Electronic Spectroscopy: Probing Electron Transitions

Electronic spectroscopy, primarily known as Ultraviolet-Visible (UV-Vis) spectroscopy, investigates the transitions of electrons between different quantized energy levels within atoms or molecules. These transitions typically involve the absorption of electromagnetic radiation in the ultraviolet (200−400 nm) and visible (400−800 nm) regions. Unlike rotational or vibrational spectroscopy, which probe molecular motion, electronic spectroscopy provides insights into the electronic structure, bonding, and conjugation within a molecule.

What is Electronic Spectroscopy?

When a molecule absorbs UV or visible light, an electron jumps from a lower energy molecular orbital (e.g., bonding or non-bonding orbital) to a higher energy, unoccupied molecular orbital (e.g., antibonding orbital). The specific wavelengths of light absorbed are characteristic of the electronic structure of the molecule. The resulting spectrum, usually plotted as absorbance versus wavelength (or wavenumber), provides a “fingerprint” that can be used for identification and quantification.

Types of Electronic Transitions

Electronic transitions involve electrons moving between different types of molecular orbitals. The energy required for these transitions varies significantly, leading to absorption at different wavelengths.

The common types of molecular orbitals involved are:

• σ (sigma) bonding orbitals: Lowest energy, formed by direct overlap of atomic orbitals, found in single bonds.
• π (pi) bonding orbitals: Higher energy than σ, formed by sideway overlap of atomic orbitals, found in double and triple bonds.
• n (non-bonding) orbitals: Lone pair electrons in orbitals, typically on heteroatoms (O, N, S, halogens). Their energy is usually between π and π∗.
• σ∗ (sigma antibonding orbitals): High energy, unoccupied orbitals corresponding to σ bonds.
• π∗ (pi antibonding orbitals): Higher energy than π, unoccupied orbitals corresponding to π bonds.

Based on these orbitals, the main types of electronic transitions are:

1. σ→σ∗ Transitions:
   • Involve the promotion of an electron from a bonding sigma (σ) orbital to an antibonding sigma (σ∗) orbital.
   • Require very high energy, typically occurring in the vacuum UV region (below 200 nm).
   • Characteristic of saturated hydrocarbons (e.g., methane, ethane).
   • Generally not observed in standard UV-Vis spectrophotometers as air absorbs in this region.
2. n →σ∗ Transitions:
   • Involve the promotion of a non-bonding (n) electron from a lone pair on a heteroatom to an antibonding sigma (σ∗) orbital.
   • Require less energy than σ→σ∗.
   • Observed in molecules with lone pairs and single bonds (e.g., alcohols, ethers, amines, alkyl halides).
   • Typically absorb in the near UV region (e.g., methanol absorbs at ∼180 nm, methyl chloride at ∼173 nm).
3. π→π∗ Transitions:
   • Involve the promotion of an electron from a pi (π) bonding orbital to a pi antibonding (π∗) orbital.
   • Require moderate energy.
   • Characteristic of molecules with unsaturated bonds (double or triple bonds) and aromatic systems (e.g., alkenes, alkynes, benzene, aldehydes, ketones).
   • Often responsible for absorption in the analytically useful UV-Vis region (200−800 nm).
   • As conjugation (alternating single and double bonds) increases, the energy gap between π and π∗ orbitals decreases, leading to absorption at longer wavelengths (red shift).
4. n →π∗ Transitions:
   • Involve the promotion of a non-bonding (n) electron from a lone pair to a pi antibonding (π∗) orbital.
   • Require the lowest energy among the common transitions.
   • Observed in molecules containing both lone pairs and π bonds (e.g., aldehydes, ketones, carboxylic acids, esters, amides, nitriles).
   • Typically weaker in intensity than π→π∗ transitions and often appear at longer wavelengths.

Energy Order of Transitions (from highest to lowest energy): σ→σ∗>n→σ∗>π→π∗≈n→π∗ (Exact order of last two can vary depending on the specific molecule).

