Auxochromes and Absorption Shifts

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Questions and Answers

What is the primary focus of spectroscopy?

Observing the color of materials under different lighting conditions.

Analyzing the structure of molecules through nuclear magnetic resonance.

Measuring the temperature of chemical reactions.

Studying the interaction of matter with electromagnetic radiation. (correct)

Which statement accurately describes the relationship between wavelength and frequency?

Increasing frequency results in longer wavelengths.

Wavelength is independent of frequency.

Wavelength and frequency are directly proportional.

Wavelength and frequency are inversely proportional. (correct)

What type of electronic transition requires the most energy?

π → π* transition

n → σ* transition

Cation to anion transition

σ → σ* transition (correct)

Which of the following compounds would most likely undergo a π → π* transition?

Alkene Signup and view all the answers

What is the equation representing Beer's-Lambert's Law?

A = ect Signup and view all the answers

What does the symbol 'η' represent in the velocity equation for electromagnetic radiation?

Wave number Signup and view all the answers

Which property is NOT required of an ideal source of radiation?

Ability to emit sound waves Signup and view all the answers

In which region of the electromagnetic spectrum would you expect to find the absorption maximum for a σ → σ* transition?

Ultraviolet region (125-135 nm) Signup and view all the answers

What does the symbol ε represent in the equation A = εct?

Molecular extinction coefficient Signup and view all the answers

What is the formula used to calculate energy associated with a photon?

E = hv Signup and view all the answers

What is a significant drawback of the Tungsten Halogen Lamp?

It emits primarily in the near IR spectrum. Signup and view all the answers

Which type of transition is characterized by an electron moving from a bonding s-orbital to an anti-bonding orbital?

σ → σ* transition Signup and view all the answers

What chemical is used in the Hydrogen Discharge Lamp for excelling UV radiation?

Hydrogen Signup and view all the answers

Which component is NOT part of the basic instrumentation setup for using Beer's-Lambert Law?

Computer processor Signup and view all the answers

What is the typical path length used in many absorption measurements?

10 mm Signup and view all the answers

The formula for ε can be expressed in which form?

ε = E1% x Molecular weight / 10 Signup and view all the answers

What is a significant disadvantage of using a mercury arc lamp?

It is not suitable for continuous spectral studies. Signup and view all the answers

What material is commonly used for lenses in the visible and near IR region?

Ordinary silicate glass Signup and view all the answers

What is the purpose of slits in a collimating system?

To resolve polychromatic radiation into monochromatic radiation. Signup and view all the answers

Which component of a monochromator is responsible for dispersing incoming radiation into its components?

Dispersing element Signup and view all the answers

What type of filter works by selectively absorbing unwanted radiation while transmitting the required radiation?

Absorption filter Signup and view all the answers

Which type of material is suitable for lenses operating below 300nm?

Fused silica Signup and view all the answers

Which of the following statements best describes a disadvantage of high-voltage sources in lamps?

They become very hot and need thermal insulation. Signup and view all the answers

Which type of monochromatic device operates based on the interference phenomenon?

Interference filter Signup and view all the answers

What is an auxochrome?

A functional group that alters wavelength and intensity of absorption. Signup and view all the answers

What does a bathochromic shift indicate?

A shift in absorption maxima to longer wavelengths. Signup and view all the answers

Which statement about Beer's Law is correct?

It indicates a direct relationship between concentration and the rate of intensity decrease. Signup and view all the answers

What is the hyperchromic effect?

An increase in the absorption intensity of a compound. Signup and view all the answers

Which of the following is NOT an auxochrome group?

-COOH Signup and view all the answers

What does Lambert's Law describe?

The intensity of light as it passes through an absorbing medium. Signup and view all the answers

Which of the following correctly describes a hypsochromic shift?

It involves a shift to shorter wavelengths. Signup and view all the answers

What is the extinction coefficient (ɛ)?

