Synthesis and stability of reactive salts of dodecafluoro-closo-dodecaborate(2-)

The synthesis and characterization of several salts of the B(12)F(12)(2-) anion are reported. The potassium salt was prepared in 72% recrystallized yield by treating K(2)B(12)H(12) with liquid HF at 70 degrees C for 14 h and 20% F(2)/N(2) in liquid HF at 25 degrees C for 72 h. The CPh(3)(+), N(n-Bu)(4)(+), NH(n-C(12)H(25))(3)(+), NH(4)(+), and Li(+) salts were prepared by metathesis reactions. The [NH(n-C(12)H(25))(3)](2)[B(12)F(12)] salt is soluble in aromatic hydrocarbon solvents. The B(12)F(12)(2-) anion is remarkably stable. The salts Li(2)B(12)F(12) and [NH(4)](2)[B(12)F(12)] were stable when heated to 450 and 480 degrees C, respectively. The B(12)F(12)(2-) anion did not react with 98% H(2)SO(4), 70% HNO(3), 3 M KOH, a 10-fold excess of Ce(NH(4))(2)(NO(3))(6) in aqueous solution, or metallic sodium in THF. In addition, B(12)F(12)(2-) did not react with metallic lithium in a mixture of ethylene carbonate and dimethyl carbonate, was not reduced at 0 V versus Li(+/0) in that solvent, and underwent a quasi-reversible oxidation at 4.9 V versus Li(+/0). The structure of [CPh(3)](2)[B(12)F(12)] was determined by single-crystal X-ray diffraction: tetragonal, space group I4(1)/acd, a = 19.102(2), b = 19.102(2), c = 20.535(3) A, V = 7492.2(2) A(3), Z = 8, T = 173(2) K, R(1) = 0.064. The B(12)F(12)(2-) anion weakly interacts with the two symmetry related CPh(3)(+) cations via F.C contacts of 3.087(2) A, which are very close to the 3.17 A sum of van der Waals radii for these two atoms. Taken together, the data suggest that B(12)F(12)(2-) may be useful as a very robust weakly coordinating anion.     

Signal irreproducibility in high-field solution magnetic resonance experiments caused by spin turbulence

Turbulent spin dynamics arising from the joint action of radiation damping and the distant dipolar field are shown to generate irreproducible measurements in popular high-field, gradient-based magnetic resonance (MR) experiments, undermining the prevailing assumption of essentially predictable observables in MR. Sizeable fluctuations in echo amplitudes are reported and numerically simulated for pulsed gradient spin echo and stimulated echo diffusion measurements. The underlying microscopic dynamical instability is characterized by analysis of the finite-time Lyapunov exponents. Perturbations to the modulated magnetization are shown to render magic-angle gradients ineffective in suppressing signal fluctuations. Alternative approaches are suggested for cancelling out the feedback interactions leading to spin turbulence.     

Reversal of enantioselectivity in the hydroformylation of styrene with [2S,4S-BDPP]Pt(SnCl3)Cl at high temperature arises from a change in the enantioselective-determining step

Deuterioformylation of styrene catalyzed by [(2S,4S)-BDPP]Pt(SnCl(3))Cl at 39 degrees C gave 3-phenylpropanal (3) and 2-phenylpropanal (2) (n:i = 1.8, 71% ee (S)-2) with deuterium only beta to the aldehyde carbonyl and in the formyl group. Small amounts of deuterium were also found in the internal (2.8%), cis terminal (1.4%), and trans terminal (1.3%) vinyl positions of the recovered styrene. Deuterioformylation of styrene at 98 degrees C gave 3- (3) and 2-phenylpropanal (2) (n:i = 2.3, 10% ee (R)-2) with deuterium both alpha and beta to the aldehyde carbonyl and in the formyl group. Deuterium was also found in the internal (20%), cis terminal (12%), and trans terminal (12%) vinyl positions of the recovered styrene. These deuterioformylation results establish that platinum hydride addition to styrene is largely irreversible at 39 degrees C but reversible at 98 degrees C. Hydroformylation of (E)- and (Z)-beta-deuteriostyrene at 40 degrees C, followed by oxidation of the aldehydes to acids, and subsequent derivitization to the (S)-mandelate esters confirmed that 84% of 2-phenylpropanal (2) arises from platinum hydride addition to the si-face of styrene, while 73% of 3-phenylpropanal (3) arises from platinum hydride addition to the re-face of styrene. At 100 degrees C, the effect of variable H(2) and CO pressure on n:i, % ee, and TOF of hydroformylation of styrene was investigated. The results are consistent with enantioselectivity not being fully determined until the final hydrogenolysis of a platinum acyl intermediate.     

