Infrared Spectroscopy: Principles and Applications in Organic Chemistry

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: $E = h\nu$ where $h$ is Planck's constant and $\nu$ is the frequency in Hz.

• Frequency and wavelength are inversely related: $\nu = \frac{c}{\lambda}$ $E = h\nu = \frac{hc}{\lambda}$

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 ($\tilde{\nu}$, in cm-1) are commonly used in IR spectroscopy and are directly proportional to energy and frequency: $\tilde{\nu} = \frac{1}{\lambda (\text{cm})}$

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 $n$ atoms have $3n - 6$ 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 ($T$), calculated as $T = \frac{I}{I_0}$, where $I$ is transmitted intensity and $I_0$ is incident intensity. $\%T = (\frac{I}{I_0}) \times 100$

• 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:

$\tilde{\nu} = \frac{1}{2\pi c} \sqrt{\frac{f(m_1 + m_2)}{m_1 m_2}}$

• $\tilde{\nu}$ = wavenumber of the stretching vibration

• $f$ = force constant (bond strength)

• $m_1, m_2$ = masses of atoms joined by the bond

• $c$ = 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

Bond	Bond Energy (kcal/mol)	Stretching Frequency (cm-1)
C–H	100 (420)	3000
C–D	100 (420)	2100
C–C	83 (350)	1200
C=C	146 (611)	1660
C≡C	200 (840)	2200

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.

Bonds to H	Triple Bonds	Double Bonds	Single Bonds
O–H single bond N–H single bond C–H single bond	C≡C C≡N	C=O C=C C=N	C–C C–N C–O
4000 cm-1	2700 cm-1	2000 cm-1	1600 cm-1 600 cm-1 (Fingerprint Region)

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.