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Date: 25-6-2019
3032
Date: 17-6-2019
1062
Date: 23-2-2018
1276
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NMR active nuclei and isotopes as tracers
Despite its low abundance , 17O has been used in studies of, for example, hydrated ions in aqueous solution and polyoxometallates. The isotope 18O is present to an extent of 0.2% in naturally occurring oxygen and is commonly used as a (nonradioactive) tracer for the element. The usual tracer for sulfur is 35S, which is made by an (n,p) reaction on 35Cl; 35S is a b-emitter with t1/2 = 87 days.
Worked example
NMR spectroscopy using 77Se and 125Te nuclei
The solution 125Te NMR spectrum of Te(cyclo-C6H11)2 at 298K shows one broad signal. On increasing the temperature to 353 K, the signal sharpens. On cooling to 183 K, the signal splits into three signals at δ 601, 503 and 381 with relative integrals of 25 :14 : 1. Rationalize these data.
Te(cyclo-C6H11)2 contains only one Te environment, but the Te atom can be in either an equatorial or axial position of the cyclohexyl ring. This leads to three possible conformers:
On steric grounds, the most favoured is the equatorial, equatorial conformer, and the least favoured is the axial, axial conformer. Signals at δ 601, 503 and 381 in the lowtemperature spectrum can be assigned to the equatorial, equatorial, axial,equatorial and axial,axial conformers respectively.
At higher temperatures, the cyclohexyl rings undergo ring inversion (ring-flipping), causing the Te atom to switch between axial and equatorial positions. This interconverts the three conformers of Te(cyclo-C6H11)2. At 353 K, the interconversion is faster than the NMR timescale and one signal is observed (its chemical shift is the weighted average of the three signals observed at 183 K). On cooling from 353 to 298 K, the signal broadens, before splitting at lower temperatures.
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تفوقت في الاختبار على الجميع.. فاكهة "خارقة" في عالم التغذية
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أمين عام أوبك: النفط الخام والغاز الطبيعي "هبة من الله"
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قسم شؤون المعارف ينظم دورة عن آليات عمل الفهارس الفنية للموسوعات والكتب لملاكاته
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