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Why does the water peak appear at different chemical shift values (ppm) in different solvents in proton NMR spectra? For example, the water peak in DMSO-d6 appears at nearly 3.33 ppm, but the same moisture peak in $\ce{CDCl3}$ appears at 1.56 ppm.

What is the reason behind it? Why should the same species ($\ce{H2O}$) give rise to two different chemical shift values?

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Consider also water in addition to the solvents you mention:

  • water-d2 (or, $\ce{D2O}$) is an extensively H-bonding polar solvent (dielectric constant $\epsilon = 79$)
  • $\ce{DMSO-d6}$) is polar ($\epsilon= 47$) but aprotic (no-H-bond donation ability)
  • chloroform-d is an apolar aprotic solvent ($\epsilon=4.81$)

Now consider the state of a trace amount of $\ce{H2O}$ in each of these solvents, giving rise to an NMR signal:

  • in $\ce{D2O}$ the trace $\ce{H}$ can exchange and forms $\ce{HDO}$ which retains an extensive H-bond network. $\ce{H}$ is strongly deshielded by bonded and H-bonded electronegative oxygen and due to the formation of oxonium ion with $\ce{H}$ carrying a significant positive partial charge. The chemical shift is $\approx \pu{4.7 ppm}$
  • in DMSO-d6, $\ce{H}$ is strongly coordinated (H-bonded) by DMSO oxygen atoms, resulting in substantial shielding. The chemical shift is $\approx \pu{3.33 ppm}$
  • in chloroform-d interactions with the solvent are comparably weak and mainly dipolar or dispersion interactions. $\ce{H}$ experiences deshielding mainly due to directly bonded oxygen. The chemical shift is $\approx \pu{1.56 ppm}$

The following table shows shifts for residual water $\ce{H}$ in different solvents ([1]; dielectric constants are for non-deuterated solvents):

$\begin{array}{c} \begin{array}{c|c|c} \text{solvent} &\text{shift(ppm)}&\epsilon\\\hline \ce{C6D6 }&0.40 & 2.28\\ \text{toluene-d8} &0.43 &2.38\\ \ce{C6D5Cl} &1.03 &5.69\\ \ce{CD2Cl2 }&1.52 &9.08\\ \ce{CDCl3} &1.56 &4.81\\ \ce{CD3CN} &2.13 &36.64\\ \text{THF-d8} &2.46 &7.52\\ \ce{(CD3)2CO} &2.84 &21.01\\ \ce{(CD3)2SO} &3.33 &47\\ \text{TFE-d3} &3.66&8.55 \\ \ce{CD3OD} &4.87 &32.6\\ \end{array} \end{array}$

Note that the most deshielding solvents are the protic H-bonding ones (the bottom two). Next in effectiveness are those with oxygen atoms with lone electron pairs available for H-bonding. Note also that the top three have an additional effect on water shifts due to the aromatic ring current.


Reference [1] Fulmer et al. Organometallics 2010, 29, 2176–2179

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Why do shifts change anyway? You might consider that different solvents may (de-)stabilize certain conformers, or result in different interactions? An example from a previous life (sadly a long time ago, and I can't recall the exact detail or even if NMR was reported in the paper I was following) is that in certain 3,4,5-trisubstituted pyridines (with non identical C3 and C5 substituents) H-2 and H-6 have similar shifts in d-CDCl3 (so it looks like an integral of 2) but they are clearly separated in d6-DMSO.

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