Variable temperature study

In a quest to increase the efficiency of olefin polymerization catalysts and their selectivity in the orientation of the polymerization, the highly effective Group IV metallocene catalysts, M(Cp)2(L)2, have been studied, since they all display high fluxionality. Following methide abstraction, the metallocene catalysts of general formula M(Cp-derivatives)2(CH3)2 (M= Ti, Zr, Hf), were turned into highly reactive M+-CH3 cationic species. The activation parameters for the methide abstraction, derived from variable temperature NMR experiments, establish a correlation between the enthalpies of methide abstraction, the chemical shift in the resulting cation, and the ethylene polymerization activities [149]. 

The Ru(iv) complex Ru(SiMe3)2(H)2(PMe3)3 is a model for dehydrogenative C-Si bond formation chemistry. Berry and coworkers have shown that, at low temperature, this complex undergoes fast reversible SiH reductive elimination on the NMR time scale (Eq. (3)), and slow H2 reductive elimination as a minor process [154]. 

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The conformational preference of the monosulfoxides of 1,2-, 1,3- and 1,4-dithianes (179-181) were determined by NMR experiments which included variable-temperature studies, double irradiation, solvent effects and the influence of lanthanide shift reagents167. For 179 and 181, the axial conformers were the dominant species in CD3OD, but for 180, the equatorial conformer was in excess. 

Equilibrium measurements measure the relative AG, and thermochemical studies generally are interested in enthalpy values, AH. The enthalpy can be obtained from AG by using the relation AG = AH — TAS. The entropy of proton transfer can either be estimated, reliably calculated using electronic structure calculations, or can be measured directly by using a Van t Hoff approach. Measuring the quantity AS requires a variable temperature study. 

Variable temperature studies again show the ring inversion (76T2339). They also indicate that a 2-methyl group shows equal preference for the quasi-axial or quasi-equatorial positions, presumably because there are no other axial substituents to cause 1,3-diaxial interactions. 

Given our ability to measure absolute rate constants for carbene additions, variable temperature studies readily afford activation parameters. Initial studies of CeHsCX additions gave very low values, 1 kcal/mol for reactions with 1-hexene and frani-pentene. Most surprisingly, the values for CgHsCCl additions to (CH3)2C=C(CH3)2 and (CH3)2C=CHCH3 were negative (—1.7 and —0.8 kcal/ mol, respectively). The reaction rates increased as temperature decreased. The preexponential (A) factors were low (2-6 x 10 s ), indicative of an unfavorable... 

Variable-temperature studies show that the back reaction is thermally activated. The dependence of AH on driving force (variations in dye formal potential) is illustrated in Fig. 13 and is consistent with normal-region reactivity [67]. If AS can be neglected, application of Eq. (1) yields values of approximately... 

There are two main approaches to excipient compatibility screening isothermal studies at an elevated temperature and variable temperature studies in which the temperature is steadily increased, as in DSC. Both approaches are valid, but it is important to note, as has been stated above, that excipient compatibility testing is not a definitive test. We cannot state that an interaction will not take place, even though one may not have been found. We can only state which excipients to avoid because there is a very obvious interaction. A typical scheme is given in Figure 1 for a DSC-based excipient compatibility study. (There are other schemes that are used successfully.)... 

Further evidence for isotropic chemical shifts was found in variable temperature studies. The aromatic carbon ortho to the oxygen resolved into two peaks at low temperatures. The aromatic carbons meta to the oxygen have two peaks, but these peaks are not as well resolved. As the temperature was raised, the splittings in both coalesced into one peak, indicating that there is rapid sampling from two magnetic environments. This is indicative of the motion of the phenyl group with respect to the backbone. At lower temperatures, the methyl resonance broadens and eventually... 

There is some evidence that while mono adducts of the j8-diketonate shift reagents may show time-averaged uniaxial symmetry, bis adducts often do not. Thus the spectra of [M(dpm)3L2], where M = Pr, Sm, Dy, Ho, Er or Yb and L = 3-picolinc or 3,5-lutidine, have been interpreted in terms of biaxial magnetic anisotropy. One magnetic axis is defined by the molecular C2 axis present in the X-ray crystal structure and the other two axes lie in similar orientations in all the adducts. A variable-temperature study of this system shows restricted rotation of the aromatic rings with a very solvent-dependent conformational equilibrium resulting from this.558-560... 

Propene on HY was, therefore, selected for the first in situ variable-temperature study using the CAVERN method. These experiments were carried out in early 1988 and published in 1989 (93). The central features of the CAVERN experiments were that the propene was introduced into the zeolite at cryogenic temperature and the sample was manipulated so that spectral acquisition could commence with an unreacted sample. Additional spectra were then acquired as the sample was slowly raised to room temperature. Detailed experiments of this sort were carried out for propene-2-l3C and propene-7-13C and less extensive experiments were performed for propene-3-13C. These experiments showed, among other things, that the 250 ppm peak was formed coincident with a second peak at ca. 156 ppm and the relative intensities of these peaks were 2 1. A careful study of the literature of carbenium ion chemistry in sulfuric acid and superacid solution media suggested the assignment of these resonances (250 and 156 ppm) to alkyl-substituted cyclopentenyl cations similar to 4. 

Recently, a variable-temperature study of the 13C-NMR of 3,4-dimethyl-hexahydro-l,3,4-oxadiazine and of the trans-fixed derivative 434 have completely clarified the conformational equilibria and allowed a rather complete energy contour to be constructed for the monocyclic oxadiazine (Fig. 17).344 The conformational analysis of other methyl-substituted tetrahydro-1,3,4-oxadiazines345,346 is consonant with these results. 

H-NMR variable-temperature studies of 2,5-dimethyltetrahydro-l,2,5-oxadiazine351-354 established a high energy barrier process (AG 14.4 + 0.1 kcal mol-1 and AS —1.2 + 0.4 kcal mol-1) comparable to that in N-methyltetrahydro-l,2-oxazine. From studies of a deuterated tetrahydro-1,2-oxazine, it was concluded that this should be attributed to slowing of 2-AT-Me inversion rather than ring reversal.353... 

Effect of pore size on the adsorption of xenon on mesoporous MCM-41 and on the l29Xe NMR chemical shifts a variable temperature study... 

The existence of tautomers and rotamers of dehydroacetic acid (239) and derived Schiff s bases has been investigated by 1H, 13C and 15N NMR spectroscopy (82JCS(P2)513). The tautomeric form (239) proposed earlier (61AK( 17)523) was confirmed and its existence as the rotamer (240) was established by variable temperature studies. 

Kozerski, L. and von Philipsbom, W., 15N chemical shifts as a conformational probe in enaminones a variable temperature study at natural isotope abundance, Org. Magn. Reson., 17, 306, 1981. 

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