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Questions and Answers
Which type of molecular vibrations may not be visible in the infrared spectrum despite having atoms of different electronegativity?
Which type of molecular vibrations may not be visible in the infrared spectrum despite having atoms of different electronegativity?
What happens to the intensity of radiation emitted by objects as temperature increases?
What happens to the intensity of radiation emitted by objects as temperature increases?
At what temperature does a human body at 37°C emit radiation primarily in the infrared range?
At what temperature does a human body at 37°C emit radiation primarily in the infrared range?
Which part of the infrared spectrum is most useful for biologists?
Which part of the infrared spectrum is most useful for biologists?
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Why might symmetric stretching not be detected in infrared spectroscopy?
Why might symmetric stretching not be detected in infrared spectroscopy?
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What is the typical wavelength range for objects emitting very little radiation at low temperatures such as 200 K?
What is the typical wavelength range for objects emitting very little radiation at low temperatures such as 200 K?
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Which molecular vibration is stated as visible in infrared spectroscopy?
Which molecular vibration is stated as visible in infrared spectroscopy?
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What is the relationship between temperature and wavelength of emitted radiation according to the provided content?
What is the relationship between temperature and wavelength of emitted radiation according to the provided content?
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Which of the following bonds is expected to absorb infrared radiation?
Which of the following bonds is expected to absorb infrared radiation?
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What is a key factor in determining if a molecule will exhibit infrared absorption?
What is a key factor in determining if a molecule will exhibit infrared absorption?
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Which molecule would NOT absorb infrared radiation?
Which molecule would NOT absorb infrared radiation?
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What is the relationship between the dipole moment and infrared absorption?
What is the relationship between the dipole moment and infrared absorption?
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Which of the following statements about IR spectroscopy is true?
Which of the following statements about IR spectroscopy is true?
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What does the term 'isotopic shift' refer to in the context of vibrational spectroscopy?
What does the term 'isotopic shift' refer to in the context of vibrational spectroscopy?
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Which of the following characteristics is NOT required for a bond to be IR active?
Which of the following characteristics is NOT required for a bond to be IR active?
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In IR spectroscopy, which factors contribute to the complexity of vibrational spectra?
In IR spectroscopy, which factors contribute to the complexity of vibrational spectra?
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What is the most important diagnostic band for proteins in the IR spectrum?
What is the most important diagnostic band for proteins in the IR spectrum?
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Which property of the hydrogen bond affects the frequency of the peptide C=O bond in proteins?
Which property of the hydrogen bond affects the frequency of the peptide C=O bond in proteins?
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What is the primary region of the IR spectrum used to identify biological molecules?
What is the primary region of the IR spectrum used to identify biological molecules?
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Which type of biological structure has weaker hydrogen bonds affecting the amide I frequency?
Which type of biological structure has weaker hydrogen bonds affecting the amide I frequency?
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What does the complexity of IR spectra allow in scientific applications?
What does the complexity of IR spectra allow in scientific applications?
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What type of bonds in proteins contribute significantly to IR absorption?
What type of bonds in proteins contribute significantly to IR absorption?
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What does the Boltzmann equation primarily describe?
What does the Boltzmann equation primarily describe?
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How do the modes of vibration in a molecule vary in intensity?
How do the modes of vibration in a molecule vary in intensity?
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What does a wavelength of 3.33 µm correspond to in terms of absorption characteristics?
What does a wavelength of 3.33 µm correspond to in terms of absorption characteristics?
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What happens to the population of energy levels as the temperature increases (indicated by an increase in T)?
What happens to the population of energy levels as the temperature increases (indicated by an increase in T)?
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In which applications can IR spectra be particularly useful?
In which applications can IR spectra be particularly useful?
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Why is the IR spectrum considered to contain more information than the UV-Visible spectrum?
Why is the IR spectrum considered to contain more information than the UV-Visible spectrum?
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Which variable in the Boltzmann equation indicates the difference in energy levels?
Which variable in the Boltzmann equation indicates the difference in energy levels?
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What effect does an increase in ΔE have on Ndown according to the content?
What effect does an increase in ΔE have on Ndown according to the content?
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What is implied about the IR spectra of different molecules?
What is implied about the IR spectra of different molecules?
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What is the relationship between the number of systems (N) and the Boltzmann constant (k_B)?
What is the relationship between the number of systems (N) and the Boltzmann constant (k_B)?
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In the provided graph of mid IR spectrum, what does the absorbance indicate?
In the provided graph of mid IR spectrum, what does the absorbance indicate?
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In the context provided, what value is suggested for an arbitrary constant to simplify calculations?
In the context provided, what value is suggested for an arbitrary constant to simplify calculations?
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What characterizes the bands observed in the IR spectrum?
What characterizes the bands observed in the IR spectrum?
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Which amino acids are indicated in the IR spectrum analysis?
Which amino acids are indicated in the IR spectrum analysis?
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Which of the following best describes the state of most systems when the temperature is low?
Which of the following best describes the state of most systems when the temperature is low?
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What is the main reason for the complexity observed in IR spectra?
What is the main reason for the complexity observed in IR spectra?
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What does a higher value of ΔE indicate about the state of energy levels in a system?
What does a higher value of ΔE indicate about the state of energy levels in a system?
