Explain hydrogen-bonding effects on chemical shift

Control OH conditions

Ethanol OH can move from δ1.6 dilute to δ4.8 neat while CH₃ stays near δ1.2.

The selective movement identifies a condition-sensitive exchangeable proton.

O–H and N–H signals are unusually sensitive to the sample conditions because hydrogen bonding and proton exchange both change their local magnetic environments.

Hydrogen-bond deshielding

Hydrogen bonding withdraws electron density from O–H and commonly raises its δ.

A distribution of associated environments and exchange can broaden the OH line.

Stronger hydrogen bonding commonly reduces electron density around the exchangeable proton and shifts it downfield. Rapid exchange broadens the signal and can average environments.

Separate variables

Concentration, temperature, solvent hydrogen-bond basicity and impurities can change OH shift and width.

Compare spectra only with the other variables controlled.

Increasing concentration can increase intermolecular hydrogen bonding, while dilution in a non-hydrogen-bonding solvent can reduce it. Temperature and traces of water may also change both shift and line width.

These trends are condition-dependent, so an O–H chemical shift should be reported as variable rather than treated as one fixed identifying number.

Read concentration data

At 298 K ethanol OH moves δ1.6→4.8 as concentration rises; CH₃ remains δ1.2.

The trend supports stronger average intermolecular association.

Compare concentrated and dilute ethanol under otherwise matched conditions. The more strongly hydrogen-bonded sample is expected to show the O–H signal further downfield, while the carbon-bound signals change much less.

Read temperature data

Supplied data: a phenol OH signal moves from δ7.1 to δ6.2 and narrows when the sample is heated from 298 K to 338 K.

Interpret the shift direction and the width change separately, and keep each conclusion to this sample and these conditions.

When a broad signal disappears after D₂O, combine that exchange evidence with the variable shift before assigning O–H or N–H.

Use solvent and D₂O controls

You suspect a broad signal is a phenol OH whose position depends on hydrogen bonding.

Plan a comparison in two solvents plus a D₂O shake. Decide which conditions must be matched, and which signals you expect to move or disappear.

State which result would support the OH assignment and which would count against it.

Common mistake: universal lookup

No single δ identifies every alcohol OH because association and exchange vary.

Use D₂O plus non-labile signals instead of forcing a variable peak.

A broad peak is not automatically an O–H signal. Water, exchange and overlapping signals can also broaden features, so use structure and an exchange test where available.

Cross-check structure

A broad δ11.8 signal lost with D₂O plus a C=O IR band supports CO2H; δ11.8 alone does not.

Next: ionisation/fragmentation.

State the controlled variable, the hydrogen-bonding change and the expected shift or width response. Avoid comparing spectra taken under unspecified different conditions.