Attenuation of Nuclear Orientation of 127In in Gd and the InGd Korringa Spin–Lattice Relaxation Time Constant

The nuclear spin–lattice relaxation of ¹²⁷In implanted into pure gadolinium metal has been measured using the method of integral attenuation of on-line nuclear orientation. The Korringa constant is determined to be C_K (¹²⁷InGd, B_pol = 0.5 T) = 0.133(18) sK. The result is compared with relaxation of In nuclei inFe.     

Acid-catalyzed nucleophilic aromatic substitution: experimental and theoretical exploration of a multistep mechanism

The mechanism for the acid-mediated substitution of a phenolic hydroxyl group with a sulfur nucleophile has been investigated by a combination of experimental and theoretical methods. We conclude that the mechanism is distinctively different in nonpolar solvents (i.e., toluene) compared with polar solvents. The cationic mechanism, proposed for the reaction in polar solvents, is not feasible and the reaction instead proceeds through a multistep mechanism in which the acid (pTsOH) mediates the proton shuffling. From DFT calculations, we found a rate-determining transition state with protonation of the hydroxyl group to generate free water and a tight ion pair between a cationic protonated naphthalene species and a tosylate anion. Kinetic experiments support this mechanism and show that, at moderate concentrations, the reaction is first order with respect to 2-naphthol, n-propanethiol, and p-toluenesulfonic acid (pTsOH). Experimentally determined activation parameters are similar to the calculated values (Delta H exp not equal =105+/-9, Delta H calcd not equal =118 kJ mol(-1); Delta G exp not equal =112+/-18, Delta G calcd not equal =142 kJ mol(-1)).     

New class of dynamics in concentrated polymer gels

We show that concentrated poly(methyl methacrylate) solution exhibits a new class of coupled dynamics, which can be regarded as an intermediate between the collective diffusion of solutions and the structural relaxations of glasses. This class of dynamics have a relaxation rate that is directly proportional to the wave vector. The transition from diffusive to coupled collective dynamics occurs at smaller length scales with increasing polymer concentration and decreasing temperature. The experimental observations can be understood by considering the contributions from physical cross-links interconnected by stiff polymer segments.     

Prediction of macroscopic properties of protic ionic liquids by ab initio calculations

We have systematically investigated combinations of anions and cations in a number of protic ionic liquids based on alkylamines and used ab initio methods to gain insight into the parameters determining their liquid range and their conductivity. A simple, almost linear, relation of the experimentally determined melting temperature with the calculated volume of the anion forming the ionic liquid is found, whereas the dependence of the melting temperature with increasing cation volume goes through a minimum for relatively short side chain length. On the basis of the present results, we propose a strategy to predict the nature of protic ionic liquids in terms of low vapor pressure and conductivity. Comparisons with previously reported strategies for prediction of melting temperatures for aprotic ionic liquids are also made.     

A narrowband optical parametric oscillator tunable over 6.8 THz through degeneracy with a transversely-chirped volume Bragg grating

An efficient nanosecond optical parametric oscillator (OPO) with output energies of 0.75 mJ using a periodically poled KTiOPO₄ crystal pumped at 532 nm and generating narrowband output continuously tunable over the range of 6.8 THz, between 1053 nm and 1075 nm, is demonstrated by employing a transversely-chirped volume Bragg grating. The tunable reflectivity spectrum of the chirped volume Bragg grating allowed a smooth transition between singly-resonant and doubly-resonant operation of the OPO without cavity rearrangement. This gave a unique possibility to experimentally verify theoretical predictions regarding the efficiency of type-I and type-0 phase matched degenerate OPOs pumped by multimode Q-switched lasers.     

Ionic liquid structure: the conformational isomerism in 1‐butyl‐3‐methyl‐imidazolium tetrafluoroborate ([bmim][BF4])

As a probe of local structure, the vibrational properties of the 1‐butyl‐3‐methylimidazolium tetrafluoroborate [bmim][BF₄] ionic liquid were studied by infrared (IR), Raman spectroscopy, and ab initio calculations. The coexistence of at least four [bmim]⁺ conformers (GG, GA, TA, and AA) at room temperature was established through unique spectral responses. The Raman modes characteristic of the two most stable [bmim]⁺ conformers, GA and AA, according to the ab initio calculations, increase in intensity with decreasing temperature. To assess the total spectral behavior of the ionic liquid both the contributions of different [bmim]⁺ conformers and the [bmim]⁺− [BF₄]⁻ interactions to the vibrational spectra are discussed. Copyright © 2008 John Wiley & Sons, Ltd.     

Enhancement of second harmonic generation efficiency: Growth and characterization of magnesium(II)-doped tetrakis(thiourea)nickel(II) chloride crystals

The influence of magnesium doping on the properties of tetrakis(thiourea)nickel(II) chloride crystals has been described. The reduction in the intensity observed in powder X-ray diffraction of doped specimen and slight shifts in vibrational frequencies confirm the lattice stress as a result of doping. Surface morphological changes due to doping of the alkaline earth metal are observed by scanning electron microscopy. The incorporation of Mg(II) into the crystal lattice was confirmed by energy dispersive X-ray spectroscopy. Lattice parameters are determined by single crystal XRD analysis. The thermogravimetric and differential thermal analysis studies reveal the purity of the materials and no decomposition is observed up to the melting point. The crystal is further characterized by Kurtz powder technique and dielectric studies.     

Optimization of capillary electrophoresis conditions for coupling to a mass spectrometer via a sheathless interface

When optimizing a capillary electrophoresis/electrospray ionization mass spectrometry (CE/ESI-MS) system, consideration has to be given not only to the separation but also to the electrospray stability. Methods developed for CE/UV analysis of drugs and peptides were considered and modified to be suitable for a CE/MS system with a robust sheathless interface. Different concentrations of the organic modifiers acetonitrile, methanol and 2-propanol were used in the separation buffer. The type and concentrations of these modifiers were also compared with reference to electrospray stability, sensitivity and time of analysis. In addition, different ionic strengths in the buffers were evaluated with reference to electrospray stability. The repeatability was used for the estimation of electrospray stability. The degree to which these parameters influenced the separation and the ESI stability was studied using a nine-peptide standard mixture and the antibiotic drugs bacampicillin and ampicillin as test substances. The analysis time and resolution were used as measures of the efficiency of the separation. A time-of-flight MS analyzer was used since it has the potential advantages of becoming a better fit for integration of CE with MS owing to the speed and sensitivity of this mass analyzer. The detection limit, i.e. 1 microM, for bacampicillin was comparable to what could be achieved with CE/MS on a quadrupole instrument using selected ion monitoring and sheath flow ESI.