Homonuclear correlation experiments for quadrupolar nuclei, spinning away from the magic angle

We present a set of homonuclear correlation experiments for half-integer quadrupolar spins in solids. In all these exchange-type experiments, the dipolar interaction is retained during the mixing time by spinning the sample at angles other than the “regular magic angle” (54.7°). The second-order quadrupolar interaction is averaged by different strategies for the different experiments. The multiple-quantum off magic angle spinning (MQOMAS) exchange experiment is essentially a regular MQMAS experiment where the quadrupolar interaction is averaged by combining magic angle spinning with a multiple- to single-quantum correlation scheme. The sample is spun at the magic angle at all times except during the mixing time which is added to establish homonuclear correlation. In the multiple-quantum P₄ magic angle spinning (MQP₄MAS) exchange experiment, the sample is spun at one of the angles at which the fourth-order Legendre polynomial vanishes (P₄ magic angle), the remaining second-order quadrupolar interaction now governed by a second-rank tensor is refocussed by the multiple to single-quantum correlation scheme. In the dynamic angle spinning (DAS) exchange experiment, the second-order quadrupolar interaction is averaged by correlating the evolution from two complementary angles. These experiments are demonstrated and compared, in view of their specific advantages and disadvantages, for ²³Na in the model compound Na₂SO₃.     

Acquisition of pure-phase diffusion spectra using oscillating-gradient spin echo

Oscillating-gradient spin echo (OGSE) diffusion experiments have long been used to measure the short-time apparent diffusion coefficient, D(app)(t), in the presence of restricted diffusion, as well as the spectrum of the slow-motion velocity autocorrelation function. In this work, we focus on two previously unexplored aspects of OGSE experiments: convection compensation and acquisition of pure-phase diffusion spectra in the presence of homonuclear scalar couplings. We demonstrate that convection compensation afforded by single-echo OGSE compares well with that in double-echo convection-compensated PGSE experiments. We also show that, in the presence of homonuclear scalar couplings, setting the OGSE echo time to 1/2J enables acquisition of pure-phase diffusion spectra and yields more reliable D estimates than mixed-phase PGSE or OGSE spectra. Pure-phase OGSE acquisition is also compatible with measurements of the apparent diffusion coefficient at an arbitrary diffusion time. These features of OGSE can be valuable in diffusion measurements of scalar-coupled small-molecule probes in cellular and other heterogeneous systems.     

Connection between deriving bridges and radial parts from multidimensional Ornstein-Uhlenbeck processes

Summary First we give a construction of bridges derived from a general Markov process using only its transition densities. We give sufficient conditions for their existence and uniqueness (in law). Then we prove that the law of the radial part of the bridge with endpoints zero derived from a special multidimensional Ornstein--Uhlenbeck process equals the law of the bridge with endpoints zero derived from the radial part of the same Ornstein--Uhlenbeck process. We also construct bridges derived from general multidimensional Ornstein--Uhlenbeck processes.     

Gaseous Chloride Complexes with Halogen Bridges—Homo-Complexes and Hetero-Complexes

The chemistry of the gaseous halide complexes which have been observed in large numbers in recent years is discussed for the example of chlorides. Homo-complexes—dimeric or polymeric chloride molecules containing chlorine-bridge linkages [e.g.(NaCl)_n, n = 2—4; (CuCl)_n, n = 2—5; (BeCl₂)_n, n = 2—4; Pd₆Cl₁₂]—occur through broad areas of the Periodic Table. Comparison of the dissociation enthalpy of such molecules with the vaporization enthalpy of the liquid chlorides is particularly informative. Hetero-complexes are formed from different chloride molecules by linkage through Cl bridges. Not only 1:1 complexes (e.g. NaAlCl₄, TIPbCl₃, KThCl₅, CdPbCl₄, AlUCl₈) but also larger molecules (e.g. CoAl₂Cl₈, CrAl₃Cl₁₂, Cu₂UCl₆, In₂UCl₁₀) are known. Types of formulas, structures, and above all the formation enthalpies of such complexes are discussed critically. Hetero-complexes are useful in chemical transport reactions, as aids in syntheses, and in gas-chromatographic separations (lanthanoids, actinoids). They also play a part in many industrial processes (chlorination of ores) and in recently developed types of high-pressure discharge tubes.     

A Mathematical Model of Suspension Bridges

We prove the existence of weak T-periodic solutions for a nonlinear mathematical model associated with suspension bridges. Under further assumptions a regularity result is also given.     

