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Questions and Answers
What is the primary type of spectroscopy that infrared spectroscopy falls under?
What is the primary type of spectroscopy that infrared spectroscopy falls under?
- Fluorescence spectroscopy
- Absorption spectroscopy (correct)
- Emission spectroscopy
- Mass spectroscopy
In which region does the most useful infrared wavelength range lie?
In which region does the most useful infrared wavelength range lie?
- 14000 - 4000 cm−1
- 670 - 400 cm−1
- 4000 - 670 cm−1 (correct)
- 400 - 10 cm−1
What is required for a molecule to absorb IR radiation?
What is required for a molecule to absorb IR radiation?
- Natural frequency must be lower than incident frequency
- No relation between natural and incident frequency
- Natural frequency must be higher than incident frequency
- Natural frequency must equal the incident frequency (correct)
What happens to molecules when they absorb infrared radiation?
What happens to molecules when they absorb infrared radiation?
When does absorption of IR occur in molecular vibrations?
When does absorption of IR occur in molecular vibrations?
Which of the following statements is true regarding dipole moment?
Which of the following statements is true regarding dipole moment?
What is the effect of the rate of vibration on IR absorption intensity?
What is the effect of the rate of vibration on IR absorption intensity?
What characterizes the far infrared region of the electromagnetic spectrum?
What characterizes the far infrared region of the electromagnetic spectrum?
What type of stretching occurs when the bond lengths change symmetrically?
What type of stretching occurs when the bond lengths change symmetrically?
Which bending type maintains the bond angle while both bonds move within the same plane?
Which bending type maintains the bond angle while both bonds move within the same plane?
What is the characteristic movement of atoms during the twisting bending motion?
What is the characteristic movement of atoms during the twisting bending motion?
Which of the following frequency ranges corresponds to sp2 C–H stretching vibrations?
Which of the following frequency ranges corresponds to sp2 C–H stretching vibrations?
What frequency range is associated with the bending vibrations of alkenes RCH-CH2?
What frequency range is associated with the bending vibrations of alkenes RCH-CH2?
Which functional group has a stretching vibration frequency range of 1710-1750 cm-1?
Which functional group has a stretching vibration frequency range of 1710-1750 cm-1?
In the context of infrared absorption frequencies, what does a frequency of 3200-3600 cm-1 correspond to?
In the context of infrared absorption frequencies, what does a frequency of 3200-3600 cm-1 correspond to?
What is the primary change involved in stretching vibrations?
What is the primary change involved in stretching vibrations?
Which of the following options is NOT a type of bending motion?
Which of the following options is NOT a type of bending motion?
What phenomenon describes the resonance that can occur between vibrations of different modes in a molecule?
What phenomenon describes the resonance that can occur between vibrations of different modes in a molecule?
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Study Notes
Infrared Spectroscopy
- Infrared (IR) spectroscopy uses the infrared region of the electromagnetic spectrum to analyze molecules.
- It involves the absorption of infrared radiation by molecules, causing vibrations within the molecule.
- The infrared region ranges from 14000–10 cm−1 (0.8–1000 μm).
- The most useful region of the IR spectrum for analysis ranges from 4000 - 670 cm−1.
- The IR spectrum is divided into three sections:
- Near infrared (NIR): 14000–4000 cm−1 (0.8–2.5 μm)
- Mid infrared (MIR): 4000–400 cm−1 (2.5–30 μm)
- Far infrared (FIR): 400–10 cm−1 (30–1000 μm)
Molecular Vibrations
- Molecules absorb IR radiation when their natural frequency matches the frequency of the incident radiation.
- Molecule vibrations cause changes in the electric dipole moment, leading to IR absorption.
Types of Molecular Vibrations
- Stretching: Change in interatomic distance along the bond axis.
- Symmetrical stretching: Both bond lengths increase or decrease symmetrically.
- Asymmetrical stretching: One bond length increases, while the other decreases.
- Bending: Change in the angle between two bonds.
- Rocking: Bond angle is maintained, but both bonds move within the plane.
- Scissoring: Bond angle decreases, with both bonds moving within the plane.
- Wagging: One atom moves above the plane, and the other moves below the plane.
- Twisting: Both atoms move to one side of the plane.
Factors Affecting IR Absorption Frequencies
- Force constant: Strength of the bond.
- Reduced mass: The weighted average of the masses of the two atoms in the bond.
IR Spectrometer Components
- Sources. Produce infrared radiation.
- Monochromators: Separate radiation into different frequencies.
- Sample preparation: Prepare the sample to allow transmission of IR radiation.
- Detectors: Sense the transmitted IR radiation.
IR Spectrum Analysis
- Functional group region: Contains peaks associated with specific functional groups.
- Fingerprint region: Contains complex peaks that are unique to each molecule, allowing for identification.
- IR spectroscopy is used to identify functional groups, analyze molecular structures, and determine the presence of specific compounds.
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