D.E Woessner

A study of molecular re-orientation in liquid acetonitrile by nuclear spin-lattice relaxation

Studies of the nitrogen, deuteron and proton relaxation rates of liquid acetonitrile at various temperatures were carried out by pulsed N.M.R. techniques. Re-orientation of the methyl group about its symmetry axis is found to be approximately ten times faster than the re-orientation of the nitrile group. Analysis of the data provides an example of a large room-temperature contribution to the intramolecular relaxation component by a spin-rotation interaction. Previously sizeable spin-rotation interactions were considered to be insignificant for organic molecules at room temperature.

An NMR investigation into the range of the surface effect on the rotation of water molecules

Abstract It is well known that the properties of water in the vicinity of a surface or interface are different from those of pure bulk water. An important question is the thickness of the water layer with properties different from bulk water, i.e., the range of the effect. Different physical properties might have different ranges. The NMR relaxation time T 1 reflects the molecular motions in the megahertz frequency range. The deuteron T 1 of D 2 O depends only on the molecular rotational properties, whereas the proton T 1 of H 2 O depends on both the translational and rotational properties. In order to determine the range of the surface effect on the rotational properties of water, the room temperature deuteron T 1 was measured as a function of water layer thickness for a carefully prepared series of hectorite-D 2 O samples. This clay was chosen because it has very low concentration of paramagnetic impurities which can complicate the experiment. The results are in complete agreement with the simple picture that at room temperature the hectorite surface influences and slows down the rotation of only the first layer or two of water molecules.

Spin-echo study of 23Na relaxation in skeletal muscle. Evidence of sodium ion binding inside a biological cell

Abstract The physicochemical state of the sodium ions in the cellular environment is studied by a pulsed NMR method. Both the spin-lattice relaxation time ( T 1 ) and the spin-spin relaxation time ( T 2 ) of 23 Na in skeletal muscle were measured with the spin-echo technique. Our data indicate that the T 1 curve follows almost a single exponential decay, while the T 2 curve is complex and can be fitted with a double exponential function. By extending the use of Hubbards theory, we have analyzed the quadrupole relaxation of the 23 Na nuclei and have found numerical evidence that most of the sodium ions are associated with the macromolecular charged sites.

Nuclear magnetic relaxation and structure in aqueous heterogenous systems

The relaxation of water in aqueous heterogeneous systems is described in terms of the effects of the substrate structure and order on the motions and arrangements of the water molecules. The genera...

Proton and deuteron spin-lattice relaxation in n-dodecane

Abstract The spin-lattice relaxation rates of protonated and deuterated n-dodecane are measured neat and in various solutions of benzene, carbon tetrachloride and carbon disulfide. An isotopic dilution technique together with an automatic data acquisition system provide a means for evaluating the inter- and intramolecular relaxation of protonated n-dodecane. The observed molecular rotational motions are to a first approximation consistent with the Stokes-Einstein hydrodynamic model corrected for the particulate nature of the solvent by a microviscosity coefficient.

Proton exchange effects on pulsed nmr signals from preferentially oriented water molecules

Abstract Preferentially oriented water molecules can give rise to doublet NMR spectra. The doublet splitting can be used as a probe to study proton exchange between H 2 O molecules. Theoretical calculations of pulsed NMR signals from powder samples of similarly oriented pairs of rigid water molecules undergoing proton exchange have been made. Under suitable conditions, these calculations can be applied to preferentially oriented water. Several different methods of determining the doublet splitting constant and the proton exchange lifetime from pulsed NMR signals are given. These methods are applied to the pulsed NMR signals from water in various cation-exchanged forms of chabazite and montmorillonoid clays. The proton exchange lifetimes in several cation-exchanged chabazites decrease in the order Ca 2+ ≈ Na + > K+ > Li + . The temperature dependences are consistent with an Arrhenius equation with an apparent activation energy 11.4 kcal/mole. The proton exchange lifetime for water in the clays depends on the clay particle size as well as on the exchanged cation. The larger particle size samples exhibit the larger exchange lifetimes.

Observation of electron-coupled nuclear spin-spin interactions in the silicon-29 MAS NMR spectrum of low albite

Silicon-29 MAS NMR spectra at a wide range of magnetic fields are presented for a well characterized feldspar, low albite. The splitting of silicon-29 resonances in these spectra is independent of magnetic field and is thus attributed to electron-coupled silicon-29 nuclear spin-spin coupling with aluminum-27. To our knowledge, this work represents the first time such a splitting has been observed for silicon-29 in the solid state. In a continuing effort to obtain high-resolution silicon-29 MAS NMR spectra of minerals and zeolites (I, 2), the spectrum of an extensively studied feldspar, low albite, was measured. Because the structure of this mineral is well characterized, a number of researchers (I, 3,4) have included albite in the development of correlations between structure and 29Si chemical shift. In addition, a recent neutron diffraction study (5) on this end member of the plagioclase feldspar group has demonstrated unequivocally that the aluminum present in low albite is highly ordered and resides almost exclusively in the T1O tetrahedral site. Thus, this material offers a well-defined system to investigate the effect of aluminum ordering on the silicon-29 MAS NMR spectrum. A sample of low albite from Amelia County, Virginia, was provided by Dr. Carl A. Francis of Harvard University. Silicon-29 MAS NMR spectra were measured at 39.6, 53.6, and 99.4 MHz on JEOL FX-200, JEOL FX-270 and Bruker AM-500 Fourier transform NMR spectrometers, respectively. Silicon-29 spin-lattice relaxation times were estimated to be between 100 and 150 s at 53.6 MHz by the T-null, inversion-recovery method. Spectra were typically measured by accumulating approximately 1000-2500 FIDs using a 30-90“ flip angle and a 30150 s recovery time. The spectra are referenced to tetramethylsilane (TMS) using solid hexamethylcyclotrisiloxane (HMCTS) as a secondary reference. The chemical shift of solid HMCTS with respect to a 30 ~01% solution of TMS in deuterochloroform was found to be -10.0 ppm.