A measure of how much light is absorbed by a compound. Signup and view all the answers

What distinguishes interference filters from absorption filters?

Interference filters provide greater transmittance and narrower band pass. Signup and view all the answers

What is a notable feature of gratings in spectrophotometers?

They provide a resolution of +/- 0.1nm. Signup and view all the answers

What is the result when white light passes through a prism?

It disperses into a rainbow of colors. Signup and view all the answers

How do diffraction gratings enhance the dispersion of light?

Through the reflection of light waves. Signup and view all the answers

Which of the following statements about absorption filters is true?

They select light with a band pass of ±20-30nm. Signup and view all the answers

What is one disadvantage of gratings compared to prisms?

Gratings tend to be more expensive. Signup and view all the answers

What is the primary construction material for prisms used in the UV spectrum?

Quartz or fused silica Signup and view all the answers

Which characteristic of an interference filter enhances its performance?

The use of a silver coating on both sides. Signup and view all the answers

Study Notes

Auxochrome

An auxochrome is a functional group that doesn't absorb light in the near UV region but alters the wavelength and intensity of absorption when attached to a chromophore.
Examples of auxochromes include: -OH, -OR, -NH₂, -NHR, -NR₂, -SH.
When an auxochrome is attached to a chromophore, it creates a new chromophore with a different absorption maximum and extinction coefficient.

Adsorption and Intensity Shifts

Bathochromic Shift (Red Shift): The absorption maximum (λ_max) of a compound moves to a longer wavelength.
Hypsochromic Shift (Blue Shift): The absorption maximum (λ_max) of a compound moves to a shorter wavelength.
Hyperchromic Effect: The absorption intensity (ε) of a compound increases.
Hypochromic Effect: The absorption intensity (ε) of a compound decreases.

Beer's Law

When a beam of monochromatic light passes through a homogeneous absorbing medium, the rate of decrease in intensity of radiation is directly proportional to the intensity of the incident light and the concentration of the absorbing species.
It can be expressed as: I = I_o e^-kc, where:
- I is the intensity of the transmitted light.
- I_o is the intensity of the incident light.
- k is a constant.
- c is the concentration of the absorbing species.

Lambert's Law

When a beam of monochromatic light passes through a homogeneous absorbing medium, the rate of decrease in intensity of radiation is directly proportional to the intensity of the incident light and the thickness of the absorbing medium.
It can be expressed as: I = I_o e^-kt, where:
- I is the intensity of the transmitted light.
- I_o is the intensity of the incident light.
- k is a constant.
- t is the thickness of the absorbing medium.

Introduction to Spectroscopy

Spectroscopy is the study of the interaction between electromagnetic radiation and matter as a function of wavelength or frequency.
It measures the electromagnetic radiation (EMR) absorbed or emitted when molecules, ions, or atoms of a sample transition between energy states.

Electromagnetic Radiation

Electromagnetic radiation consists of discrete packets of energy called photons.
Each photon has an oscillating electric field (E) and an oscillating magnetic field (M) that are perpendicular to each other.

Properties of Electromagnetic Radiation

Wavelength: The distance between two successive maxima on an electromagnetic wave. Measured in m, cm, mm, nm, and micrometers.
Frequency: The number of wavelength units passing through a given point in unit time. Denoted by "ν," measured in cycles per second (Hertz).
Wave Number: The number of waves per cm in a vacuum. Calculated as ν = 1/wavelength.
Velocity: The product of wavelength and frequency, equal to the velocity of the wave in the medium. V = η χλ.
Energy: The energy of a photon is directly proportional to its frequency. E = h ν = hc/λ, where:
- h is Planck's constant.
- ν is the frequency of the radiation.
- c is the speed of light.
- λ is the wavelength of the radiation.

