Heike Schmitt

The dynamics of ND+4 and NH3D+ groups in ammonium persulphate studied by deuteron nuclear magnetic resonance. I. Tunneling and stochastic reorientations at low temperatures

We report on a single-crystal deuteron nuclear magnetic resonance (NMR) spectroscopy study of the low-temperature dynamics of ND4+ and NH3D+ ions in the title compound. The most prominent feature of the dynamics of ND4+ ions is uniaxial rotational (primary) tunneling of three deuterons about the N–D bond of the fourth. At T<25 K, the latter deuteron gets localized on the time scale of 10−3 s. We identify the direction of this unique N–D bond. The low-temperature limit of the primary tunneling frequency is 1.6 MHz. In the form of splittings of NMR resonances, our spectra also contain clear evidence of secondary tunneling which, again, is uniaxial. Again we say about which of the other three N–D bonds it takes place. The secondary tunneling frequency is only 4.5 kHz. The deuteron of NH3D+ ions gets localized at T<25 K. It can reside in any of the four sites available to the hydrogens of the ion. The dynamics of the three protons depends strongly on which site the deuteron occupies. If it is the site which w...

o-Terphenyl: flips of the end rings in the crystal phase

We demonstrate by selective saturation deuteron NMR experiments on a crystal of selectively deuterated o-terphenyl (OTP) that both end rings I and II attached to the central ring undergo thermally activated flip motions. In crystals of OTP, the end rings I and II are not symmetry related, their dynamics can be different and, in fact, is different, although mutual steric hindrance of the rings suggests a strong correlation. We measured the rates k I and k II of the flips of both end rings I and II as a function of temperature. We find that, independent of the temperature, k I exceeds k II by roughly a factor of 100. This result excludes correlated flips of rings I and II in the sense that a flip of one ring necessarily entails a flip of the other. The activation energies EaI and EaII of the two flip processes turn out to be equal (80.5 kJ mol−1) within experimental errors. This is taken as a hint that the flips are, after all, related to each other. A mechanism is proposed of how this is possible under the constraint k I » k II.

ND+4 and NH3D+ dynamics in ammonium persulphate. II. Transition from low-to-high-temperature regime

The transition of the dynamics of ND4+ ions in ammonium persulphate, dominated at low temperatures by coherent uniaxial rotational tunnelling about one specific N–D bond (the preferred bond) and, at high temperatures, by frequent stochastic jumps about all N–D bonds is elucidated with deuteron spin-lattice relaxation measurements, selective saturation experiments and deuteron NMR line shape analyses. Between 20 and 35 K, the coherent uniaxial tunnelling is superseded by thermally activated stochastic jumps about the same bond with kinetic parameters kdyn0=10(11.5±0.5) s−1 and Edyna=(3.6±0.3) kJ/mol. At higher temperatures, thermally activated stochastic jumps about the other N–D bonds set in. Their kinetic parameters are kst0=10(12.2±0.5) s−1 and Esta=(7.8±0.5) kJ/mol. From the primary and secondary tunnelling observed at low temperatures we infer the heights of the potentials which hinder rotations of the ND4+ ions about the preferred and any other N–D bond. These heights, minus the rotator’s ground stat...

Orientational disorder in 1,2,3-trichloro-4,5,6-trimethylbenzene. A single crystal deuterium NMR study of the site populations and dynamics

Deuterium NMR measurements on powder and single crystal samples of 1,2,3-trichloro-4,5,6-trimethylbenzene (TCTMB) specifically deuterated at the central methyl group (TCTMB-d3) are reported. The compound exhibits three solid phases: (I) A high temperature phase, just below the melting point (400 to 499 K), of unknown structure. (II) An intermediate monoclinic phase (268 to 400 K). (III) A low temperature triclinic phase (<268 K). Earlier X-ray investigations of the latter two phases indicate that they are, respectively, well ordered (Phase III at 173 K), and orientationally disordered, with the TCTMB molecules statistically aligned along the six possible orientations of the benzene core (Phase II at 298 K). In contrast to these observations the deuterium NMR results show a continuous growth, with temperature, of orientational disorder already in Phase III. The transition to Phase II involves a discontinuous increase in the disorder, but a considerable degree of order is retained in this phase even at high temperatures. This ordering reflects the non-equal population distribution of the molecular orientation in the crystal lattice sites. A quantitative analysis of the deuterium NMR spectra provided detailed information on this distribution and its temperature dependence in the solid phases of TCTMB. The deuterium NMR spectra also exhibit characteristic dynamic effects. As a function of increasing temperature, four dynamic regions can be distinguished: (i) Below 15 K, the spectrum exhibits features typical of coherent quantum-mechanical tunneling of the CD3 groups. (ii) Above 15 K, up to about 170 K, there is fast classical (incoherent) reorientation of the methyl groups, but otherwise a rigid molecular lattice. (iii) Between 180 and 250 K, the spectra exhibit dynamic lineshape changes reflecting the onset of six-fold jumps of the TCTMB molecules. (iv) Above 250 K the spectra correspond to a dynamically averaged, but temperature dependent, quadrupolar splitting, reflecting the changes in the orientational distribution of the TCTMB molecules. This region includes the phase transitions, III to II and II to I. T1 relaxation measurements over the entire temperature range yielded kinetic parameters for the molecular six-fold jumps (k(260 K) = 7 × 106 s−1, ΔE = 38.5 kJ mol−1), as well as for the incoherent methyl group reorientation (k(25 K) = 1 × 109 s−1, ΔE = 2.8 kJ mol−1).

Dynamics in molecular crystals: An application of proton NMR to selectively protonated biphenyl. Do the rings flip together or independently?

AbstractThe synthesis of a specific isotopomer, C6D4H(ortho)-H(ortho)D4C6 of biphenyl is reported. The intramolecular dipolar coupling of the protons leads to a well-resolved single-crystal proton nuclear magnetic resonance (NMR) spectrum and allows one to study the dynamics of the phenyl rings in a unique way. At room temperature and above, the most conspicuous dynamical mode consists of 180° ring flips. The present data together with previous measurements of the total flip rate allow us to conclude that the rings flip almost exclusively independently of each other. Between the incommensurate (IC) phase transition of biphenyl at 38 K andT=250 K, the prominent namical mode consists of oscillatory twists ϕ(t) of the two rings. The data allow us to infer the mean square, (φ2), of these twists. (φ2) is found to grow linearly withT for 50<T<200 K. From the slope of (φ2) vs.T the frequency (the wave number

The proton nuclear magnetic shielding tensors in biphenyl: Experiment and theory

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The shape and information content of high−field solid−state proton NMR spectra of methyl groups

) is derived. The result is