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Subject: search.filters.subject.530Institute: search.filters.UBSInstitut.Sonstige EinrichtungAuthor: search.filters.author.Schweitzer, DieterAuthor: search.filters.author.Spiess, Hans WolfgangStart date: 1970End date: 1979

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Now showing 1 - 7 of 7
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    Nuclear magnetic shielding tensors for 1H, 13C, and 15N in organic solids
    (1975) Spiess, Hans Wolfgang; Haeberlen, Ulrich; Kempf, Jürgen; Schweitzer, Dieter
    A survey will be given of nuclear magnetic shielding tensors obtained by multiple pulse techniques for 1H in carboxylic acids and by high field NMR for 13C in carbonyl- and carboxyl groups of aromatic compounds and for 15N in pyridine and nitrobenzene.
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    Nitrogen-15 NMR of pyridine in high magnetic fields
    (1974) Schweitzer, Dieter; Spiess, Hans Wolfgang
    The 15N NMR of pyridine was studied in the liquid and solid state. In the liquid the spin-lattice relaxation time T1 was studied from −60 to +55°C at 14 and 30 MHz. At low temperature, the two most important relaxation mechanisms are relaxation due to anisotropic chemical shift and intermolecular dipole-dipole interaction. The small contribution of intermolecular dipole-dipole interaction of 15N with protons was determined by making use of correlation times τc obtained from 13C relaxation rates, which were also measured. At higher temperatures, relaxation by spin-rotation interaction becomes important. Analysis of the relaxation data shows that the anisotropy of the motion in liquid pyridine is rather small. The principal elements of the chemical shift tensor were obtained from powder spectra at −105°C by FT NMR: σxx = −313 ± 10 ppm, σyy = −94 ± 10 ppm, σzz =+469 ± 10 ppm relative to liquid pyridine. From these values the following spin-rotation components were calculated: Cxx = 16.5 ± 1 kHz, Cyy = 11.5 ± 1 kHz, Czz = −0.6 ± 1 kHz. In both cases the z-axis is perpendicular to the molecular plane.
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    Spin rotation interaction and anisotropic chemical shift in 13CS2
    (1971) Spiess, Hans Wolfgang; Schweitzer, Dieter; Haeberlen, Ulrich; Hausser, Karl H.
    The 13C nuclear spin-lattice relaxation time T1 was studied in liquid CS2 from -106°C to +35°C at resonance frequencies of 14, 30, and 62 MHz. The relaxation is caused by anisotropic chemical shift and spin-rotation interaction. It is shown that for linear molecules the spin-rotation constant C and the anisotropy of the chemical shift Δσ can be obtained from the relaxation rates without use of adjustable parameters. The analysis yields: C = -13.8 ± 1.4 kHz and Δσ = 438 ± 44 ppm.
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    Molecular motion in liquid toluene from a study of 13C and 2D relaxation times
    (1973) Spiess, Hans Wolfgang; Schweitzer, Dieter; Haeberlen, Ulrich
    The 13C nuclear spin-lattice relaxation times for ring and methyl carbons in liquid toluene were studied from −95°C to +60°C at frequencies of 14 and 61 MHz. Data were taken for protonated as well as deuterated toluene. The results were analyzed in terms of three relaxation mechanisms: intramolecular dipole-dipole coupling, spin-rotation interaction, and anisotropic chemical shift. The last mechanism gives a significant contribution only to the relaxation rate of the ring carbons of the deuterated species at 61 MHz and low temperatures. A tentative value of Δσ = 295 ppm is obtained in this case. In order to separate the contributions of the dipole-dipole and spin-rotation interactions the 13C data are compared with deuteron relaxation times. Comparison of the 13C data in the protonated and deuterated form of toluene shows that the correlation times for the ring differ by 20% and an even larger effect of isotopic substitution is found for the methyl group. It is demonstrated that the fast internal motion of the methyl group cannot be studied quantitatively using deuteron or 13C intramolecular dipole-dipole relaxation rates alone because of the sensitivity of the results to the angle, varpi, the Z-axis of the electric field gradient, or the internuclear vector, respectively, forms with the C3 axis. Analysis of the relaxation rates due to spin-rotation interaction yields τj (internal), the correlation time of angular momentum of the internal motion directly. The correlation time of reorientation τc (internal) is calculated from τj (internal) using Gordon's extended diffusion model which is applied to a symmetric rotor with a fixed axis. It is found that both τj (internal) and τc (internal) are of the same magnitude as the correlation time of the free rotor. The ratio of correlation times of the overall and internal reorientation ranges from approximately 200 at the melting point to approximately 13 at +60°.
