“19^F- NMR of SF4 gives two triplets [1:2:1] in cryoscopic conditions but it gives only one broad peak on heating”, explain the difference.[ I for 19^F=1/2]
It sounds like interchange (and therefore equivalence) between axial and equatorial fluorine atoms is being facilitated by a higher temperature. Under cryoscopic conditions, the coupling of equatorial with axial fluorine atoms (and vice versa) will give you a pair of triplets, each with a 1:2:1 ratio.
http://pubs.acs.org/doi/abs/10.1021/ed083p336.2
The above gives a fairly good explanation of the behaviour. In NMR changes in spectra are common across a range of species, NMR is a slow technique so at higher temperatures what you see are the average results when you have fluxional behaviour.
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Is the single peak in the higher temperature at a chemical shift between the two peaks at the lower temperature? If so then it's almost certainly chemical exchange between different conformations. If not then you probably have some intermolecular process taking place that gives rise to the result. There's some old stuff on chemical exchange that you might want to look at that. Also you can record the spectrum at a range of temperatures between the two and if in chemical exchange regime you will see the peaks broaden then coalesce as you raise the temperature.
There are two axis F and two equatorial F. Each type of F was split by the other 2 F, so there are two triplet.
Two types of F atoms will exchange. When exchange rate is faster than NMR time scale, then one peak observed (in your case, high temp); when exchange rate is slower than NMR time scale, then two peaks observed (in your case, without heating).
A structure is attached, it may be helpful.
coupling is noticed at lower temperature, however, at higher temp. it might change rapidly from alpha to beta state and vice versa. or in another explanation sulfur atom exceed the octet rule, this means that d-orbitals sharing this bonding, thus upon gaining energy the electrons around sulfur migrate to higher orbital levels decreasing the differences between axial and equatorial fluorine
Classic case of chemical exchange in my opinion. The first answer is fine.
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