Dynamic NMR studies

Temperature-dependent (dynamic) NMR studies are suited to the study of processes with rate constants between 10 and 10 s Some applications are shown in Table 2.13 and in problems 13 and 14. 

It must be emphasized that all time-dependent chemical phenomena, such as tautomerism, are sensitive to temperature changes. In this section, we treat classic DNMR (dynamic NMR) studies and cases of changes in the NMR spectra with temperature together. In Table XI we have classified these studies according to the physical state of the sample and to the nuclei... 

The conformational preference of 1,3,5-trithiane-l-oxide has been determined in solution and in the solid state . and dynamic NMR studies indicated that the S=0 bond is equatorial (182) in solution, as did molecular mechanics calculations. Surprisingly, the axial conformation (183) is preferred in the crystalline state. 

Although the drug delivery to the lipid bilayer membrane is just the first step for bioactivities and phopholipid vesicles are rather simple in view of the composite structure of biomembranes, the unambiguous specification of the preferential location of the drug is essential the successive processes of the action are expected to be induced via the delivery site in membranes. We expect more advances in the dynamic NMR study, so that we can get insight into the mechanism of DD in membranes. 

Earlier dynamic NMR studies had already demonstrated that the substituted semibullvalenes [87], [88] and [89] were indeed rapidly equilibrating Cope systems and not ground-state homoaromatics (Anet and Schenck, 1970 Moriarty et al., 1972 Russell et al., 1973). The bis(trifluoromethyl)semi-bullvalenes [90] and [91] had also been formulated as non-equilibrating Cope systems (Liu, 1968 Liu and Krespan, 1969). 

Strong additional support for the assignment of the metapara-cyclophane structure 129 was obtained from dynamic NMR studies The temperature dependence of the proton resonance of this compound is analogous to that of the parent [2.2]metaparacyclophane 3>. The multiplets observed for the protons of the p-phenyl nucleus gradually broaden with increasing temperature, disappear completely at 150 °C, and reappear at 180 °C as a midway peak. The isomer of 129, the 13-methoxy[2]paracyclo[2](l,4)naphthalenophane (132), formation of which by the mechanism outlined above seems equally feasible, does not appear to occur. 

Neutral lanthanide complexes are convenient models for the cationic zirconocene systems and avoid complications due to the presence of counteranions and the limited solubility of ionic compounds. Dynamic NMR studies on yttrium complexes 44-46 has allowed the determination of the alkene binding enthalpy, the activation enthalpy of alkene dissociation, and the relative rates of dissociation and alkyl site exchange (site epimerisation) (Scheme 8.20). Compared to the Zr... 

Anomeric effects are evident from dynamic NMR studies on at least one substrate, N-benzyloxy-Af-chlorobenzamide (2c) ". In acetone-de the benzyl aromatic signal (S 7.85) de-coalesced into two signals (ratio 2 1) close to 200 K, corresponding to a free energy barrier of ca 10-11 kcalmoH Amide isomerization appeared to be faster than N—0 rotation since benzoyl resonances were largely unaffected. 

Although an unsuccessful optical resolution21 of ( )-cyclodecene suggested its optical instability, Robert s dynamic NMR studies 22) of the racemization process in deuterated ( )-cycloalkenes 13 succeeded in providing a t1/2 value of 104 sec (room temperature). This optical instability, found in the parent compound, could explain Westen s fruitless attempt to prepare the chiral ( )-cyclodecenone 16 from the (+)-methanesulfonate 15 23). 

An X-ray analysis 34a) of ( ),( ),( )-l,5,9-cyclododecatriene (38) suggested that this molecule is most comfortable in a twisted chiral conformation of D3 symmetry, and a dynamic NMR study and a force-field calculation Mb) estimated AG = 8.6 kcal/mol for the racemization process between two enantiomeric D3 conformers. A pseudochair conformation of similar D3 symmetry was also suggested35) for 1,5,9-cyclodo-decatriyne (39), following an ab initio STO-3G calculation and photoelectron spectroscopy. 

Klein, O., Bonvehi, M. M., Aguilar-Parrilla, F., Jagerovic, N., Elguero, J., and Limbach, H.-Fl. (1999). Flydrogen bond compression during triple proton transfer in crystalline pyrazoles. A dynamic NMR study. Israel J. Chem. 39, 291-9. 

By careful control of the conditions of the reaction one can obtain preferentially oxidation of sulfur to its four coordinate oxidation state and by using a second set of conditions one can obtain the oxidation to the six-coordinate sulfur species. The dynamic NMR study of SF3CF2SF3 is currently in progress in collaboration with A. H. Cowley (60). This differentiation of oxidation states is extremely promising, and work in progress shows that this is not at all an isolated situation. Mercaptans and other organosulfur compounds definitely exhibit this capacity in fluorine reactions. 

This observation, that the aptitude of X to be displaced with inversion, is parallel to the tendency of the Si—X bond to be stretched under the influence of an attacking nucleophile, does not agree with the apicophilic-ity order stated from dynamic NMR studies on stable pentacoordinate phosphoranes (42), which is as follows ... 

Of special interest are results of the dynamic NMR study of the conformational mobility of type-1 1-heteracyclohexanes (73JA4634) and their 3,5-naphtho analogs of type 4 (82CC333). Free energy barriers to ring inversion of these compounds are given in Table II. Values for compounds 1 decrease in the order O > S > Se > Te, whereas in their naphtho analogs, they increase in the same order. 

Dynamic NMR studies [7-10] have shown that two exchange processes occur in these complexes (i) Interconversion between the conformations of the ethylene bridges (66 AX). This process can be achieved by a rearrangement in the coordination polyhedron and is characterized by a AG/ value between 49 and 57 kj mol-1. It should be noted that the shuffle in the coordination polyhedra converts 1 and 4 into their mirror images (1 1 and 4 4 ), whereas 2 and 3 are converted into each others mirror images (2 3 and 3 2 ). (ii) A relatively slow racemization of the terminal N-atoms (AG/ = 65-71 kj mol-1), requiring decoordination of the terminal N-atoms and their pending arms. 

Allyl complexes (pseudo-rotations, dynamic NMR studies, 1, 416 with tungsten carbonyls and isocyanides, 5, 688-689 rc-Allyl complexes with Cr, 5, 305 with Cr(II), 5, 300 with Cr(III), 5, 300 and cyclodextrins, 12, 789 in enyne carbometallation, 10, 328 with rhodium, 7, 220-221 (j-Allyl complexes, with iron, 6, 98 5-Allyldiisopinocampheylboranes, in asymmetric allylboration, 9, 198... 

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