UV-Vis Spectroscopy Quiz
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Questions and Answers

In UV-Vis spectroscopy, what is the range of wavelengths considered as visible light?

  • 380 – 780 nm (correct)
  • < 180 nm
  • 180 – 380 nm
  • 800 – 1000 nm

What happens to π and n electrons when high energy radiation is absorbed by molecules?

  • They are promoted to higher energy levels. (correct)
  • They are converted into kinetic energy.
  • They are lost from the molecule entirely.
  • They become vibrational energy.

Which equation represents the overall energy of a molecule considering various transitions?

  • Eoverall = Eelectronic + Evibrational
  • Eoverall = Eground + Eexcited
  • Eoverall = Evis + Eir
  • Eoverall = Eelectronic + Evibrational + Erotational + Enuclear (correct)

What is the result of a photon absorption in a molecule during the electronic transition?

<p>It results in the promotion of electrons to a high energy state. (B)</p> Signup and view all the answers

What is the wavelength range of vacuum ultraviolet (VUV) radiation?

<p>&lt; 180 nm (C)</p> Signup and view all the answers

What type of transition is primarily responsible for peaks in the UV spectrum of vanillin?

<p>Electronic transition (B)</p> Signup and view all the answers

Which part of the vanillin structure is likely responsible for its UV absorption?

<p>Carbonyl group (B), Aromatic ring system (C)</p> Signup and view all the answers

Which instrument is typically used to measure the UV spectrum?

<p>UV-Visible spectrophotometer (B)</p> Signup and view all the answers

In the context of the UV spectrum, what does a peak represent?

<p>The energy absorbed during an electronic transition (B)</p> Signup and view all the answers

What factor does NOT affect the UV absorption peaks of a compound like vanillin?

<p>Molecular weight of adjacent compounds (C)</p> Signup and view all the answers

What type of radiation has a wavelength range of 180-380 nm?

<p>Ultraviolet radiation (A)</p> Signup and view all the answers

Which energy transition is NOT associated with electronic levels?

<p>Nuclear spin transitions (A), Rotational transitions (C), Vibrational transitions (D)</p> Signup and view all the answers

Which of the following transitions specifically applies to gas molecules?

<p>Rotational transitions (A)</p> Signup and view all the answers

What effect does the absorption of infrared radiation have on molecules?

<p>Changes in the amplitude of vibration (C)</p> Signup and view all the answers

What is the energy formula for total molecular energy?

<p>Etotal = Eelec + Evib + Erot + Enucl (D)</p> Signup and view all the answers

Within which type of spectroscopy are electronic transitions primarily observed?

<p>Ultraviolet-visible spectroscopy (C)</p> Signup and view all the answers

In relation to energy levels, which statement is true for organic molecules?

<p>They are polyatomic with more energy levels than atoms. (C)</p> Signup and view all the answers

What is the wavelength range associated with far-infrared radiation?

<p>$100 ext{ µm} - 10 ext{ cm}$ (B)</p> Signup and view all the answers

What is a chromophore in organic molecules?

<p>A functional group capable of absorbing UV-visible radiation (D)</p> Signup and view all the answers

What does ε indicate about a chromophore?

<p>The magnitude of light absorption at a specific wavelength (A)</p> Signup and view all the answers

Which of the following statements is true regarding molecular and atomic spectra?

<p>Molecular spectra provide a continuous broad band spectrum. (C)</p> Signup and view all the answers

The electronic transition during UV-Vis absorption primarily involves electrons moving from which orbitals?

<p>From the HOMO to the LUMO. (B)</p> Signup and view all the answers

What characterizes strongly absorbing chromophores in terms of molar absorptivity (ε)?

<p>ε values greater than 10,000 L mol-1 cm-1 (A)</p> Signup and view all the answers

Which type of molecular orbital results from the overlap of two atomic orbitals?

<p>Both bonding and antibonding orbitals (C)</p> Signup and view all the answers

Which types of compounds can typically contain chromophores?

