$\text{}^1\text{H}$ $\text{NMR}$ spectroscopy is a powerful technique chemists use to determine the structure of organic molecules by mapping the environment of hydrogen atoms. Standard protons bonded to carbon atoms, such as those in a methyl ($\text{CH}_3$) group, exhibit predictable signals. However, the proton attached to an oxygen atom in an alcohol or carboxylic acid, known as the hydroxyl ($\text{OH}$) proton, presents a unique challenge in the spectrum. Unlike other protons, the $\text{OH}$ signal is highly variable, sometimes appearing as a broad, featureless hump or even seeming to vanish entirely. This unpredictable behavior stems from fundamental chemical processes that interfere with the $\text{NMR}$ measurement timescale.
The Exchangeable Nature of the Hydroxyl Proton
The unusual spectral behavior of the $\text{OH}$ proton is rooted in its ability to undergo rapid chemical exchange. This means the proton can quickly transfer between the hydroxyl group of one molecule and a neighboring molecule, often catalyzed by trace amounts of acid or base impurities. Because the rate of this exchange is typically faster than the timescale of the $\text{NMR}$ experiment, the instrument records an average signal.
This rapid exchange fundamentally impacts the proton’s interaction with adjacent hydrogen atoms on the carbon backbone. Normally, the magnetic influence of neighboring protons causes a signal to split, known as spin-spin coupling. When the $\text{OH}$ proton exchanges quickly, it effectively decouples itself from the adjacent $\text{C-H}$ protons, as it does not stay in one magnetic environment long enough for stable coupling to be registered.
Hydrogen bonding also contributes to the proton’s instability. The oxygen atom’s high electronegativity draws electron density away from the proton, making it susceptible to forming transient bonds with other electronegative atoms. The constant formation and breaking of these temporary hydrogen bonds alters the electron density around the $\text{OH}$ proton, resulting in a highly variable chemical shift.
Where and How the OH Signal Appears
When the hydroxyl proton appears in the $\text{}^1\text{H}$ $\text{NMR}$ spectrum, its visual characteristics are distinct from $\text{C-H}$ protons. The most common appearance is a broad, unsplit singlet, resulting directly from the rapid exchange process averaging out potential coupling. The signal’s broadness relates to an intermediate exchange rate, meaning the proton is not exchanging fast enough for a sharp peak, nor slow enough to resolve coupling.
The chemical shift ($\delta$, measured in $\text{ppm}$) for the $\text{OH}$ proton is highly variable, lacking a narrow, consistent range. For typical aliphatic alcohols, the signal can appear from $0.5$ $\text{ppm}$ to $5.0$ $\text{ppm}$, extending downfield to $8$ $\text{ppm}$ for phenols or beyond $10$ $\text{ppm}$ for acidic species like carboxylic acids. Because other functional groups have reliable ranges, the position of the $\text{OH}$ signal should never be the sole basis for structure confirmation. The lack of splitting and the broad shape are the primary indicators that a signal belongs to an exchangeable proton.
How Experimental Conditions Change the Signal
The position and shape of the $\text{OH}$ signal are highly sensitive to experimental conditions. Concentration is a primary factor, as it directly influences intermolecular hydrogen bonding. In a concentrated solution, strong hydrogen bonds maximize, pulling electron density away from the $\text{OH}$ proton. This causes the proton to become deshielded and shift downfield to a higher $\text{ppm}$ value.
Conversely, diluting the sample reduces molecule-to-molecule interactions, breaking up hydrogen-bonded clusters. As bonds weaken, the proton becomes more shielded, and its signal shifts upfield toward a lower $\text{ppm}$ value. This concentration dependence is a unique characteristic used to confirm an $\text{OH}$ peak; dilution and re-acquisition of the spectrum will show a marked shift in the peak’s position.
Temperature also influences the signal, as raising the temperature disrupts hydrogen bonds. When heated, reduced hydrogen bonding causes the $\text{OH}$ signal to shift upfield, similar to dilution.
Furthermore, the choice of solvent can dramatically alter the signal, particularly aprotic solvents like $\text{DMSO-d}_6$. In $\text{DMSO}$, the solvent strongly accepts hydrogen bonds, stabilizing the $\text{OH}$ proton and slowing the exchange rate considerably. This slower exchange can sometimes resolve the coupling, causing the $\text{OH}$ signal to appear as a recognizable multiplet, such as a doublet or triplet.
Confirming the Presence of the OH Proton
Because the $\text{OH}$ signal is variable, chemists use the $\text{D}_2\text{O}$ (Deuterium Oxide) shake test to confirm its presence. This method takes advantage of the proton’s exchangeable nature. First, the initial $\text{}^1\text{H}$ $\text{NMR}$ spectrum is acquired to identify all proton signals, including the ambiguous one suspected to be the hydroxyl proton.
A small amount of $\text{D}_2\text{O}$ is then added to the $\text{NMR}$ tube and briefly shaken. The $\text{OH}$ proton rapidly exchanges with the deuterium atoms in the $\text{D}_2\text{O}$ to form an $\text{OD}$ group.
Since deuterium ($\text{D}$) is essentially invisible in a standard $\text{}^1\text{H}$ $\text{NMR}$ spectrum, the signal corresponding to the $\text{OH}$ proton disappears entirely from the re-acquired spectrum. The peak’s disappearance provides conclusive evidence that the signal belongs to an exchangeable proton, confirming its identity as an $\text{OH}$ or $\text{NH}$ group.