Chemical Spectroscopy Overview

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Questions and Answers

Which type of electrons is primarily involved in saturated bonds?

π-electrons

d-electrons

σ-electrons (correct)

n-electrons

What is the relationship between n-π* transitions and π-π* transitions in terms of energy requirement?

n-π* transitions require less energy than π-π* transitions. (correct)

Both transitions require equal energy.

n-π* transitions require more energy than π-π* transitions.

π-π* transitions do not require any energy.

Which of the following energy transitions is considered a high-energy transition?

n-π* transition

π-π* transition (correct)

σ-σ* transition (correct)

σ-π* transition

Where in the electromagnetic spectrum is the visible light range typically found?

400 to 700 nm Signup and view all the answers

What happens to a beam of light when it is passed through a prism?

It forms a continuous spectrum of colors. Signup and view all the answers

What makes it difficult to observe absorptions having λmax < 200 nm?

Everything, including quartz glass and air, absorbs in this spectral region. Signup and view all the answers

Which type of compound would contain π-electrons?

Unsaturated hydrocarbons Signup and view all the answers

Which transition is associated with a maximum wavelength (m{ ext{λmax}}) of 135 nm?

σ-σ* transition Signup and view all the answers

What is the main reason the n to pi* transition is considered 'forbidden'?

It is weaker than the pi to pi* transition. Signup and view all the answers

Which transition has the longest wavelength among the given examples?

n to pi* at 279 nm Signup and view all the answers

What does the symbol ε represent in the context of electronic transitions?

The extinction coefficient Signup and view all the answers

How does the absorption of radiation relate to chemical species in a medium?

It selectively attenuates certain frequencies of electromagnetic radiation. Signup and view all the answers

Which of the following statements about the electronic transitions is NOT correct?

n to sigma* transitions are stronger than pi to pi* transitions. Signup and view all the answers

What is the correct equation representing the energy difference between the ground and excited state of electrons?

E = Hν Signup and view all the answers

Which types of transitions contribute to molecular absorption spectra?

Electronic, vibrational, and rotational transitions Signup and view all the answers

In terms of relative energy, how do electronic, vibrational, and rotational energies compare?

EEL > EVIB > EROT Signup and view all the answers

What type of molecular spectra arises from transitions in the ultraviolet-visible region?

Electronic spectra Signup and view all the answers

At room temperature, where do most elementary particles typically exist?

In their lowest energy level (E0) Signup and view all the answers

What is required for an atom to transition to a higher energy state?

A photon that matches the energy difference exactly Signup and view all the answers

What is the wavelength range for rotational spectra?

10^{-3} – 0.67 nm Signup and view all the answers

Which mathematical expression relates energy difference to wavelength?

ΔE = HC/λ Signup and view all the answers

Study Notes

Electronic Transitions

When an electron absorbs a photon, it moves from a lower energy level to a higher energy level
The energy difference between these levels is given by: E = hν
The total energy of a molecule is the sum of its electronic, vibrational, and rotational energies: E = Eele + Evib + Erot

Molecular Absorption

Molecules absorb and emit light in a wider range of wavelengths compared to atoms
Therefore, they show spectral bands instead of sharp lines
This is due to quantized transitions in three types of molecular energy levels: electronic, vibrational, and rotational
The relative energies of these transitions can be classified as: EEL > EVIB > EROT (10,000:100:1)

Types of Molecular Absorption Spectra

Electronic Spectra: Due to changes in electronic transitions, and associated vibrational and rotational transitions
- Occur in the ultraviolet-visible region (200-800nm)
Vibrational Spectra: Due to vibrational and rotational transitions
- Occur in the infrared region (2.5-15 µm [400-4000cm-1])
Rotational Spectra: Due to rotational transitions only
- Occur in the microwave region (10-3 - 0.67 nm)

Quantum Theory

Every particle (atom, ion, or molecule) exists in discrete energy states: E0, E1, E2, E3, etc.
At room temperature, most particles are in their lowest energy level, E0 (ground state)
When atoms absorb photons, they can be promoted to higher energy levels: E1, E2, E3, etc.
This occurs only if the photon's energy exactly matches the energy difference between the ground state and an excited state: ΔE = (EN - E0) = hν = hc/λ.
The promoted atoms are said to be in excited states: M + hν → M*

UV-Visible Spectroscopy

Measures the absorbance or transmittance of radiation in the UV-visible range of the electromagnetic spectrum
Arise from electron transitions
Stage 1: Excitation of a species (M) by absorbing a photon
Stage 2: Relaxation of the excited species (M*) by converting it into a new species through a photochemical reaction

Types of Electrons in Organic Molecules

σ-Electrons: Involved in saturated bonds, found in carbon-hydrogen in paraffins. Require high energy to excite, more than the energy provided by UV light
π-Electrons: Involved in unsaturated hydrocarbons, present in alkenes, alkynes, and aromatic compounds
n-Electrons: Non-bonding electrons, not involved in molecular bonding. Found in heteroatoms like oxygen, nitrogen, etc.

UV Absorption Process

σ-σ and σ-π transitions**: High energy transitions, accessible in vacuum UV (λmax).
π-π transitions*: Energetically lower than σ-σ* and σ-π*. Relevant for organic molecules, especially conjugated systems.
n-π transitions*: Lowest energy transition, typically in the UV region. Involves non-bonding electrons (n) and π* orbitals.

Visible Spectrum

When white light passes through a prism, it separates into a band of colors (continuous spectrum).
Each color corresponds to a specific wavelength of electromagnetic radiation.
The visible range is a small part of the entire electromagnetic spectrum.

Molecular Structure and UV-Visible Spectrum

Ethane (C2H6) undergoes a σ-σ* transition at a very short wavelength (135 nm)
Ethylene (C2H4) has a π-π* transition at a slightly longer wavelength (165 nm), indicating lower energy required for this transition.
The n-π* transition in molecules with lone pairs is even lower in energy and typically occurs in the UV region.
The strength of absorption (ε) can be categorized as:
- Strong: ε > 10,000
- Moderate: 100 < ε < 10,000
- Weak: ε < 100

Absorption of Radiation

Absorption is a process where certain frequencies of electromagnetic radiation are selectively absorbed by a chemical species in a transparent medium.
This absorption weakens the intensity of those frequencies, leading to a decrease in the transmitted light.

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Description

This quiz covers the fundamentals of electronic transitions and molecular absorption spectra. It explains how energy levels affect absorption and emission in molecules, and details the types of molecular absorption spectra, including electronic and vibrational spectra. Enhance your understanding of molecular energy transitions and their spectral representations.