Silly NMR Question

bomba923
Acidic chemical shift--(NMR Question)

*Well, the greater the deshielding on a proton, the greater the chemical shift (as measured I believe from $Si\left( {CH_3 } \right)_4$), with smaller electron density to reduce the effect of the external magnetic field on THAT proton

Aside from organic molecules, let's take the acidic proton on periodic acid $HIO_4$. Now, due to the massive deshielding from periodate, I would expect the proton to have very great chemical shift.

What quantitatively is the average (or just the range of normal values) chemical shift of the acidic proton in periodic acid ($HIO_4$) ??

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bomba923
Please---any numbers will be greatly appreciated!

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Staff Emeritus
Gold Member
bomba923 said:
Aside from organic molecules, let's take the acidic proton on periodic acid $HIO_4$. Now, due to the massive deshielding from periodate, I would expect the proton to have very great chemical shift.
I know virtually nothing about NMR, so if the following is too naive or just plain stupid, please consider this a disclaimer.

Now, from my tiny understanding of things, the chemical shift from deshielding comes from a sort of "diamagnetic effect" from the H-atom's electron (and possibly other electrons - in alkali metal hydrides, for instance, but I could be totally wrong - in comparable proximity to the proton). So, in general, factors that increase the (negative) charge density around the proton, increase the shielding. Factors that reduce the charge density cause deshielding, which leads to a related chemical shift. The latter should see a strong correlation (I would think) with factors that increase the acidic nature of that proton, since this is essentially the same thing. So, I would imagine that you would see large chemical shifts in strong acids.

What I'm leading up to, is boldly (or naively) drawing a correlation between the chemical shift and the Ka for the acid. Since the proton chemical shift for most carboxylic acids is itself of order 10 or more, I would imagine the chemical shifts for the strong inorganic acids (like HIO4) to be bigger still.

Of course, the whole thing probably gets complicated because we are now talking about aqueous solutions where a large fraction of the protons are actually free, and there is a dynamic equilibrium between the free protons and the bound protons.