Structure and NMR spectra of cyclophanes with unsaturated bridges (cyclophenes)

The calculated structures of several known and hypothetical cyclophanes with ethylene bridges (cyclophenes) are reported together with experimental and calculated values of their NMR parameters. Of the exchange‐correlation functionals and basis sets used in this work, only the ωB97X‐D/6‐311++G(2d,2p) and ωB97X‐D/cc‐pVQZ yielded values of the C_sp3–C_sp3 bond length close to the experimental data, although significant differences still remain. As far as the NMR parameters are concerned, except for close‐lying signals, chemical shifts and coupling constants calculated at the ωB97X‐D/cc‐pVQZ level reproduce in most cases the experimental trends. Contrary to the calculations of geometries, an agreement between the values of the NMR parameters obtained at ωB97X‐D/cc‐pVQZ level and the experimental ones is the poorest compared with that of the ωB97X‐D/6‐311++G(2d,2p) one. Taking into account that the results of the different calculations show the same qualitative trends in most cases, we believe that they correctly describe the structure and properties of the hypothetical molecules studied here. Copyright © 2012 John Wiley & Sons, Ltd.     

Thermal stability of polybenzoxazines with lanthanum chloride and their crosslinked structures

Lanthanum chloride (LaCl₃) was incorporated into five kinds of benzoxazines by different preparation methods. The thermal stability and the structures of polybenzoxazines were characterized by thermogravimetric analysis (TGA) and Fourier transform infrared (FTIR) spectroscopy. The evolved gases from the degradation process of polybenzoxazines were analyzed by FTIR. The results showed that the thermal stability and char yields of three kinds of polybenzoxazines containing LaCl₃ can be improved obviously. LaCl₃ has an important effect on the polymerization reactions of benzoxazines. More stable arylamine Mannich bridges were observed in the chemical structures of the polybenzoxazines. It is these structures that can effectively retard the volatilization of aniline derivatives and result in the improvement of the thermal stability of the polybenzoxazines.     

Aplicación de métodos computacionales en la evaluación de la respuesta aeroelástica de puentes soportados por cables

The possibilities of computational methods for assessing the response of cable supported bridges under wind action are considered in this work. The main objective is to study the possibilities of substituting wind tunnel campaigns by computer based analyses, particularly at the early design stage. The preliminary proposed design for a continuous cable-stayed bridge with two main spans of 650 m and a single box girder deck has been considered as a case study. The force coefficients of the deck cross-section have been computed and the unsteady response associated to vortex-shedding has been simulated using CFD commercial software. Furthermore, an in-house piece of software has been employed to obtain the response for flutter and buffeting phenomena adopting the hybrid approach; with that purpose the experimental flutter functions of a similar box girder deck were adopted. The computational results have been validated by comparison with similar experimental results published by other researchers. It has been verified that the set of adopted methods offers reliable results with moderate costs; therefore, the proposed approach is very suitable at the early design stage of long span bridges or at conceptual design works.     

Structure and Magnetic Properties of a Manganese Complex with 1-Carboxymethylpyridinium-3-carboxylate

One complex with a zwitterionic dicarboxylate ligand,[Mn(L)(N3)]n1(L=1-carboxymethylpyridinium-3-carboxylate),was synthesized and structurally characterized.Complex 1crystallizes in triclinic,space group P1 with a=7.570(2),b=7.808(2),c=9.232(3),=94.450(19)o,V=489.0(3)3,Z=2,Mr=277.11,Dc=1.882 g/cm3,F(000)=278,μ=1.359 mm-1,S=1.021,the final R=0.0509 and wR=0.1149 for 2236 observed reflections with I2(I).Complex 1 consists of two-dimensional coordination layers in which the anionic uniform chains with mixed triple bridges(one end-on(EO)azide and two syn-syn carboxylates)are cross-linked by the flexible cationic N-methylpyridinium spacers.Magnetic studies demonstrated that the triple bridge in 1 transmits antiferromagnetic coupling between the Mn(II)ions.     

Systematic Studies on Homo- and Heteronuclear Doubly Bonded Compounds of Heavier Group 15 Elements

The first stable phosphabismuthene (R 1 --P=Bi--R 2 ) and stibabismuthene (R 1 --Sb=Bi--R 2 ) were successfully synthesized by taking advantage of efficient steric protection groups, 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (Tbt), 2,6-bis[bis(trimethylsilyl)-methyl]-4-[tris(trimethylsilyl)methyl]phenyl (Bbt), and 2,4,6-tri- t -butylphenyl (Mes*). Their spectroscopic properties and structural parameters were systematically compared with those of previously reported stable, homonuclear, doubly bonded systems, such as diphosphene, diarsene, distibene, and dibismuthene.