Interpret IR Spectrum

Analyze infrared absorption spectra to identify functional groups, assess hydrogen bonding, and compile a comprehensive inventory of structural features present in the sample.

When to Use

• Identifying functional groups in an unknown compound as a first screening step
• Confirming the presence or absence of specific functional groups (e.g., verifying a reaction converted an alcohol to a ketone)
• Monitoring reaction progress by tracking the appearance or disappearance of characteristic absorptions
• Distinguishing between similar compounds that differ in functional group content
• Complementing NMR and mass spectrometry data with vibrational information

Inputs

• Required: IR spectrum data (absorption frequencies in cm-1 with intensities, either as %Transmittance or Absorbance plot)
• Required: Sample preparation method (KBr pellet, ATR, Nujol mull, thin film, solution cell)
• Optional: Molecular formula or expected compound class
• Optional: Known structural fragments from other spectroscopic data
• Optional: Instrument parameters (resolution, scan range, detector type)

Procedure

Step 1: Establish Spectrum Quality and Format

Verify that the spectrum is suitable for interpretation before analyzing peaks:

1. Check y-axis format: Determine whether the spectrum is plotted in %Transmittance (%T, peaks point down) or Absorbance (A, peaks point up). All subsequent analysis assumes consistent convention.
2. Verify wavenumber range: Confirm the spectrum covers at least 4000--400 cm-1 for a standard mid-IR analysis. Note any truncation.
3. Assess baseline: A good baseline should be relatively flat and near 100%T (or 0 Absorbance) in regions with no absorption. Sloping or noisy baselines reduce reliability.
4. Check resolution: Adjacent peaks separated by less than the instrumental resolution cannot be distinguished. Typical FTIR resolution is 4 cm-1.
5. Identify preparation artifacts: KBr pellets may show a broad O-H band from absorbed moisture (~3400 cm-1). Nujol mulls obscure C-H stretches. ATR spectra show intensity distortion at low wavenumbers. Note any artifacts that limit interpretation.

Expected: Spectrum confirmed as suitable for analysis, with format, range, and artifacts documented.

On failure: If the spectrum has severe baseline problems, saturation (flat-bottomed peaks from too-concentrated samples), or preparation artifacts obscuring critical regions, note the limitation and flag affected spectral regions as unreliable.

Step 2: Scan the Diagnostic Region (4000--1500 cm-1)

Systematically analyze the high-frequency region where most functional groups produce characteristic absorptions:

1. O-H stretches (3200--3600 cm-1): Look for broad absorptions. A sharp peak near 3600 cm-1 indicates free O-H; a broad band centered at 3200--3400 cm-1 indicates hydrogen-bonded O-H (alcohols, carboxylic acids, water).
2. N-H stretches (3300--3500 cm-1): Primary amines show two peaks (symmetric and asymmetric stretch); secondary amines show one peak. These are typically sharper than O-H bands.
3. C-H stretches (2800--3300 cm-1):

Frequency (cm-1)	Assignment
3300	sp C-H (alkyne, sharp)
3000--3100	sp2 C-H (aromatic, vinyl)
2850--3000	sp3 C-H (alkyl, multiple peaks)
2700--2850	Aldehyde C-H (two peaks from Fermi resonance)

4. Triple-bond region (2000--2300 cm-1):

Frequency (cm-1)	Assignment	Notes
2100--2260	C triple-bond C	Weak or absent if symmetric
2200--2260	C triple-bond N	Medium to strong
~2350	CO2	Atmospheric artifact, disregard

5. Carbonyl region (1650--1800 cm-1) -- the most diagnostic single region in IR:

Frequency (cm-1)	Assignment
1800--1830, 1740--1770	Acid anhydride (two C=O stretches)
1770--1780	Acid chloride
1735--1750	Ester
1700--1725	Carboxylic acid
1705--1720	Aldehyde
1705--1720	Ketone
1680--1700	Conjugated ketone / alpha-beta unsaturated
1630--1690	Amide (amide I band)

6. C=C and C=N stretches (1600--1680 cm-1): Alkene C=C appears at 1620--1680 cm-1 (weak to medium). Aromatic C=C shows multiple peaks near 1450--1600 cm-1. C=N (imine) appears at 1620--1660 cm-1.

Expected: All absorptions in the diagnostic region identified, with functional group assignments and confidence levels (strong, tentative, absent).

On failure: If the carbonyl region is obscured (e.g., water absorption in KBr, atmospheric CO2), note the gap. If an expected functional group absorption is absent, confirm with a second preparation method before concluding it is truly absent.

Step 3: Analyze the Fingerprint Region (1500--400 cm-1)

Examine the lower-frequency region for confirmatory and structural detail:

1. C-O stretches (1000--1300 cm-1): Ethers, esters, alcohols, and carboxylic acids produce strong C-O stretching absorptions. Esters show a characteristic strong band near 1000--1100 cm-1 in addition to the carbonyl.
2. C-N stretches (1000--1250 cm-1): Amines and amides; overlap with C-O makes assignment tentative without other evidence.
3. C-F, C-Cl, C-Br stretches:

Frequency (cm-1)	Assignment
1000--1400	C-F (strong)
600--800	C-Cl
500--680	C-Br

4. Aromatic substitution pattern (700--900 cm-1): Out-of-plane C-H bending reveals substitution:

Frequency (cm-1)	Pattern
730--770	Mono-substituted (+ 690--710)
735--770	Ortho-disubstituted
750--810, 860--900	Meta-disubstituted
790--840	Para-disubstituted

5. Overall fingerprint comparison: The fingerprint region is unique to each compound. If a reference spectrum is available, overlay and compare this region for identity confirmation.

