UV-Vis Spectroscopy MCQ Quiz - Objective Question with Answer for UV-Vis Spectroscopy - Download Free PDF

CONCEPT:

Beer-Lambert Law

• The Beer-Lambert Law states that the absorbance (A) of a solution is directly proportional to the concentration (c) of the solute, the path length (b) of the cell, and the molar extinction coefficient (ε).
• The law is mathematically expressed as:

  A = εbc

EXPLANATION:

• According to the Beer-Lambert Law:

  A = εbc

  • ε (molar extinction coefficient): A constant that indicates how strongly a substance absorbs light at a particular wavelength.
  • b (path length): The distance that the light travels through the solution, typically measured in centimeters.
  • c (concentration): The amount of solute present in a given volume of solution.
• Absorbance is influenced by:
  • The concentration of the solute (c).
  • The path length of the cell (b).
  • The molar extinction coefficient (ε).
• The absorbance of a solution is measured at a specific wavelength (λmax) where the solute has its maximum absorbance.

Therefore, the absorbance of a solution of a solute is not a function of λmax.

Concept:

UV-Visible Spectroscopy:

• Molecular spectroscopy involves the study of the interaction of Ultraviolet (UV)-Visible radiation with molecules.
• Ultraviolet light and visible light have just the right energy to cause an electronic Transition of electrons from one filled orbital to another of higher Energy unfilled orbital When a molecule absorbs light of an appropriate wavelength and an electron is promoted to a higher energy molecular orbital, the molecule is then in an excited state.

Structure Determination:

• The effect of substituent groups can be reliably quantified from empirical observation of known conjugated structures and applied to new systems
• This quantification is referred to as the Woodward-FieserRuleswhich we will apply to three specific chromophores:

1. Conjugated dienes
2. Conjugated dienones
3. Aromatic systems

Conjugated Dienes

• Woodward and the Fieser performed extensive studies of terpene and steroidal alkenes and noted similar substituents and structural features would predictably lead to an empirical prediction of the wavelength for the lowest energy π-π*electronic transition.

Woodward-Fieser Rules for Acyclic Dienes:

• The rules begin with a base value for the max of the chromophore being observed:

  (acyclic butadiene = 217 nm)

• The incremental contribution of substituents is added to this base value from the group tables:

Woodward-Fieser Rules for Cyclic Dienes:

Explanation:

These rules predict the UV absorption maximum of compounds.

Conclusion:

So, the  of the given compound is 283 nm.

The correct answer is  The decrease in the energy of the n and π* orbitals, with the n orbital experiencing a greater decrease, leading to a hypsochromic shift.

Concept:-

• Hypsochromic Shift: This term refers to the shift of an absorption band toward shorter wavelengths (higher energy) in the UV-visible spectrum. It happens in this case because the energy gap between the ground and excited states of the molecule increases when changing the solvent from non-polar hexane to polar methanol.
• Solvent Effects on UV/Vis Spectra: Solvent polarity and the ability to form hydrogen bonds can significantly influence the electronic transitions observed in UV/Vis spectroscopy. Polar solvents, particularly those that can form hydrogen bonds with the solute, can stabilize certain electronic states of the solute molecule, thereby affecting the energy required for electronic transitions.
• n-π Transitions*: In UV/Vis spectroscopy, an n-π* transition involves the excitation of an electron from a non-bonding orbital (n) to an anti-bonding pi orbital (π*). These transitions are sensitive to the environment around the molecule, including solvent interactions that can stabilize or destabilize the involved orbitals.
• Orbital Stabilization and Electronic Transitions: The energy levels of molecular orbitals, including non-bonding (n) and anti-bonding (π*), can be influenced by interactions with the solvent. The stabilization of these orbitals through interactions such as hydrogen bonding can modify the energy required for electronic transitions, resulting in shifts in the observed UV/Vis absorption spectrum.