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    Spin echo experiments on 13C, 2H, 1H, and 19F in some small molecules in the liquid phase
    (1972) Haeberlen, Ulrich; Spiess, Hans Wolfgang; Schweitzer, Dieter
    The dependence on the π pulse repetition rate (2τ)−1 of the Carr-Purcell (CP) spin echo decay constant R is studied for four nuclei in C6H6, C6D6, C6F6, C6H12, C6H5CH3, CH3I, H2O, D2O, and CS2. Both deuteron resonances, the proton resonances of CH3I, of extremely pure H2O and C6H6 and the 13C resonance of CS2 yield straight lines when R is plotted vs. (2τ)2, i.e., R is governed by transverse relaxation and diffusion. However, in some unexpected cases, T2 is found to be smaller than T1. The H and F resonances of C6H6, C6F6, and H2O with traces of impurities do not give straight-line plots of R vs. (2τ)2. An oscillatory dependence of R on the pulse repetition rate is found for the 13C resonances of C6H6 and C6H5CH3. It can be shown to be due to the J coupling of the 13C spins to the directly bonded protons. The theory developed for exchange of chemically shifted spins can be applied and is extended for the slow exchange limit to an AX3 system in an effort to explain the results of methyl 13C quantitatively. Because of the sensitivity of CP measurements on instrumental effects a detailed description is given of the measurement procedures and of the equipment of which a superconducting solenoid is an essential part. A connection between Carr-Purcell measurements, of the Gill-Meiboom version, and spin-locking experiments is pointed out.
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    Anisotropic chemical shifts and spin rotation constants of 15N from liquid and solid state NMR: Nitrobenzene
    (1974) Schweitzer, Dieter; Spiess, Hans Wolfgang
    The study of the 15N spin-lattice relaxation time T1 in nitrobenzene at 14 and 32 MHz from −10 to 60°C showed that at 32 MHz relaxation due to anisotropic chemical shift is predominant. At low frequencies, the relaxation is caused mainly by spin rotation and at low temperature also by intermolecular dipole-dipole interaction. From the powder spectrum in solid nitrobenzene, the principal elements of the shielding tensor σ were obtained: σxx = −273 ± 10 ppm, σyy = +94 ± 10 ppm, and σzz = +156 ± 10 ppm relative to liquid nitrobenzene, Δσ = σ xx - ½(σ yy + σ zz) = -398 ± 20ppm. From the almost axially symmetric σ -tensor, the spin rotation constants were calculated: C‖ = 11.4 ± 1.5 kHz and C⊥ = 1.35 ± 0.5 kHz, where C‖ is the component parallel to the twofold axis of the molecule. These values for Δσ and the spin rotation constants are in excellent agreement with those obtained by analysis of the relaxation data. A comparison of anisotropic chemical shifts and spin rotation constants for 15N and 13C in isoelectronic compounds is given.
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    15N and 13C spin lattice relaxation in neat liquids at high magnetic fields
    (1975) Schweitzer, Dieter; Spiess, Hans Wolfgang
    General features of 15N spin lattice relaxation in nitrobenzene, pyridine and triethanolamine are discussed and compared with l3C relaxation mechanisms.