors, the lifetimes in the case of meso diastereomers can further be controlled by increasing the energy spacing between the T-1 and T-2 states, since beta-phenyl quenching occurs from the latter for compounds 2 and 3. Through a rational conformational control, a surprisingly long triplet lifetime (300 ns) has been measured for the first time for a purely n,pi* triplet-excited beta-phenylpropiophenone dimer (1-rac).

Phase transitions in triammonium hydrogen disulphate: Crystal structure of an intermediate stable phase at-90 degrees C

Triammonium hydrogen disulphate, (NH4)(3)H(SO4)(2), belongs to the family of crystal structures M3H(XO4)(2) (with M = NH4, K, Rb, Cs, and X = S, Se) which display super protonic phases at elevated temperatures, while at room temperature these are relatively poor proton conductors. The crystal structure of triammonium hydrogen disulphate has been determined by X-ray diffraction at -90 degrees C and the variation in the characteristics of the hydrogen bond is discussed in comparison with that of the structures at -110 degrees C and room temperature. It is concluded that the mechanics involving the proton migration in such systems is realised in terms of the variations in the hydrogen bond features with temperature.

Synthesis and cation binding properties of new bile acid-based crown ethers

A series of bile acid-based crown ethers (7a-c,12 and 13) were easily constructed from readily available precursors. Measurement of association constants (K-a) with alkali metal picrates in CHCl3 showed that azacrown ethers 7a-c and Chola-Cuowns 12 and 13 show greater binding towards Rb+ and K+. The presence of the aromatic moieties showed subtle changes in the binding properties. Insight II minimized structures show very different conformations of aromatic units in 7a-b and 13.

Direct and sensitized (energy and electron transfer) geometric isomerization of stilbene within zeolites: a comparison between solution and zeolite as reaction media

The trans- and cis-stilbenes upon inclusion in NaY zeolite are thermally stable. Direct excitation and triplet sensitization results in geometric isomerization and the excited state behavior under these conditions are similar to that in solution. Upon direct excitation, a photostationary state consisting of 65% cis and 35% trans isomers is established. Triplet sensitization with 2-acetonaphthone gave a photostationary state consisting of 63% cis and 37% trans isomers. These numbers are similar to the ones obtained in solution. Thus, the presence of cations and the confined space within the zeolite have very little influence on the overall chemistry during direct and triplet sensitization. However, upon electron transfer sensitization with N-methylacridinium (NMA) as the sensitizer within NaY, isomerization from cis-stilbene radical cation to trans-stilbene occurs and the recombination of radical ions results in triplet stilbene. Prolonged irradiation gave a photostationary state (65% cis and 35% trans) similar to triplet sensitization. This behavior is unique to the zeolite and does not take place in solution. Steady state fluorescence measurements showed that the majority of stilbene molecules are close to the N-methylacridinium sensitizer. Diffuse reflectance flash photolysis studies established that independent of the isomer being sensitized only trans radical cation is formed. Triplet stilbene is believed to be generated via recombination of stilbene radical cation and sensitizer radical anion. One should be careful in using acidic HY zeolite as a medium for photoisomerization of stilbenes. In our hands, in these acidic zeolites isomerization dominated the photoisomerization. (C) 2002 Elsevier Science B.V. All rights reserved.

Light-induced geometric isomerization of 1,2-diphenylcyclopropanes included within Y zeolites: Role of cation-guest binding

Through a systematic study of several diphenylcyclopropane derivatives, we have inferred that the cations present within a zeolite control the excited-state chemistry of these systems. In the parent 1,2-diphenylcylopropane, the cation binds to the two phenyl rings in a sandwich-type arrangement, and such a mode of binding prevents cis-to-trans isomerization. Once an ester or amide group is introduced into the system (derivatives of 2beta,3beta-diphenylcyclopropane-1alpha-carboxylic acid), the cation binds to the carbonyl group present in these chromophores and such a binding has no influence on the cis-trans isomerization process. Cation-reactant structures computed at density functional theory level have been very valuable in rationalizing the observed photochemical behavior of diphenylcyclopropane derivatives included in zeolites. While the parent system, 1,2-diphenyleylopropane, has been extensively investigated in the context of chiral induction in solution, owing to its failure to isomerize from cis to trans, the same could not be investigated in zeolites. However, esters of 2beta,3beta-diphenylcyclopropane-1alpha-carboxylic acid could be studied within zeolites in the context of chiral induction. Chiral induction as high 20% ee and 55% de has been obtained with selected systems. These numbers, although low, are much higher than what has been obtained in solution with the same system or with the parent system by other investigators (maximum similar to10% ee).

Precise measurement of hyperfine intervals using avoided crossing of dressed states

We demonstrate a technique for precisely measuring hyperfine intervals in alkali atoms. The atoms form a three-level system in the presence of a strong control laser and a weak probe laser. The dressed states created by the control laser show significant linewidth reduction. We have developed a technique for Doppler-free spectroscopy that enables the separation between the dressed states to be measured with high accuracy even in room temperature atoms. The states go through an avoided crossing as the detuning of the control laser is changed from positive to negative. By studying the separation as a function of detuning, the center of the level-crossing diagram is determined with high precision, which yields the hyperfine interval. Using room temperature Rb vapor, we obtain a precision of 44 kHz. This is a significant improvement over the current precision of similar to1 MHz.

