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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?
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What is the principle underlying charge transfer complexes?
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What is the λmax of I2 in benzene?
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What is required for n to π* and π to π* transitions to occur?
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What is the result of the overlap of filled π-orbitals in the donor molecule with the depleted orbital in the acceptor molecule?
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What is the condition for the gross selection rule for vibrational spectra?
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What happens to the energy levels at high vibrational excitations due to anharmonicity?
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What is the relationship between the vibrational frequency and the wavenumber in the IR region?
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What is the most important spectral transition in vibrational spectroscopy at room temperature?
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What is the expression for the energy of a vibrational level in terms of the vibrational quantum number v?
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What is the expression for the vibrational frequency ʋ in terms of the force constant k and the reduced mass µ?
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What is the result of the absorption of UV-VIS light by a molecule in electronic transitions?
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What is the Franck-Condon principle in the context of electronic transitions?
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What is the relationship between absorbance and transmittance in UV-Vis spectroscopy?
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What is the unit of molar absorptivity (e) in UV-Vis spectroscopy?
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What is the role of HOMO and LUMO in electronic transitions?
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What is the formula for absorbance in terms of molar absorptivity, path length, and concentration?
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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.