Explanation:-

• The blue shift (hypsochromic shift) in the UV absorption spectrum of pyridine when changing the solvent from hexane to methanol is a result of solvent effects on the electronic transitions of pyridine. Hexane is a non-polar solvent, while methanol is a polar solvent capable of forming hydrogen bonds.
• For pyridine, the observed peaks in the 320-380 nm range correspond to the n-π* transition. These transitions involve the excitation of a non-bonding electron (n) on the nitrogen atom to an anti-bonding π* orbital. When the solvent is changed from hexane to methanol, hydrogen bonds form between the lone pair on the nitrogen atom of pyridine and the hydrogen atom of the hydroxyl group in methanol. This interaction stabilizes the non-bonding electron, effectively lowering its energy.
• These are the lowest energy peaks in the spectrum and correspond to the n-π* transition in pyridine. Hexane (C₆H₁₄) is a non-polar hydrocarbon. Methanol (CH₃OH) is a polar solvent with the ability to form hydrogen bonds. For pyridine, the hydrogen atom of the hydroxyl group of methanol will form hydrogen bonds with the lone pair on the nitrogen atoms, as shown in Figure

Hydrogen bond between methanol and pyridine.
. Hexane cannot form such hydrogen bonds.

As these hydrogen bond interactions stabilize the n (non-bonding) orbital more than the π* (anti-bonding) orbital, the difference in energy between the n and π* orbitals increases upon changing the solvent to methanol. As a consequence, the energy required for the n-π* transition increases, which corresponds to a shift toward the blue (higher energy) end of the spectrum.

Conclusion:-

So, This phenomenon can be attributed to the decrease in the energy of the n and π* orbitals, with the n orbital experiencing a greater decrease, leading to a hypsochromic shift.

The correct answer is CH₃CH₂CHO

Explanation:

The given spectral data and match it to the possible structures:

• UV Spectrum:
  • λ_max 229 nm: This transition in the UV region is often indicative of conjugation or the presence of a carbonyl group.
• IR Spectrum:
  • 1738 cm^-1 (strong absorption): This is characteristic of a carbonyl (>C=O) stretch.
  • 2720 cm^-1 (weak absorption): This is indicative of an aldehyde C-H stretch.

 each option to see which one matches the spectral data:

1. CH₃CH₂CH₂OH (Propyl alcohol):
   • No strong carbonyl stretch at 1738 cm^-1
   • No weak aldehyde C-H stretch at 2720 cm^-1
   • This structure is not consistent with the given spectral data.
2. CH₃COCH₃ (Acetone):
   • Has a carbonyl group (C=O) which could show absorption around 1700 cm^-1, but not exactly 1738 cm^-1
   • No aldehyde group to provide a stretch at 2720 cm^-1
   • This structure is not consistent with the given spectral data.
3. CH₃CH₂OCH₃ (Ethyl methyl ether):
   • No carbonyl stretch at 1738 cm^-1
   • No aldehyde group to provide a stretch at 2720 cm^-1
   • This structure is not consistent with the given spectral data.
4. CH₃CH₂CHO (Propanal):
   • Carbonyl group (>C=O) is present and should show a strong absorption around 1738 cm^-1
   • Aldehyde C-H stretch typically around 2720 cm^-1
   • This structure is consistent with the given spectral data.

So, the IR absorptions at 1738 cm^-1 and 2720 cm^-1, the most likely structure is CH₃CH₂CHO (Propanal), which perfectly fits the observed spectral data.

The correct answer is 

Concept: -

Bathochromic shift: In spectroscopy, the position shift of a peak or signal to longer wavelength (lower energy). Also called a red shift. A hypsochromic shift is the shift of a peak or signal to shorter wavelength (higher energy). Also called a blue shift.

Explanation: -

1. n-π transitions- These involve the movement of an electron from a non-bonding (n) orbital to an antibonding (π*) orbital. Increasing solvent polarity destabilizes the n-π* transition, resulting in a hypsochromic (blue) shift.