X-ray structure and DFT study of C1-C60(CF3)12. A high-energy, kinetically-stable isomer prepared at 500 degrees C

The title compound, prepared from C(60) and CF(3)I at 500 degrees C, exhibits an unusual fullerene(X)12 addition pattern that is 40 kJ mol(-1) less stable than the previously reported C(60)(CF(3))12 isomer.     

Aspect ratio dependence of the enhanced fluorescence intensity of gold nanorods: experimental and simulation study

Experimental observations and theoretical treatments are carried out for the band shape and relative intensity of the emission from gold nanorods of various aspect ratios in the range between 2.25 (1.5 theory) and 6.0 (9 theory). The calculation of the fluorescence spectra requires knowledge of the nanorod size distribution, the enhancement factors, and the shape of the unenhanced fluorescence spectrum. The size distribution is determined from the fit of the observed absorption spectrum for each value of aspect ratio studied to the theoretical model of Gans. The theory by Boyd and Shen is used for calculating the enhancement of the fluorescence spectrum of the previously observed weak emission of bulk gold, which originates from the interband transition. This is carried out for nanorods of different aspect ratios. To compare theory to the observed nanorod fluorescence spectra, which suffer from self-absorption, the calculated nanorod fluorescence spectra are corrected for this effect using the observed absorption spectra. The comparison between the observed and the calculated fluorescence band shapes is found to be good. The calculated changes in the relative intensities upon changing the aspect ratios are found to be much greater than that observed. This is due to the fact that for the observed emission of all the nanorods studied nonradiative processes dominate the relaxation mechanism of the excited state, a fact that was not included in the theoretical treatments.     

Synthesis and structural characterisation of rhodium hydride complexes bearing N-heterocyclic carbene ligands

Addition of excesses of N-heterocyclic carbenes (NHCs) IEt₂Me₂, IⁱPr₂Me₂ or ICy (IEt₂Me₂ = 1,3-diethyl-4,5-dimethylimidazol-2-ylidene; IⁱPr₂Me₂ = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene; ICy = 1,3-dicyclohexylimidazol-2-ylidene) to [HRh(PPh₃)₄] (1) affords an isomeric mixture of [HRh(NHC)(PPh₃)₂] (NHC = IEt₂Me₂ (cis-/trans-2), IⁱPr₂Me₂ (cis-/trans-3), ICy (cis-/trans-4) and [HRh(NHC)₂(PPh₃)] (IEt₂Me₂(cis-/trans-5), IⁱPr₂Me₂ (cis-/trans-6), ICy (cis-/trans-7)). Thermolysis of 1 with the aryl substituted NHC, 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene (IMesH₂), affords the bridging hydrido phosphido dimer, [{(PPh₃)₂Rh}₂(μ-H)(μ-PPh₂)] (8), which is also the reaction product formed in the absence of carbene. When the rhodium precursor was changed from 1 to [HRh(CO)(PPh₃)₃] (9) and treated with either IMes (=1,3-dimesitylimidazol-2-ylidene) or ICy, the bis-NHC complexes trans-[HRh(CO)(IMes)₂] (10) and trans-[HRh(CO)(ICy)₂] (11) were formed. In contrast, the reaction of 9 with IⁱPr₂Me₂ gave [HRh(CO)(IⁱPr₂Me₂)₂] (cis-/trans-12) and the unusual unsymmetrical dimer, [(PPh₃)₂Rh(μ-CO)₂Rh(IⁱPr₂Me₂)₂] (13). The complexes trans-3, 8, 10 and 13 have been structurally characterised.     

