The interior water pool of aerosol OT (AOT) reverse micelles tends toward bulk water properties as the micelle size increases. Thus, deviations from bulk water behavior in large reverse micelles are less expected than in small reverse micelles. Probing the interior water pool of AOT reverse micelles with a highly charged decavanadate (V(10)) oligomer using (51)V NMR spectroscopy shows distinct changes in solute environment. For example, when an acidic stock solution of protonated V(10) is placed in a reverse micelle, the (51)V chemical shifts show that the V(10) is deprotonated consistent with a decreased proton concentration in the intramicellar water pool. Results indicate that a proton gradient exists inside the reverse micelles, leaving the interior neutral while the interfacial region is acidic.

Baruah B, Gabriel GJ, Akbashev MJ, Booher ME. 2013. Facile synthesis of silver nanoparticles stabilized by cationic polynorbornenes and their catalytic activity in 4-nitrophenol reduction. Langmuir 29(13):4225-34. ABSTRACT: We report the facile one-pot single-phase syntheses of silver nanoparticles stabilized by norbornene type cationic polymers. Silver nanoparticles (AgNPs) stabilized by polyguanidino oxanorbornenes (PG) at 5 and 25 kDa and polyamino oxanorbornenes (PA) at 3 and 15 kDa have been synthesized by the reduction of silver ions with NaBH 4 in aqueous solutions at ambient temperature. The four different silver nanoparticles have been characterized by UV−vis spectroscopy, Fourier transform infrared spectroscopy (FTIR), dynamic light scattering (DLS), and transmission electron microscopy (TEM) for their particle size distributions. Interestingly, PG stabilizes the silver nanoparticles better than PA as evident from our spectroscopic data. Furthermore, the AgNP-PG-5K (5K = 5 kDa) was found to serve as an effective catalyst for the reduction of 4-nitrophenol to 4-aminophenol in the presence of NaBH 4 . The reduction has a pseudo-first-order rate constant of 5.50 × 10 −3 s −1 and an activity parameter of 1375 s −1 g −1 , which is significantly higher than other systems reported in the literature.

The location and interactions of solutes in microheterogeneous environments, such as reverse micelles, critically influence understanding of many phenomena that utilize probe molecules to characterize properties in chemical, biological, and physical systems. The information gained in such studies depends substantially on the location of the probe used. Often, intuition leads to the assumption that ionic probe molecules reside in the polar water pool of a system. In this work, the location of a charged polar transition metal coordination complex in a reverse micellar system is determined using NMR spectroscopy. Despite the expected Coulomb repulsion between the surfactant headgroups and the negatively charged complex, the complex spends significant time penetrating into the hydrophobic portion of the reverse micellar interface. These results challenge the assumption that ionic probe molecules reside solvated by water in microheterogeneous environments and suggest that probe molecule location be carefully considered before interpreting data from similar systems.

Simple procedures and characterization of a series of well-defined precursors are described for preparation of a unique microenvironment in nanoreactors, reverse micelles. The Na(+), K(+), Rb(+), Cs(+), and Mg(2+) surfactants were prepared using liquid-liquid ion exchange using chloride and nitrate salts. The surfactants were characterized using (1)H NMR spectroscopy and a variety of other techniques. (1)H NMR spectroscopy was found to be a sensitive probe for characterization of the size of the nanoreactor as well as its water content. (1)H NMR spectra can be used for detailed characterization of reactions in confined environments when counterion effects are likely to be important. (1)H NMR spectroscopy revealed two separate peaks corresponding to water in Mg(AOT)2 samples; one peak arises from water coordinated to the Mg(2+) ion while the other peak arises from bulk water. The two water signals arise directly from the slow exchange of the water coordinated to Mg(2+) in these microemulsions with water in the water pool, and provide an opportunity to study hydration of Mg(2+). This work thus extends the potential use of MAOT microemulsions for applications such as in green chemistry.