Chromophores and Auxochromes

Certain groups within a molecule are primarily responsible for absorbing UV-Vis radiation.

• Chromophore: A covalently unsaturated group responsible for electronic absorption in the UV-Vis region. It contains π bonds and/or non-bonding electrons.
  • Examples: C=C (alkene), C=O (carbonyl), C≡N (nitrile), N=N (azo), aromatic rings.
  • The more conjugated a chromophore (more alternating double and single bonds), the longer the wavelength of absorption (λmax​). This is because the π electron system is delocalized over a larger area, lowering the energy gap between π and π∗ orbitals.
• Auxochrome: A saturated group that, when attached to a chromophore, alters both the wavelength and intensity of absorption. Auxochromes typically contain lone pair electrons but no π bonds.
  • Examples: −OH (hydroxyl), −NH2​ (amino), −OR (alkoxy), −X (halogen).
  • Auxochromes extend the conjugation of the chromophore by allowing their lone pair electrons to interact with the π electron system, lowering the π→π∗ energy gap and often increasing intensity.

Shifts in λmax​ and Intensity

• Bathochromic Shift (Red Shift): A shift of λmax​ to a longer wavelength (lower energy). Usually caused by increasing conjugation or the presence of an auxochrome.
• Hypsochromic Shift (Blue Shift): A shift of λmax​ to a shorter wavelength (higher energy). Can be caused by removal of conjugation or a change in solvent polarity.
• Hyperchromic Effect: An increase in the intensity of absorption (increase in εmax​).
• Hypochromic Effect: A decrease in the intensity of absorption (decrease in εmax​).

Beer-Lambert Law

The Beer-Lambert Law (also known as Beer’s Law) is a fundamental relationship in UV-Vis spectroscopy that links the absorbance of a solution to the concentration of the absorbing species and the path length of the light through the solution.

A=εbc

Where:

• A = Absorbance (unitless)
• ε = Molar absorptivity (or molar extinction coefficient), a constant characteristic of the absorbing substance at a particular wavelength (units: L mol−1 cm−1 or M−1 cm−1)
• b = Path length of the sample cell (usually in cm)
• c = Concentration of the absorbing substance (usually in mol L−1 or M)

Key points about Beer-Lambert Law:

• It assumes monochromatic light, a homogeneous solution, and no association/dissociation of the absorbing species.
• It is widely used for quantitative analysis to determine unknown concentrations.

Instrumentation: UV-Vis Spectrophotometer

A UV-Vis spectrophotometer measures the amount of light absorbed by a sample across the UV and visible regions.

Basic components:

1. Light Source: Provides radiation in the UV ($ \text{Deuterium lamp})andVisible( \text{Tungsten-halogen lamp}$) regions.
2. Monochromator: Selects a narrow band of wavelengths (monochromatic light) from the broadband source. Typically uses gratings or prisms.
3. Sample Compartment: Holds the sample in a cuvette (quartz for UV, glass or plastic for visible). A reference cuvette with solvent is also used.
4. Detector: Converts transmitted light into an electrical signal (e.g., photomultiplier tube, photodiode array).
5. Readout/Computer: Processes and displays the spectrum.

Types of Spectrophotometers:

• Single-beam: Light passes through the sample, then the reference. Less stable for long-term measurements.
• Double-beam: Light is split and passes simultaneously through the sample and reference cuvettes. This compensates for variations in source intensity and provides more stable and accurate measurements.

Selection Rules for Electronic Transitions

While we observe many electronic transitions, not all are equally probable. Selection rules, derived from quantum mechanics, dictate which transitions are “allowed” (high probability, intense bands) and which are “forbidden” (low probability, weak bands).