2. π-π transitions- These transitions involve the excitation of an electron from a π bonding orbital to a π* antibonding orbital. Increasing solvent polarity stabilizes the π-π* transition, leading to a bathochromic (red) shift.

Conclusion: -

In the UV-visible absorption spectrum of an α, β-unsaturated carbonyl compound, with increasing solvent polarity, n-π* transitions undergo hypsochromic shift, π-π* undergo bathochromic shift.

Concept:

Absorption spectroscopy is a powerful analytical technique used in chemistry, physics, and various other fields to study the interaction of electromagnetic radiation with matter. It provides valuable information about the electronic structure, chemical composition, and concentration of substances in a sample. 

Given:

• ε280 (Tyrosine) =1500 M-1cm-1
• ε280(Tryptophan) = 5000 M-1cm-1
• Absorbance (A) = 0.59
• Concentration (C) = 10µM or 10-5 M
• Path Length (l) = 2 cm 

Explanation:

According to Lambert-Beer Law 

A = εlc 

A_Tyr= 3 X 1500 X 2 X 10^-5

A_Trp= n X 5000 X 2 X 10^-5

∴ A_Total = n₁A_Tyr + n₂A_Trp

0.59 = (2 x 10^-5)(4500 + 5000n₂)

29500 = 4500+5000n₂

n₂ = 5

Conclusion:

The number of tryptophan residues in the protein will be 5.

The correct answer is  The decrease in the energy of the n and π* orbitals, with the n orbital experiencing a greater decrease, leading to a hypsochromic shift.

Concept:-

• Hypsochromic Shift: This term refers to the shift of an absorption band toward shorter wavelengths (higher energy) in the UV-visible spectrum. It happens in this case because the energy gap between the ground and excited states of the molecule increases when changing the solvent from non-polar hexane to polar methanol.
• Solvent Effects on UV/Vis Spectra: Solvent polarity and the ability to form hydrogen bonds can significantly influence the electronic transitions observed in UV/Vis spectroscopy. Polar solvents, particularly those that can form hydrogen bonds with the solute, can stabilize certain electronic states of the solute molecule, thereby affecting the energy required for electronic transitions.
• n-π Transitions*: In UV/Vis spectroscopy, an n-π* transition involves the excitation of an electron from a non-bonding orbital (n) to an anti-bonding pi orbital (π*). These transitions are sensitive to the environment around the molecule, including solvent interactions that can stabilize or destabilize the involved orbitals.
• Orbital Stabilization and Electronic Transitions: The energy levels of molecular orbitals, including non-bonding (n) and anti-bonding (π*), can be influenced by interactions with the solvent. The stabilization of these orbitals through interactions such as hydrogen bonding can modify the energy required for electronic transitions, resulting in shifts in the observed UV/Vis absorption spectrum.

Explanation:-

• The blue shift (hypsochromic shift) in the UV absorption spectrum of pyridine when changing the solvent from hexane to methanol is a result of solvent effects on the electronic transitions of pyridine. Hexane is a non-polar solvent, while methanol is a polar solvent capable of forming hydrogen bonds.
• For pyridine, the observed peaks in the 320-380 nm range correspond to the n-π* transition. These transitions involve the excitation of a non-bonding electron (n) on the nitrogen atom to an anti-bonding π* orbital. When the solvent is changed from hexane to methanol, hydrogen bonds form between the lone pair on the nitrogen atom of pyridine and the hydrogen atom of the hydroxyl group in methanol. This interaction stabilizes the non-bonding electron, effectively lowering its energy.
• These are the lowest energy peaks in the spectrum and correspond to the n-π* transition in pyridine. Hexane (C₆H₁₄) is a non-polar hydrocarbon. Methanol (CH₃OH) is a polar solvent with the ability to form hydrogen bonds. For pyridine, the hydrogen atom of the hydroxyl group of methanol will form hydrogen bonds with the lone pair on the nitrogen atoms, as shown in Figure

Hydrogen bond between methanol and pyridine.
. Hexane cannot form such hydrogen bonds.

As these hydrogen bond interactions stabilize the n (non-bonding) orbital more than the π* (anti-bonding) orbital, the difference in energy between the n and π* orbitals increases upon changing the solvent to methanol. As a consequence, the energy required for the n-π* transition increases, which corresponds to a shift toward the blue (higher energy) end of the spectrum.

Conclusion:-

So, This phenomenon can be attributed to the decrease in the energy of the n and π* orbitals, with the n orbital experiencing a greater decrease, leading to a hypsochromic shift.

Concept:-

Beer-Lambert law: This law states that the absorbance of light by a substance is directly proportional to its concentration, path length, and its molar absorption coefficient.

A = εcl

Molar Absorption Coefficient (or Extinction Coefficient): This is the measure of how strongly a chemical species absorbs light at a particular wavelength. It is a characteristic property of each substance.

Transmittance and Absorbance: Transmittance is the fraction of incident light that is transmitted through the sample, while absorbance is a measure of the quantity of light absorbed by a sample. Both are integral parts of spectrophotometry and are inversely related to each other.

Explanation:-

The Beer-Lambert law is as follows:

A = εcl

Where: A is the absorbance, ε is the molar extinction coefficient (in M^-1 cm^-1) c is the concentration (in M) l is the path length (in cm)

Here, we have three compounds 'P', 'Q', and 'R' with given concentrations and molar absorption coefficients. Also, all compounds are non-interacting, so their respective contributions to the total absorbance will be additive (we essentially sum up the individual absorbances).

Now, Calculate the individual absorbance for P, Q, and R:

A_P = ε_P  c_P  l = (1 x 10⁵ M^-1 cm^-1) x (1 x 10^-6 M) x (1 cm) = 0.1
A_Q = ε_Q  c_Q  l = (2 x 10⁵ M^-1 cm^-1) x (2 x 10^-6 M) x (1 cm) = 0.4
A_R = ε_R  c_R  l = (3 x 10⁵ M^-1 cm^-1) x (3 x 10^-6 M) x (1 cm) = 0.9

The total absorbance (A_total) is then the sum of the absorbances of P,Q and R:

A_total = A_P + A_Q + A_R = 0.1 + 0.4 + 0.9 = 1.4

Next, the percent transmittance (T%) is calculated from the absorbance by:

T% = 10^(-At_ot) x 100% = 10^(-1.4) x 100% = 3.98%

This indicates that about 3.98% of the incident light at a wavelength of 300 nm transmits through the solution. 

Conclusion:-

So, The % transmittance at 300 nm is 3.98%

CONCEPT:

Electronic Transitions and Energy Requirements

• In electronic transitions, electrons move from one molecular orbital to another due to the absorption of energy.
• The energy required for a transition depends on the energy gap between the molecular orbitals involved.
• Common electronic transitions in molecules include:
  • \(\sigma \rightarrow \sigma^*\): Transition between bonding and antibonding sigma orbitals. This requires the highest energy because sigma bonds are the strongest.
  • \(n \rightarrow \sigma^*\): Transition from a non-bonding orbital to an antibonding sigma orbital. This requires less energy than \(\sigma \rightarrow \sigma^*\).
  • \(\pi \rightarrow \pi^*\): Transition between bonding and antibonding pi orbitals. This requires less energy than the above two transitions.
  • \(n \rightarrow \pi^*\): Transition from a non-bonding orbital to an antibonding pi orbital. This requires the least energy among the listed transitions.

EXPLANATION:

• The energy required for a transition decreases as the energy gap between the orbitals decreases.
• Among the given options:
  • \(\sigma \rightarrow \sigma^*\) involves the largest energy gap, so it requires the most energy.
  • \(n \rightarrow \sigma^*\) involves a smaller energy gap than \(\sigma \rightarrow \sigma^*\).
  • \(\pi \rightarrow \pi^*\) involves an even smaller energy gap than the above two transitions.
  • \(n \rightarrow \pi^*\) involves the smallest energy gap, so it requires the least energy.