The possibility that a negatively charged organic molecule penetrates the lipid interface in a reverse micellar system is examined using UV-vis absorption and NMR spectroscopy. The hypothesis that deprotonated forms of dipicolinic acid, H(2)dipic, such as Hdipic(-) and dipic(2-), can penetrate the lipid interface in a microemulsion is based on our previous finding that the insulin-enhancing anionic [VO(2)dipic](-) complex was found to reside in the hydrophobic layer of the reverse micelle (Crans et al. J. Am. Chem. Soc. 2006, 128, 4437-4445). Penetration of a polar and charged compound, namely Hdipic(-) or dipic(2-), into a hydrophobic environment is perhaps unexpected given the established rules regarding the fundamental properties of compound solubility. As such, this work has broad implications in organic chemistry and other disciplines of science. These studies required a comprehensive investigation of the different dipic species and their association in aqueous solutions at varying pH values. Combining the aqueous studies using absorption and NMR spectroscopy with those in microemulsions defines the differences observed in the heterogeneous environment. Despite the expected repulsion between the surfactant head groups and the dianionic probe molecule, these studies demonstrate that dipic resides deep in the hydrophobic portion of the reverse micellar interface. In summary, these results provide evidence that ionic molecules can reside in nonpolar locations in microheterogeneous environments. This suggests that additional factors such as solvation are important to molecule location. Documented ability to penetrate lipid surfaces of similar charge provides a rationale for why specific drugs with less than optimal hydrophobicity are successful even though they violate Lipinski's rules.

Using 51 V magic angle spinning solid-state NMR, SSNMR, spectroscopy and quantum chemical DFT calculations we have characterized the chemical shift and quadrupolar coupling parameters of a series of 8 hydroxylamido vanadium(V) dipicolinate complexes of the general formula VO(dipic)(ONR 1 R 2 )(H 2 O) where R 1 and R 2 can be H, CH 3 , or CH 2 CH 3 . This class of vanadium compounds was chosen for investigation because of their seven coordinate vanadium atom, a geometry for which there is limited 51 V SSNMR data. Furthermore, a systematic series of compounds with different electronic properties are available and allows for the effects of ligand substitution on the NMR parameters to be studied. The quadrupolar coupling constants, C Q , are small, 3.0 to 3.9 MHz, but exhibit variations as a function of the ligand substitution. The chemical shift tensors in the solid state are sensitive to changes in both the hydroxylamide substituent and the dipic ligand, a sensitivity which is not observed for isotropic chemical shifts in solution. The chemical shift tensors span approximately 1000 ppm, and are nearly axially symmetric. Based on DFT calculations of the chemical shift tensors, one of the largest contributors to the magnetic shielding anisotropy is an occupied molecular orbital with significant vanadium d z 2 character along the V=O bond.

Microemulsions form in mixtures of polar, nonpolar, and amphiphilic molecules. Typical microemulsions employ water as the polar phase. However, microemulsions can form with a polar phase other than water, which hold promise to diversify the range of properties, and hence utility, of microemulsions. Here microemulsions formed by using a room-temperature ionic liquid (RTIL) as the polar phase were created and characterized by using multinuclear NMR spectroscopy. (1)H, (11)B, and (19)F NMR spectroscopy was applied to explore differences between microemulsions formed by using 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF(4)]) as the polar phase with a cationic surfactant, benzylhexadecyldimethylammonium chloride (BHDC), and a nonionic surfactant, Triton X-100 (TX-100). NMR spectroscopy showed distinct differences in the behavior of the RTIL as the charge of the surfactant head group varies in the different microemulsion environments. Minor changes in the chemical shifts were observed for [bmim](+) and [BF(4)](-) in the presence of TX-100 suggesting that the surfactant and the ionic liquid are separated in the microemulsion. The large changes in spectroscopic parameters observed are consistent with microstructure formation with layering of [bmim](+) and [BF(4)](-) and migration of Cl(-) within the BHDC microemulsions. Comparisons with NMR results for related ionic compounds in organic and aqueous environments as well as literature studies assisted the development of a simple organizational model for these microstructures.

No abstract