1. Spin Selection Rule (ΔS=0):
   • For a transition to be allowed, the spin multiplicity of the initial and final electronic states must be the same.
   • Example: Singlet (S0​) to Singlet (S1​) transitions are allowed. Singlet (S0​) to Triplet (T1​) transitions are spin-forbidden, hence very weak (e.g., phosphorescence).
2. Laporte Selection Rule (For Centrosymmetric Molecules):
   • Applies to molecules that possess a center of inversion (centrosymmetric, e.g., octahedral complexes like Fe(CN)64−​).
   • Transitions between states of the same parity (g ↔ g, u ↔ u) are forbidden.
   • Transitions between states of different parity (g ↔ u) are allowed.
   • For example, in transition metal complexes, d-d transitions are often Laporte forbidden because d orbitals are centrosymmetric.

Vibronic Coupling (Intensity Stealing)

Even “forbidden” transitions can appear weakly in a spectrum due to vibronic coupling. This occurs when a molecular vibration (which can break the symmetry of the molecule or distort the electron cloud) couples with the electronic transition. This temporary distortion can relax the selection rules, allowing the forbidden transition to gain some intensity (“steal” intensity from allowed transitions). This is often seen as fine structure or broadened bands in spectra.

Solvent Effects

The polarity of the solvent can significantly affect the position (λmax​) and intensity of electronic absorption bands.

• For π→π∗ Transitions:
  • Increasing solvent polarity generally causes a bathochromic shift (red shift). This is because polar solvents can stabilize both the ground and excited states, but often stabilize the excited state more, leading to a smaller energy gap and longer λmax​.
• For n →π∗ Transitions:
  • Increasing solvent polarity generally causes a hypsochromic shift (blue shift). This is because non-bonding electrons are often more stabilized by hydrogen bonding with polar solvents in the ground state than in the excited state. This increases the energy gap, shifting absorption to shorter wavelengths.

Related Phenomena: Fluorescence and Phosphorescence

Electronic spectroscopy is closely related to luminescence phenomena:

• Fluorescence: Emission of light that occurs when an excited electron (typically in S1​) rapidly (nanoseconds) returns to the ground electronic state (S0​), often after losing some vibrational energy. The emitted light is usually at a longer wavelength than the absorbed light (Stokes shift).
• Phosphorescence: Emission of light that occurs when an excited electron undergoes intersystem crossing (ISC) from S1​ to a triplet state (T1​) and then slowly (microseconds to seconds) returns to S0​. This transition is spin-forbidden, hence the long lifetime.

These processes are often visualized using a Jablonski diagram, which illustrates the various radiative (fluorescence, phosphorescence) and non-radiative (vibrational relaxation, internal conversion, intersystem crossing) pathways for excited electrons.

Applications of Electronic Spectroscopy

UV-Vis spectroscopy is a widely used and versatile analytical technique:

1. Quantitative Analysis: The most common application. Using the Beer-Lambert Law, the concentration of an unknown substance in solution can be determined by measuring its absorbance at a specific wavelength (its λmax​) and using a known molar absorptivity (ε) or a calibration curve.
2. Qualitative Analysis (Compound Identification): While not as definitive as NMR or Mass Spectrometry, the unique UV-Vis spectrum (pattern of λmax​ and εmax​ values) can help identify a compound, especially when compared to known reference spectra. It’s particularly useful for identifying compounds with conjugated systems or aromatic rings.
3. Functional Group Identification: The presence or absence of specific chromophores (e.g., carbonyls, aromatic rings, conjugated double bonds) can be inferred from their characteristic absorption peaks.
4. Kinetic Studies (Reaction Monitoring): Changes in concentration of reactants or products over time can be monitored by observing the increase or decrease in absorbance at specific wavelengths. This allows for the determination of reaction rates and mechanisms.
5. Determination of pKa​ Values: The pKa​ of an acid-base indicator or a weak acid/base can be determined by monitoring changes in its UV-Vis spectrum as a function of pH, due to changes in its electronic structure upon protonation/deprotonation.
6. Analysis of Biological Molecules: Widely used in biochemistry to quantify proteins (e.g., using Bradford assay, or inherent tryptophan/tyrosine absorption), nucleic acids (DNA, RNA quantification at 260 nm), and study enzyme kinetics.
7. Ligand Field Theory in Coordination Chemistry: UV-Vis spectra of transition metal complexes provide information about the crystal field splitting energy (Δo​ or Δt​), aiding in understanding bonding and electronic structure.
8. Purity Checks: The ratio of absorbances at certain wavelengths (e.g., A260​/A280​ for nucleic acids) can indicate sample purity.