Therefore, the electronic transition that requires the least energy is \(n \rightarrow \pi^*\).

CONCEPT:

\(\lambda\) max (Wavelength of Maximum Absorption)

• \(\lambda\) max of a compound refers to the wavelength at which the compound absorbs the maximum light in a UV-Vis spectrum.
• The electronic transitions in a molecule, especially the transitions of π-electrons or non-bonding electrons (n), determine the \(\lambda\) max value.
• Conjugation and functional groups significantly affect the \(\lambda\) max of a compound. More conjugation generally shifts the \(\lambda\) max to a higher wavelength (red shift).

EXPLANATION:

• To determine which compound has a \(\lambda\) max at 273 nm, we need to consider the structure and conjugation of each option.
• Option 1 has a specific arrangement of conjugated double bonds or functional groups that absorb light at 273 nm.
• Other options might have either less conjugation (resulting in a lower \(\lambda\) max) or different substituents that affect the absorption wavelength.
• For example:
  • Option 1 might have a benzene ring or a conjugated system that absorbs UV light near 273 nm.
  • Options 2, 3, and 4 might lack sufficient conjugation or have functional groups that shift the \(\lambda\) max to a different region.

Therefore, the compound in Option 1 has \(\lambda\) max at 273 nm because of its specific conjugation and structure.

CONCEPT:

Beer-Lambert Law

• The Beer-Lambert Law states that the absorbance (A) of a solution is directly proportional to the concentration (c) of the solute, the path length (b) of the cell, and the molar extinction coefficient (ε).
• The law is mathematically expressed as:

  A = εbc

EXPLANATION:

• According to the Beer-Lambert Law:

  A = εbc

  • ε (molar extinction coefficient): A constant that indicates how strongly a substance absorbs light at a particular wavelength.
  • b (path length): The distance that the light travels through the solution, typically measured in centimeters.
  • c (concentration): The amount of solute present in a given volume of solution.
• Absorbance is influenced by:
  • The concentration of the solute (c).
  • The path length of the cell (b).
  • The molar extinction coefficient (ε).
• The absorbance of a solution is measured at a specific wavelength (λmax) where the solute has its maximum absorbance.

Therefore, the absorbance of a solution of a solute is not a function of λmax.

Concept:

UV-Visible Spectroscopy:

• Molecular spectroscopy involves the study of the interaction of Ultraviolet (UV)-Visible radiation with molecules.
• Ultraviolet light and visible light have just the right energy to cause an electronic Transition of electrons from one filled orbital to another of higher Energy unfilled orbital When a molecule absorbs light of an appropriate wavelength and an electron is promoted to a higher energy molecular orbital, the molecule is then in an excited state.

Structure Determination:

• The effect of substituent groups can be reliably quantified from empirical observation of known conjugated structures and applied to new systems
• This quantification is referred to as the Woodward-FieserRuleswhich we will apply to three specific chromophores:

1. Conjugated dienes
2. Conjugated dienones
3. Aromatic systems

Conjugated Dienes

• Woodward and the Fieser performed extensive studies of terpene and steroidal alkenes and noted similar substituents and structural features would predictably lead to an empirical prediction of the wavelength for the lowest energy π-π*electronic transition.

Woodward-Fieser Rules for Acyclic Dienes:

• The rules begin with a base value for the max of the chromophore being observed:

  (acyclic butadiene = 217 nm)

• The incremental contribution of substituents is added to this base value from the group tables:

Woodward-Fieser Rules for Cyclic Dienes:

Explanation:

These rules predict the UV absorption maximum of compounds.

Conclusion:

So, the  of the given compound is 283 nm.

