NMR line shape analysis

Weis [85AHC(38)1, pp. 70-73] has also studied the kinetics of 1,4-dihydro to 1,6-dihydro transformation quantitatively using H NMR line-shape analysis. The results indicated that two mechanisms (monomolecular and bimolecular reactions) are involved in the process, for which all the kinetic parameters were calculated. 

NMR line shape analysis, 260-263 Negative activation enthalpy, 162 Noncompetitive inhibitor, 93 Nucleophilic catalysis, 237-238 Numerical simulations, 112-119... 

Similarly, 39K nmr line-shape analysis has been applied to the study of the interaction of K+ with 18-crown-6 in a range of solvent mixtures (Schmidt Popov, 1983). In most instances, the cation exchange proceeds via a normal dissociative process (as found in water) however, in 1,3-dioxolane, metal exchange switches to a bimolecular exchange mechanism. 

This reaction has been reexamined using optical, IR and NMR spectroscopic methods to probe NO reactions with Fe(TPP)(NO) and the more soluble Fe(TmTP)(NO) (92). These studies confirmed the formation of Fe(Por)(NO)2 in toluene-dg at low temperature (Eq. (43)). NMR line shape analysis was used to calculate K43 = 23 M-1 at 253 K (3100 M-1 at 179 K, AH° = —28kJmol 1) (92). The failure of the Fen(Por) complexes to promote NO disproportionation, in contrast to the behavior of the respective Ru(II) and Os(II) analogs, may find its origin partly in the relatively low stability of the dinitrosyl intermediate (K52 estimated to be 2.8 M-1 at 298 K) and unfavorable kinetics of subsequent reaction of this species with NO. 

Organomagnesium compounds undergo fast intermolecular carbon-magnesium bond exchange in solution. One such process in THF solution, (equation 14) was studied by NMR line-shape analysis ... 

NMR is a powerful technique for providing information about the distribution and dynamics of local RFs, characteristic of such systems. While the quadrupole-perturbed NMR line shape analysis gives details about the distribution of local RFs, spin-lattice relaxation (SLR) studies can give information on the dynamics in the frustrated state of these systems. From the literature, it can be seen that most of the NMR experiments have been carried out in RADP mixed systems and its deuterated analogues. ETFI group published a number of results21-23 on various mixed crystals. 

Dynamics of typical reorganizing systems that have been investigated using NMR line shape analysis include first-order degenerate processes such as degenerate bond rotations (equation 1), first-order interconversions where A and B are different species (equation 2), bimolecular group transfer (equation 3) and mutual exchange (equation 4). 

The parameters that are extricated from a line shape analysis are the reciprocal mean lifetimes, l/rsp(sp = species), between successive exchanges via each particular step. In chemical usage l/rsp is preferably written as kj, the pseudo-first-order rate constant. The results of NMR line shape analysis provide kinetic information via the well-known relationship given in equation 57. A term such as in equation 5 is used for each separate exchange step. 

NMR line shape analysis of the intermolecular exchanging system was handled in two ways, which gave very similar results, by use of the 6Li resonances of l-propyllithium-6Li and by calculating the 13C NMR of lithium bound carbon in hexamer under conditions when hexamer prevailed in the equilibrium. 

As noted, the sample of neopentyllithium in diethyl ether-dio, described above, contained neopentyllithium dimer solvated by diethyl ether-dio, 0.125 M, in addition to the 14 PMDTA monomer 0.34 M. Averaging of the 6Li NMR for these two species indicated a fast mutual exchange of lithiums between PMDTA coordinated monomer and ether solvated dimer. NMR line shape analysis of the 6Li resonance gave AH = 12 kcalmol-1 and AS = +10 eu for this exchange process. It is interesting that at 230 K the pseudo-first-order rate constants for inversion in 14-PMDTA and exchange between the latter monomer and dimeric etherate are, respectively, 5.06 s 1 and 2.57 s-1. This implies that the two processes may be mechanistically linked and that nitrogen inversion in 14 PMDTA alone must be a much slower process. 

Thomas and coworkers showed that f-butyllithium in pentane consists exclusively of cubic tetramers. Below 251 K, the 13C NMR of 6Li bound carbon consists of a 1 3 6 7 6 3 1 multiplet with 7(13C,6 Li) = 5.1 Hz, the familiar signature of a cubic tetramer24. On increasing the temperature above 251 K, this resonance broadens and resolves again by 268 K into a nonet with splitting of 4.1 Hz due to fast intraaggregate C—Li exchange. Carbon-13 NMR line shape analysis established AH = 25 1 kcalmol-1 and AS1 = 44 eu. 

Carbon-13 chemical shifts are typically 75, 106, 128.5 and 86 for, respectively, a, o, m and p carbons indicating delocalized anions within contact ion-pairs. At low temperature, all the ring hydrogens are nonequivalent and, with increasing temperature, the resonances for the two ortho protons average as do those for the meta protons due to increasingly faster rotation around the Ca—Q bonds. The e(ae) elements which account for the changes in the proton NMR line shape analysis are described in equation 27. 

TABLE 10. Dynamics of reorganization of allylic lithium TMEDA complexes, 0.3 M in diethyl ether-4io from 13C NMR line shape analysis... 

With increasing temperature, these two doublets progressively average to single lines at their respective centers by 298 K. This behavior was ascribed to a lithium walk around one face of the plane of the dianion. NMR line shape analysis of these data gave rise to AH and A.S values for the lithium walk of 12.6 kcal mol 1 and +4.5 eu, respectively. 

We have shown how organolithium compounds adopt a variety of structures which differ in state of aggregation and degree of solvation. These species interconvert rapidly at equilibrium by different mechanisms, such as intermolecular C—Li exchange ligand transfer and dissociation-recombination processes as well as first-order reorganizations such as inversion and rotation. Dynamics of many of these processes have been determined by our methods of NMR line shape analysis. 

The 13C-NMR line-shape analysis at T < TN in (TMTTF)2Br has proved that the periodicity of the magnetic modulation is commensurate with the underlying lattice [52]. The spin lattice relaxation is activated in the SDW ground state since there exists a finite energy gap in the phason modes of this commensurate SDW phase (Fig. 8). 

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