Conclusion

Electronic spectroscopy, particularly in the UV-Vis region, is an indispensable tool in chemistry, biology, and materials science. By analyzing how molecules absorb light due to electronic transitions, we can gain profound insights into their fundamental electronic structure, the extent of conjugation, and the presence of specific functional groups. Its straightforward principles, versatile applications, and ability to perform both qualitative and quantitative analysis make it one of the most widely employed spectroscopic techniques in research and industry.

Electronic Spectroscopy: Multiple Choice Questions

Instructions: Choose the best answer for each question. Explanations are provided after each question.

1. Electronic spectroscopy primarily involves transitions in which part of the electromagnetic spectrum? a) Microwave and Infrared b) Infrared and Visible c) Ultraviolet and Visible d) X-ray and Gamma-ray e) Radio waves and Microwaves

Explanation: Electronic transitions typically require higher energy than rotational or vibrational transitions, falling into the UV and Visible light regions.

2. Which of the following electronic transitions requires the highest energy? a) π→π∗ b) n→π∗ c) σ→σ∗ d) n→σ∗ e) All require similar energy

Explanation: σ bonds are the strongest and lowest energy bonding orbitals. Promoting an electron from a σ orbital to a σ∗ orbital requires a large amount of energy.

3. What is a “chromophore”? a) A saturated group that modifies absorption. b) A non-bonding electron responsible for fluorescence. c) A covalently unsaturated group responsible for electronic absorption in the UV-Vis region. d) A solvent molecule that causes a red shift. e) A component of a UV-Vis spectrophotometer.

Explanation: A chromophore is the part of a molecule that contains double or triple bonds or aromatic rings, and is directly responsible for absorbing UV or visible light.

4. A shift of λmax​ to a longer wavelength is known as a: a) Hypsochromic shift b) Hyperchromic effect c) Bathochromic shift d) Hypochromic effect e) Stokes shift

Explanation: “Batho” means deep or long, and “chromic” refers to color. A bathochromic shift means the absorption moves to a longer (redder) wavelength.

5. What is the Beer-Lambert Law equation? a) E=hν b) A=εbc c) A=2B(J+1) d) ΔG=ΔH−TΔS e) ν=2π1​μk​​

Explanation: The Beer-Lambert Law (A=εbc) relates absorbance (A) to molar absorptivity (ε), path length (b), and concentration (c).

6. Which component of a UV-Vis spectrophotometer is responsible for selecting a narrow band of wavelengths? a) Light source b) Cuvette c) Detector d) Monochromator e) Readout

Explanation: A monochromator uses gratings or prisms to disperse the light and select only a specific range of wavelengths to pass through the sample.

7. According to the spin selection rule, which transition is allowed? a) Singlet to Triplet b) Triplet to Singlet c) Singlet to Singlet d) Triplet to Quintet e) Any transition involving a change in spin multiplicity

Explanation: The spin selection rule states that for an electronic transition to be allowed, the total spin quantum number of the molecule must not change (ΔS=0). Singlet to Singlet maintains the same spin multiplicity.

8. Which effect causes a decrease in the intensity of absorption (εmax​)? a) Bathochromic effect b) Hypsochromic effect c) Hyperchromic effect d) Hypochromic effect e) Vibronic coupling

Explanation: “Hypo” means under or less, referring to a decrease in intensity.

9. For a centrosymmetric molecule, transitions between states of the same parity (g ↔ g) are forbidden by which rule? a) Spin selection rule b) Beer-Lambert Law c) Laporte selection rule d) Franck-Condon principle e) Mutually exclusive rule

Explanation: The Laporte selection rule specifically applies to centrosymmetric molecules and states that transitions where the electronic state’s parity (g or u) does not change are forbidden.