<p>Dienes, aromatics, and conjugated ketones (C)</p> Signup and view all the answers

How does the size of a chromophore affect its ability to absorb light?

<p>Larger chromophores generally enhance the probability of absorption. (A)</p> Signup and view all the answers

Which type of overlap results in the formation of a σ bond?

<p>Two s atomic orbitals (D)</p> Signup and view all the answers

In a triple bond, how many σ and π bonds are present?

<p>1 σ bond and 2 π bonds (D)</p> Signup and view all the answers

What is the energy characteristic of a σ bonding molecular orbital?

<p>Lowest energy orbital (D)</p> Signup and view all the answers

Which of the following orbitals does not contribute to π bonding?

<p>Hybridized sp3 orbital (D)</p> Signup and view all the answers

Which type of electrons is typically excited to higher energy levels during electronic transitions?

<p>Ï€ electrons (B)</p> Signup and view all the answers

What is an example of a molecule containing nonbonding (n) orbitals?

<p>Formaldehyde (A)</p> Signup and view all the answers

What is the result of a σ → σ∗ transition in molecular orbital theory?

<p>An electron is promoted from a bonding σ orbital to an antibonding σ orbital (A)</p> Signup and view all the answers

What is characteristic of π orbitals compared to σ orbitals in terms of electron density?

<p>Clouds of electron density above and below the internuclear axis (B)</p> Signup and view all the answers

What is the energy requirement difference between σ → σ* transitions and n → σ* transitions?

<p>σ → σ* transitions require more energy than n → σ* transitions. (D)</p> Signup and view all the answers

In what wavelength range do n → σ* transitions typically occur?

<p>150 – 250 nm (C)</p> Signup and view all the answers

What is the molar absorptivity range for n → π* transitions?

<p>10 – 100 L mol-1 cm-1 (B)</p> Signup and view all the answers

Which transition is primarily responsible for most absorption spectroscopy of organic compounds?

<p>n → π* and π → π* (D)</p> Signup and view all the answers

Why can't transitions to the π* orbital occur in molecules without π bonds?

<p>The molecule lacks the necessary π orbitals. (D)</p> Signup and view all the answers

What property generally characterizes π → π* transitions compared to n → π* transitions?

<p>Higher molar absorptivity for π → π* transitions. (A)</p> Signup and view all the answers

Which of the following transitions is likely to happen in the range of 200-780 nm?

<p>n → π* (C)</p> Signup and view all the answers

What is the nature of the absorbance maximum for methane due to σ → σ* transitions?

<p>Absorbance at 125 nm. (B)</p> Signup and view all the answers

In the context of chromophores, what does a small number of organic functional groups with n → σ* peaks indicate?

<p>They are rare in organic compounds. (B)</p> Signup and view all the answers

What is the primary result of UV-visible radiation interacting with metals?

<p>Change of electron distribution in valence electrons (D)</p> Signup and view all the answers

Which energy transitions are primarily observed in infrared spectroscopy?

<p>Vibrational transitions (A)</p> Signup and view all the answers

What does the equation $E_{total} = E_{elec} + E_{vib} + E_{rot} + E_{nucl}$ represent?

<p>The energy of a molecule considering various transitions (D)</p> Signup and view all the answers

Which of the following correctly describes rotational transitions?

<p>They apply to gas molecules rotating about their axes (B)</p> Signup and view all the answers

What is primarily observed in the UV-visible region of molecular spectroscopy?

<p>Electronic transitions of bound electrons (C)</p> Signup and view all the answers

Which wavelength range corresponds to visible light?

<p>380-780 nm (A)</p> Signup and view all the answers

What happens to molecules when they absorb infrared radiation?

<p>They transition to higher rotational states (B)</p> Signup and view all the answers

What is the energy requirement and wavelength range for σ → σ* transitions?

<p>Large energy, λ &lt; 150 nm (C)</p> Signup and view all the answers

Which of the following statements is true about n → σ* transitions?

