Chemical Spectroscopy Overview

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Questions and Answers

Which type of electrons is primarily involved in saturated bonds?

  • Ï€-electrons
  • d-electrons
  • σ-electrons (correct)
  • n-electrons

What is the relationship between n-π* transitions and π-π* transitions in terms of energy requirement?

  • n-Ï€* transitions require less energy than Ï€-Ï€* transitions. (correct)
  • Both transitions require equal energy.
  • n-Ï€* transitions require more energy than Ï€-Ï€* transitions.
  • Ï€-Ï€* transitions do not require any energy.

Which of the following energy transitions is considered a high-energy transition?

  • n-Ï€* transition
  • Ï€-Ï€* transition (correct)
  • σ-σ* transition (correct)
  • σ-Ï€* transition

Where in the electromagnetic spectrum is the visible light range typically found?

<p>400 to 700 nm (C)</p> Signup and view all the answers

What happens to a beam of light when it is passed through a prism?

<p>It forms a continuous spectrum of colors. (D)</p> Signup and view all the answers

What makes it difficult to observe absorptions having λmax < 200 nm?

<p>Everything, including quartz glass and air, absorbs in this spectral region. (C)</p> Signup and view all the answers

Which type of compound would contain π-electrons?

<p>Unsaturated hydrocarbons (D)</p> Signup and view all the answers

Which transition is associated with a maximum wavelength (m{ ext{λmax}}) of 135 nm?

<p>σ-σ* transition (B)</p> Signup and view all the answers

What is the main reason the n to pi* transition is considered 'forbidden'?

<p>It is weaker than the pi to pi* transition. (A), It involves a change in spin multiplicity. (C)</p> Signup and view all the answers

Which transition has the longest wavelength among the given examples?

<p>n to pi* at 279 nm (A)</p> Signup and view all the answers

What does the symbol ε represent in the context of electronic transitions?

<p>The extinction coefficient (D)</p> Signup and view all the answers

How does the absorption of radiation relate to chemical species in a medium?

<p>It selectively attenuates certain frequencies of electromagnetic radiation. (D)</p> Signup and view all the answers

Which of the following statements about the electronic transitions is NOT correct?

<p>n to sigma* transitions are stronger than pi to pi* transitions. (A)</p> Signup and view all the answers

What is the correct equation representing the energy difference between the ground and excited state of electrons?

<p>E = Hν (A)</p> Signup and view all the answers

Which types of transitions contribute to molecular absorption spectra?

<p>Electronic, vibrational, and rotational transitions (A)</p> Signup and view all the answers

In terms of relative energy, how do electronic, vibrational, and rotational energies compare?

<p>EEL &gt; EVIB &gt; EROT (B)</p> Signup and view all the answers

What type of molecular spectra arises from transitions in the ultraviolet-visible region?

<p>Electronic spectra (A)</p> Signup and view all the answers

At room temperature, where do most elementary particles typically exist?

<p>In their lowest energy level (E0) (D)</p> Signup and view all the answers

What is required for an atom to transition to a higher energy state?

<p>A photon that matches the energy difference exactly (C)</p> Signup and view all the answers

What is the wavelength range for rotational spectra?

<p>10^{-3} – 0.67 nm (A)</p> Signup and view all the answers

Which mathematical expression relates energy difference to wavelength?

<p>ΔE = HC/λ (B)</p> Signup and view all the answers

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Study Notes

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
  • Ï€-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-Ï€ transitions**: High energy transitions, accessible in vacuum UV (λmax).
  • Ï€-Ï€ transitions*: Energetically lower than σ-σ* and σ-Ï€*. Relevant for organic molecules, especially conjugated systems.
  • n-Ï€ transitions*: Lowest energy transition, typically in the UV region. Involves non-bonding electrons (n) and Ï€* 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 Ï€-Ï€* transition at a slightly longer wavelength (165 nm), indicating lower energy required for this transition.
  • The n-Ï€* 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.

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