Spectroscopy: Spectroscopic Analysis and EMR

• Spectroscopy is the study of the interactions between radiation and matter.

Spectroscopic Analysis

• Analytical methods measure the amount of radiation absorbed or produced by molecular or atomic species.
• These methods include:
  • Molecular absorption spectroscopy
  • Molecular fluorescence spectroscopy
  • Atomic absorption spectroscopy (AAS)
  • Atomic emission spectroscopy

Electromagnetic Radiation (EMR)

• EMR is a form of radiant energy possessing both particle and wave properties.
• Light strictly refers to visible radiation, but UV, Visible, and sometimes IR radiation can be referred to as light.
• Electromagnetic radiation is divided into different regions based on the type of atomic or molecular transition which gives rise to the absorption or emission of photons.
• The boundaries describing the electromagnetic spectrum are not rigid, and spectral regions can overlap.

Wave Properties of EMR

• Wavelength refers to the distance traversed by one complete cycle, measured in cm.

Frequency

• Frequency is the number of wave cycles passing a fixed point per second, measured in cycles/sec.
• 1 cycle/sec (s⁻¹) = 1 Hz
• ν = c / λ
• C is the velocity of light, 3 × 10¹⁰ cm/sec or 3 × 10⁸ m/sec

Wave Number

• Wave number (ύ) is the number of cycles or waves per cm and is the reciprocal of wavelength.
• The formula of wave number is: ύ = 1/λ = ν/c (cm⁻¹)

Photon Properties of EMR

• In radiation/matter interaction, it is useful to consider light as photons or quanta.
• The relationships between the energy of a photon (E), its wavelength, frequency, and wave number are: E = hν = hc/λ = hcύ.
• h is Planck's constant (6.63 × 10⁻³⁴ J s).
• ν and ύ are directly proportional to E, while λ is inversely proportional to E.
• Shorter wavelengths have greater energy and are more powerful.
• The UV range has shorter wavelengths, carrying more energy photons than visible light.

Interaction of Matter with EMR

• Planck’s constant (h) is that describes the behavior of particles and waves on the atomic scale, along with the particle component of light, in the mathematical form of quantum mechanics.
• The absorption of UV or visible radiation generally results from exciting bonding electrons.
• Because of this, absorption spectroscopy is valuable for identifying functional groups in molecules.
• Bonding electrons and non-bonding electrons are both present.
• When a substance is irradiated with EMR, the energy of incident photons may transfer to atoms or molecules, raising them from the ground state to an excited state.
• The process of raising them into an excited state is called absorption.
• The energy of transition can be described as: ΔE = Eₛ – E₉ = hν, where ν is the frequency of EMR and is characteristic for each molecule.
• Absorbed energy is rapidly lost as heat, light, or through molecular collisions.
• When excited electrons return to the ground state, light or heat is emitted.

Electronic Transitions

• Electronic transitions include σ → σ*, which is not observed in the UV region, as it requires a large energy corresponding to radiant frequency in the short UV region.
• n→σ* transitions can occur in saturated compounds with unshared electrons and can be induced by EMR in the 125 – 250 nm region, resulting in absorption peaks under 200 nm.
• Most applications of absorption spectroscopy in organic compounds are based upon the ∏→∏* transition, corresponding to absorption peaks in the 200 – 700 nm spectral region.
• The greater the number of conjugated multiple bonds in a compound, the longer the wavelength of light the compound will absorb resulting in a smaller energy requirement for the transition from the highest occupied MO to the lowest unoccupied MO.

Monochromatic and Polychromatic Light

• A beam of light carrying only one discrete wavelength (λ) is monochromatic.
• A beam carrying radiation of several wavelengths (λs) is polychromatic or heterochromatic.
• The color of light absorbed by a molecule is not the color we see; it is the color we do not see.

Theory of Light Absorption

• When a beam of light of intensity I₀ passes through a transparent cell containing a solution of an absorbing substance, the intensity of the light may be reduced.
• A portion of the incident light is reflected at the cell faces (Iᵣ).
• A portion is absorbed by the solution (Iₐ).
• A portion is scattered (Iₛ).
• A portion is refracted (Iₙ).
• The remainder is transmitted (Iₜ).
• Thus, I₀ = Iₐ + Iₜ + Iₛ + Iᵣ + Iₙ + .....
• For a clear solution, Iₛ = 0, and using a blank consisting of a similar cell filled with the solvent used, Iᵣ and Iₙ will be cancelled.
• Therefore, I₀ = Iₐ + Iₜ
• Chemical analysis measures the amount of light absorbed (Iₐ) or transmitted (Iₜ) by a solution, relating this to the concentration of a particular ion or compound in the solution.