<p>Occur in the wavelength range of 150 – 250 nm (C)</p> Signup and view all the answers

What is the range of molar absorptivity (ε) for π → π* transitions?

<p>1,000 – 10,000 L mol-1 cm-1 (A)</p> Signup and view all the answers

What is the primary condition for the occurrence of π → π* transitions in organic molecules?

<p>Presence of unsaturated groups (D)</p> Signup and view all the answers

Which electronic transition requires the least energy among the following?

<p>n → σ* transitions (C)</p> Signup and view all the answers

What is the primary effect of high energy radiation absorption on electrons in molecules?

<p>Electrons are promoted to higher energy levels in antibonding orbitals. (C)</p> Signup and view all the answers

Which represents the formula for overall energy changes in molecules during electronic transitions?

<p>Eoverall = Eelectronic + Evibrational + Erotational + Enuclear (C)</p> Signup and view all the answers

In which wavelength range is conventional UV-Vis spectroscopy conducted?

<p>180 - 780 nm (C)</p> Signup and view all the answers

What must occur for a molecule to transition from its ground state to an excited state during UV-Vis absorption?

<p>The molecule must absorb a photon. (D)</p> Signup and view all the answers

What is the incorrect interpretation of the process of electronic transitions in molecules?

<p>Electrons can descend to lower energy levels without energy input. (B)</p> Signup and view all the answers

Which of the following energy levels is NOT typically associated with ground and excited states of molecules?

<p>Gravitational energy levels (B)</p> Signup and view all the answers

How are π and n electrons predominantly affected during the absorption of high energy radiation?

<p>They are promoted to antibonding molecular orbitals. (B)</p> Signup and view all the answers

What happens in the vacuum ultraviolet (VUV) region of the spectrum?

<p>It includes wavelengths less than 180 nm. (A)</p> Signup and view all the answers

Which molecular transitions are primarily responsible for peaks observed in electronic spectroscopy?

<p>Electronic transitions (D)</p> Signup and view all the answers

Which of the following statements accurately describes the electronic transition process in molecules?

<p>The energy of UV-Vis photons can lead to the promotion of electrons from the HOMO to the LUMO. (C)</p> Signup and view all the answers

What factor does NOT influence the molar absorptivity (ε) of a chromophore?

<p>Color of the compound (B)</p> Signup and view all the answers

Which of the following best describes a chromophore?

<p>An atom or group that absorbs UV-visible radiation due to π or n electrons. (B)</p> Signup and view all the answers

What type of spectrum is produced by the electronic transitions of molecules?

<p>Continuous broad band spectrum (B)</p> Signup and view all the answers

What term describes the highest occupied molecular orbital involved in electronic transitions?

<p>HOMO (A)</p> Signup and view all the answers

Which type of chromophore generally shows large molar absorptivity (ε) values?

<p>Strongly absorbing chromophores with ε &gt; 10,000 L mol-1 cm-1 (D)</p> Signup and view all the answers

Which electronic transition is less likely to occur in molecules without double or triple bonds?

<p>π → π* transition (D)</p> Signup and view all the answers

What is the primary characteristic of molecular orbital theory regarding bonding molecular orbitals?

<p>They are always more stable and lower in energy than their corresponding antibonding orbitals. (D)</p> Signup and view all the answers

How does the magnitude of molar absorptivity (ε) reflect the nature of the chromophore?

<p>It corresponds directly to the chromophore's light absorption probability. (D)</p> Signup and view all the answers

Which statement best differentiates the molecular spectrum from the atomic spectrum?

<p>Molecular spectra give continuous broad bands, while atomic spectra are typically lines. (D)</p> Signup and view all the answers

What type of atomic orbital overlap results in the formation of a σ bond?

<p>End to end overlap of two 's' orbitals (C)</p> Signup and view all the answers

In a molecule containing a triply bonded nitrogen, how many σ and π bonds are present?

<p>One σ bond and two π bonds (A)</p> Signup and view all the answers

What energy characteristic is true for a σ bonding molecular orbital?