10. What phenomenon allows “forbidden” electronic transitions to appear weakly in a spectrum? a) Fluorescence b) Phosphorescence c) Intersystem crossing d) Vibronic coupling e) Internal conversion

Explanation: Vibronic coupling involves the interaction between electronic and vibrational motions, which can temporarily break symmetry rules and allow weakly forbidden transitions to gain some intensity.

11. Increasing solvent polarity generally causes a blue shift (hypsochromic shift) for which type of transition? a) σ→σ∗ b) π→π∗ c) n→π∗ d) d-d transitions e) All transitions

Explanation: For n→π∗ transitions, the non-bonding electrons are often more stabilized by polar solvents in the ground state than in the excited state, increasing the energy gap and shifting absorption to shorter wavelengths.

12. What is the most common application of the Beer-Lambert Law in UV-Vis spectroscopy? a) Determining molecular weight b) Identifying functional groups c) Quantitative analysis (determining concentration) d) Studying reaction mechanisms e) Measuring bond lengths

Explanation: The Beer-Lambert Law directly links absorbance to concentration, making it ideal for determining the unknown concentration of a known substance.

13. Which type of light source is typically used for the UV region in a UV-Vis spectrophotometer? a) Tungsten-halogen lamp b) Deuterium lamp c) Xenon arc lamp d) Incandescent bulb e) LED

Explanation: Deuterium lamps produce stable, continuous radiation in the ultraviolet region.

14. What describes the emission of light from a molecule that occurs rapidly (nanoseconds) after absorption, usually at a longer wavelength than the absorbed light? a) Phosphorescence b) Chemiluminescence c) Fluorescence d) Bioluminescence e) Rayleigh scattering

Explanation: Fluorescence is the rapid emission of light from an excited singlet state back to the ground singlet state, with a characteristic Stokes shift (emitted light is lower energy/longer wavelength).

15. Which of the following would NOT typically act as a chromophore? a) C=C (alkene) b) C=O (carbonyl) c) Benzene ring d) C-H single bond e) C≡N (nitrile)

Explanation: A C-H single bond is saturated and contains only σ electrons, which absorb in the high-energy vacuum UV region, not typically in the standard UV-Vis range.

16. As the number of conjugated double bonds in a molecule increases, what typically happens to its λmax​? a) It shifts to shorter wavelengths (blue shift). b) It shifts to longer wavelengths (red shift). c) It remains unchanged. d) Its intensity decreases. e) It disappears.

Explanation: Increasing conjugation delocalizes the π electrons over a larger area, lowering the energy gap between the highest occupied molecular orbital (π) and lowest unoccupied molecular orbital (π∗), which leads to absorption at longer wavelengths.

17. What is an “auxochrome”? a) A group that absorbs light independently. b) A saturated group that, when attached to a chromophore, alters its absorption wavelength and intensity. c) A substance used to clean cuvettes. d) A type of detector in a spectrophotometer. e) A compound that does not absorb UV-Vis light.

Explanation: Auxochromes contain lone pair electrons that can interact with the π system of a chromophore, modifying its electronic transitions.

18. If a solution shows an absorbance of 0.5 and the molar absorptivity is 5000 L mol−1 cm−1 in a 1 cm cuvette, what is the concentration of the solution? a) 0.0001 M b) 0.001 M c) 0.01 M d) 0.1 M e) 1 M

Explanation: Using Beer-Lambert Law A=εbc, we rearrange to c=A/(εb). So, c=0.5/(5000×1)=0.0001 M.

19. What is the primary advantage of a double-beam UV-Vis spectrophotometer over a single-beam one? a) It is more portable. b) It uses less sample. c) It compensates for variations in source intensity and provides more stable measurements. d) It can measure fluorescence. e) It is cheaper to operate.

Explanation: Double-beam instruments simultaneously compare the light passing through the sample and a reference, correcting for any fluctuations in the light source.

