A Low Temperature Dynamic 1 H, 13 C and 77 Se NMR Study of 2,2'‐Selenodicyclohexanol

1,3-Diisopropyl-2,2-dimethyl-1,3,2,4λ2-diazasilastannetidine (1b) and -plumbetidine (2b) are dimeric [(1b)2, (2b)2] in solution. At room temperature the structure of (1b)2 is fluxional. The dynamic behaviour is interpreted - on the basis of 1H. 13C, 29Si and 119Sn NMR data - as an intramolecular process in which the four-membered rings keep their identity. Such a process involves either concerted opening of the two coordinative Sn - N bonds and mutual slippage of the two rings, or consecutive cleavage of one of the coordinative Sn - N bonds and rotation about the other one. At room temperature the dimer (2b)2 is in equilibrium with its monomer 2b, whereas at low temperature the dynamic process corresponds to that established for (1b)2. In solutions which contain a mixture of the dimers (1b)2 and (2b)2, the presence of the mixed dimer 1b/2b can be proved unambiguously by consistent 29Si, 119Sn and 207Pb NMR data.

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Mécanisme de la réaction de glycosylation par réversion acide en présence d'anhydride trifluorométhane sulfonique comme agent de condensation

Canadian Journal of Chemistry ◽

10.1139/v81-069 ◽

1981 ◽

Vol 59 (2) ◽

pp. 482-489 ◽

Cited By ~ 20

Author(s):

André A. Pavia ◽

Sak N. Ung-Chhun

Keyword(s):

Low Temperature ◽

Necessary Conditions ◽

Free Acid ◽

Trifluoromethanesulfonic Acid ◽

Nmr Study ◽

Trifluoromethanesulfonic Anhydride ◽

Glycosylation Reaction ◽

Comme Agent ◽

Mechanism Of The Reaction

The acid-catalysed mechanism of the reaction using trifluoromethanesulfonic anhydride to synthesize symmetrical 1,1′-glycosyl-glycosides such as trehalose and galacto, manno, arabino, xylo etc. analogues has been elucidated. The authors were able to prove by 19F nmr study at low temperature that the efficiency of the system is due to the peculiar property of trifluoromethanesulfonic acid generated in situ to form a stable, insoluble hydroxonium trifluoromethanesulfonate CF3SO3− H3O+. The procedure satisfies the two necessary conditions for a glycosylation reaction being workable: (i) the presence in catalytic amounts of free acid which is regenerated during the condensation step; (ii) a very efficient trapping of the water liberated during the protonation step which displaces the equilibrium towards the formation of the glycoside. The α-stereoselectivity of the reaction is discussed in terms of anomerisation of the β-anomers on to the thermodynamically more stable α ones, through a cyclic oxonium intermediate.

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Low-Temperature Spin Fluctuations beyond Spin Waves

Solid State Phenomena ◽

10.4028/www.scientific.net/ssp.233-234.20 ◽

2015 ◽

Vol 233-234 ◽

pp. 20-24 ◽

Cited By ~ 2

Author(s):

N.B. Melnikov ◽

B.I. Reser

Keyword(s):

Low Temperature ◽

Spin Fluctuation ◽

Functional Integral ◽

Spin Fluctuations ◽

Integral Formalism ◽

Ferromagnetic Metals ◽

Wave Approximation ◽

Dynamic Spin ◽

Functional Integral Formalism ◽

Temperature Dynamic

A simple low-temperature dynamic spin-fluctuation theory of ferromagnetic metals is developed. The theory is based on the functional integral formalism for the multiband Hubbard Hamiltonian and takes into account both single-site and nonlocal spin fluctuations. We show that our approach correctly reproduces the T3/2 law at low temperatures. The calculated results of magnetic properties for Fe and Fe0.65Ni0.35 Invar demonstrate that the approach works on a much wider temperature interval than the spin-wave approximation.

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