27A1-NMR

The rf pulses for 27A1 NMR experiments were calibrated using an aqueous solution of A1C13. For the rf power level attenuated by 10 dB, the duration of the 180 "-pulse of the broadband probe was 60 ps. All solids imaging experiments were performed with t => 300 ps and the nominal flip angle a = 90°/(J + 1/2). The two pulses had the same amplitude and for 27A1 MRI were 10- and 20-ps long, respectively. 

These behaviors are discussed from 27A1 NMR data giving the fraction of Al ions bound on the polymer and from of a model of electrostatic interactions. 

In a first part of this paper, we will discuss results of a 27A1 NMR study of the binding of Al ions on acrylamide-acrylic acid copolymers as a function of pH, at the light of a simple electrostatic model. The second part deals with the phase diagrams, physical gelation and precipitation phenomenon, for different copolymer compositions and under various conditions of concentrations, pH and salinity. 

Bottero et Al. (11,12) have shown that 27A1 NMR allows to... 

Adsorption vs. polymerization, corrosion inhibition, 644-646 27A1 NMR, 127-129 Alcohols, structure, 172/ ... 

Analysis of the 27A1 NMR spectra has confirmed the structure of the aluminium di-Schiff bases complexes being derivatives of trans-1,2-dia-minocyclohexane. The A1 chemical shift values of 6.57 and 39.48 ppm indicated the presence of dynamic equilibrium between five- and six-coordinated states in CDC13 solution. 

Figure 1.31P and 27A1 NMR spectra of F127- and F108-templated large-pore mesoporous... 

Multiwall carbon nanotubes (MWCNTs) have been synthesized by catalytic chemical vapor deposition (CCVD) of ethylene on several mesoporous aluminosilicates impregnated with iron. The aluminosilicates were synthesized by sol-gel method optimizing the Si/Al ratios from 6 to 80. The catalysts are characterized by nitrogen adsorption, X-ray diffraction, 27A1 NMR, thermogravimetric analysis (TGA) and infrared. The MWCNTs are characterized by TGA and transmission and scanning electron microscope. 

The effect of probe molecules on the 27A1 NMR has attracted some attention recently. In particular, the determination of the quadrupole coupling constant, Cq, is a sensitive means to learn more about the bonding situation at the aluminum in acid sites, and how it reflects the interaction with basic probe molecules. If one of the four oxygen atoms in an AIO4 tetrahedral coordination is protonated, as in a zeolitic acid site, the coordination is somewhat in between a trigonal and a tetrahedral A1 environment [232]. The protonated oxygen decreases its bond order to A1 to approximately half of its size compared to an unprotonated zeolite. 

An important result of the multinuclear NMR investigations of 23— 25, 27, 28, and 31-35 is that the structures, in contrast to aggregates of monometalated secondary phosphanes and arsanes, are retained in solution. Thus, the phosphandiide derivatives here discussed show resonance signals in their 31P NMR spectra that are independent of concentration and temperature (see Table III). The 31P and 27A1 NMR chemical shifts of 23-25, 27, and 31-35 differ with respect to ring size of the clusters and electronic influences by the substituents at phosphorus and aluminum. 

Two new amidine compounds (G6H2Ph3)G(N1Pr)2Al(CH3)2 and (G6H2Ph3)G(NCy)2Al(CH3)2 (Figure 21(c)) were prepared from the reaction of w-terphenyl-substituted amidines A,A -diisopropyl(or dicyclohexyl) benzamidine with trimethylaluminum.146 The 27A1 NMR showed broad peaks between <5=184 and 170 ppm. The coordination environment around the aluminum atoms is distorted tetrahedral. The acute N-Al-N (—69°) angle indicated considerable strain around the aluminum. 

Additionally, the outer ten A1 atoms are bonded to two bromine atoms each and saturated by a donor molecule (THF, THP). The apex and base atoms in the AI12 icosahedron are not naked they are shielded by one donor molecule each. Despite the great sensitivity of these Al22 subhalides 65 and 66, it was possible to obtain solid-state 27A1 NMR spectra and XPS measurements, which showed indeed that three electronically different types of A1 atoms are present [100]. The type of... 

J.W. Wiench, V.P. Balema, V.K. Pecharsky, M. Pruski, Solid-state 27A1 NMR investigation of thermal decomposition of LiAlH , J. Solid State Chem. 177 (2004) 648-653. 

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