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Questions and Answers
What is the energy range required to change the electron distribution in molecules?
What is the energy range required to change the electron distribution in molecules?
What is the principle that explains why electronic transitions occur faster than nuclear responses?
What is the principle that explains why electronic transitions occur faster than nuclear responses?
Which type of molecular orbitals is involved in the strongest transition in molecular spectroscopy?
Which type of molecular orbitals is involved in the strongest transition in molecular spectroscopy?
What type of chromophore is responsible for the ultraviolet absorption at 180 nm in ethylene?
What type of chromophore is responsible for the ultraviolet absorption at 180 nm in ethylene?
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What is the name of the theory that explains the splitting of d-orbitals in transition metal complexes?
What is the name of the theory that explains the splitting of d-orbitals in transition metal complexes?
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Which type of transition is responsible for the absorption of light in carbonyl compounds?
Which type of transition is responsible for the absorption of light in carbonyl compounds?
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What is the name of the law that relates the absorbance of light to the concentration of a molecule?
What is the name of the law that relates the absorbance of light to the concentration of a molecule?
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What is the term for the difference in energy between the t2g and eg orbitals in an octahedral complex?
What is the term for the difference in energy between the t2g and eg orbitals in an octahedral complex?
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Which of the following is an application of UV-Vis spectroscopy?
Which of the following is an application of UV-Vis spectroscopy?
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What happens to the energies of the d orbitals in an octahedral complex according to Ligand Field Theory?
What happens to the energies of the d orbitals in an octahedral complex according to Ligand Field Theory?
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What is the process in which the excited molecule discards its excitation energy as a photon?
What is the process in which the excited molecule discards its excitation energy as a photon?
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What type of decay involves spontaneous emission of radiation over a long period of time?
What type of decay involves spontaneous emission of radiation over a long period of time?
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What is the term for the transfer of energy from an excited molecule to surrounding molecules?
What is the term for the transfer of energy from an excited molecule to surrounding molecules?
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What is the purpose of fluorescent dyes in fluorescence microscopy?
What is the purpose of fluorescent dyes in fluorescence microscopy?
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What is the term for the intensity of fluorescence depending on the capability of solvent molecules to accept the transition energy?
What is the term for the intensity of fluorescence depending on the capability of solvent molecules to accept the transition energy?
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What type of transition occurs in phosphorescence?
What type of transition occurs in phosphorescence?
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What is the term for the non-radiative transition from a singlet to a triplet state?
What is the term for the non-radiative transition from a singlet to a triplet state?
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What is the purpose of Jablonsky diagrams?
What is the purpose of Jablonsky diagrams?
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What is the term for the spectrum that is similar in structure to the absorption spectrum, but with a larger wavelength?
What is the term for the spectrum that is similar in structure to the absorption spectrum, but with a larger wavelength?
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Why does phosphorescence occur with heavy elements such as sulfur?
Why does phosphorescence occur with heavy elements such as sulfur?
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Study Notes
Molecular Spectroscopy Fundamentals
- Electronic transitions require significant energy, approximately 100 kJ/mol, corresponding to visible light wavelengths like blue (470 nm).
- Transition occurs rapidly compared to nuclear response, leading to vertical transitions from the ground state to excited states.
- Different vibrational states exist, resulting in multiple absorption frequencies in molecular spectra.
Electronic Spectrum of SO2(g)
- Polyatomic molecules exhibit specific absorption spectra due to molecular orbital (MO) theories.
- Main types of MOs include bonding (σ, π), anti-bonding (σ*, π*), and non-bonding (n).
- Dominant transitions involve π* ← π and σ* ← n, with the strongest transition occurring from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO).
Chromophores and Color
- Chromophores are groups with unique optical absorption characteristics; their transition positions depend on the molecular structure.
- Conjugation in carbon-carbon double bonds affects wavelength: unconjugated bonds absorb UV light (e.g., λ=180 nm in ethylene); conjugated systems absorb longer wavelengths, often in the visible spectrum.
- Carbonyl groups undergo transitions like π* ← π and π* ← n, with notable absorption near 290 nm.
Transition Metal Complexes
- Complexes of transition metals exhibit vivid colors due to ligand interactions, as seen in compounds like Cu(NH3)4²⁺.
- Ligand field theory explains energy variations in d orbitals upon ligand approach, influencing colors based on ligand field splitting parameters (∆0).
UV/Vis Spectroscopy Importance
- UV/Visible spectroscopy is vital for quantitative analysis, applying Lambert-Beer law: A=εcl, where ε is the molar absorptivity.
- Concentrations are determined using wavelengths where only the target analyte exhibits high absorption.
Fate of Excited States
- Molecules absorbing photons enter excited states; three possible outcomes follow:
- Non-radiative decay: Energy dissipates as thermal motion among surrounding molecules.
- Radiative decay: Energy is released as light, transitioning the molecule back to a lower energy state.
- Possible involvement in photochemical reactions.
Radiative Decay Types
- Fluorescence: Rapid photon emission occurs within nanoseconds after excitation, resulting in lower energy (longer wavelength) radiation.
- Phosphorescence: Delayed emission can last from seconds to hours, functioning as an energy reservoir.
Fluorescence Mechanism
- A molecule excited from a ground state can release energy via spontaneous emission to lower energy states.
- Emission spectra resemble absorption spectra but with larger wavelengths and different vibrational characteristics.
- Fluorescence intensity can diminish due to solvent interactions — solvents capable of accepting transition energy (like water) can quench fluorescence.
Applications of Fluorescence
- Fluorescent dyes absorb UV light, emitting visible light, utilized in applications such as fluorescence microscopy for biological imaging.
- Fluorescence microscopy enables visualization of proteins, nucleic acids, and membranes in cells.
Phosphorescence Mechanism
- Involves non-radiative transitions leading to a triplet state, which is more stable and has a 'forbidden' transition back to the singlet state, resulting in weak emission.
- Jablonsky diagrams visually represent energy levels and transitions during fluorescence and phosphorescence.
- Phosphorescence is often observed in materials with heavy elements and is not typical in lighter elements like phosphorous.
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Description
This quiz covers the concepts of molecular spectroscopy, including electronic transitions, ultraviolet and visible spectra, and the Frank-Condon principle. It explores the energies required to change electron distributions and the relationship between electronic transitions and nuclear responses. Test your knowledge of these fundamental principles in physical chemistry!