<p>Lowest energy orbital (A)</p> Signup and view all the answers

What is the shape of electron density in σ orbitals?

<p>Cylindrical symmetrical around the internuclear axis (D)</p> Signup and view all the answers

Which statement accurately describes π bonds?

<p>They involve clouds of electron density above and below the internuclear axis (C), They have a higher energy than corresponding σ bonds (D)</p> Signup and view all the answers

During an electronic transition in molecular orbital theory, when does an electron move from a σ bonding orbital?

<p>To the corresponding antibonding σ orbital (A)</p> Signup and view all the answers

Which of the following is true regarding nonbonding (n) orbitals?

<p>They consist of unshared electron pairs in an atomic-like orbital (A)</p> Signup and view all the answers

What is the primary reason for the electronic excitation of electrons in molecular orbitals?

<p>Absorption of high-energy radiation (C)</p> Signup and view all the answers

Which type of molecular orbital transition occurs as a result of a π bond?

<p>π → π* (A)</p> Signup and view all the answers

In molecular orbital theory, which hybridized orbital combination can form σ bonds?

<p>Any combination of 's' and 'spx' hybridized orbitals (D)</p> Signup and view all the answers

Flashcards

UV-Vis Spectroscopy

A technique used to analyze the interaction of UV or visible light with molecules.

Electronic Transitions (Eelec)

Energy changes associated with movement of electrons between different energy levels within the molecule.

Vibrational Transitions (Evib)

Energy changes related to changes in the vibration of atoms within a molecule.

Rotational Transitions (Erot)

Energy changes due to the rotation of molecules.

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Infrared Spectroscopy range

The range of wavelengths, often low energy or long wavelengths, used in spectroscopy studies of vibrational energy transitions of molecules. Ranges approx. 780nm – 3.75µm(low energy).

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Total Molecular Energy (Etotal)

The sum of electronic, vibrational, rotational, and nuclear energy levels of a molecule.

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Atomic Absorption/Emission Spectroscopy

Techniques that detect atoms by measuring the absorption or emission of light of specific wavelengths.

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UV-Vis range

A range of electromagnetic radiation that includes ultraviolet and visible light.

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Electronic Transition

The process where electrons move from a lower energy level to a higher one in a molecule, absorbing light.

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UV-Vis Wavelength Range

The range of wavelengths studied in conventional UV-Vis spectroscopy is 180–780 nanometers.

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Absorption of Radiation

When molecules absorb light, the energy of the light promotes the bonding or lone-pair electrons to higher levels.

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Excited State

The state of a molecule when it has absorbed light and its electrons have moved to a higher energy level.

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Sigma (σ) Orbital

An orbital with cylindrical electron density around the internuclear axis. Formed by the overlap of atomic orbitals like s, p, and hybridized orbitals.

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Sigma (σ) Bond

A single bond formed by the overlap of atomic orbitals. Single bonds in molecules are sigma bonds. Double and triple bonds also contain a sigma bond.

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σ* Antibonding Molecular Orbital

A higher energy molecular orbital formed from the combination of atomic orbitals. When an electron is promoted to this orbital, it weakens the bond. This is a higher energy state.

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Ï€* Antibonding Molecular Orbital

A higher energy molecular orbital formed from the combination of π orbitals. Promotion of an electron to this orbital weakens the π bond.

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Fluorescence Decay Process

Molecules absorb high-energy UV/Vis light, become excited (M*), then release lower-energy Vis light as they return to their ground state (Mo).

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Molecular Spectrum

A continuous, broad band spectrum produced by electronic transitions in molecules, contrasted with the atomic line spectrum

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Chromophore

An atom or group in a molecule with pi or n electrons that can absorb UV-Vis light. These groups are often unsaturated.

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Molar Absorptivity (ε)

A measure of how strongly a molecule absorbs UV-Vis light.

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Electronic Transition (Molecules)

The movement of an electron from a lower energy level to a higher energy level in a molecule, typically triggered by absorbing light.

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HOMO

Highest Occupied Molecular Orbital, and electron starts at this energy level in light absorption.

