Accurate and absolute diffusion measurements of Rhodamine 6G in low-concentration aqueous solutions by the PGSE-WATERGATE sequence.

A pulsed field gradient spin-echo nuclear magnetic resonance (NMR) sequence with solvent suppression (PGSE-WATERGATE) was applied to accurately measure the diffusion coefficients of Rhodamine 6G (Rh6G) in low-concentration aqueous solutions. Three samples with Rh6G concentrations of CRh6G = 1, 4.5, and 25 µM were investigated. The precise determination of the diffusion coefficients in this low-concentration range was made possible by using a cryogenically cooled NMR probe and by the effective solvent suppression of the PGSE-WATERGATE sequence. The present results bridge the gap between diffusion data measured by fluorescence correlation spectroscopy in the single molecule limit and diffusivities obtained by pulsed field gradient NMR (PFG-NMR) without solvent suppression at higher concentrations. To further extend the concentration range, the diffusion coefficient of Rh6G was also measured on a sample with CRh6G = 410 µM by PFG-NMR. The overall concentration dependence of the Rh6G diffusion at 25 °C is discussed in terms of dimerization of the Rh6G molecules. The concentration-dependent monomer/dimer proportion is deduced from the diffusion data.

Characterization of the fluorescence correlation spectroscopy (FCS) standard rhodamine 6G and calibration of its diffusion coefficient in aqueous solutions.

Precise diffusion measurements of rhodamine 6G (Rh6G) dissolved in D2O at concentrations between 50 and 200 µM were carried out in the temperature range from 280 to 320 K using pulsed field gradient nuclear magnetic resonance (PFG-NMR). The obtained diffusion coefficients can be used as a calibration reference in fluorescence correlation spectroscopy (FCS). Besides measuring the diffusivity of Rh6G, the diffusion coefficient of the solvent in the same system could be determined in parallel by PFG-NMR as the resonances of water and Rh6G are well separated in the (1)H NMR spectrum. To analyze the differences due to the isotope effect of the solvent (D2O vs. H2O), the correlation time τD of Rh6G was measured by FCS in both D2O and H2O. The obtained isotopic correction factor, τD(D2O)/τD(H2O) = 1.24, reflects the isotope effect of the solvent´s self-diffusion coefficients as determined previously by PFG-NMR.

WITHDRAWN: Erratum to "Proton NMR Studies of the NaAlH4 Structure" [J. Magn. Reson. 200 (2009) 280-284].

The Publisher regrets that this article is an accidental duplication of an article that has already been published, doi:10.1016/j.jmr.2010.03.017. The duplicate article has therefore been withdrawn.

Proton NMR studies of the NaAlH4 structure.

Advanced nuclear magnetic resonance (NMR) techniques were applied to study the local environments of hydrogen in NaAlH(4). Through a combined application of the magic echo (ME) and the magic Hahn echo (MHE) sequences the hetero- and homonuclear contributions to the dipolar second moment (M(2)) were determined separately. The obtained values are compared with the second moments calculated by the van Vleck formulae, using structural data determined by neutron scattering on NaAlD(4). This comparison indicates structural differences between NaAlH(4) and NaAlD(4). A model is suggested for the orientation of the [AlH(4)](-) tetrahedra in NaAlH(4), for which the calculated second moments are in good agreement with the experimentally observed values.