Using ligand bite angles to control the hydricity of palladium diphosphine complexes

A series of [Pd(diphosphine)(2)](BF(4))(2) and Pd(diphosphine)(2) complexes have been prepared for which the natural bite angle of the diphosphine ligand varies from 78 degrees to 111 degrees. Structural studies have been completed for 7 of the 10 new complexes described. These structural studies indicate that the dihedral angle between the two planes formed by the two phosphorus atoms of the diphosphine ligands and palladium increases by over 50 degrees as the natural bite angle increases for the [Pd(diphosphine)(2)](BF(4))(2) complexes. The dihedral angle for the Pd(diphosphine)(2) complexes varies less than 10 degrees for the same range of natural bite angles. Equilibrium reactions of the Pd(diphosphine)(2) complexes with protonated bases to form the corresponding [HPd(diphosphine)(2)](+) complexes were used to determine the pK(a) values of the corresponding hydrides. Cyclic voltammetry studies of the [Pd(diphosphine)(2)](BF(4))(2) complexes were used to determine the half-wave potentials of the Pd(II/I) and Pd(I/0) couples. Thermochemical cycles, half-wave potentials, and measured pK(a) values were used to determine both the homolytic ([HPd(diphosphine)(2)](+) --> [Pd(diphosphine)(2)](+) + H*) and the heterolytic ([HPd(diphosphine)(2)](+) --> [Pd(diphosphine)(2)](2+) + H(-)) bond-dissociation free energies, Delta G(H*)* and Delta G(H-)*, respectively. Linear free-energy relationships are observed between pK(a) and the Pd(I/0) couple and between Delta G(H-)* and the Pd(II/I) couple. The measured values for Delta G(H*)* were all 57 kcal/mol, whereas the values of Delta G(H-)* ranged from 43 kcal/mol for [HPd(depe)(2)](+) (where depe is bis(diethylphosphino)ethane) to 70 kcal/mol for [HPd(EtXantphos)(2)](+) (where EtXantphos is 9,9-dimethyl-4,5-bis(diethylphosphino)xanthene). It is estimated that the natural bite angle of the ligand contributes approximately 20 kcal/mol to the observed difference of 27 kcal/mol for Delta G(H-)*.     

Analysis of DNA restriction fragments and polymerase chain reaction products by capillary electrophoresis

Capillary electrophoresis (CE) is a new, high-resolution tool for the analysis of DNA restriction fragments and DNA amplified by the polymerase chain reaction (PCR). By combining many of the principles of traditional slab gel methods in a capillary format, it is possible to perform molecular size determinations of human and plant PCR amplification products and DNA restriction fragments. DNA restriction fragments and PCR products were analyzed by dynamic sieving electrophoresis (DSE) and capillary gel electrophoresis (CGE). As part of this study, sample preparation procedures, injection modes, and the use of molecular mass markers were evaluated. Optimum separations were performed using the uPage-3 (3% T, 3% C) CGE columns with UV detection at 260 nm. Membrane dialysis and ultrafiltration/centrifugation proved to be nearly equivalent methods of sample preparation. Reproducibility studies demonstrated that blunt-ended, non-phosphorylated markers (specifically allele generated markers) provide the most accurate calibration for PCR product analysis. This study demonstrates that CE offers a high-speed, high-resolution analytical method for accurately determining molecular size and/or allelic type as compared with traditional methodologies.     

The impact of processing on the nutritional quality of food proteins

During many food processing regimens, food proteins may undergo a variety of chemical modifications. Despite the enormous consumption of processed foods worldwide, much remains to be learned about the exact nature of these modifications. This is partly due to the complex nature of the changes involved, and partly to the problems of analysis imposed by the intractable nature of the food matrix. Such difficulties are compounded by the paucity of chemically based analytical tests that accurately measure amino acid availability in biologically relevant terms. In this review, we explore the known changes that proteins and amino acids undergo during food processing and the consequences of these changes on the physical and nutritional qualities of the food. We also examine the impact of these protein derivatizations for the analysis of food proteins and amino acids, and highlight areas that require future research.