The Kinetics of Intramolecular Reactions from Relaxation Time Measurements

Abstract

Analysis of spin-lattice relaxation ( T I ) provides alternatives to the standard line shape procedures for the investigation of intramolecular reactions; furthermore it expands the NMR range at both the high and the low energy barrier limits. Dipole-dipole-and quadrupole-relaxation times are sensitive to very rapid processes such as methyl rotation. Analysis of relaxation in the rotating frame (T,& provides kinetics for many sorts of processes, particularly those in the dynamic range that is too rapid for line shape methods. Saturation transfer and the coalescence of relaxation times at high temperatures can be used to measure high energy barriers. The scope and limitations of these methods are described.

Artificial Molecular Rotors

“…The most common technique for the study of solution-state rotors is dynamic NMR (or DNMR) spectroscopy. [159][160][161][162][163][164][165][166][167] More detailed discussions on 2-D NMR techniques are available in recent reviews. [168][169][170] NMR experiments for observing dynamical processes in solids are also described in other sources.…”

“…675 Several half-sandwich complexes with cyclooctadiene (COD) "stools" have been measured by Mann and co-workers 676 using 13 C spin-lattice relaxation experiments. For the rhodium complex (162), these authors obtained a barrier (∆G q ) of ∼1.9 kcal mol -1 , and for the iridium complex (163) it was ∼1.7 kcal mol -1 . Recently, Mynott and co-workers 677 found restricted rotation of an unsubstituted Cp ring in (η 5 -Cp)M + (Ph 3 P) 2 -(η 2 -CH 2 CHPh) complexes for M ) Ru and Os (164).…”

13C Nuclear Spin Relaxation Study of Haloethynyltrimethylsilanes. Carbon–Halogen Interaction

“…Within the adopted diffusion model, the Direct measurements of relaxation times of such nuclei are R DD (C Me ) contribution to the total relaxation rate is related to an even more challenging task. One of the possible solutions all three diffusion constants by the functional dependence (2) to this problem relies on the application of the indirect method based on the interpretation of the scalar relaxation R DD (C Me ) of the second kind of spin 1 2 in the molecule adjacent to the quadrupole nucleus. As was mentioned above in the case …”

“…Assuming that the extreme-narrowing condition holds, the ap-ppm for C a and C b in chloro, bromo, and iodo derivatives, respectively (5). Thus, it may be expected that the SA mechpropriate relationship has the form (2) anism contributes substantially to the total relaxation of these carbons, especially at higher magnetic fields. Because of the R DD (CH) Å (h/h max )R, [1] symmetry of the molecules studied, the SA relaxation of acetylenic carbons depends only on the rotational diffusion where R is the total relaxation rate and h max Å 1.988.…”

“…In the extremenarrowing limit such small contributions are field indepenlaxation mechanisms. These compounds are in many respects analogous to the parent ethynyltrimethylsilane studied dent (1)(2)(3).…”