Alexander J. Vega

Quadrupole echo distortion as a tool for dynamic NMR: Application to molecular reorientation in solid trimethylamine

Deuterium NMR quadrupole echo spectra of trimethylamine‐d9 (TMA‐d9) were recorded in the solid phase from about −150 °C to the melting point (−117 °C). The spectra exhibit pronounced line shape variation with temperature which is interpreted in terms of two independent dynamic processes, (i) threefold methyl group reorientation about the respective C3 axes, and (ii) threefold whole molecule reorientation about the molecular C’3 axis. To derive quantitative kinetic parameters for these processes the experimental spectra are compared with simulated traces, placing particular emphasis on the effect of line shape distortion due to the delay time τ between the π/2 pulses in the quadrupole echo experiment. This approach improves the sensitivity of the fitting method and allows accurate determination of the kinetic parameters for both the C3 and C3 reorientation processes. The results are k(−150 °C)=3.0×104 s−1, k’(−150 °C)=0.1×104 s−1, ΔE=3.0±0.6 kcal/mol, and ΔE’=8.0±1.6 kcal/mol, respectively. In Appendix A a...

Dynamic quadrupole echo NMR of deuterium in liquid crystalline solutions: The effect of the chemical shift

The problem of calculating dynamic deuterium quadrupole echo spectra in ordered systems in which exchange takes place between noninteracting nuclei with different quadrupole interactions and chemical shifts is solved. For the solution a specific set of basic spin I=1 matrices is chosen which allows casting the solution in a form having the same dimensionality as the Bloch–McConnell equations of spin I=1/2 nuclei. The theory is applied to two dynamic systems, viz., the ring inversion of cyclohexane and the Cope rearrangement in bullvalene, both in liquid crystalline solution. It is shown that even in ordered systems the time interval between the π/2 pulses in the quadrupole echo experiment may strongly affect the shape and intensity of the spectrum, thus providing an additional experimental variable for analyzing dynamic spectra. When large chemical shifts or off‐resonance pulses are involved, distorted spectra are obtained. This distortion can be removed by including π pulses in the quadrupole echo sequen...

Neutron powder diffraction study and physical characterization of zeolite D-RHO shallow-bed calcined at 773 Κ and 873 Κ

Zeolite RHO samples, shallow-bed calcined at 773 Κ and at 873 K, respectively, were studied by time-of-flight neutron powder diffraction, by infrared and by magic angle spinning nuclear magnetic resonance * Permanent address: Mineralogisches Institut der Universität Würzburg, Am Hubland, D-8700 Würzburg, Bundesrepublik Deutschland. ** Contribution no. 4110. 282 W. Η. Baur et al. spectroscopy. The chemical composition of the starting material was (NH4)10.2Cs0.2Al10.4Si37.6O96 · 43H20. After calcination, deuteration and dehydration the framework composition, as determined by NMR, IR, microcalorimetry and neutron powder diffraction, was approximately D9Al9Si39096 (D-RHO-S773 and D-RHO-S873) and 95% and 93% respectively, of the sample was accessible to methanol. About 2 to 4 AI atoms per unit cell were removed from the framework by the calcination. Both samples crystallize in the centrosymmetric space group Imbm at room temperature and at 623 Κ with cell constants a around 15.08 A. One of the samples shows no thermal expansion; the other has a negative thermal expansion coefficient. After correcting the powder patterns for irregular background effects, 250 Bragg peaks were utilized in the refinement. This is more than was previously possible in powder refinements of a zeolite RHO. We found the resulting geometry of the (Si,Al)02 framework to be more regular than in previous crystal structure refinements of zeolite RHO. Nonframework aluminum species could not be located in either sample at either temperature. In D-RHO-S 873 evidence for a deuterium atom bonded to oxygen atom O(l) was found. In zeolites H-RHO and D-RHO crystallizing in space group Imbm the cell constant is strictly dependent on the Τ—Ο bond lengths (where Τ is Si, Al), because of symmetry restrictions and because the angles Τ — Ο — Τ vary only slightly. Therefore the amount of aluminum per unit cell c(Al), in these zeolites can be determined from the known size of the unit cell length a by the relation c(Al) = [36.47a -542.3],

Neutron powder diffraction study and physical characterization of zeolite D-ZK—5 deep-bed calcined at 500°C and 650°C

Abstract Zeolite ZK—5 samples, deep-bed calcined at 500°C and at 650°C, respectively, were studied by neutron powder diffraction, i.r. and n.m.r. In addition their methanol sorption was measured. The chemical composition of the starting material was (NH4)xCs4.5K0.003Al22.5Si73.5 o192. After calcination at 500°C and deuteration the composition was D8.5Cs4.5Al13Si83O192 (DZK500); 75% of the pore space was accessible to methanol. Sample DZK500 crystallizes in space group lm 3m , a = 18.633 A . The sample calcined at 650°C and subsequently deuterated (DZK650) had the composition D3.5Cs4.5Al8Si88O192, and only 44% of its pore space was accessible to methanol. Sample DZK650 crystallizes in space group lm 3m , a = 18.440 A . l.r. and n.m.r. studies show that 9.5–20.2 Al/u.c. in DZK500 and 14.5–19.2 Al/u.c. in DZK650 are removed from the framework. The neutron diffraction study shows Cs atoms located in eight rings and shows a peak in the difference Fourier map at 0, ∼0.45, and ∼0.07 in the γ cage which has been attributed to non-framework Al O atoms. The discrepancy between the number of Al O in the γ cage and that found by Al and Si n.m.r. is attributed to the presence of non-framework Al O in other sites or randomly distributed condensed AlOOH clusters.

Characterization of the sorption of mono-, di-, and trimethylamine in zeolite H-rho by deuterium n.m.r.

The sorption complexes of monomethylamine (MMA), dimethylamine (DMA), and trimethylamine (TMA) in H-rho are studied by means of deuterium n.m.r. of the methyl deuterated compounds over the temperature range −180 to +150 °C. The uptake rate of these sorbates from the gas phase at room temperature decreases strongly with the molecular weight and is negligible for TMA. Sorption is, however, enhanced by increasing the temperature and pressure. In MMA, strong chemisorption hinders the diffusion of the sorbate through the cavities, and thermal activation is required to reach a homogeneous distribution. In all cases, a wide dispersity of adsorbed species is observed, coarsely divided into chemisorbed species, which are assumed to be associated with the Bronsted acid sites, and weakly bound physisorbed species. The n.m.r. spectra show that at low loadings and below room temperature the molecules are predominantly chemisorbed while local motions take place in DMA (π-flips) and in TMA (threefold rotations). At high temperatures, the molecules begin to diffuse in the channels. When the number of molecules exceeds that of the available acid sites (12 per u.c.), most of the sorbate appears to be in a physisorbed state. Magic angle spinning (MAS) n.m.r. measurements of the 27 Al, 29 Si, and 13 C nuclei in some of the samples used for the deuterium measurements are also reported. These measurements indicate that structural changes in the zeolite framework occur upon sorption of the methylamine sorbates, analogous to those observed in water and methanol sorbed H-rho.

Studies on Organoaluminum Precursors OP Aluminum Nitride Fibers

A melt-spinnable precursor of aluminum nitride fibers derived from triethylaluminum and ammonia contains AlNH, AlNH 2 groups, and a small number of AlN units characteristic of aluminum nitride. The molecular weight of a spinnable composition is 070, corresponding to an average molecular weight of 13 organoaluminum groups. Ammonia, a curing agent for the fibers, accelerates elimination of ethane from the material, and decreases its solubility in toluene.