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Questions and Answers
How does the presence of impurities influence the UV spectrum of a compound?
How does the presence of impurities influence the UV spectrum of a compound?
What is the effect of steric hindrance on the absorption wavelength of isomer A?
What is the effect of steric hindrance on the absorption wavelength of isomer A?
Which of the following best describes the term 'bathochromic shift'?
Which of the following best describes the term 'bathochromic shift'?
What is the effect of polar solvents on the π − π* transitions compared to n − π* transitions?
What is the effect of polar solvents on the π − π* transitions compared to n − π* transitions?
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Which of the following statements is true regarding electronic spectroscopy?
Which of the following statements is true regarding electronic spectroscopy?
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What is the term for the shift that results in absorption moving toward longer wavelengths?
What is the term for the shift that results in absorption moving toward longer wavelengths?
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Which statement correctly describes auxochromes?
Which statement correctly describes auxochromes?
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Which transition requires the least energy among the following?
Which transition requires the least energy among the following?
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How does conjugation affect the wavelength of absorption?
How does conjugation affect the wavelength of absorption?
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In a polar solvent, π-π* transitions usually result in which type of spectral shift?
In a polar solvent, π-π* transitions usually result in which type of spectral shift?
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What effect does a hyperchromic shift have on absorption intensity?
What effect does a hyperchromic shift have on absorption intensity?
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Which of the following is NOT a characteristic of chromophores?
Which of the following is NOT a characteristic of chromophores?
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What is the primary effect of auxochromes attached to chromophores?
What is the primary effect of auxochromes attached to chromophores?
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What does the HOMO-LUMO gap indicate about a molecule's conjugation?
What does the HOMO-LUMO gap indicate about a molecule's conjugation?
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Which statement correctly describes Absorbance in UV-visible spectroscopy?
Which statement correctly describes Absorbance in UV-visible spectroscopy?
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In the context of electronic transitions, what does a σ−σ* transition involve?
In the context of electronic transitions, what does a σ−σ* transition involve?
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Which of the following is NOT a characteristic of Ultraviolet (UV) spectroscopy?
Which of the following is NOT a characteristic of Ultraviolet (UV) spectroscopy?
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What does Beer-Lambert law express in UV-visible spectroscopy?
What does Beer-Lambert law express in UV-visible spectroscopy?
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What type of transition is less energy-intensive than σ−σ* transition?
What type of transition is less energy-intensive than σ−σ* transition?
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In the formula of absorbance, which variable represents the intensity of incident light?
In the formula of absorbance, which variable represents the intensity of incident light?
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Which of the following compounds is likely to undergo an n−π* transition?
Which of the following compounds is likely to undergo an n−π* transition?
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What effect does a polar solvent have on n − π* transitions?
What effect does a polar solvent have on n − π* transitions?
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Which of the following base values for conjugated dienes is correct?
Which of the following base values for conjugated dienes is correct?
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What is the increment for adding a double bond that extends conjugation in α,β-unsaturated carbonyl compounds?
What is the increment for adding a double bond that extends conjugation in α,β-unsaturated carbonyl compounds?
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How does the presence of an -NR2 group affect the λmax for α,β-unsaturated carbonyl compounds?
How does the presence of an -NR2 group affect the λmax for α,β-unsaturated carbonyl compounds?
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What is the effect of increasing the extent of conjugation in compounds?
What is the effect of increasing the extent of conjugation in compounds?
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What is the base value for a 5-membered cyclic ketone according to Woodward-Fieser rules?
What is the base value for a 5-membered cyclic ketone according to Woodward-Fieser rules?
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Which of the following polar groupings has the smallest increment for λmax?
Which of the following polar groupings has the smallest increment for λmax?
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What determines whether compound A will appear at a longer wavelength than compound B in terms of conjugation?
What determines whether compound A will appear at a longer wavelength than compound B in terms of conjugation?
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Study Notes
Ultraviolet and Visible Spectroscopy
- UV and visible spectroscopy, also known as electronic spectroscopy, measures the number of conjugated double bonds and aromatic conjugation within a molecule.