20. Phosphorescence involves an electronic transition from which state to the ground state? a) S0​→S1​ b) S1​→S0​ c) T1​→S0​ d) S1​→T1​ e) T1​→T0​

Explanation: Phosphorescence is a spin-forbidden transition from an excited triplet state (T1​) back to the ground singlet state (S0​), leading to a long emission lifetime.

21. Which type of cuvette is typically used for measurements in the UV region? a) Glass cuvette b) Plastic cuvette c) Quartz cuvette d) Ceramic cuvette e) Metal cuvette

Explanation: Glass and plastic absorb UV radiation. Quartz cuvettes are transparent to UV light, making them suitable for UV measurements.

22. If a molecule undergoes a transition from a non-bonding orbital to a pi antibonding orbital, it is a(n): a) σ→σ∗ transition b) π→π∗ transition c) n→σ∗ transition d) n→π∗ transition e) d-d transition

Explanation: This is the definition of an n→π∗ transition, common in molecules with lone pairs and double/triple bonds.

23. What is the primary use of UV-Vis spectroscopy in biochemistry for nucleic acids? a) Determining their sequence. b) Measuring their molecular weight. c) Quantifying their concentration (e.g., DNA, RNA). d) Identifying their specific bonding types. e) Analyzing their three-dimensional structure.

Explanation: Nucleic acids (DNA, RNA) strongly absorb UV light at 260 nm due to their conjugated aromatic bases, allowing for their quantitative determination using Beer-Lambert Law.

24. Which phenomenon refers to an increase in absorption intensity? a) Hypochromic effect b) Bathochromic effect c) Hypsochromic effect d) Hyperchromic effect e) Solvatochromism

Explanation: “Hyper” means over or more, indicating an increase in intensity.

25. A shift of λmax​ to a shorter wavelength (higher energy) is known as a: a) Bathochromic shift b) Hypsochromic shift c) Hyperchromic effect d) Hypochromic effect e) Stokes shift

Explanation: “Hypso” means up or shorter, referring to a shift to shorter wavelengths (higher energy).

26. In transition metal complexes, d-d transitions are often forbidden by the Laporte selection rule. Why do they sometimes still appear in spectra? a) Due to very high concentration. b) Due to strong ligand field effects. c) Due to vibronic coupling. d) Due to spin-orbit coupling. e) Due to the presence of an auxochrome.

Explanation: Vibronic coupling allows nominally forbidden d-d transitions to gain some intensity by temporarily distorting the molecule’s symmetry through vibrations.

27. What is the typical wavelength range for the visible region in UV-Vis spectroscopy? a) 10−100 nm b) 200−400 nm c) 400−800 nm d) 800−2000 nm e) 2000−4000 nm

Explanation: The visible region of the electromagnetic spectrum spans approximately 400 nm (violet) to 800 nm (red).

28. Which of the following accurately describes the relationship between the energy gap of π and π∗ orbitals and conjugation? a) As conjugation increases, the energy gap increases. b) As conjugation decreases, the energy gap decreases. c) As conjugation increases, the energy gap decreases. d) Conjugation has no effect on the energy gap. e) Only σ orbitals are affected by conjugation.

Explanation: Increased delocalization of π electrons through extended conjugation lowers the energy of the π∗ orbital and raises the energy of the π orbital, thereby decreasing the overall π→π∗ energy gap.

29. What type of lamp is commonly used as a light source for the visible region in a UV-Vis spectrophotometer? a) Deuterium lamp b) Xenon arc lamp c) Mercury vapor lamp d) Tungsten-halogen lamp e) LED array

Explanation: Tungsten-halogen lamps provide stable, continuous radiation across the visible and near-infrared regions.

30. Which biological molecule is commonly quantified at 280 nm using UV-Vis spectroscopy due to the absorption of aromatic amino acids? a) DNA b) RNA c) Proteins d) Carbohydrates e) Lipids

Explanation: Proteins absorb UV light at 280 nm primarily due to the aromatic amino acids tryptophan and tyrosine.

