Search

The article presents a science experiment to help college students measure the solution viscosity via diffusion-ordered nuclear magnetic resonance (NMR) spectroscopy. In this experiment, students explore the intermolecular forces including viscosity through numerous methods. Emphasis is on the investigation of the viscosity, a physical property based on molecular structure, using advanced NMR techniques.

The dimeric structure is characterized for a chiral amide base complex consisting of an (S)-N-isopropyl-O-triisopropylsilyl valinol ligand and lithium. The complex is characterized by a variety of NMR techniques, including multinuclear one- and two-dimensional NMR experiments and diffusion ordered NMR spectroscopy (DOSY) as well as diffusion coefficient-formula weight (D-fw) correlation analyses. Spartan calculations are presented which support the structural assignment. This structural characterization leads to an explanation of the behavior and the reactivity of these complexes in solution. [ABSTRACT FROM AUTHOR]

The tetrahydrofuran tetrasolvated dimeric lithium dienolate derived from 2,2,7,7-tetramethyloctan-3,6-dione is characterized in the solid state by X-ray diffraction analysis and in solution by diffusion NMR. This dienolate was reacted with tropanone to yield two new products that are also described. [ABSTRACT FROM AUTHOR]

The utility of recently introduced 6Li diffusion-ordered NMR spectroscopy (DOSY) is demonstrated. 6Li DOSY results can be correlated to 1H data through 2-D 6Li{1H} heteronuclear Overhauser effect NMR spectroscopy (HOESY) experiments. 6Li DOSY quickly confirms 1H DOSY results and allows unambiguous assignment of resonances to specific aggregates. Well known aggregates of lithium-6 n-butyllithium (n-Bu6Li) are examined in deuterated tetrahydrofuran (THF-d8) solution as an example, and to reaffirm the previous literature conclusions about these complexes. [ABSTRACT FROM AUTHOR]

Formula weight (FW) information is important to characterize the composition, aggregation number, and solvation state of reactive intermediates and organometallic complexes. We describe an internal reference correlated DOSY method for calculating the FW of unknown species in different solvents with different concentrations. Examples for both the small molecule (DIPA) and the organometallic complex (aggregate 1) yield excellent correlations. We also found the relative diffusion rate is inversely proportional to the viscosity change of the solution, which is consistent with the theoretical Stokes-Einstein equation. The accuracy of the least-squares linear prediction r2 and the percentage difference of FW prediction are directly related to the density change; greater accuracy was observed with decreasing density. We also discuss the guidelines and other factors for successful application of this internal reference correlated DOSY method. This practical method can be conveniently modified and applied to the characterization of other unknown molecules or complexes. [ABSTRACT FROM AUTHOR]

Article A metastable, polymorphic hexameric crystal structure of lithium pinacolone enolate (LiOPin) is reported along with three preparation methods. NMR-based structural characterization implies that the lithium pinacolate hexamer deaggregates to a tetramer in toluene but retains mainly the hexameric structure in nonaromatic hydrocarbon solvents such as cyclohexane. Moreover, the presence of a small amount of lithium aldolate (LiOA) dramatically influences the aggregation state of LiOPin by forming a mixed aggregate with a 3:1 ratio (LiOPin3⋅LiOA). [ABSTRACT FROM AUTHOR]

This work deals with synthesis and characterization of polyaniline and titanium dioxide nanocomposites for explosive detection and mitigation. The titanium dioxide nanotube array was prepared through electrochemical oxidation of pure titanium in a fluorine ion-containing ethylene glycol water solution followed by annealing at 450 °C in air. Polyaniline was obtained by electrochemical polymerization from an aniline and sulfuric acid solution. Both polyaniline and the nanotube show sorption of 2,4,6-trinitrotoluene (TNT) vapor at 60 °C. Polyaniline modified by alginic acid sodium salt caused color change of TNT solutions. Polyaniline-based sensor showed decrease of electrical resistance in TNT acetonitrile solutions. Ultraviolet light response tests revealed that TNT caused significant drop in open circuit voltage of the titanium dioxide nanotube. In addition, the polyaniline/titanium oxide nanocomposites show colorimetric responses in the explosive solution, which makes them have multiple response mechanisms for nitro-aromatic explosive detection and mitigation. [ABSTRACT FROM AUTHOR]