Temperature dependence, *H NMR

Likewise, temperature dependent H NMR spectroscopic studies on yV-phenyl-3//-azepin-2-amine indicate that one of the C3 protons is highly shielded, the other strongly deshielded.82... 

Reliable information on the thermodynamic stability of group 13/15 adducts is usually obtained by gas phase measurements. However, due to the lability of stibine and bismuthine adducts in the gas phase toward dissociation, temperature-dependent H-NMR studies are also useful for the determination of their dissociation enthalpies in solution [41b], We focussed on analogously substituted adducts t-BusAl—E(f-Pr)3 (E = P 9, As 10, Sb 11, Bi 12) since they have been fully characterized by single crystal X-ray diffraction, allowing comparisons of their thermodynamic stability in solution with structural trends as found in their solid state structures. 

The di-tert. butyl substituted calicene 230 was calculated to possess considerable non-bonding (steric) interactions in the planar geometry184. The relief of strain when going to a perpendicular transition state is reflected by the low coalescence temperature of tert. butyl signals found on temperature-dependent H-NMR spectroscopy187. ... 

Figure 10.3 Temperature-dependent H NMR spectra of in the presence of Re(PiPr3)2 (N0)(C0)H2 of CF3COOH in tolnene-dg. (Reprodnced with permission from ref. 7.)...

Triazole can be considered to exist in a tautomeric equilibrium between (la) and (lb), in which the hydrogen is transferred from the hydrazine to amine nitrogen (Equation (1)). The chemistry of 1,2,4-triazole suggests that (la) predominates, while ab initio theoretical studies <90JPC5499> and a temperature dependent H NMR study <84JCS(P2)1025> suggest that it is the exclusive form. 

We have been able to demonstrate by synthesizing the unsymmetrical compound 87 and recording its temperature-dependent H NMR spectra that, in addition to the equilibrium between the enantiomers 86a and 86b, further ligand rearrangements need also to be considered. 

Triarylsilanes have also been shown to adopt propeller conformations in the solid state 45 and in solution. 46> Trimesitylsilane (8) exhibits a temperature dependent -H-nmr spectrum which indicates rapid stereoisomerization at ambient temperature. Thus, at 40 °C the spectrum features two singlets in the methyl region in a ratio of 1 2, assigned to the para and ortho methyl substituents, respectively. Upon cooling the sample, the ortho methyl proton signal broadens and splits into two signals of equal intensity, a result consistent with a propeller conforma-... 

The unsymmetrical diene ligands in 85b and 85c may form two dia-stereomers with the less substituted double bond in the cis or trans configuration with respect to the Mo—C a bond. For 85b, only the isomer with the less substituted double bond cis to this bond is found. Compound 85c forms both isomers, but the cis isomer is distinctly preferred. In contrast with the fluxional >j2-diene complexes 84a-84c, compounds 85a-85c do not show temperature-dependent H-NMR spectra for the //4-diene ligands. However, this does not disprove a hindered rotation of the f/4-diene ligands when one of the two conformers (o or u) is energetically more favored than the other. 

Pyridinoparacyclophane (30)23) exhibits a temperature-dependent H NMR spectrum in which 30 is certainly undergoing conformational flipping. The AG value for 30 is 10.7 kcal/mole and is significantly smaller than the value of 20.6 kcal/mole for the carbocyclic analog 31. 

Figure 3 Temperature dependent H NMR spectra of 5-benzyl-l-[(4-nitrophenyl) amino]-4-phenyl-l,3-dihydro-2H-imidazole-2-thione 26.

Examples come from Hahn et al. [491] who synthesised the pincer imidazolium salt from a,a -dibromolutidine and A-alkyl-benzimidazole using a standard protocol. Synthesis of the palladium(ll) complex was then achieved with palladium(ll) acetate in DMSO. The structure is similar to that of the palladium(II) pincer carbene complexes with nonannulated pincer ligands. Temperature dependant H-NMR spectroscopy... 

As shown by temperature-dependent H NMR experiments, the two distinguishable species present in solutions of 4-6 were found to interconvert f 1 ... 

Fig. 2. Temperature dependent H-NMR spectrum (60 MHz, toluene-dg) of [s-cis-T/ -l,2,5,6-tetramethyl-3,4-bis(methylene)-tricyclo[3.1.0.0 ]hexane]zirconocene (51).

Another example for a new titanium complex [Cp2Ti(As2Me4)]2[AsFg]4 contains a six-membered metallacycle. The temperature-dependent H NMR spectra indicate dynamic behaviour and a chair conformation (40). 

Fig. 3. Temperature-dependent H NMR spectra of alcohol 8 in FSO3H/SO2CIF...

B. The Temperature Dependent H NMR of Uranocene and Octamethyluranocene. Our initial interest was in verifying the temperature dependence of the isotropic shift in uranocene and the reported non-zero intercept at T =0. Recent laser Raman studies by Spiro and co-workers (41) have established that the first excited state in uranocene is 466 cm- above the ground state. Thus, the isotropic shift may not vary linearly with the inverse of the temperature from -100°C to 100°C. Indeed, below 100°K some controversy exists concerning the temperature dependence of the magnetic moment in uranocene (42,43). 

A higher value of 16.4 kcal mol has been reported,but a more recent " line shape analysis of the temperature dependent H NMR spectrum of 4 confirms the earlier value obtained from the coalescence temperature approximation. 

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