31. The Jablonski diagram illustrates pathways for excited electrons. Which process involves a spin flip from a singlet to a triplet state? a) Fluorescence b) Phosphorescence c) Internal conversion (IC) d) Intersystem crossing (ISC) e) Vibrational relaxation

Explanation: Intersystem crossing (ISC) is a non-radiative process where an electron changes its spin multiplicity from a singlet state to a triplet state.

32. What is the unit for molar absorptivity (ε) in the Beer-Lambert Law? a) cm b) mol L−1 c) Unitless d) L mol−1 cm−1 e) nm

Explanation: By rearranging the Beer-Lambert Law (A=εbc), ε=A/(bc). Since A is unitless, b is in cm, and c is in mol/L, the units for ε are L mol−1 cm−1.

33. If a compound appears blue to the human eye, what color of light is it primarily absorbing? a) Blue b) Green c) Red d) Yellow e) Orange

Explanation: The color we perceive is the color that is transmitted or reflected. If a substance appears blue, it is absorbing the complementary color, which is orange/red.

34. Which of the following is a limitation of the Beer-Lambert Law? a) It only works for colored solutions. b) It requires a double-beam spectrophotometer. c) It assumes monochromatic light. d) It is not applicable to organic compounds. e) It only works at high concentrations.

Explanation: One key assumption of Beer-Lambert Law is that the light passing through the sample is monochromatic (a single wavelength). Deviations occur with polychromatic light.

35. What is the relationship between fluorescence and phosphorescence lifetimes? a) Fluorescence is always shorter than phosphorescence. b) Phosphorescence is always shorter than fluorescence. c) They have similar lifetimes. d) Lifetimes depend only on temperature. e) Lifetimes depend only on the solvent.

Explanation: Fluorescence involves an allowed S1​→S0​ transition and is very fast (nanoseconds). Phosphorescence involves a forbidden T1​→S0​ transition, making it much slower (microseconds to seconds).

36. A sample has an absorbance of 1.0 at 300 nm. If the concentration is halved, what will the new absorbance be, assuming Beer-Lambert Law holds? a) 2.0 b) 1.0 c) 0.5 d) 0.25 e) 0.0

Explanation: Absorbance is directly proportional to concentration. If the concentration is halved, the absorbance will also be halved.

37. Which process on a Jablonski diagram represents a non-radiative transition between electronic states of the same spin multiplicity? a) Fluorescence b) Phosphorescence c) Internal Conversion (IC) d) Intersystem Crossing (ISC) e) Absorption

Explanation: Internal Conversion (IC) is a non-radiative process where an excited electron transitions from a higher vibrational level of one electronic state to a higher vibrational level of a lower electronic state of the same spin multiplicity.

38. What type of quantitative analysis is a standard application of UV-Vis spectroscopy? a) Elemental analysis b) Determining reaction rates c) Molecular weight determination d) Isotopic ratio analysis e) NMR spectral analysis

Explanation: Monitoring the change in absorbance over time to determine how fast a reactant is consumed or a product is formed is a direct application of UV-Vis for kinetic studies.

39. Which of the following statements about π→π∗ transitions is true? a) They are characteristic of saturated hydrocarbons. b) They involve lone pair electrons. c) They typically absorb at very high energy (vacuum UV). d) They are responsible for absorption in molecules with unsaturated bonds. e) They are usually weaker in intensity than n→π∗ transitions.

Explanation: π→π∗ transitions involve the delocalized electrons in double or triple bonds, making them characteristic of unsaturated and aromatic compounds.

40. What does the term “Stokes shift” refer to in fluorescence spectroscopy? a) The shift in absorption wavelength due to solvent polarity. b) The difference in energy between the absorbed and emitted photons, where emitted is lower energy. c) The splitting of energy levels in a magnetic field. d) The decrease in fluorescence intensity with increasing temperature. e) The shift in wavelength caused by instrumental error.

Explanation: The Stokes shift is the difference in wavelength (or energy) between the peak of the absorption spectrum and the peak of the emission (fluorescence) spectrum, where the emitted light is always at a longer wavelength (lower energy) due to vibrational relaxation in the excited state.