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In relation to sample analysis, what is a limitation of the IR spectrum?
In relation to sample analysis, what is a limitation of the IR spectrum?
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What characteristic of electron clouds contributes to the formation of temporary dipoles?
What characteristic of electron clouds contributes to the formation of temporary dipoles?
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How do IR and Raman spectroscopies differ in measuring chemical bonds?
How do IR and Raman spectroscopies differ in measuring chemical bonds?
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Why is water considered a strong absorber in IR spectroscopy?
Why is water considered a strong absorber in IR spectroscopy?
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Which statement best characterizes the Raman spectrum of water?
Which statement best characterizes the Raman spectrum of water?
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What is a major advantage of using IR and Raman spectroscopy together?
What is a major advantage of using IR and Raman spectroscopy together?
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What aspect of water affects its interactions in IR spectroscopy?
What aspect of water affects its interactions in IR spectroscopy?
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Why does water not interfere with Raman signals from biomolecules?
Why does water not interfere with Raman signals from biomolecules?
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Which characteristic is not true about vibrational Raman spectra compared to IR spectra?
Which characteristic is not true about vibrational Raman spectra compared to IR spectra?
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Study Notes
Infrared and Raman Basics
- Infrared (IR) and Raman spectroscopy are used to study vibrational properties of molecules.
- IR spectroscopy relies on the change in dipole moment during vibration, while Raman spectroscopy relies on the change in polarizability.
- Molecules with a changing dipole moment absorb IR radiation, and thus are 'IR active'.
- Molecules with a changing polarizability scatter light at different wavelengths, demonstrating 'Raman activity'.
- Water is a strong absorber in the infrared, interfering with the analysis of other molecules.
- Short pathlengths are used to minimise water interference in IR analysis.
- Attenuated Total Reflection (ATR) cells are used for short pathlength measurements.
- Raman spectroscopy is a versatile technique for analysing various samples, including biological specimens.
Conditions for IR absorption
- Molecules can absorb IR radiation when their dipole moment changes during vibration. This depends on atoms with differing electronegativity.
- Molecules like H2, N2, and O2 have no change in dipole moment during vibration and thus do not absorb IR radiation.
- Some vibrations might be 'IR inactive', even with unequal electronegativity atoms, if the vibration's dipole moment does not change.
Vibrational Raman spectroscopy
- Raman spectroscopy can be rotational or vibrational.
- Rotational Raman spectroscopy studies molecular rotation, mainly used for gaseous samples.
- Vibrational Raman spectroscopy, similarly to vibrational IR spectroscopy, analyses the vibrational frequencies, but through a different mechanism.
- Vibrational Raman is applicable to gases, liquids and solids, and important for biological studies.
- Water is a weak Raman scatterer, making it suitable for observing other molecules in aqueous solutions.
Discovery of Raman Radiation
- Sir Chandrasekhara Venkata Raman discovered the Raman effect, where light scattered by a substance changes wavelength.
- Raman scattering is caused by small changes in the energy of photons interacting with the sample.
- The Raman effect is much weaker than Rayleigh scattering, with only a very small fraction of scattered light changing wavelength.
- A laser is used in Raman scattering experiments due to its monochromatic and high intensity properties.
How Raman radiation is measured
- In Raman spectroscopy, the light scattered by the sample is measured.
- Only the photons emitted by the sample are measured, and not the ones transmitted.
Raman Radiation: Stokes and Anti-Stokes photons
- Incident light interacts with a sample during Raman scattering, an interaction that may lead to vibrational energy changes.
- Stokes photons have lower energy and longer wavelength than the incident light.
- Anti-Stokes photons have higher energy and shorter wavelength compare to the incident light.
- Rayleigh photons have same wavelength as the incident light.
Comparison of IR and Raman spectroscopy
- IR spectroscopy and Raman spectroscopy give similar spectra for their respective 'active' molecules.
- However, the mechanisms behind them differ: IR depends on change in dipole moment; Raman depends on change in polarizability.
- Water is a weak scatterer and thus does not interfere with Raman spectra as much as it does with IR spectra.
The Vibrational Raman spectrum
- The Raman spectrum shows bands representing vibrational modes of molecules.
- The Stokes side of the spectrum is generally much more intense than the anti-Stokes.
- To increase the intensity of the Stokes side in Raman, longer exposure times are needed.
Numeric Example of Raman Shift
- The change in wavenumber represents the difference between the incoming and outcoming light during the Raman scattering process.
The amide I frequency maximum
- The frequency maximum of the amide I band in proteins depends on the secondary structure of the protein.
- This is due to differences in hydrogen bond strengths between different structures (e.g., α-helices and β-sheets).
Summary
- IR radiation arises from hot bodies.
- IR absorption is related to changes in dipole moments.
- Water obscures other molecules in IR spectra; short path lengths or ATR cells can help.
- Raman spectroscopy depends on changes in polarizability.
- Both techniques have their strengths and weaknesses; they can inform structural information combined.
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Description
Explore the fundamentals of Infrared (IR) and Raman spectroscopy in this quiz. Understand how these techniques study vibrational properties of molecules and the conditions for IR absorption. Learn about the importance of dipole moment changes and the impact of water interference in IR analysis.
