Chemical Spectroscopy Overview

Electronic Transitions

• When an electron absorbs a photon, it moves from a lower energy level to a higher energy level
• The energy difference between these levels is given by: E = hν
• The total energy of a molecule is the sum of its electronic, vibrational, and rotational energies: E = Eele + Evib + Erot

Molecular Absorption

• Molecules absorb and emit light in a wider range of wavelengths compared to atoms
• Therefore, they show spectral bands instead of sharp lines
• This is due to quantized transitions in three types of molecular energy levels: electronic, vibrational, and rotational
• The relative energies of these transitions can be classified as: EEL > EVIB > EROT (10,000:100:1)

Types of Molecular Absorption Spectra

• Electronic Spectra: Due to changes in electronic transitions, and associated vibrational and rotational transitions
  • Occur in the ultraviolet-visible region (200-800nm)
• Vibrational Spectra: Due to vibrational and rotational transitions
  • Occur in the infrared region (2.5-15 µm [400-4000cm-1])
• Rotational Spectra: Due to rotational transitions only
  • Occur in the microwave region (10-3 - 0.67 nm)

Quantum Theory

• Every particle (atom, ion, or molecule) exists in discrete energy states: E0, E1, E2, E3, etc.
• At room temperature, most particles are in their lowest energy level, E0 (ground state)
• When atoms absorb photons, they can be promoted to higher energy levels: E1, E2, E3, etc.
• This occurs only if the photon's energy exactly matches the energy difference between the ground state and an excited state: ΔE = (EN - E0) = hν = hc/λ.
• The promoted atoms are said to be in excited states: M + hν → M*

UV-Visible Spectroscopy

• Measures the absorbance or transmittance of radiation in the UV-visible range of the electromagnetic spectrum
• Arise from electron transitions
• Stage 1: Excitation of a species (M) by absorbing a photon
• Stage 2: Relaxation of the excited species (M*) by converting it into a new species through a photochemical reaction

Types of Electrons in Organic Molecules

• σ-Electrons: Involved in saturated bonds, found in carbon-hydrogen in paraffins. Require high energy to excite, more than the energy provided by UV light
• π-Electrons: Involved in unsaturated hydrocarbons, present in alkenes, alkynes, and aromatic compounds
• n-Electrons: Non-bonding electrons, not involved in molecular bonding. Found in heteroatoms like oxygen, nitrogen, etc.

UV Absorption Process

• σ-σ and σ-π transitions**: High energy transitions, accessible in vacuum UV (λmax).
• π-π transitions*: Energetically lower than σ-σ* and σ-π*. Relevant for organic molecules, especially conjugated systems.
• n-π transitions*: Lowest energy transition, typically in the UV region. Involves non-bonding electrons (n) and π* orbitals.

Visible Spectrum

• When white light passes through a prism, it separates into a band of colors (continuous spectrum).
• Each color corresponds to a specific wavelength of electromagnetic radiation.
• The visible range is a small part of the entire electromagnetic spectrum.

Molecular Structure and UV-Visible Spectrum

• Ethane (C2H6) undergoes a σ-σ* transition at a very short wavelength (135 nm)
• Ethylene (C2H4) has a π-π* transition at a slightly longer wavelength (165 nm), indicating lower energy required for this transition.
• The n-π* transition in molecules with lone pairs is even lower in energy and typically occurs in the UV region.
• The strength of absorption (ε) can be categorized as:
  • Strong: ε > 10,000
  • Moderate: 100 < ε < 10,000
  • Weak: ε < 100

Absorption of Radiation

• Absorption is a process where certain frequencies of electromagnetic radiation are selectively absorbed by a chemical species in a transparent medium.
• This absorption weakens the intensity of those frequencies, leading to a decrease in the transmitted light.