Questions: Infrared Spectroscopy for Qualitative Analysis

Question 1

Two unknown compounds both show a strong carbonyl absorption at 1715 cm⁻¹ and no O-H or N-H peaks in their IR spectra. How can IR spectroscopy distinguish between them?

• A. It cannot — the functional-group region is identical, so the compounds cannot be differentiated by IR
• B. Compare the fingerprint region (1500–400 cm⁻¹) against reference spectra — two distinct molecules will show different patterns even with identical functional-group peaks
• C. Measure the exact carbonyl peak position more precisely — structural isomers always differ by at least 10 cm⁻¹
• D. Use ATR sampling instead of transmission — ATR changes relative band positions and may reveal the difference

Answer: (B)

The functional-group region identifies which classes of bonds are present, but the fingerprint region (1500–400 cm⁻¹) contains complex combinations of C-C, C-O, and C-N stretches and bends that are unique to each molecule's full structure. Two structural isomers may share all major functional-group absorptions yet have completely different fingerprint patterns, enabling library matching to distinguish them. This two-region strategy — functional-group region first, then fingerprint region — is the core analytical workflow.

Question 2

An ATR spectrum of a compound shows that the carbonyl band at 1715 cm⁻¹ appears relatively weaker compared to bands near 1000 cm⁻¹ than it does in the corresponding transmission spectrum. What explains this?

• A. The diamond ATR crystal absorbs in the carbonyl region, reducing the observed peak intensity
• B. ATR evanescent wave penetration depth increases at lower wavenumbers, so longer-wavelength absorptions appear relatively stronger in ATR vs transmission
• C. The sample was not pressed firmly enough against the crystal, reducing signal only in the high-wavenumber region
• D. ATR inverts band intensities compared to transmission — this is an artifact to be corrected by background subtraction

Answer: (B)

The evanescent wave generated at the ATR crystal surface penetrates further into the sample at lower wavenumbers (longer wavelengths). This means absorptions at lower wavenumbers (1000 cm⁻¹) benefit from deeper penetration and appear relatively stronger compared to high-wavenumber bands (1715 cm⁻¹). ATR spectra therefore show systematically different relative band intensities compared to transmission spectra. ATR-specific reference databases or software corrections account for this when performing library matching.

Question 3

The absence of an O-H absorption band in the 3200–3600 cm⁻¹ region definitively confirms that the compound contains no hydroxyl groups.

• T. True
• F. False

Answer: False

Absence of a band is not always diagnostic because some vibrations are IR-inactive. For a vibration to absorb IR radiation, it must involve a change in dipole moment. Highly symmetrical vibrations — particularly in molecules with centers of symmetry — can be IR-inactive while remaining Raman-active. Additionally, in practice, weak or overlapping bands can be missed. The correct interpretation is that the absence of an O-H band is consistent with no hydroxyl groups, but additional evidence (NMR, MS, Raman) is needed for a definitive conclusion.

Question 4

The fingerprint region of two different molecules will differ even when both molecules contain identical functional groups.

• T. True
• F. False

Answer: True

The fingerprint region (1500–400 cm⁻¹) arises from complex coupled vibrations involving the entire molecular framework — C-C, C-O, and C-N stretches, bending modes, and skeletal deformations — that are uniquely sensitive to molecular structure beyond just functional group type. Two structural isomers sharing the same functional groups will have the same major peaks in the functional-group region but distinct fingerprint patterns, just as two people may have the same eye color but different fingerprints. This makes the fingerprint region essential for confirming identity, not just classification.

Question 5

Explain why the fingerprint region is essential for distinguishing structural isomers, even when their functional-group region spectra appear identical.

Model answer: The functional-group region (4000–1500 cm⁻¹) reports which bond types are present — O-H, C=O, N-H, C-H — without fully encoding the molecular framework connecting them. Structural isomers share the same functional groups but differ in connectivity. The fingerprint region (1500–400 cm⁻¹) contains absorptions from coupled skeletal vibrations (C-C, C-O, ring deformations, bending modes) that are sensitive to the complete three-dimensional arrangement of atoms. These vibrational patterns are complex enough to be unique to each molecule, like a fingerprint. Library matching algorithms compare this entire pattern rather than individual peaks, allowing isomers with identical functional-group regions to be reliably distinguished.

A practical example: pentanone isomers (2-pentanone and 3-pentanone) both show a C=O stretch near 1715 cm⁻¹, C-H stretches, and no other heteroatom bands — their functional-group regions look nearly identical. But their fingerprint patterns diverge substantially, enabling identification by library comparison.