Infrared Spectroscopy

Introduction to Spectroscopy

Spectroscopy is an essential analytical technique in organic chemistry, used to determine the structure of molecules by analyzing their interaction with electromagnetic radiation. It is a non-destructive method that measures the amount of light absorbed by a sample as the wavelength is varied.

• Spectroscopy studies the interaction of matter with the electromagnetic spectrum.

• Electromagnetic radiation exhibits both particle and wave properties.

• A photon is a particle of electromagnetic energy.

• The energy of a photon is proportional to its frequency: where is Planck's constant and is the frequency in Hz.

• Frequency and wavelength are inversely related:

Types of Spectroscopy

• Infrared (IR) Spectroscopy: Measures bond vibration frequencies to identify functional groups.

• Mass Spectrometry (MS): Fragments molecules and measures the masses of the fragments.

• Nuclear Magnetic Resonance (NMR) Spectroscopy: Detects signals from hydrogen atoms to distinguish isomers.

• Ultraviolet (UV) Spectroscopy: Uses electron transitions to determine bonding patterns.

The Electromagnetic Spectrum and Molecular Effects

• Different regions of the electromagnetic spectrum interact with molecules in distinct ways:

• High energy (short wavelength) regions cause ionization and electronic transitions (e.g., UV, X-rays).

• Infrared (IR) region (wavelengths 2.5–25 μm) is associated with molecular vibrations.

• Wavenumbers (, in cm-1) are commonly used in IR spectroscopy and are directly proportional to energy and frequency:

The IR Spectroscopic Process

• Quantum mechanical energy levels in IR correspond to molecular vibrations, perceived as heat.

• Covalent bonds behave like vibrating springs, with two main types of vibrations:

  • Stretching: Vibration along the bond axis (symmetric or asymmetric).

  • Bending: Vibration not along the bond axis (scissoring, rocking, twisting, wagging).

• Nonlinear molecules with atoms have fundamental vibrational modes.

• When IR radiation matches the frequency of a molecular vibration, energy is absorbed, increasing the amplitude of vibration (not frequency).

• The spectrometer detects the emitted photon as the molecule returns to its ground state, producing an IR spectrum.

The IR Spectrum

• X-axis: Wavenumbers (cm-1), typically 400–4000 cm-1 (mid-infrared).

• Y-axis: Percent transmittance (), calculated as , where is transmitted intensity and is incident intensity.

• Peaks are Gaussian distributions representing the average energy of transitions.

• High frequencies and wavenumbers correspond to higher energy transitions.

Detecting Different Bonds: Hooke's Law

Bond vibrations can be modeled using Hooke's law:

• = wavenumber of the stretching vibration

• = force constant (bond strength)

• = masses of atoms joined by the bond

• = speed of light

Key Points:

• Stronger bonds and lighter atoms give rise to higher wavenumbers.

• Frequency decreases with increasing atomic weight and increases with increasing bond energy.

Stretching Frequencies Table

Peak Intensities

• Strong (s): Tall peak, low transmittance

• Medium (m): Mid-height peak

• Weak (w): Short peak, high transmittance

• Broad (br): Abnormally broad Gaussian distribution (spans many energies)

Fingerprint of a Molecule

• Whole-molecule vibrations and bending vibrations are quantized.

• No two molecules (except enantiomers) have identical IR spectra.

• Simple stretching: 1600–3500 cm-1

• Complex vibrations: 600–1400 cm-1 ("fingerprint region")

IR-Active and Inactive Bonds

• A polar bond is usually IR-active.

• A non-polar bond in a symmetrical molecule absorbs weakly or not at all.

Instrumentation

An Infrared Spectrometer

• Consists of a glowing wire source, monochromator, sample and reference cells, rotating mirrors, and a detector.

• Produces a spectrum of light transmitted versus wavenumber.

FT-IR Spectrometer

• Uses an interferometer for better sensitivity and faster scans (1–2 seconds).

• Multiple scans are averaged for accuracy.

• Laser calibration ensures precision.

Infrared Group Analysis

• The primary use of IR spectroscopy is to detect functional groups.

• Functional groups are identified by their characteristic absorption bands.

• Most organic functional groups show multiple IR bands due to multiple bonds.

Characteristic Absorptions

Carbon-Carbon Bond Stretching

• C–C: 1200 cm-1

• C=C: 1660 cm-1

• C≡C: 2200 cm-1 (weak or absent if internal)

• Conjugation lowers the frequency:

  • Isolated C=C: 1640–1680 cm-1

  • Conjugated C=C: 1620–1640 cm-1

  • Aromatic C=C: ~1600 cm-1

Carbon-Hydrogen Stretching

• Bonds with more s character absorb at higher frequency:

  • sp3 C–H: just below 3000 cm-1

  • sp2 C–H: just above 3000 cm-1

  • sp C–H: at 3300 cm-1

Examples of IR Spectra

• Alkane: C–H stretch (just below 3000 cm-1), C–H bending (1400–1500 cm-1).

• Alkene: C–H stretch (just above 3000 cm-1), C=C stretch (1640–1680 cm-1).

• Alkyne: C≡C stretch (2100–2260 cm-1), sp C–H stretch (3300 cm-1).

• Aromatics: Pair of sharp bands at 1500 and 1600 cm-1, C–H stretch at 3000–3100 cm-1.

Substitution Patterns in Aromatics

• The region 1667–2000 cm-1 (overtone of bending) can indicate substitution patterns (mono-, ortho-, meta-, para-).

• Out-of-plane bending vibrations (650–900 cm-1) help identify substitution on aromatic rings and alkenes.

Ethers

• Strong band for antisymmetric C–O–C stretch at 1050–1150 cm-1.

• Other bands dominated by hydrocarbon component.

O–H and N–H Stretching

• Both occur around 3300 cm-1 but differ in appearance:

  • Alcohol O–H: broad, rounded tip

  • Secondary amine (R2NH): broad, one sharp spike

  • Primary amine (RNH2): broad, two sharp spikes

  • No signal for tertiary amine (R3N)

Alcohol IR Spectrum

• Strong, broad O–H stretch (3200–3400 cm-1), one of the most recognizable IR bands.

• C–O stretch (1050–1260 cm-1), position varies with substitution.

• Broad shape due to hydrogen bonding.

Amine IR Spectrum

• N–H stretch for NH2 as a doublet (3200–3500 cm-1).

• Deformation band for NH2 (1590–1650 cm-1).

• "Wag" band at 780–820 cm-1 (not diagnostic).

Additional info: These notes cover the core concepts of IR spectroscopy as outlined in Chapter 12 of a typical Organic Chemistry curriculum, including principles, instrumentation, and interpretation of IR spectra for functional group identification.