NMR spectra variable temperature

Three different conformations are possible for monomeric chalcogen diimides (Eig. 10.1). Variable-temperature NMR spectra indicate that the cis,trans isomer of S(NR)2 is most stable in solution for small organic groups (R = Me, Bu). With bulkier organic substituents, small amounts of the trans,trans isomer exist in equilibrium with the cis,cis isomer. " The cis,cis isomer is observed in solutions of certain sulfur diimides with... 

Rate constants and activation parameters for 6 interconversion for the chelate rings in [Fe(CN)4(diamine)] anions with diamine = 1,2-ethanediamine, (2R,35)-butanediamine, and R,2S -di-cyclohexanediamine have been determined from line shape analysis of variable temperature NMR spectra. AH values are 25 kJmoN, 30kJ mol , and 43 kJ mol , values 0 J K mol , -3 J K ... 

The carborane is bonded to the rhodium center via a thiolate anion and a bridging B-H-Rh bond. Complex 95 undergoes B-H/B-H-Rh interchange coupled with an apparent rotation of the Rh(COD)+ fragment. Variable temperature NMR spectra were recorded between 293 and 179 K. The apparent rotation of 1, 5-COD is found in a rather large number of Rh-complexes [152]. 

Figure 1.24 Variable temperature NMR spectra of 96 in CD2Cl2/d -toluene, and the proposed dynamic process responsible for the exchange of the nuclei.

Figure 7.8 Variable-temperature NMR spectra of CpNbHs in the presence of (CF3)2CH0H (1 1) in toluene-ds- The less intense line belongs to the central hydride ligand. (Reproduced with permission from ref. 12.)...

Benzoannelation of the oxepane ring system leads to an increase in the barrier to ring inversion. Thus variable temperature NMR spectra on l,2,4,5-tetrahydro-3-benzoxepin (2) suggested that a chair conformation was preferred with a barrier to ring inversion of ca. 39.7 kJ mol"1. 

Figure 4.8 Variable-temperature + NMR spectra of / /.A/-dimethylformamide in DMSO-dg coalescence temperature, + 413 K...

The variable-temperature NMR spectra help to explain the catalytic properties of the dppp complex system which were outlined previously in Table I. The reduced catalytic activity compared with the tris(triphenylphosphine) complex system is apparently due to the reduced dissociation of the cyclic complexes. For example, the 90°C spectra of Figures 3 and 13, clearly show that the ligand-exchange rate is much slower in the case of dppp. However, temperature-dependent ligand exchange of the monocyclic complex occurs and leads to cis-bisphosphine species that catalyze the hydroformylation of olefins at minimal partial pressures of CO. The hydroformylation rate of the dppp system is faster at 1 atm CO pressure than that of the dppe system. Of course, such hydroformylations are nonselective due to the cis-stereochemistry. 

Figure 6. Variable-temperature NMR spectra of 3d in CD2C12 solution at 500 MHz.

In an effort to evaluate conformational properties, pyridinophane 3 was synthesized 7-9) and compared to cyclophanes 2 and 4. Perusal of the variable temperature NMR spectra of 2-4 demonstrates that 2 is the only cyclophane in this series which shows a temperature dependence the AG for 2 has been found to be >27 kcal/ mole. The H NMR spectra of 3 and 4 remain unchanged to 200 °C the bridging methylenes in 3 exhibit and ABCD pattern whereas in 4 and A2B2 pattern is observed. Comparison of these data with results reported by Vogtle10) indicates the relative order of steric interactions (hydrogen-hydrogen, hydrogen-A-lone pair, and jV-lone pair-JV-lone pair) to be 2 < 3 < 4 3i 7). 

More recent developments have utilized metal vapor synthesis (method F) involving cocondensation at low temperature of the metal, cyclopentadiene (or other dienes), and PF3. The hydrido-cyclooctadienyl complex [Cr( 5-C8Hn)(PF3)3H] 125) (see Section IX) made in this fashion exhibits variable-temperature NMR spectra which establish the existence of an exchange between the hydrido atom and a methylenic hydrogen bound to an sp3 carbon adjacent to the diene unit. 

Fig. 5.78. Variable-temperature NMR spectra of the lithium or-thosilicate/metasilicate mixture. Typically, narrow spectra required 64 pulses and broader spectra 500 pulses. The line broadening used in processing each spectrum was less than 10% of the total line width. (Reprinted from I. Faman and J. F. Stebbins, J. Am. Chem. Soc. 112 32,1990.)...

Intermolecular electron-transfer rates have been studied for uranocene and substituted derivatives of uranium, neptunium, and plutonium by examining the variable-temperature NMR spectra of mixtures of (CgH8)2An and [(C8Hg)2An]. In all cases, electron-transfer rates are rapid. Specific rates could not be derived for uranium and plutonium derivatives owing to the small chemical shift differences between analogous An(fV) and An(III) compounds, but in the case of (f-BuC8H7)2Np, the rate has been estimated to be of the same order of magnitude as comparable lanthanide cyclooctatetraene compounds ( 10 s ). ... 

Fig. 1-19. Variable temperature NMR spectra (36.23 MHz) of [Rh(dppf-P, P )2][BPhJ (adapted from [65]).

A striking difference between the variable temperature NMR spectra of 14 and of 15-17 is observed. Although, in the heteroleptic diorganotin(II) compounds 15-17 coalescence occurs for the diastereotopic NMe2 resonances, the AB pattern of the benzylic methylene resonances remains intact up to + 120°C (the highest temperature studied). This points to a process involving Sn—N dissociation, while the Sn lone pair remains stereochemically active and rotation around the —Sn bond is blocked. 

Variable-temperature NMR spectra were used to probe the fluxional behaviour of Mo(q3-C3H4Me)(q2-S2CNC4H8)(CO)(q2-diphos), where diphos = dppm, dppe. Detailed assignments to endo- and exo-isomers were made from and... 

Compounds of Group 10. Dynamic NMR spectra for (192) and (193) gave measurements of barriers to n-a allyl conversion.946 Similar studies were made on (dippe)Ni(ri3-allyl)(CN), where dippe = bis(di-isopropylphosphino) ethane.947 All 4 diastereotropic isopropyl groups in the Z-conformer of (194) (Ar = 2,6-1Pr2C6H3) give separate resonances. TOCSY experiments gave evidence on ZjE isomerisation.948 Variable-temperature NMR spectra for [2-(r 2 r 2-... 

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