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Questions and Answers
How does bond length affect bond strength?
How does bond length affect bond strength?
- Longer bond lengths increase bond strength.
- Shorter bond lengths weaken bond strength.
- Bond strength is unaffected by bond length.
- Longer bond lengths weaken bond strength. (correct)
What effect does electronegativity difference have on bond strength?
What effect does electronegativity difference have on bond strength?
- It increases bond strength. (correct)
- It decreases bond strength.
- It only affects single bonds.
- It has no effect on bond strength.
How does the percentage of S character affect bond strength?
How does the percentage of S character affect bond strength?
- Increased S character decreases frequency.
- S character has no effect on frequency.
- All bonds have the same S character.
- Increased S character increases frequency. (correct)
What is the effect of electron withdrawing groups on wavenumber?
What is the effect of electron withdrawing groups on wavenumber?
What happens to H-bonds in a non-polar solvent?
What happens to H-bonds in a non-polar solvent?
What is the measurement unit for infrared spectroscopy?
What is the measurement unit for infrared spectroscopy?
Which type of molecular vibration does NOT affect bond length?
Which type of molecular vibration does NOT affect bond length?
Which statement about infrared (IR) spectroscopy is FALSE?
Which statement about infrared (IR) spectroscopy is FALSE?
What primarily occurs in the fingerprint region of an IR spectrum?
What primarily occurs in the fingerprint region of an IR spectrum?
How does the mass of bonded atoms affect the frequency and wavenumber in IR spectroscopy?
How does the mass of bonded atoms affect the frequency and wavenumber in IR spectroscopy?
Which of the following represents a type of out-of-plane bending vibration?
Which of the following represents a type of out-of-plane bending vibration?
What type of IR-active vibration leads to a change in dipole moment?
What type of IR-active vibration leads to a change in dipole moment?
Which type of stretching vibration is characterized by no change in dipole moment?
Which type of stretching vibration is characterized by no change in dipole moment?
What is the approximate wavelength range for the fingerprint region in an IR spectrum?
What is the approximate wavelength range for the fingerprint region in an IR spectrum?
Which functional group has an IR absorption peak around 3500-2500 cm⁻¹ due to strong hydrogen bonding?
Which functional group has an IR absorption peak around 3500-2500 cm⁻¹ due to strong hydrogen bonding?
What differentiates the IR spectrum of a primary amine from that of a secondary amine?
What differentiates the IR spectrum of a primary amine from that of a secondary amine?
At what wavelength does the C=O bond of simple ketones, aldehydes, and carboxylic acids typically absorb?
At what wavelength does the C=O bond of simple ketones, aldehydes, and carboxylic acids typically absorb?
Which type of C-H bond appears in the range of 3100-3000 cm⁻¹?
Which type of C-H bond appears in the range of 3100-3000 cm⁻¹?
In the context of IR spectroscopy, what distinguishes a mono-substituted benzene from a di-substituted benzene?
In the context of IR spectroscopy, what distinguishes a mono-substituted benzene from a di-substituted benzene?
Which of the following statements is true regarding tertiary amines in IR spectroscopy?
Which of the following statements is true regarding tertiary amines in IR spectroscopy?
What is the primary purpose of Proton Nuclear Magnetic Resonance Spectroscopy (¹H-NMR)?
What is the primary purpose of Proton Nuclear Magnetic Resonance Spectroscopy (¹H-NMR)?
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Study Notes
Infrared (IR) Spectroscopy
- Measures the absorption of infrared radiation by a sample.
- Used to identify functional groups in a molecule.
- Unit of measurement is wavenumber (cm⁻¹), which is inversely proportional to wavelength.
- Displays as %T (percent transmittance) versus wavenumber.
Molecular Vibrations
- Changes in the shape of a molecule, including stretching and bending of bonds.
- Stretching: Changes the distance between two atoms along the bond axis.
- Symmetric stretch: No change in dipole moment, IR inactive.
- Asymmetric stretch: Change in dipole moment, IR active.
- Bending: Changes the bond angle between two atoms.
- In-plane bending: Scissoring and rocking motions.
- Out-of-plane bending: Twisting and wagging motions.
- Both are IR active.
Application of IR Spectroscopy
- 4000-1400 cm⁻¹: Region primarily associated with stretching vibrations, useful for identifying functional groups.
- 1400-650 cm⁻¹: Fingerprint region, dominated by bending vibrations, unique to each molecule.
Factors Affecting Wavenumber
- Mass of bonded atoms: Greater mass results in lower frequency and wavenumber.
- Bond strength (Force constant, K):
- Bond length: Longer bond lengths result in weaker bonds.
- Electronegativity difference: Higher difference leads to stronger bonds and higher wavenumber.
- S character: Increase in s character increases bond strength and frequency.
- Resonance effects: Higher resonance results in lower frequency.
- Hydrogen bonding (H-bonds):
- Broadens the peak and lowers frequency due to weakened bond strength.
- Intermolecular H-bonds can be broken by dilution in non-polar solvents.
- Intramolecular H-bonds are not affected by dilution.
- Electronic effects:
- Electron-withdrawing groups (-I or -M effects) increase wavenumber.
- Electron-donating groups (+I or +M effects) decrease wavenumber.
Fingerprint Region
- