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Vibrational Spectroscopy Flashcards
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Vibrational Spectroscopy Flashcards
Lecture 4
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1
Question
What are the two main types of vibrational spectroscopy discussed and what do they do?
Answer
Infrared (IR) Spectroscopy and Raman Spectroscopy. Capture the vibrational energy related to molecular bonds or lattice vibrations.
2
Question
When do various molecular and atomic absorption and emission events occur?
Answer
when EM radiation of different energy interacts with matter
3
Question
What two main spectrums are used in FITR?
Answer
Mid-IR spectrum (200–4000cm⁻¹) mostly matches the vibrational energy spectrum of molecules. Far-IR spectrum (10-200 cm⁻¹) also used at times.
4
Question
What is the equation relating vibrational energy E_vib to the force constant K and bond displacement Δr?
Answer
E_vib = (1/2) K (Δr)² = (1/2) K (r - r_e)², where r_e is the equilibrium bond length.
5
Question
How is the vibrational energy quantized in terms of vibrational frequency ν_vib?
Answer
ν is the vibrational quantum number, h is Planck's constant. K: Force constant (bond stiffness) Δr: Displacement of the bond length from equilibrium r: Bond length at any instant re: Equilibrium bond length
6
Question
Formula for harmonic oscillator analogy used in molecular vibrational frequency analysis.
Answer
K as the force constant and μ as the reduced mass of the oscillating atoms.
7
Question
What is the general relationship between vibrational frequency (wavenumber) regions and type of bonds?
Answer
8
Question
What is the relationship between dipole moment (μ), polarizability (α), and external electric field (E) relevant to IR sensitivity?
Answer
9
Question
What condition must a molecular vibration satisfy to be Raman active?
Answer
The vibration must be associated with a change in polarizability, meaning the derivative of polarizability with respect to the normal coordinate Q is not zero (∂α/∂Q ≠ 0).
10
Question
What is the difference between infrared (IR) sensitivity and Raman sensitivity in molecular vibrations?
Answer
IR sensitivity requires a change in dipole moment, while Raman sensitivity requires a change in polarizability.
11
Question
What are 'normal vibration modes' in molecules?
Answer
Normal vibration modes are vibrational patterns where all atoms oscillate at the same frequency and phase about their equilibrium positions without moving the molecule's center of gravity or causing translation or rotation.
12
Question
How many vibrational modes does a molecule with N atoms have, and how does this differ for linear molecules?
Answer
A general molecule has 3N-6 vibrational modes, while a linear molecule has 3N-5 vibrational modes, accounting for rotations and translations.
13
Question
Explain why some normal modes are IR active while others are Raman active, using CO₂ as an example.
Answer
Modes that involve a change in dipole moment (like CO₂'s anti-symmetric stretch) are IR active. Those that involve changes in polarizability but not dipole moment (like CO₂'s symmetric stretch) are Raman active.
14
Question
What is the fundamental principle behind combining multiple wavelengths of light to form an interferogram in FTIR spectroscopy?
Answer
Multiple wavelengths of light create a unique interference pattern (interferogram) when combined, as their differing frequencies and phases interact over varying optical path differences.
15
Question
How does adding more frequencies affect the combined signal waveform in spectroscopy?
Answer
Adding more frequencies increases the complexity of the combined waveform, producing constructive and destructive interference patterns that encode information about the component wavelengths.
16
Question
What is the purpose of performing a Fourier Transform on an interferogram in FTIR spectroscopy?
Answer
The Fourier Transform converts the time (or optical path difference) domain interferogram into a frequency domain spectrum, which separates the individual wavelength components for analysis.
17
Question
Describe the Michelson interferometer setup and how it produces an interferogram.
Answer
In a Michelson interferometer, the initial light beam is split into two paths: one with fixed path length and the other with a movable mirror changing path length. When recombined, these produce interference patterns (interferograms) as the optical path difference varies during mirror movement.
18
Question
In FTIR spectra, what does a shift in peak positions indicate about molecular structure?
Answer
Shifts in peak positions reflect changes in bonding environments or molecular structures, such as differences in bond types or neighboring atoms influencing vibrational frequencies.
19
Question
What evidence from the signal sampling plots demonstrates the importance of sampling rate for accurate Fourier analysis?
Answer
Signals sampled at higher sampling rates (Sampling Rate 2 and 3) display clearer sinusoidal patterns closely matching the true signals, while signals at a low sampling rate (Sampling Rate 1) appear noisy and distorted. This shows that adequate sampling is necessary to capture signal characteristics accurately for reliable frequency analysis.