INFRARED SPECTROSCOPY
Functional Group Absorption Reference
PMC 431 — Compiled from COOU Lecture Notes
I. REGIONS OF THE IR SPECTRUM
Region Wavenumber Range (cm⁻¹) Contents
Near-IR (NIR) 12,800 – 4,000 Combination / overtone bands
Mid-IR — Diagnostic / 4,000 – 1,000 Fundamental bands for organic molecules
Functional Group
Fingerprint Region < 1,000 Complex; unique to each molecule — used
for identity confirmation
Far-IR 200 – 10 Inorganics and organometallics
II. GENERAL DIAGNOSTIC REGION OVERVIEW (4000–1000 cm⁻¹)
Sub-Region Range (cm⁻¹) Bond Types
X–H Stretching 4000–2500 O–H, N–H, C–H
Triple Bond Stretching 2500–2000 C≡C, C≡N
Double Bond 2000–1500 C=O, C=C, C=N
Stretching
Fingerprint Region < 1500 C–O, C–N, C–X, bending modes
III. VIBRATIONAL MODE TYPES
Stretching — change in bond length
Symmetrical stretch: both atoms move equally away from / toward each other.
Asymmetrical stretch: atoms move in opposite, unequal directions.
Bending — change in bond angle
Mode Plane Description
Scissoring In-plane Two bonds close like scissors
Rocking In-plane Both bonds rock in the same direction
Wagging Out-of-plane Both bonds wag out of plane together
Twisting Out-of-plane Bonds twist in opposite out-of-plane directions
, IV. FUNCTIONAL GROUP ABSORPTION FORMULAE
A. ALKANES (n-Alkanes)
Vibration Frequency (cm⁻¹) Type Region
CH₃ C–H stretch 2962 Asymmetrical stretch Diagnostic
CH₃ C–H stretch 2872 Symmetrical stretch Diagnostic
CH₂ C–H stretch 2926 Asymmetrical stretch Diagnostic
CH₂ C–H stretch 2853 Symmetrical stretch Diagnostic
CH₃ C–H bend ~1375 Bending Fingerprint
CH₂ C–H bend ~1465 Bending Fingerprint
C–C stretch 1200–800 Weak; not diagnostically useful Fingerprint
C–C bend ~500 Out of spectral window —
Key: C–H stretches appear BELOW 3000 cm⁻¹ for sp³ carbon (alkanes). Any C–H ABOVE 3000 cm ⁻¹ indicates sp²
or sp hybridisation.
B. UNCONJUGATED ALKENES (Linear)
Vibration Frequency (cm⁻¹) Type Region
C=C–H stretch ≥ 3000 Stretch Diagnostic
C=C stretch 1680–1600 Moderate to weak Diagnostic
C=C–H bend 1000–650 Out-of-plane bend Fingerprint
C. CYCLIC ALKENES
Vibration Frequency (cm⁻¹) Notes Region
C=C stretch Sensitive to ring size/strain Smaller ring → higher Diagnostic
frequency
Key: Ring strain shifts C=C stretch upward compared to open-chain (unconjugated) alkenes.
D. CONJUGATED ALKENES
Vibration Frequency (cm⁻¹) Type Region
C–H stretch ~3090 Stretch Diagnostic
Symmetrical C=C stretch ~1640 Weak Diagnostic
Asymmetrical C=C stretch ~1598 Strong Diagnostic
Out-of-plane C=C–H bends 990, 892 Out-of-plane bend Fingerprint
Key: Conjugation LOWERS C=C stretch frequency and INCREASES intensity vs. unconjugated alkenes.
Key: Symmetrical conjugated molecules (e.g. butadiene): only the asymmetric C=C stretch is observed.
Asymmetrical molecules (e.g. 1-methylbutadiene): two C=C stretches appear.
Functional Group Absorption Reference
PMC 431 — Compiled from COOU Lecture Notes
I. REGIONS OF THE IR SPECTRUM
Region Wavenumber Range (cm⁻¹) Contents
Near-IR (NIR) 12,800 – 4,000 Combination / overtone bands
Mid-IR — Diagnostic / 4,000 – 1,000 Fundamental bands for organic molecules
Functional Group
Fingerprint Region < 1,000 Complex; unique to each molecule — used
for identity confirmation
Far-IR 200 – 10 Inorganics and organometallics
II. GENERAL DIAGNOSTIC REGION OVERVIEW (4000–1000 cm⁻¹)
Sub-Region Range (cm⁻¹) Bond Types
X–H Stretching 4000–2500 O–H, N–H, C–H
Triple Bond Stretching 2500–2000 C≡C, C≡N
Double Bond 2000–1500 C=O, C=C, C=N
Stretching
Fingerprint Region < 1500 C–O, C–N, C–X, bending modes
III. VIBRATIONAL MODE TYPES
Stretching — change in bond length
Symmetrical stretch: both atoms move equally away from / toward each other.
Asymmetrical stretch: atoms move in opposite, unequal directions.
Bending — change in bond angle
Mode Plane Description
Scissoring In-plane Two bonds close like scissors
Rocking In-plane Both bonds rock in the same direction
Wagging Out-of-plane Both bonds wag out of plane together
Twisting Out-of-plane Bonds twist in opposite out-of-plane directions
, IV. FUNCTIONAL GROUP ABSORPTION FORMULAE
A. ALKANES (n-Alkanes)
Vibration Frequency (cm⁻¹) Type Region
CH₃ C–H stretch 2962 Asymmetrical stretch Diagnostic
CH₃ C–H stretch 2872 Symmetrical stretch Diagnostic
CH₂ C–H stretch 2926 Asymmetrical stretch Diagnostic
CH₂ C–H stretch 2853 Symmetrical stretch Diagnostic
CH₃ C–H bend ~1375 Bending Fingerprint
CH₂ C–H bend ~1465 Bending Fingerprint
C–C stretch 1200–800 Weak; not diagnostically useful Fingerprint
C–C bend ~500 Out of spectral window —
Key: C–H stretches appear BELOW 3000 cm⁻¹ for sp³ carbon (alkanes). Any C–H ABOVE 3000 cm ⁻¹ indicates sp²
or sp hybridisation.
B. UNCONJUGATED ALKENES (Linear)
Vibration Frequency (cm⁻¹) Type Region
C=C–H stretch ≥ 3000 Stretch Diagnostic
C=C stretch 1680–1600 Moderate to weak Diagnostic
C=C–H bend 1000–650 Out-of-plane bend Fingerprint
C. CYCLIC ALKENES
Vibration Frequency (cm⁻¹) Notes Region
C=C stretch Sensitive to ring size/strain Smaller ring → higher Diagnostic
frequency
Key: Ring strain shifts C=C stretch upward compared to open-chain (unconjugated) alkenes.
D. CONJUGATED ALKENES
Vibration Frequency (cm⁻¹) Type Region
C–H stretch ~3090 Stretch Diagnostic
Symmetrical C=C stretch ~1640 Weak Diagnostic
Asymmetrical C=C stretch ~1598 Strong Diagnostic
Out-of-plane C=C–H bends 990, 892 Out-of-plane bend Fingerprint
Key: Conjugation LOWERS C=C stretch frequency and INCREASES intensity vs. unconjugated alkenes.
Key: Symmetrical conjugated molecules (e.g. butadiene): only the asymmetric C=C stretch is observed.
Asymmetrical molecules (e.g. 1-methylbutadiene): two C=C stretches appear.