Variable temperature x-ray crystal structure analysis of a type I langbeinite: Rb2Cd2(SO4)(3)

The structure of a type I langbeinite, Rb2Cd2(SO4)(3), displays three different phases, cubic with a = 10.378(5) Angstrom (space group P2(1)3) at room temperature, monoclinic at 120 K with a = 10.328(3), b = 10.322(3), c = 10.325(3) Angstrom, beta = 89.975(1)degrees (space group P2(1)), and orthorhombic at 85 K with a = 10.319(2), b = 10.321(2), c = 10.320(2) Angstrom (space group P2(1)2(1)2(1)), respectively. Precise single-crystal analyses of these phases indicate that Rb2Cd2(SO4)(3) distorts initially from cubic to monoclinic upon cooling followed by a significant reorientation of the SO4 tetrahedra, resulting in an orthorhombic symmetry upon further cooling. The three structures have been established unequivocally using the same crystal. There is no indication of the formation of an intermediate triclinic phase or any lattice disorder as conjectured in several earlier reports on compounds belonging to the type I langbeinite. The bond valence sum analyses of the coordination around the Rb sites indicate asymmetry in the bond strengths which could be the driving force of the ferroelectric behavior in these materials.

Axially homogeneous, zero mean flow buoyancy-driven turbulence in a vertical pipe

We report an experimental study of a new type of turbulent flow that is driven purely by buoyancy. The flow is due to an unstable density difference, created using brine and water, across the ends of a long (length/diameter=9) vertical pipe. The Schmidt number Sc is 670, and the Rayleigh number (Ra) based on the density gradient and diameter is about 108. Under these conditions the convection is turbulent, and the time-averaged velocity at any point is ‘zero’. The Reynolds number based on the Taylor microscale, Reλ, is about 65. The pipe is long enough for there to be an axially homogeneous region, with a linear density gradient, about 6–7 diameters long in the midlength of the pipe. In the absence of a mean flow and, therefore, mean shear, turbulence is sustained just by buoyancy. The flow can be thus considered to be an axially homogeneous turbulent natural convection driven by a constant (unstable) density gradient. We characterize the flow using flow visualization and particle image velocimetry (PIV). Measurements show that the mean velocities and the Reynolds shear stresses are zero across the cross-section; the root mean squared (r.m.s.) of the vertical velocity is larger than those of the lateral velocities (by about one and half times at the pipe axis). We identify some features of the turbulent flow using velocity correlation maps and the probability density functions of velocities and velocity differences. The flow away from the wall, affected mainly by buoyancy, consists of vertically moving fluid masses continually colliding and interacting, while the flow near the wall appears similar to that in wall-bound shear-free turbulence. The turbulence is anisotropic, with the anisotropy increasing to large values as the wall is approached. A mixing length model with the diameter of the pipe as the length scale predicts well the scalings for velocity fluctuations and the flux. This model implies that the Nusselt number would scale as Ra1/2Sc1/2, and the Reynolds number would scale as Ra1/2Sc−1/2. The velocity and the flux measurements appear to be consistent with the Ra1/2 scaling, although it must be pointed out that the Rayleigh number range was less than 10. The Schmidt number was not varied to check the Sc scaling. The fluxes and the Reynolds numbers obtained in the present configuration are much higher compared to what would be obtained in Rayleigh–Bénard (R–B) convection for similar density differences.

Finding a Box Representation for a Graph in $O(n^2\Delta^2\ln n)$ time

An axis-parallel box in $b$-dimensional space is a Cartesian product $R_1 \times R_2 \times \cdots \times R_b$ where $R_i$ (for $1 \leq i \leq b$) is a closed interval of the form $[a_i, b_i]$ on the real line. For a graph $G$, its boxicity is the minimum dimension $b$, such that $G$ is representable as the intersection graph of (axis-parallel) boxes in $b$-dimensional space. The concept of boxicity finds application in various areas of research like ecology, operation research etc. Chandran, Francis and Sivadasan gave an $O(\Delta n^2 \ln^2 n)$ randomized algorithm to construct a box representation for any graph $G$ on $n$ vertices in $\lceil (\Delta + 2)\ln n \rceil$ dimensions, where $\Delta$ is the maximum degree of the graph. They also came up with a deterministic algorithm that runs in $O(n^4 \Delta )$ time. Here, we present an $O(n^2 \Delta^2 \ln n)$ deterministic algorithm that constructs the box representation for any graph in $\lceil (\Delta + 2)\ln n \rceil$ dimensions.

Subnatural linewidth in a strongly-driven degenerate two-level system

We observe linewidths below the natural linewidth for a probe laser on a degenerate two-level F -> F' transition, when the same transition is driven by a strong control laser. We take advantage of the fact that each level of the transition is made of multiple magnetic sublevels, and use the phenomenon of electromagnetically induced transparency (EIT) or absorption ( EIA) in multilevel systems. Optical pumping by the control laser redistributes the population so that only a few sublevels contribute to the probe absorption, an explanation which is verified by a density-matrix analysis of the relevant sublevels. We observe more than a factor of 3 reduction in linewidth in the D(2) line of Rb in room-temperature vapor. Such subnatural features vastly increase the scope of applications of EIT, such as high-resolution spectroscopy and tighter locking of lasers to atomic transitions, since it is not always possible to find a suitable third level. (C) 2011 Elsevier B.V. All rights reserved.