The correct answer is CH₃CH₂CHO

Explanation:

The given spectral data and match it to the possible structures:

• UV Spectrum:
  • λ_max 229 nm: This transition in the UV region is often indicative of conjugation or the presence of a carbonyl group.
• IR Spectrum:
  • 1738 cm^-1 (strong absorption): This is characteristic of a carbonyl (>C=O) stretch.
  • 2720 cm^-1 (weak absorption): This is indicative of an aldehyde C-H stretch.

 each option to see which one matches the spectral data:

1. CH₃CH₂CH₂OH (Propyl alcohol):
   • No strong carbonyl stretch at 1738 cm^-1
   • No weak aldehyde C-H stretch at 2720 cm^-1
   • This structure is not consistent with the given spectral data.
2. CH₃COCH₃ (Acetone):
   • Has a carbonyl group (C=O) which could show absorption around 1700 cm^-1, but not exactly 1738 cm^-1
   • No aldehyde group to provide a stretch at 2720 cm^-1
   • This structure is not consistent with the given spectral data.
3. CH₃CH₂OCH₃ (Ethyl methyl ether):
   • No carbonyl stretch at 1738 cm^-1
   • No aldehyde group to provide a stretch at 2720 cm^-1
   • This structure is not consistent with the given spectral data.
4. CH₃CH₂CHO (Propanal):
   • Carbonyl group (>C=O) is present and should show a strong absorption around 1738 cm^-1
   • Aldehyde C-H stretch typically around 2720 cm^-1
   • This structure is consistent with the given spectral data.

So, the IR absorptions at 1738 cm^-1 and 2720 cm^-1, the most likely structure is CH₃CH₂CHO (Propanal), which perfectly fits the observed spectral data.

The correct answer is 

Concept: -

Bathochromic shift: In spectroscopy, the position shift of a peak or signal to longer wavelength (lower energy). Also called a red shift. A hypsochromic shift is the shift of a peak or signal to shorter wavelength (higher energy). Also called a blue shift.

Explanation: -

1. n-π transitions- These involve the movement of an electron from a non-bonding (n) orbital to an antibonding (π*) orbital. Increasing solvent polarity destabilizes the n-π* transition, resulting in a hypsochromic (blue) shift.

2. π-π transitions- These transitions involve the excitation of an electron from a π bonding orbital to a π* antibonding orbital. Increasing solvent polarity stabilizes the π-π* transition, leading to a bathochromic (red) shift.

Conclusion: -

In the UV-visible absorption spectrum of an α, β-unsaturated carbonyl compound, with increasing solvent polarity, n-π* transitions undergo hypsochromic shift, π-π* undergo bathochromic shift.

The correct option is option 2.

Concept: -

Electronic excitation of cis-trans alkenes leads to two distinct states, the trans-excitation state usually being of lower energy than the cis excited state. Furthermore, the molar extinction coefficient of a cis alkene is generally low (cis-stilbens = 935 m² mol-¹, 278 nm) than that of the trans (trans-stilbene = 2400 m² mol¹, 294 nm) so that more of the trans isomer is excited at a given time. As a result the trans alkene is more heavily populated either by direct or through sensitization. These excited states may either interconvert through a rotational process or return to the respective electronic states.

The triplet sensitized cis-trans isomerization of alkenes leads to an equilibrium mixture called the photostationary state which is dependent on the triplet energy of the photosensitizer.

Explanation: -

The cis-trans isomerization of stilbene has been well studied. Photolyzed isomerization of either isomer leads to an equilibrium mixture of cis-trans stilbenes, the ratio of which is 60 percent cis and 40 percent trans in the presence of high-energy triplets. Photolysis of stilbene can be achieved by an excited molecule having triplet energy above 60 kcal/mole. cis (E₁ = 57 kcal/mole) and trans (E₁ = 48 kcal/mole). Photoisomerization then takes place via interconversion to the triplet state of the other or via a common intermediate [*] called the phantom triplet which collapses to the ground state isomers.

Conclusion: -

So, we can say that the extinction coefficient of trans-stilbene is greater than cis-stilbene at an exciting wavelength.