NMR temperature dependent

If the amount of the sample is sufficient, then the carbon skeleton is best traced out from the two-dimensional INADEQUATE experiment. If the absolute configuration of particular C atoms is needed, the empirical applications of diastereotopism and chiral shift reagents are useful (Section 2.4). Anisotropic and ring current effects supply information about conformation and aromaticity (Section 2.5), and pH effects can indicate the site of protonation (problem 24). Temperature-dependent NMR spectra and C spin-lattice relaxation times (Section 2.6) provide insight into molecular dynamics (problems 13 and 14). 

The temperature-dependent NMR spectrum of the ion can be analyzed to show that there is a barrier (8.4 kcal/mol) for the ring flip that interchanges the two hydrogens of the methylene group. The C-NMR chemical shift is also compatible with the homoaromatic structure. MO calculations are successful in reproducing the structural and spectroscopic characteristics of the cation and are consistent with a homoaromatic structure. ... 

Cotton, F. A., Frenz, B. A., Deganello, G. Shaver, A. (1973) Fluxional organometallic molecules. XXXIV. The structure and temperature-dependent NMR spectrum of cis-(l,2,6-trihap/o- 3,4,5-fri/iapto-bicyclo[6.2.0]deca-l,3,5-lriene)hexacarbonyldi iron(Fe—Fe)," J. Organomet. Chem. 50,227-240. 

Reaction of iron atoms with cycloheptatriene to form [Fe( r) -C7H7)-(t7 -C7H9)] was confirmed by another group 15) these workers determined the crystal structure of the species, demonstrating a sandwich structure with the open faces of the two 7j -systems skewed to each other. The temperature-dependent NMR spectrum of this species (16) indicated two types of fiuxional behavior in solution. Evidence for a 1,-2-shift mechanism of the l-5-i7-cycloheptatrienyl moiety in the structure shown. 

A major goal was to investigate the solid state structures of such compounds by single crystal X-ray diffraction. It was found that Lewis acid-base adducts R3M—ER3 show general structural trends, which allow estimations on the relative stability of the adducts. The experimental results were confirmed by computational calculations, giving even deeper insights into the structural parameters and the thermodynamic stability of simple Lewis acid-base adducts. In addition, their thermodynamic stability in solution was investigated by temperature-dependent NMR spectroscopy. 

Fig. 3. Dissociation enthalpies of adducts f-Bu3Al-E( -Pr)3 as obtained from temperature-dependent NMR Studies...

Examples in organometallic systems are known. Reaction of thiuram disulfides, (R2NCS2)2, with Co(Cp)(CO)2 produces dithiocarbamato pseudo-octahedral cobalt(III) complexes Co(Cp)(dtc)2 with one chelated and one monodentate dtc, also accessible via Co(Cp)I(dtc).1050 Fluxional behavior, including monodentate chelate exchange, was observed for some complexes in temperature-dependent NMR studies. The Co(Cp)I(dtc) complex was defined in a crystal structure. 

In a rare example which demonstrates the possibilities of the approach Biirgi and Dubler-Steudler (1988a) have recently combined structure and reactivity data in a detailed study of the ring-inversion reaction of a homogeneous set of organometallic compounds. The reaction is the auto-merization of zircocene and hafnocene complexes [73 M = Zr or Hf, X = C or O], known from temperature-dependent NMR measurements to undergo the equilibration [73]—s.[73 ]. Principal-component analysis of... 

Asymmetrically substituted thioamides 24e-h exist in equilibrium between rotamers 24-1 and 24-11. The NMR spectrum of AT-benzyl-iV-methylmeth-acrylthioamide 24e showed the distribution of 24e-I 24e-II was 55 45 (Scheme 13 and Table 8) [47]. The free energy of activation of the rotational barrier was measured by the temperature-dependent NMR spectra in DMSO at various temperatures. The free energy of activation of the rotation of the (C=S)-N bond could be estimated as 22.7 kcal mokh Furthermore, Table 8 shows the ratio of I II, and all three thioamides 24f-h favor the conformation I with the situation that a benzyl group was placed closely to the thiocarbonyl group on... 

Temperature-dependent lineshape changes were observed in an early study of the fluo-renyllithium(TMEDA) complex. A detailed study by lineshape analysis, which was also applied to the TMEDA complex of 2,3-benzofluorenyllithium(TMEDA) (Figure 29f, yielded barriers AG (298) of 44.4 and 41.9 kJmoD for the 180° ring flip in these systems, respectively . A second dynamic process, which was detected via the temperature dependence of, the spin-lattice relaxation time in the rotating frame, is characterized by barriers of 35.1 and 37.6 kJmoD, respectively, and may be ascribed to the ring inversion process. For the fluorenyl complex, a barrier AG (298) of 15.9 kJmoD for the methyl rotation in the TMEDA hgand was determined from temperature-dependent NMR spectra of the deuteriated system. 

The aggregation behaviour and the structure in solution of two closely related dilithium compounds 90 and 91 (Figure 16) was studied by Gunther, Maercker and coworkers, using one- and two-dimensional NMR techniques ( H, C, Li) . While in diethyl ether, both compounds exist as a dimer, the dimeric structure is partly broken up in THF or by the addition of TMEDA. Also, activation barriers and thermodynamic parameters of aggregate exchanges were determined by temperature-dependent NMR studies. 

The reversibility of oxidative addition of these Ir complexes to Cjq was proven with temperature-dependent NMR measurements [19], These measurements were carried out with Cgo[lr(CO)Cl(PEt3)2]2 [19], or in another study with a dendritic complex derived from Ir(CO)Cl(PPh2R)2 vvith R being a Frechet-type dendron of the first or second generation [21],... 

Four-membered heterocycles (B). They have the negatively charged nitrogen atoms of the hydrazido ligands in a bridging position between two metal atoms. Both N—N bonds are exocyclic and adopt a trans position in the solid state. In solution, an equilibrium between cisitrans isomers was detected by temperature dependent NMR spectroscopy in many cases. 

Kinetic data on the oxepin-benzene oxide equilibration have been obtained from the temperature-dependent NMR studies. Low values were observed for the enthalpy of isomerization of oxepin (7.1 kJ mol-1) and 2-methyloxepin (1.7 kJ mol-1) to the corresponding benzene oxides (67AG(E)385). The relatively small increase in entropy associated with oxepin formation (5-11 J K 1 mol-1) is as anticipated for a boat conformation in a rapid state of ring inversion. Thermal racemization studies of chrysene 1,2- and 3,4-oxides have allowed accurate thermodynamic parameters for the oxepin-arene oxide equilibration process in the PAH series to be obtained (81CC838). The results obtained from racemization of the 1,2- (Ea 103.7 kJ mol-1, AS 3.7 JK-1 mol-1 and 3,4- (Ea 105.3 kJmoF1, AS 0.7 J K"1 mol ) arene oxides of chrysene are as anticipated for the intermediacy of the oxepins (31) and (32) respectively. 

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