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LUMO

Lowest Unoccupied Molecular Orbital, the electron moves to during electronic excitation.

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σ → σ* transitions

Transitions that involve promoting an electron from a bonding sigma orbital to an antibonding sigma orbital. These require high energy and occur in the vacuum UV region (λ < 150 nm).

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n → σ* transitions

Transitions that involve promoting an electron from a non-bonding orbital (n) to an antibonding sigma orbital (σ*). These occur in the far UV region (150-250 nm) and are characteristic of saturated compounds with lone pairs.

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π → π* transitions

Transitions that involve promoting an electron from a bonding pi orbital to an antibonding pi orbital. These occur in the UV-Vis region (200-780 nm) and are characteristic of unsaturated compounds.

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n → π* transitions

Transitions that involve promoting an electron from a non-bonding orbital (n) to an antibonding pi orbital (Ï€*). These occur in the UV-Vis region (200-780 nm) and are characteristic of compounds with lone pairs and double bonds.

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λmax

The wavelength at which a molecule absorbs the maximum amount of light. It is characteristic of a particular chromophore.

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Solvent Effect

The influence of the solvent on the absorption spectrum of a molecule. Different solvents can shift λmax.

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Vacuum UV Region

The region of the electromagnetic spectrum below 150 nm. It is used for studying transitions that require very high energy.

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UV Spectrum Peaks

The peaks observed in the UV spectrum of a molecule correspond to different types of electronic transitions, specifically those related to the excitation of electrons within the molecule.

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UV Spectrum of Vanillin

The UV spectrum of vanillin shows various peaks that correspond to different electronic transitions within the molecule. The peaks are attributed to the excitation of electrons in specific functional groups, like the aromatic ring and the carbonyl group.

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Functional Groups in UV Spectrum

Different functional groups in a molecule contribute to specific peaks in the UV spectrum. For example, the aromatic ring in vanillin shows π → π* transitions, while the carbonyl group contributes to both n → π* and π → π* transitions.

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Vibrational Transitions

Changes in the way atoms vibrate within a molecule, usually caused by the absorption of infrared (IR) radiation. These affect the energy levels of the molecule.

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Rotational Transitions

Changes in the rotational energy of molecules, caused by the absorption of low energy radiation. These transitions are important for analyzing gaseous samples.

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Total Energy of a Molecule

The sum of energies from different sources: electronic, vibrational, rotational, and nuclear. These different energy levels contribute to the overall energy state of the molecule.

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Organic Molecules and Energy Levels

Organic molecules have more energy levels than atoms due to their complex structure with multiple atoms bonded together. This leads to broader absorption spectra.

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What is UV-Vis spectroscopy?

A technique that studies how molecules interact with ultraviolet and visible light.

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What does UV-Vis spectroscopy measure?

It measures the absorption of photons by molecules, causing electrons to jump to higher energy levels.

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What are the different types of energy transitions in molecules?

Electronic transitions (electrons moving), vibrational transitions (bonds stretching/bending), and rotational transitions (molecule spinning) contribute to the overall energy of a molecule.

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What are π and n electrons?

Ï€ electrons are found in double or triple bonds, while n electrons are non-bonding electrons in atoms like oxygen or nitrogen.

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What is the difference between the ground state and excited state?

In the ground state, electrons are in their lowest energy levels. When a molecule absorbs light, electrons move to higher energy levels, reaching an excited state.

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What are chromophores?

Specific parts of a molecule that absorb UV-Vis light.

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What is λmax?

The wavelength at which a molecule absorbs the maximum amount of light.

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How does the solvent affect UV-Vis spectra?

Different solvents can shift the λmax, affecting how much light a molecule absorbs.

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How is UV-Vis spectroscopy useful?

It helps identify and quantify substances by their unique absorption patterns, aiding in research and industrial processes.

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σ bond

A strong chemical bond formed by the direct overlap of atomic orbitals along the internuclear axis. It is the strongest type of covalent bond and is present in all single bonds.

