Protonation-rotation

With octalin (55), rotation to the trans chair form (59) followed by protonation from the outside face of the double bond at the less substituted carbon atom leads to the tertiary cation (60) with net equatorial introduction of the proton. Rotation to a trans boat form would lead to net axial protonation<84) ... 

Jonas et al. measured the proton rotating frame spin-lattice relaxation time (Tip) at pressures from 1 bar to 5000 bar and at temperatures of 50 to 70 °C for DPPC and at 5 to 35 °C for POPC. If intermolecular dipolar interactions modulated by translational motion contribute significantly to the proton relaxation, the rotating frame spin-lattice relaxation rate (1/Tip) is a function of the square root of the spin-locking field angular frequency... 

NMR Nuclear (proton) rotation Most structurally informative technique Costly and requires considerable infrastructure Only low-field instruments suitable... 

R. R. Knispel and H. E. Petch, Proton rotating frame relaxation in lithium hydrazinium sulfate, Li... 

CsSH is the same as that of CsBr shows that SH is behaving as a spherically symmetrical ion with the same radius as Br". In contrast to the low-temperature forms of the hydroxides of Na, K, and Rb, which are described in (b), the hydrosulphides have a rhombohedral structure at room temperature. In this calcite-Iike structure the SH" ion has lower symmetry than in the high-temperature forms and appears to behave like a planar group. It is probable that the proton rotates around the S atom in a plane forming a disc-shaped ion like a rotating CO3 " ion. 

For these and other reasons it seems improbable that H2S" is a correct formulation for either of these radicals. Possible alternatives include H2S+, isoelectronic with PH2, and species such as H2S2", H2S2 or more complex polysulfide radicals. The radical H2S+ is a possible candidate for the species in alkali halide crystals (56), which apparently contains only one sulfur atom, except that some restricted rotational motion would need to be invoked to explain the equivalence of the two protons. Rotation within the molecular plane would achieve this, and would also account for the form and magnitude of the anisotropic proton coupling. The 33S tensor is also accommodated reasonably well, as also is the positive g-shift. However, the very large value for Ag is more difficult to reconcile with this formulation. 

Measurement of relaxation is usually easier then but also time consuming. It should be borne in mind that spin-lattice relaxation of is bi-exponential [86] unless protons are decoupled during the relaxation delay. at natural abundance and nuclei are often used as probe of dynamic processes. Parameters such as and spin-lattice relaxation times and Tj), carbon and proton-rotating-frame relaxation... 

A proton rotates in a circie of radius 5.00 X 10 " m. What are the first three rotationai energy ieveis ... 

The protons rotate around their oxide ion hosts. The activation barrier for rotational diffusion is generally low so that these rotations are easy [18], but they lead to no long-range proton migration. The stretching vibrations, on the other hand, may lead to a jump to the next oxide ion. The diffusivity of protons may be expressed ... 

Fig. 13.3 Proton diffusion path in orthorhombic SrCeOs as obtained from a quantum molecular dynamics simulations [40]. The proton rotational diffusion around 01 is distinctly biased toward the neighboring 01 (b), which is part of a one-dimensional proton diffusion path formed by 01 oxygen only (a). Proton transfer between 01 dark) and the less basic and more frequent 02 (grey) is anticipated to control long-range, isotropic proton diffusion (see text)...

In simple imines there are two reasonable meehanisms to explaining the Z/E isomerization. Mechanism 1 (protonation-rotation) eonsists on the protonation of the nitrogen atom followed by rotation about the C-N bond. Sinee protonation eould deerease the C=N bond order, the rotation around the C-N bond axis seems reasonable (Seheme 27.2). 

The calculations show that protonated hydroximoyl chlorides 4 and 5 have a rotation barrier considerably higher than the corresponding hydroximates 6 (differences of about 17 and 10 kcal mol , respectively). A higher energy rotation barrier in the iminium intermediate should disable the protonation-rotation pathway, favoring the alternative nucleophilic attack. This is in agreement with the nucleophile catalysis mechanism proposed previously for the E/Z isomerization of hydroximoyl chlorides 1 and 2 (Scheme 27.5). 

The cross-polarization times are short compared to the shortest proton rotating-frame relaxation time, for the carbons (if there is extensive motion in the mid-kiloHertz range, then becomes approximately 10 s and does not meet this criterion). 

Fig. 5.5 Quantum mechanical simulations show that the formation of an oxygen vacancy in BaCeOs results in a displacement of the oxygen ions towards it and of the cerium ions away from it (l.h.s.) [119]. On ocupation of the vacancy by OH [120] (see Section 5.6) the Ce relaxation is substantially reduced (r.h.s.). The increased displacement of the oxygens towards the OHq defect is caused by the directive hydrogen bridges (here partitioned between 8 oxygen neighbours). The time resolution under consideration is coarse compared to the fast proton rotation, which is sketched schematicaJly. The jump of the proton occurs on an even coarser time scale (see Fig. 6.13, pitge 291). Detailed discussion is given in Section 6.2.1. According to Ref. [120].

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