Electronic Transitions

σ → σ transitions:* An electron in a bonding σ-orbital is excited to the corresponding anti-bonding σ* orbital. This occurs in saturated compounds and requires a lot of energy. For example, methane, with only C-H bonds, has an absorption maximum at 125 nm.
n → σ transitions:* Saturated compounds containing atoms with lone pairs (non-bonding electrons) like O, N, S, and halogens are capable of n → σ* transitions. These transitions require less energy than σ → σ* transitions and are initiated by light with wavelengths in the range of 150-250 nm.
π → π transitions:* A π electron in a bonding π-orbital is excited to the corresponding anti-bonding π* orbital. This occurs in conjugated compounds with multiple bonds, like alkenes, alkynes, carbonyl compounds, nitriles, and aromatic compounds. For example, alkenes typically absorb in the region of 170 to 205 nm.

Beer-Lambert's Law

The mathematical equation for Beer-Lambert's Law is: A = εct, where:
- A = Absorbance.
- ε = Molecular extinction coefficient (a measure of how strongly a substance absorbs light at a particular wavelength).
- c = Concentration of the sample.
- t = Path length (typically 10 mm or 1 cm).

Instrumentation in Spectroscopy

Spectrophotometers use a combination of components to analyze the interaction between electromagnetic radiation and matter. These key components include:
- Source of radiation: Provides the electromagnetic radiation used in the analysis.
- Collimating system: Makes the light beams parallel and focused.
- Monochromator: Isolates radiation of the desired wavelength from the continuous spectrum.
- Sample holder: Contains the sample to be analyzed.
- Detector: Senses the transmitted or emitted radiation after it passes through the sample.
- Amplifier: Increases the signal from the detector.
- Read-out devices: Display the results of the analysis.

Source of Radiation

Requirements of an ideal source:
- Stability: Constant output without fluctuations.
- Continuous spectrum: Emits a wide range of wavelengths.
- High intensity: Sufficient light for detection.
- Durable: Long lifespan and minimal fatigue.
Common Radiation Sources:
- Tungsten Halogen Lamp: Used for visible and near infrared regions.
- Hydrogen Discharge Lamp: Emits ultraviolet radiation in the near UV region.
- Xenon Discharge Lamp: Emits intense radiation across a wide range of wavelengths, including UV, visible, and near infrared.
- Mercury arc Lamp: Provides discrete lines of radiation, not suitable for continuous studies.

Collimating System

The collimating system aligns and focuses the light beams from the source. Key components include:
- Lenses: Made of materials transparent to the desired wavelength range.
- Mirrors: Reflect and focus light beams.
- Slits: Control the width of the light beam, affecting resolution.

Monochromators

Monochromators isolate the desired wavelength of light from the continuous spectrum emitted by the source. Key components include:
- Entrance Slit: Defines the incoming light beam.
- Dispersing Element: Separates the light into its different wavelengths (e.g., prism, grating).
- Exit Slit: Allows a narrow band of wavelengths to pass through.
Types of Monochromators:
- Filters: Absorb or reflect specific wavelengths.
  - Absorption Filters: Selectively absorb unwanted wavelengths.
  - Interference Filters: Use interference phenomena to reflect unwanted wavelengths.
- Prisms: Disperse light based on the refractive index of the prism material.
- Gratings: Use diffraction to separate light into its wavelengths.
  - Diffraction Gratings: High-resolution gratings that use reflection to separate wavelengths.
  - Transmission Gratings: Similar to diffraction gratings but use refraction to separate wavelengths.

Advantages of Different Monochromator Types

Interference Filters: Provide greater transmittance, narrower bandwidth, and are less expensive.
Absorption Filters: Simple, cheap, and easy to use but less accurate with broader bandwidth.
Prisms: Useful for wide wavelength ranges but less efficient than gratings.
Gratings: Provide higher resolution and linear dispersion, making them suitable for precise wavelength analysis.

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Description

This quiz covers the concepts of auxochromes and their impact on chromophores, including bathochromic and hypsochromic shifts. Additionally, it discusses Beer’s Law and the effects on absorption intensity. Test your understanding of how these principles affect light absorption in chemical compounds.