- It involves the promotion of electrons from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO).
- The HOMO-LUMO gap decreases as the conjugation increases.
- The ultraviolet region corresponds to 400-200 nm, and the visible region to 800-400 nm.
- UV-visible spectroscopy is based on Beer-Lambert's law: log (I₀/I) = ε.l.c or ε = A/c.l
- I₀ = intensity of incident light
- I = intensity of transmitted light
- ε = molar absorptivity
- l = path length of the sample
- c = concentration of the sample
- A = Absorbance
Absorbance and Transmittance
- Absorbance (A) is the reciprocal of transmittance.
- Transmittance (T) is the fraction of incident light transmitted.
Electronic Transitions
- Electronic transitions occur when a molecule absorbs electromagnetic radiation in the UV-visible region, and its electrons are promoted from the ground state to the excited state, or from the bonding orbital to the anti-bonding orbital.
Types of Electronic Transitions
- σ - σ Transition*: Transition of an electron from a bonding sigma orbital (σ) to an anti-bonding sigma orbital (σ*). Examples include alkanes due to sigma bonds holding all atoms together.
- n - σ & n - π Transition**: Transitions from a non-bonding molecular (n) orbital to an anti-bonding sigma orbital or anti-bonding pi orbital (π*). These transitions require less energy than σ - σ* transitions. Examples include alkyl halides, aldehydes, and ketones.
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π - π Transition*: Typically observed in unsaturated molecules such as alkenes, alkynes, aromatics, and carbonyl compounds. This transition requires less energy compared to n - σ* transitions.
- Energy order: n-π* < π-π* < n-σ* < σ-σ*
Absorption, Intensity Shift, and UV Spectrum
- Bathochromic shift (Red shift): Absorption shifts towards longer wavelengths (λmax).
- Hypsochromic shift (Blue shift): Absorption shifts towards shorter wavelengths (λmax).
- Hyperchromic shift: The intensity of the absorption maximum (εmax) increases.
- Hypochromic shift: The intensity of the absorption maximum (εmax) decreases.
Chromophores and Auxochromes
- Chromophores: Covalently unsaturated groups that absorb electromagnetic radiation in the UV-visible region, giving the compound its color. Common examples are C=C, C≡C, benzene, and NO2.
- Auxochromes: Saturated groups containing lone pairs of electrons. When attached to a chromophore, they cause the absorption to shift towards a longer wavelength. Common examples are -OH, -SH, and -NH2.
Factors Affecting Absorbance and Intensity
- Conjugation: As the number of double bonds (chromophores) in conjugation increases, the absorption shifts towards longer wavelengths.
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Effect of solvent:
- In polar solvents, π - π* transitions shift red due to dipole interactions lowering the energy of the excited state.
- In polar solvents, n - π* transitions shift blue due to dipole interactions lowering the energy of the ground state.
The Woodward-Fieser Rules
- For Conjugated Dienes: A set of rules to predict the λmax (nm) for conjugated dienes, based on the base value and increments for different substituents and structures.
- For α, β-Unsaturated Carbonyl Compounds: Similar rules predict the λmax (nm) for α, β-unsaturated carbonyl compounds based on the base value and increments for different substituents and structures.
Applications of UV Spectroscopy
- Extent of conjugation: Determines the degree of conjugation based on the wavelength of absorption.
- Distinguishing conjugated and non-conjugated compounds: Conjugated compounds absorb at longer wavelengths than their non-conjugated counterparts.
- Studying geometric isomerism: Steric hindrance can affect the UV spectrum, allowing for the distinction of geometric isomers in a compound.
- Detecting impurities: Impurities can lead to additional peaks in the UV spectrum, providing information about the compound's purity.
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Description
Test your understanding of ultraviolet and visible spectroscopy concepts, including HOMO-LUMO transitions, Beer-Lambert's law, and the significance of absorbance and transmittance. This quiz covers essential principles and applications in analytical chemistry. Challenge your knowledge and see how well you grasp these critical topics.
