IR, UV-Vis, and NMR

Why it matters

Spectroscopy uses the interaction of light (electromagnetic radiation) with matter to identify compounds and measure how much is present. Three techniques do most of the everyday work: infrared (IR) for functional groups and identity tests, UV-Visible for quantitation, and nuclear magnetic resonance (NMR) for full structure. A pharmacopoeia identity test, for example, often requires the sample's IR spectrum to match a reference standard's. Analysts need to read all three at a basic level and know which one answers which question.

IR: which bonds are present?

IR light makes bonds vibrate. Each type of bond absorbs at a characteristic wavenumber (cm^-1). Key regions:

Below about 1500 cm^-1 is the fingerprint region: too complex to assign peak by peak, but unique to each compound, which is why it is used for identity matching. Most labs now use ATR-FTIR, which needs only a drop or a few milligrams pressed on a crystal.

UV-Vis: how much is present?

UV-Vis light promotes electrons to higher energy levels. Molecules with conjugated double bonds or aromatic rings absorb in the UV or visible range; more conjugation shifts absorption to longer wavelengths (that is why highly conjugated dyes are colored). UV-Vis spectra are broad and not very specific, but quantitation is excellent through the Beer-Lambert law:

A = epsilon x b x c

with absorbance A, molar absorptivity epsilon (L mol^-1 cm^-1), path length b (cm) and concentration c (mol/L). The law holds best at absorbances of roughly 0.1 to 1. Calibration curves were covered in lab-techniques-instrumentation and lab-data-quality; HPLC UV detectors apply the same law.

NMR: how are the atoms connected?

In a strong magnetic field, nuclei such as 1H and 13C absorb radio waves at frequencies that depend on their chemical surroundings. A 1H NMR spectrum tells you three things:

1. Chemical shift (ppm): the environment. Alkyl C-H about 0.9 to 1.5; H next to C=O about 2.0 to 2.5; H on a carbon bearing oxygen about 3.3 to 4.5; alkene H about 4.5 to 6.5; aromatic H about 6.5 to 8; aldehyde H about 9 to 10; carboxylic acid H about 10 to 12.
2. Integration: the area of each signal is proportional to the number of H atoms.
3. Splitting (n + 1 rule): a signal is split into n + 1 lines by n equivalent neighboring H atoms. A CH3 next to a CH2 is a triplet; the CH2 next to a CH3 is a quartet.

Worked example: matching spectra to structures

Two isomers of C2H6O: ethanol (CH3CH2OH) and dimethyl ether (CH3OCH3).

• IR: ethanol shows a broad O-H band near 3300 cm^-1; dimethyl ether has none.
• 1H NMR: dimethyl ether has all six H in one environment, giving one singlet (about 3.2 ppm). Ethanol gives three signals: a triplet (3H, about 1.2 ppm), a quartet (2H, about 3.7 ppm) and an OH signal (1H, position varies).

A second case: a compound shows a strong IR band at 1715 cm^-1, no O-H, and a 1H NMR singlet of 6H near 2.1 ppm. A ketone with two equivalent CH3 groups next to C=O: acetone.

Which technique for which question?

• "Is this the right material?" IR against a reference spectrum.
• "How much is in the solution?" UV-Vis (or HPLC-UV).
• "What is the structure of this unknown or impurity?" NMR, with MS for mass and IR for functional groups.

Common mistakes

• Trying to assign every peak in the IR fingerprint region.
• Reading an absorbance above about 1.5 and trusting Beer-Lambert linearity.
• Forgetting that NMR integration gives relative numbers of H.
• Counting neighbors across an oxygen in the n + 1 rule (coupling through O-H is often not seen because the OH proton exchanges).

Quick recap

• IR: functional groups and identity; C=O near 1700, O-H broad near 3300.
• UV-Vis: quantitation with A = epsilon b c; conjugation shifts absorbance to longer wavelengths.
• 1H NMR: shift (environment), integration (how many H), splitting (n + 1 neighbors).
• Combine IR, NMR and MS to determine a structure.

Try it

Open the Do tab for the quiz: match spectra to structures.