Measurement of Absorbance and Beer-Lambert's Law
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Questions and Answers

What is the range of wavelengths in which n to π* and π to π* transitions occur?

200 - 700 nm

What type of compounds have small peaks in the UV region due to n to σ* transitions?

Saturated compounds containing atoms with lone pairs

What is the order of increasing molar absorbtivities in electronic transitions?

n→∏* < ∏→∏* < n→σ* < σ→σ*

What is the result of the absorption of UV or visible radiation in electronic transitions?

<p>The excitation of outer electrons</p> Signup and view all the answers

What is the principle underlying charge transfer complexes?

<p>The transfer of electronic charge through space from an electron-rich molecule to an electron-deficient molecule</p> Signup and view all the answers

What is the λmax of I2 in benzene?

<p>255 nm</p> Signup and view all the answers

What is required for n to π* and π to π* transitions to occur?

<p>An unsaturated group in the molecule to provide the p electrons</p> Signup and view all the answers

What is the result of the overlap of filled π-orbitals in the donor molecule with the depleted orbital in the acceptor molecule?

<p>Two new molecular orbitals are generated</p> Signup and view all the answers

What is the condition for the gross selection rule for vibrational spectra?

<p>The electric dipole moment of the molecule must change during the vibration.</p> Signup and view all the answers

What happens to the energy levels at high vibrational excitations due to anharmonicity?

<p>The energy levels become less widely spaced.</p> Signup and view all the answers

What is the relationship between the vibrational frequency and the wavenumber in the IR region?

<p>The vibrational frequency is in the range of 1013-1014 Hz, which corresponds to a wavenumber range of 300-3000 cm-1.</p> Signup and view all the answers

What is the most important spectral transition in vibrational spectroscopy at room temperature?

<p>The transition from v = 0 to v = 1.</p> Signup and view all the answers

What is the expression for the energy of a vibrational level in terms of the vibrational quantum number v?

<p>E = (v + 0.5)hʋ.</p> Signup and view all the answers

What is the expression for the vibrational frequency ʋ in terms of the force constant k and the reduced mass µ?

<p>ʋ = 1/2π(k/µ)½.</p> Signup and view all the answers

What is the result of the absorption of UV-VIS light by a molecule in electronic transitions?

<p>electronic excitation of the molecule</p> Signup and view all the answers

What is the Franck-Condon principle in the context of electronic transitions?

<p>The absorption spectrum consists of many lines instead of a single, sharp, and purely electronic absorption line.</p> Signup and view all the answers

What is the relationship between absorbance and transmittance in UV-Vis spectroscopy?

<p>A = -log10 T</p> Signup and view all the answers

What is the unit of molar absorptivity (e) in UV-Vis spectroscopy?

<p>L mol-1 cm-1</p> Signup and view all the answers

What is the role of HOMO and LUMO in electronic transitions?

<p>HOMO is the electronic ground state and LUMO is the higher energy state that the electron is promoted to during electronic excitation.</p> Signup and view all the answers

What is the formula for absorbance in terms of molar absorptivity, path length, and concentration?

<p>A = εlc</p> Signup and view all the answers

Study Notes

Vibrational Spectroscopy

  • The vibrational energy (E) of a molecule is given by E = (v + 0.5)hʋ, where v is the vibrational quantum number.
  • The vibrational frequency (ʋ) is given by ʋ = 1/2π(k/μ)½, where k is the force constant and μ is the reduced mass.
  • The vibrational frequency range is 1013-1014 Hz, which corresponds to the IR region (300-3000 cm⁻¹).
  • The selection rule for vibrational spectra is Δv = ±1, which means that the electric dipole moment of the molecule must change during the vibration.

Anharmonicity

  • At high vibrational excitations, the energy levels become less widely spaced, and the energy (Ev) is given by Ev = (v + 1/2)hʋ - (v + 1/2)²hʋxe + ..., where xe is the anharmonicity constant.

Measurement of Absorbance

  • The Beer-Lambert Law is used to measure absorbance, which is given by A = -log10 T, where T is the transmittance.
  • The transmittance (T) is given by T = I/I₀, where I₀ is the power in and I is the power out.
  • The absorbance (A) is also given by A = εlc, where ε is the molar absorptivity, l is the path length, and c is the concentration.

Electronic Transitions

  • Electronic transitions involve the promotion of an electron from the electronic ground state to a higher energy state, usually from the HOMO to the LUMO.
  • The Franck-Condon principle states that instead of a single sharp absorption line, the absorption spectrum consists of many lines.
  • The energy range for electronic transitions is 200-700 nm, which corresponds to the UV-VIS region.

Types of Electronic Transitions

  • n to σ* transitions: occur in saturated compounds with atoms having lone pairs, and have relatively low energy.
  • n to π* and π to π* transitions: occur in unsaturated compounds with p electrons, and have relatively higher energy.
  • The energy requirement for these transitions is n→π* < π→π* < n→σ* < σ→σ*.

Charge Transfer Complexes

  • Charge transfer complexes involve the transfer of electronic charge from an electron-rich molecule to an electron-deficient molecule.
  • The filled π-orbitals in the donor molecule overlap with the depleted orbital in the acceptor molecule, generating two new molecular orbitals.
  • The λmax of I₂ in benzene is 255 nm, and in hexane is 500 nm.

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Description

This quiz covers the measurement of absorbance and the concept of Beer-Lambert's Law, including transmittance and absorbance calculations.

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