Infrared (IR) and Mass Spectroscopy

Introduction to Spectroscopy and Chemical Analysis

Spectroscopy is a fundamental analytical technique in organic chemistry, allowing for the determination of molecular structure through the interaction of electromagnetic radiation with matter. Prior to the 1950s, chemical structure elucidation relied on chemical reactions, but the advent of spectroscopic methods such as Nuclear Magnetic Resonance (NMR), Infrared (IR), and Ultraviolet-Visible (UV-Vis) spectroscopy revolutionized the field. Mass spectrometry (MS), while not a true spectroscopic technique, is also essential for molecular analysis.

• Spectroscopy: Study of the interaction between electromagnetic radiation (light) and matter.

• Mass Spectrometry: Analytical technique measuring the mass-to-charge ratio (m/z) of ions, not involving light absorption or emission.

Principles of Molecular Spectroscopy

Light exhibits both wave-like and particle-like properties (wave-particle duality). In spectroscopy, the wave properties—specifically wavelength (λ) and frequency (ν)—are used to study molecular interactions. The energy of a photon is directly proportional to its frequency and inversely proportional to its wavelength.

• Frequency (ν): Number of wave cycles per second (Hz).

• Wavelength (λ): Distance between two consecutive peaks or troughs.

• Energy of a photon:

• Relationship between speed, wavelength, and frequency:

• Energy and wavelength:

Quantization of Molecular Energies

Molecular energies are quantized, meaning molecules can only occupy discrete energy levels. Absorption of electromagnetic radiation causes transitions between these levels, with the energy difference () matching the energy of the absorbed photon.

• Absorption occurs when .

• The specific energies absorbed depend on molecular structure.

• A spectrum is a graph of absorption versus frequency or wavelength.

Types of Molecular Transitions

Depending on the energy of the absorbed light, molecules can undergo various transitions:

• Electronic transitions (UV-Vis region)

• Vibrational transitions (IR region)

• Rotational transitions (Microwave region)

• Ionization (X-ray, gamma-ray region)

Infrared (IR) Spectroscopy

Principles and Vibrational Modes

IR spectroscopy is based on the absorption of infrared light, which excites molecular vibrations. Atoms in molecules vibrate like masses connected by springs, and these vibrations are quantized.

• Vibrational Modes: Ways in which the atoms in a molecule can vibrate.

• Types: Stretching (change in bond length) and Bending (change in bond angle).

• Number of fundamental vibrational modes:

  • Nonlinear molecule:

  • Linear molecule:

  • Example: Water (H2O, nonlinear, n=3): modes

IR-Active and IR-Inactive Vibrations

For a vibration to be IR-active, it must cause a change in the dipole moment of the molecule. Symmetrical molecules or vibrations that do not alter the dipole moment are IR-inactive.

• IR-active: Vibration changes dipole moment; absorption observed.

• IR-inactive: No dipole moment change; no absorption.

• Intensity of absorption depends on the magnitude of dipole moment change.

Factors Affecting Vibrational Frequency

• Bond strength (force constant, k): Stronger bonds vibrate at higher frequencies.

• Reduced mass (μ): Heavier atoms vibrate at lower frequencies.

• Vibrational frequency equation:

• Examples:

  • C–H stretch: ~3000 cm−1

  • C–D stretch: ~2200 cm−1

  • C–F stretch: ~1100 cm−1

  • C–Cl stretch: ~700 cm−1

  • C–Br stretch: ~500 cm−1

Infrared Spectroscopy and Wavenumbers

IR spectra are typically plotted as percent transmittance versus wavenumber (cm−1), which is the reciprocal of wavelength in centimeters. Wavenumber is directly proportional to energy.

• Wavenumber (\( \tilde{\nu} \)): (in cm−1)

• Most useful IR region: 4000–400 cm−1

• Fingerprint region: 600–1400 cm−1, unique for each molecule

• Functional group region: 1600–3500 cm−1, used to identify functional groups

Interpretation of IR Spectra

Key absorption bands in IR spectra correspond to specific bond vibrations and functional groups:

• Hydrocarbons (C–C, C=C, C≡C):

  • C–C: ~1200 cm−1

  • C=C: ~1660 cm−1

  • C≡C: <2200 cm−1

  • Conjugation lowers frequency (aromatic C=C: ~1600 cm−1)

• C–H Stretching:

  • sp3: 2800–3000 cm−1

  • sp2: 3000–3100 cm−1

  • sp: ~3300 cm−1 (sharp)

• O–H and N–H Stretching:

  • O–H (alcohol): broad, ~3300 cm−1

  • N–H (amine): broad, with one (secondary) or two (primary) spikes, ~3300 cm−1

  • No signal for tertiary amines (no N–H bond)

• Carbonyl Compounds (C=O):

  • Strong, sharp peak around 1700 cm−1

  • Ketones: ~1710 cm−1

  • Aldehydes: ~1725 cm−1

  • Carboxylic acids: ~1710 cm−1 (broad O–H, 2500–3500 cm−1)

• Carbon–Nitrogen Stretching:

  • C–N: ~1200 cm−1

  • C=N: ~1660 cm−1

  • C≡N: >2200 cm−1 (stronger than C≡C)

Summary Table: IR Stretching Frequencies

Practice: Functional Group Identification from IR Spectra

Given IR spectra, identify the major functional groups by analyzing the position and shape of the absorption bands above 1600 cm−1.

Mass Spectrometry (MS)

Principles and Instrumentation

Mass spectrometry is a destructive analytical technique that measures the mass-to-charge ratio (m/z) of ions. It is used to determine the molecular weight and structure of compounds.

• Ion Source: Sample is ionized, usually to cations by electron impact or other methods.

• Mass Analyzer: Ions are separated based on their m/z ratio.

• Detector: Measures and records the abundance of each ion.

• Common ionization techniques: Electron Impact (EI), Electrospray Ionization (ESI), Matrix-Assisted Laser Desorption Ionization (MALDI).

Mass Spectrum Interpretation

• Base Peak: Most intense peak, assigned 100% abundance.

• Molecular Ion (M+): Highest-mass ion, corresponds to the unfragmented molecule.

• Fragmentation: Molecular ion breaks into smaller ions, producing a characteristic pattern.

• Most ions are singly charged, so m/z ≈ mass.

Fragmentation Patterns

• Homolytic Cleavage: Produces a cation and a radical; only cations are detected.

• Alpha Cleavage: C–C bond next to a heteroatom or carbonyl breaks, forming resonance-stabilized ions.

• McLafferty Rearrangement: β-cleavage in carbonyl compounds, producing a neutral alkene and a resonance-stabilized ion.

Isotopes in Mass Spectra

Isotopic variants of elements produce additional peaks in the mass spectrum. The relative abundance of these peaks provides information about the presence of elements such as Cl, Br, and S.

Summary

• IR spectroscopy identifies functional groups by their characteristic absorption bands.

• Mass spectrometry provides molecular weight and structural information through fragmentation patterns and isotopic peaks.

• Both techniques are essential for the elucidation of organic molecular structures.