Silly NMR Question

  • Thread starter bomba923
  • Start date
Acidic chemical shift--(NMR Question)

*Well, the greater the deshielding on a proton, the greater the chemical shift (as measured I believe from [itex] Si\left( {CH_3 } \right)_4 [/itex]), 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 [itex] HIO_4 [/itex]. 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 ([itex] HIO_4 [/itex]) ??
 
Last edited:
Please---any numbers will be greatly appreciated!
 
Last edited:

Gokul43201

Staff Emeritus
Science Advisor
Gold Member
6,987
14
bomba923 said:
Aside from organic molecules, let's take the acidic proton on periodic acid [itex] HIO_4 [/itex]. 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.
 

movies

Science Advisor
283
1
I think that a better approximation of aqueous acid is that there is very little free H+ and a lot of H3O+. To really have "free" protons you'd have to have more H+ than H2O. Water is more basic than things like Cl-.

At any rate, I asked my professor a question like this once and he said that, theoretically, the proton that would have the greatest chemical shift would be a free proton. It is, of course, impossible to get that data in solution phase because you never have a free proton in solution. You'd have to use gas phase NMR.
 

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving

Hot Threads

Top