Department of Chemistry

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Chemical exchange in solution state has been investigated traditionally by both 1D and 2D NMR, permitting the extraction of kinetic parameters (e.g. the spin-lattice relaxation time T 1, the exchange rate constant k and the activation parameters). This work demonstrates a simple 1D NMR approach employing multiply selective excitation to study multi-site exchange processes in solution, applying it to systems that exhibit three-site exchange. This approach involves simultaneous excitation of all - or a chosen subset of - the exchanging sites by using an appropriately modulated shaped radiofrequency pulse. The pulse sequence, as well as analysis is summarized. Significant features of the experiment, which relies on sign labelling of the exchanging sites, include considerably shorter experiment time compared to standard 2D exchange work, clear definition of the exchange time window and uniform pulse non-ideality effects for all the exchanging sites. Complete kinetic information is reported in the study of dynamic processes in superacid solutions of two weak bases, studied by 1H NMR. An analytical solution, leading to the determination of four rate parameters, is presented for proton exchange studies on these systems, which involve a mixture of two weak bases in arbitrary concentration ratio, and stoichiometric excess of the superacid. © Indian Academy of Sciences.

Fulltext

Journal of Chemical Sciences

Study of multi-site chemical exchange in solution state by NMR: 1D experiments with multiply selective excitation

AuNP/PDMS nanocomposites have been synthesized in the form of gels, foams, and films with distinctive structure and morphology. A simple in situ process in aqueous medium for the formation of such composite materials is described. The nanoparticles are held firmly within the PDMS while still being chemically accessible to substances soluble in PDMS. We demonstrate the utility of this property for water purification applications such as removing aromatic solvents and sulfur-containing contaminants from water. The contaminants can be freed from the composite with a simple thermal treatment, allowing the material to be reused. We also demonstrate chemically selective uptake and release of a fluorescent dye by the nanocomposite as a drug delivery model system. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

World University root node

Macromolecular Chemistry and Physics

A simple water-based synthesis of Au nanoparticle/PDMS composites for water purification and targeted drug release

We compute the entropy and transport properties of water in the hydration layer of dipalmitoylphosphatidylcholine bilayer by using a recently developed theoretical scheme [two-phase thermodynamic model, termed as 2PT method; S.-T. Lin, J. Chem. Phys. 119, 11792 (2003)] based on the translational and rotational velocity autocorrelation functions and their power spectra. The weights of translational and rotational power spectra shift from higher to lower frequency as one goes from the bilayer interface to the bulk. Water molecules near the bilayer head groups have substantially lower entropy (48.36 J/mol/K) than water molecules in the intermediate region (51.36 J/mol/K), which have again lower entropy than the molecules (60.52 J/mol/K) in bulk. Thus, the entropic contribution to the free energy change (TΔS) of transferring an interface water molecule to the bulk is 3.65 kJ/mol and of transferring intermediate water to the bulk is 2.75 kJ/mol at 300 K, which is to be compared with 6.03 kJ/mol for melting of ice at 273 K. The translational diffusion of water in the vicinity of the head groups is found to be in a subdiffusive regime and the rotational diffusion constant increases going away from the interface. This behavior is supported by the slower reorientational relaxation of the dipole vector and OH bond vector of interfacial water. The ratio of reorientational relaxation time for Legendre polynomials of order 1 and 2 is approximately 2 for interface, intermediate, and bulk water, indicating the presence of jump dynamics in these water molecules. © 2010 American Institute of Physics.

Journal of Chemical Physics

Entropy and dynamics of water in hydration layers of a bilayer

We study the nature of highly excited eigenstates of strongly coupled multimode systems with three degrees of freedom. Attempts to dynamically assign the eigenstates using classical-quantum correspondence techniques poses a considerable challenge, due to both the number of degrees of freedom and the extensive chaos in the underlying classical phase space. Nevertheless, we show that sequences of localized states interspersed between delocalized states can be identified readily by using the parametric variation technique proposed earlier by us. In addition, we introduce a novel method to lift the quantum eigenstates onto the classical resonance web using a wavelet-based local time-frequency approach. It is shown that the lifting procedure provides clear information on the various dominant nonlinear resonances that influence the eigenstates. Analyzing the spectroscopic Hamiltonians for CDBrClF and CF 3CHFI as examples, we illustrate our approach and demonstrate the consistency between state space and phase space perspectives of the eigenstates. Two exemplary cases of highly mixed quantum states are discussed to highlight the difficulties associated with their assignment. In particular, we provide arguments to distinguish between the states in terms of their predominantly classical or quantum nature of the mixing. © 2009 American Chemical Society.

