Ulrich Haeberlen

The dynamics of hydrogens in double well potentials : the transition of the jump rate from the low temperature quantum-mechanical to the high temperature activated regime

The temperature dependence of the deuteron spin lattice relaxation time T1 in four carboxylic acids, which were deuterated in the carboxyl groups, is presented. These data allow determination of the rate Γ of the hydrogen transfer between the two minima of the double well potential in which the hydrogens move along the hydrogen bonds. The temperature dependence of Γ has clearly distinct low temperature quantum‐mechanical and high temperature thermally activated regimes. Evidence is presented that the transfer of hydrogens in dimers of carboxylic acids is strongly influenced by a high‐frequency mode which is strongly coupled to the hydrogens. This supports the conclusions recently put forward by the ETH group [J. Chem. Phys. 93, 1502 (1990)]. A model based on the spin–Boson–Hamiltonian with a bath consisting of acoustic phonons plus a unique high frequency mode explains fully the jump rate in the quantum mechanical regime of all four compounds studied. The way in which the unique bath mode influences the h...

A stable superconducting state at 8 K and ambient pressure in\(\alpha _t - (BEDT - TTF^ \star )_2 I_3 ^1 \)

We report bulk superconductivity at 8 K and ambient pressure in crystals of αt(BEDT-TTF)2I3. In contrast to the earlier observed metastable superconducting state at 8 K in crystals of β-(BEDT-TTF)2I3 here the superconducting state is stable and the crystals can be prepared by tempering α-(BEDT-TTF)2I3 above 70 °C for several days. ac-susceptibility measurements show that the observed superconducting state at 8 K is a bulk property of the crystals. Resistivity measurements indicate a sharp superconducting transition at 8 K with an onset temperature of about 9 K. The upper critical fields Hc2 at 1.3 K lie between 3 and 11 T depending on the direction of the magnetic field with respect to the crystal axes. ESR- as well as NMR-measurements indicate a total transformation of the α-phase crystals into the new superconducting αt-crystals after tempering.

Spin rotation interaction and anisotropic chemical shift in 13CS2

Abstract 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.

Molecular motion in liquid toluene from a study of 13C and 2D relaxation times

Abstract 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, ϖ, 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 Gordons 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°.

A new method for suppressing baseline distortions in FT NMR

Abstract A new method called BCF (baseline cosine fitting) which drastically reduces the distortions of the Fourier absorption spectrum arising as a consequence of the spectrometer dead time Tdead is described. It is able to reconstruct reliably the unknown segment from t = 0 to t = Tdead of the digitized FID from the initially distorted spectrum. Within the procedure a new method for baseline recognition is presented. BCF works with only three parameters, default values of which can handle most cases fully automatically. The capability of MEM and LPSVD to overcome the dead-time problem is investigated and compared with BCF. The range of applicability of all three methods is explored.

Improved measurement of the positive muon anomalous magnetic moment

A new measurement of the positive muons anomalous magnetic moment has been made at the Brookhaven Alternating Gradient Synchrotron using the direct injection of polarized muons into the superferric storage ring. The angular frequency difference omega (a) between the angular spin precession frequency omega (s) and the angular orbital frequency omega (c) is measured as well as the free proton MMR frequency omega (p). These determine R = omega (a)/omega (p) = 3.707 201(19) x 10(-3). With mu (mu)/mu (p) = 3.183 345 39(10) this gives a(mu+) = 11 659 191(59) x 10-(10) (+/-5 ppm), in good agreement with the previous CERN and BNL measurements for mu (+) and mu (-), and with the standard model prediction.

Design and construction of a high homogeneity rf coil for solid-state multiple-pulse NMR

Abstract A prescription is given for construction of a coil having an optimal rf homogeneity desired, especially, in multiple-pulse solid-state NMR. The approach is to use a coil with variable pitch. The field in a helical coil and in the coil with variable pitch are compared. The field distribution across the sample and some experimental aspects associated with the nonuniformity of the rf field are also discussed.

A high precision magnetometer based on pulsed NMR

Abstract A magnetometer based on pulsed proton magnetic resonance has been developed and constructed. The system will be employed for an accurate measurement of the absolute magnetic field in the region of 1.45 T in a precision experiment on the muons anomalous magnetic moment at the Brookhaven National Laboratory (BNL, USA), where a knowledge of the magnetic field is required with 1 × 10−7 relative accuracy. The performance of the magnetometer has been tested in a large bore superconducting magnet and a precision of one part in 108 was achieved.

1H Multiple−Pulse Study of a Single Crystal of trans−Diiodoethylene: Example of Self − Decoupling

Abstract The proton NMR in single-crystal spheres of trans-diiodoethylene has been studied by multiple-pulse techniques at room temperature. In certain orientations the linewidth was found to be as low as 115 Hz despite the presence of the abundant 1211 Spins giving rise to strong dipole-dipole interactions with the protons. If these interactions were static, the resulting linewidths in the 1H multiple-pulse spectra would be of the order of 10 kHz. The narrow lines observed show that the quadrupolar relaxation of the 127I spins effectively reduces these dipolar interactions resulting in self-decoupling. The self-decoupling efficiency, however, shows a marked angular dependence, as evidenced by the angular variation of the proton linewidth, which has a maximum value of 410 Hz. To understand this, the dipolar coupling between a spin I= 1 2 and a spin S= 5 2 experiencing a strong quadrupole interaction, as is the case for 1271 in trans-diiodoethylene, is discussed in detail. It is shown that the angular dependence of the self-decoupling is not due to differences in the static 1H-127I dipolar coupling, but reflects the angular variation of the 127I relaxation rates. The data were analyzed to yield the proton shielding tensor σ in trans-diiodoethylene : σxx = −4.1, σyy = −5.9, σzz = −8.6 ppm relative to TMS. The least-shielded direction, the z axis, is perpendicular to the molecular plane, and the x and y axes both form an angle of 45° with the CC double bond.

13C Anisotropic chemical shift in a single crystal of benzophenone

Abstract The 13 C resonance of the carbonyl carbon in a single crystal of benzophenone was studied by Fourier transform NMR at room temperature. Rotation patterns about the three cyrstallographic axes yielded the orientations of the major axis systems of the chemical shift tensor σ relative to the crystallographic axes for the four molecules in the unit cell. The principal elements of σ were found to be: σ xx = −79 ± 4, σ yy = −36 ± 4, and σ zz = +94 ± 4 ppm , relative to CS 2 . The Z axis of the nearly axially symmetric σ tensor was found to be perpendicular to the plane spanned by the carbonyl carbon and the carbon and oxygen atoms directly bound to it.