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Ï€ bond

A weaker chemical bond formed by the sideways overlap of atomic orbitals above and below the internuclear axis. These bonds are found in double and triple bonds.

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Molecular Orbital

A region of space where electrons are likely to be found in a molecule, formed by the combination of atomic orbitals.

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Antibonding Molecular Orbital

A higher energy molecular orbital formed from the combination of atomic orbitals that results in a weakening of the chemical bond.

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Nonbonding (n) Orbital

An atomic orbital that is not directly involved in bonding, consisting of unshared electron pairs on electronegative atoms.

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σ* Antibonding Orbital

A high-energy antibonding orbital formed by the combination of σ orbitals. Promotion of an electron to this orbital weakens the σ bond.

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Ï€* Antibonding Orbital

A high-energy antibonding orbital formed by the combination of π orbitals. Promotion of an electron to this type of orbital weakens the π bond.

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Study Notes

Molecular Absorption Theory: UV-Visible Spectroscopy

  • UV-visible spectroscopy analyzes how molecules absorb ultraviolet and visible light.
  • The interaction of UV-visible radiation with molecules results in changes in electron distribution.
  • UV light range is 180-380 nm, visible light range is 380-780 nm
  • The specific wavelengths absorbed by a molecule depend on its structure.
  • Molar absorptivity (ε) is useful for detecting λmax.
  • Vacuum UV spectroscopy occurs at wavelengths shorter than 180 nm.

Atomic Absorption/Emission Spectroscopy

  • Type of radiation:
    • UV: 180-380 nm
    • Visible: 380-780 nm
  • Interaction with metals: Results in electron distribution changes.

Internal Energy of Molecules

  • Etotal = Eelec + Evib + Erot + Enucl
  • Eelec: Electronic transitions—UV-vis, X-ray spectroscopy
  • Evib: Vibrational transitions—infrared spectroscopy
  • Erot: Rotational transitions—microwave spectroscopy
  • Enucl: Nucleus spin—nuclear magnetic resonance spectroscopy
  • Organic molecules have more energy levels than atoms

Vibrational and Rotational Energy States Within Electronic Levels

  • Molecules in excited states have vibrational and rotational energy levels
  • Rotational energy levels within electronic levels are very closely spaced.

Energy Transitions for Molecules

  • Rotational:
    • Uses far-infrared, microwave, or radio waves λ = 100 µm – 10 cm
    • Gas molecules rotate about their axes
    • Absorption of low energy causes the energy of molecules to increase leading to transitions to higher rotational states in the ground state
    • No electron promotion
  • Vibrational:
    • Infrared light λ= 780nm – 3.75 µm
    • Absorption causes changes in vibrational amplitude (stretching or bending)
    • No electron promotion
  • Molecules in excited states have vibrational and rotational energy levels

Electronic Transitions

  • UV-Vis Spectroscopy:
    • Conventional UV λ= 180-380 nm
    • Visible λ= 380-780 nm
    • Absorption of high energy radiation causes the promotion of bonded/lone pairs of electrons to higher energy levels
    • Ï€ and n electrons are promoted to antibonding orbitals

Summary of Energy Transitions

  • Eoverall = Eelectronic + Evibrational + Erotational + Enuclear
  • Energy changes occur during absorption of IR, VIS, and UV radiation by organic molecules
  • Ground state and excited states contain vibrational and rotational energy levels

Electronic (Molecular) Spectroscopy

  • Phenomenon of UV and visible light interaction with molecules
  • Conventional UV-Vis spectroscopy: 180-780 nm
  • Vacuum UV region: <180 nm
  • Absorption of photon results in electronic transitions
    • Electrons promoted to higher states from ground state

Molecular Spectrum

  • Result of electronic transitions: continuous broad band spectrum
  • Contrast to atomic absorption line spectrum
    • Limited structural information
    • Unique for a given compound (fingerprint)
    • λmax can be read

