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Introduction to Molecular Spectroscopy

• Molecular Spectroscopy: Study of the interaction of electromagnetic radiation with matter.
• Key Components:
  • Electromagnetic Force and Molecules

Electromagnetic Radiation

• Key Terms:
  • Wavelength (λ): Length of one wave.
  • Wavenumber (σ): Number of waves per meter; defined by the formula $ext{σ} = \frac{1}{ ext{λ}}$.
  • Frequency (ν): Number of waves per second.
  • Speed of Light (c): $c = ext{λ} imes ext{ν}$, where $c = 2.998 imes 10^8 ext{ m/s}$.
• Energy Relationships:
  • Planck's Law: $E = h<br />\nu$, where $h = 6.626 imes 10^{-34} ext{ J∙s}$.
  • Energy Units: Joules (J), kilocalories (kcal), electron volts (eV).

Electromagnetic Spectrum

• General Characteristics:
  • High Energy: Short wavelength → High frequency → High wavenumber.
  • Low Energy: Long wavelength → Low frequency → Low wavenumber.

Techniques in Spectroscopy

UV-Vis Spectroscopy

• Wavelength Range: 190 - 800 nm.
• Effect: Energy absorption by bound and free electrons in a molecule.
• Information Obtained: Type of electrons and binding type in molecules (e.g. O2N, OH).

Infrared (IR) Spectroscopy

• Wavelength Range: 2.5 - 16 µm (4000 - 625 cm⁻¹).
• Effect: Energy absorption leading to changes in vibrational states of molecular bonds.
• Information Obtained: Type of functional groups (e.g. O2N, OH).

Nuclear Magnetic Resonance (NMR) Spectroscopy

• Frequency Range: 60 - 900 MHz.
• Effect: Energy absorption due to changes in spin states of atomic nuclei with magnetic properties (e.g. 1H, 13C, 31P).
• Information Obtained: Number of unique atomic nuclei in a molecule and the environment around each nucleus.

Radiation Absorption

• Interaction: Energy from electromagnetic radiation is absorbed by the molecule, promoting it to an excited state.
• Process: For example, an electron can be excited to a higher orbital.
• Energy Contributions:
  • $E<em>{ ext{Total}} = E</em>{ ext{Electrons}} + E<em>{ ext{Vibration}} + E</em>{ ext{Rotation}} + …$
  • The order of energy contribution: E{ ext{Electrons}} >> E{ ext{Vibration}} >> E_{ ext{Rotation}}.

Quantization of Energy

• Basic Spectrum Representation:
  • $ext{ΔE} = E<em>1 - E</em>0 = ext{ΔE} = h<br />\nu$ (Planck)
• Absorption Condition: Energy absorption occurs only when the energy of the electromagnetic radiation matches $ext{ΔE}$ exactly, related to either $ext{λ}$, $<br />\nu$ or $σ$.
• Absorption Characteristics: Related to the excited state of the molecule, considering both intensity and transmission characteristics.