Expected: Confirmatory assignments for functional groups identified in Step 2, plus additional structural detail (substitution patterns, C-O/C-N assignments).

On failure: The fingerprint region is inherently complex and overlapping. If assignments are ambiguous, flag them as tentative and rely on the diagnostic region and other spectroscopic data for final conclusions.

Step 4: Assess Hydrogen Bonding and Intermolecular Effects

Evaluate how sample state and intermolecular interactions affect the spectrum:

1. Hydrogen bonding broadening: Compare the width and position of O-H and N-H bands. Free O-H is sharp and near 3600 cm-1; hydrogen-bonded O-H is broad and shifted to 3200--3400 cm-1. Carboxylic acid dimers show a very broad O-H from 2500--3300 cm-1.
2. Concentration and state effects: Solution spectra at different concentrations can distinguish intramolecular (concentration-independent) from intermolecular (concentration-dependent) hydrogen bonds.
3. Fermi resonance: Two overlapping bands can interact to split into a doublet. The classic example is the aldehyde C-H pair near 2720 and 2820 cm-1. Recognize Fermi resonance to avoid misassigning extra peaks as separate functional groups.
4. Solid-state effects: KBr pellets and Nujol mulls reflect solid-state packing, which broadens bands and can shift frequencies by 10--20 cm-1 relative to solution spectra. ATR spectra are closest to the neat liquid state.

Expected: Hydrogen bonding state characterized, preparation-method artifacts accounted for, and any anomalous band shapes explained.

On failure: If hydrogen bonding effects cannot be resolved (e.g., overlapping O-H and N-H bands), note the ambiguity. A D2O exchange experiment or variable-temperature study can help, but these require additional data.

Step 5: Compile Functional Group Inventory

Assemble all findings into a structured report:

1. List confirmed functional groups: Groups with strong, unambiguous absorptions in the diagnostic region (e.g., sharp C=O at 1715 cm-1 = ketone or aldehyde).
2. List tentative assignments: Groups with weaker evidence or overlapping absorptions that could be explained by more than one functional group.
3. List absent functional groups: Groups whose characteristic strong absorptions are clearly missing from the spectrum (e.g., no broad O-H band means no free alcohol or carboxylic acid).
4. Note discrepancies: Any absorptions that do not fit the proposed functional group set, or expected absorptions that are missing.
5. Cross-reference: Compare the IR-derived functional group inventory with information from other techniques (NMR, MS, UV-Vis) if available.

Expected: A complete functional group inventory categorized by confidence level, with specific frequencies and intensities cited as evidence for each assignment.

On failure: If the inventory is incomplete or contradictory, identify which additional experiments (ATR vs. KBr comparison, variable concentration, D2O exchange) would resolve the ambiguities.

Validation

• Spectrum quality assessed (baseline, resolution, artifacts, y-axis format)
• Solvent, preparation-method, and atmospheric artifacts identified and excluded
• All absorptions in the diagnostic region (4000--1500 cm-1) assigned or flagged
• Carbonyl region analyzed with specific sub-type assignment where possible
• Fingerprint region examined for confirmatory evidence
• Hydrogen bonding effects evaluated and their influence on peak shape/position documented
• Functional group inventory compiled with confidence levels
• Absent functional groups explicitly noted (negative evidence is informative)
• Assignments cross-referenced with other available spectroscopic data

Common Pitfalls

• Ignoring preparation artifacts: KBr moisture (broad 3400 cm-1), Nujol C-H (2850--2950 cm-1), and ATR intensity distortion at low wavenumbers all mimic or obscure real sample absorptions. Always consider the preparation method.
• Over-interpreting the fingerprint region: The region below 1500 cm-1 is complex and overlapping. Use it for confirmation, not primary identification. Avoid assigning every peak.
• Confusing atmospheric CO2 with sample peaks: The sharp doublet near 2350 cm-1 is almost always atmospheric CO2, not a sample absorption. Background subtraction should remove it, but verify.
• Neglecting band intensity and width: A strong, broad absorption has different diagnostic value than a weak, sharp peak at the same frequency. Report intensity (strong/medium/weak) and shape (sharp/broad) alongside frequency.
• Single-peak assignments: Never identify a functional group from a single absorption alone. Carbonyl groups, for example, should be supported by additional bands (C-O for esters, N-H for amides, C-H for aldehydes).
• Assuming absence from weak absorption: Some functional groups produce inherently weak IR absorptions (symmetric C=C, triple bonds in symmetric alkynes). Absence of a peak does not always mean absence of the group.

Related Skills

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• interpret-mass-spectrum

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• interpret-uv-vis-spectrum

  -- characterize chromophores complementing IR functional group data

• interpret-raman-spectrum

  -- obtain complementary vibrational data for IR-inactive modes

• plan-spectroscopic-analysis

  -- select and sequence spectroscopic techniques before data acquisition