Molecular Absorption

  • Absorption of UV/visible light in organic molecules restricted to certain unsaturated functional groups (chromophores)
  • Chromophore: atom or functional group containing Ï€ or n electrons capable of low excitation energy absorption.
  • UV/Vis used to detect chromophores
    • Dienes
    • Aromatics
    • Polyenes
    • Conjugated ketones

Molar Absorptivity for Chromophores

  • Large ε for strongly absorbing chromophores (> 10,000 L mol−1 cm−1)
  • Small ε for weakly absorbing chromophores (10 - 100 L mol−1 cm−1)
  • Magnitude of ε reflects size of chromophore and probability of light absorption.
  • ε useful for detection of λmax

Molecular Absorption

  • Absorption of UV/vis photons by molecules results in electronic excitation.
  • Excitation of molecules that contain chromophoric groups.
  • Electronic transitions involve promotion of electrons to higher orbitals from the HOMO to the LUMO.

HOMO/LUMO

  • HOMO: Highest occupied molecular orbital
  • LUMO: Lowest unoccupied molecular orbital

Molecular Orbital Theory

  • Overlap of 2 atomic orbitals = 2 new molecular orbitals: bonding (lower energy) and antibonding (higher energy).

Sigma (σ) Orbitals

  • Cylindrical symmetrical electron density around internuclear axis
  • Formed by s atomic orbitals or p atomic orbitals overlapped end-to-end

Molecular Orbitals (H2)

  • Electrons in sigma bonds not normally excited to higher energy levels.
  • Ï€ and n electrons are excited.

Molecular Orbital Theory

  • Ï€ orbitals have clouds of electron density above and below internuclear axis
  • Double bonds = 1 σ bond, 1 Ï€ bond
  • Triple bonds = 1 σ bond, 2 Ï€ bonds
  • Unhybridized p orbitals overlap to form bonding (Ï€) and antibonding (Ï€*) molecular orbitals.

Nonbonding (n) Orbitals

  • Consist of unshared electron pairs on electronegative atoms
  • Don't affect molecular energy.

σ→σ* Transitions

  • Involving excitation of electrons from bonding σ orbitals to corresponding antibonding σ* orbitals.
  • Requires high energy, usually in vacuum UV region (λ<150nm); ε = 10-10,000 L mol−1 cm−1.

n→σ* Transitions

  • Involving excitation from non-bonding electrons (n) to antibonding σ* orbitals.
  • Requires less energy than σ→σ* transitions (150-250nm); ε = 200-2000 L mol−1 cm−1.

n→πand π→πTransitions

  • Most UV/Vis spectroscopy focuses on transitions of n or Ï€ electrons to Ï€* excited state.
  • Occurs in 180-780 nm wavelength region.
  • Transition needs unsaturated group to form Ï€ electrons.
  • Molar absorptivity (ε) is relatively low for n→π* transitions (10-100 L mol-1 cm-1); relatively high for π→π* transitions (1000-10,000 L mol-1 cm-1)

Common Electronic Transitions of Chromophores

  • Specific groups in molecules (chromophores) have characteristic absorption peaks.
  • Different transitions have different λmax (maximum absorption wavelength).

UV Spectrum of Isoprene

  • Isoprene has a Ï€ → Ï€* transition at 222 nm, which peaks in the UV part of the spectrum.

UV Spectrum of Benzoic Acid

  • Benzoic acid has peaks at specific wavelengths, including n→π* at 230 nm, π→π* at 272 nm, and another π→π* at 282 nm.

UV Spectrum of Vanillin

  • Vanillin has absorption peaks corresponding to specific types of transitions (n → Ï€* or π→ Ï€*).

Up Next: UV-Visible Spectrophotometer

  • Double beam grating spectrometer: diagram shows the components of a UV-Vis instrument.

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Unit 3.5 - Molecular Absorption Theory PDF

Description

Test your knowledge on UV-Vis spectroscopy concepts including the range of visible light, transitions of electrons, and the role of specific compounds like vanillin. Explore how these principles apply to molecular absorption and spectroscopy analysis.

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