# Proton NMR chemical shift of water peak in different solvents

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?

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

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.