Journal of Physical Chemistry A

Decoding the dynamical information embedded in highly excited vibrational eigenstates: State space and phase space viewpoints

Fluorescence resonance energy transfer (FRET) from coumarin 480 (C480) to rhodamine 6G (R6G) is studied in a room temperature ionic liquid (RTIL) microemulsion by picosecond and femtosecond emission spectroscopy. The microemulsion is comprised of the RTIL 1-pentyl-3-methylimidazolium tetraflouroborate, [pmim][BF4], in TX-100/ benzene. We have studied the microemulsion with and without water. The time constants of FRET were obtained from the risetime of the acceptor (R6G) emission. In the RTIL microemulsion, FRET occurs on multiple time scales: 1, 250, and 3900 ps. In water containing RTIL microemulsion, the rise components are 1.5, 250, and 3900 ps. The 1 and 1.5 ps components are assigned to FRET at a close contact of donor and acceptor (RDA ∼ 12 A). This occurs within the highly polar (RTIL/water) pool of the microemulsion. With increase in the excitation wavelength (Aex) from 375 to 435 nm, the relative contribution of the ultrafast component of FRET (∼1 ps) increases from 4% to 100% in the RTIL microemulsion and 12% to 100% in the water containing RTIL microemulsion. It is suggested that at Xex = 435 nm, mainly the highly polar RTIL pool is probed where FRET is very fast due to the close proximity of the donor and the acceptor. The very long 3900 ps (RDA ∼ 45 A) component may arise from FRET from a donor in the outer periphery of the microemulsion to an acceptor in the polar RTIL pool. The 250 ps component (RDA ∼ 29 A) is assigned to FRET from a donor inside the surfactant chains. © 2009 American Chemical Society.

Ultrafast fret in a room temperature ionic liquid microemulsion: A femtosecond excitation wavelength dependence study

(Chemical Equation Presented) The first example of a catalytic enantioselective intramolecular hydride shift/ring closure reaction is reported. This redox neutral reaction cascade allows for the efficient formation of ring-fused tetrahydroquinolines in high enantioselectivities. © 2009 American Chemical Society.

Journal of the American Chemical Society

Catalytic enantioselective intramolecular redox reactions: Ring-fused tetrahydroquinolines

(Chemical Equation Presented) Cyclic aminals were prepared through a Brønsted acid-promoted reaction. This redox neutral process involves iminium ion formation, 1,5 H-transfer, followed by ring closure. © 2009 American Chemical Society.

Journal of Organic Chemistry

Facile formation of cyclic aminals through a Brønsted acid-promoted redox process

Polycyclic tetrahydroquinolines were prepared by an efficient Lewis acid catalyzed 1,5-hydride shift, ring closure sequence. Gadolinium triflate was identified as a catalyst that is superior to scandium triflate as well as other Lewis acids. An approach toward a catalytic enantioselective variant is also described. © 2009 American Chemical Society.

Organic Letters

Lewis acid catalyzed formation of tetrahydroquinolines via an intramolecular redox process

We report molecular dynamics simulations of bilayers using a united atom model with explicit solvent molecules. The bilayer consists of the single tail cationic surfactant behenyl trimethyl ammonium chloride (BTMAC) with stearyl alcohol (SA) as the cosurfactant. We study the gel to liquid crystalline transitions in the bilayer by varying the amount of water at fixed BTMAC to SA ratio as well as by varying the BTMAC to SA ratio at fixed water content. The bilayer is found to exist in the tilted, Lβ′ phase at low temperatures, and for the compositions investigated in this study, the L β′ to Lα melting transition occurred in the temperature range 330-338 K. For the highest BTMAC to SA composition (2:3 molar ratio), a diffuse headgroup-water interface is observed at lower temperatures, and an increase in the d-spacing occurs prior to the melting transition. This pretransition swelling is accompanied by a sharpening in the water density variation across the headgroup region of the bilayer. Signatures of this swelling effect which can be observed in the alkane density distributions, area per headgroup, and membrane thickness are attributed to the hydrophobic effect. At a fixed bilayer composition, the transition temperature (&gt;338 K) from the L β′ to LR transition obtained for the high water content bilayer (80 wt %) is similar to that obtained with low water content (54.3 wt %), confirming that the melting transition at these water contents is dominated by chain melting. © 2009 American Chemical Society.

Journal of Physical Chemistry B

The influence of bilayer composition on the gel to liquid crystalline transition

Deuterium isotope effect on the solvation dynamics and fluorescence anisotropy decay of coumarin 153 (C153) bound to dimethyl Β -cyclodextrin (DMB) and trimethyl Β -cyclodextrin (TMB) is studied using femtosecond upconversion. In D2 O, there is a marked increase in the steady state emission quantum yield and fluorescence lifetime of C153 bound to DMB and TMB. This suggests strong coupling between C153 and D2 O inside the cyclodextrin cavity. In D2 O, average solvation time of C153 in DMB is about 1.7 times slower compared to that in water. For TMB in D2 O, solvation is 1.5 times slower. The deuterium isotope effect on solvation dynamics at long time arises mainly from the longer excited state lifetime. The longest components of solvation dynamics are ascribed to self-diffusion of C153 out of the cyclodextrin cavity. The nearly 1.5 times slower anisotropy decay of C153 bound to DMB and TMB in D2 O (compared to H2 O) is attributed to higher viscosity of D2 O. © 2009 American Institute of Physics.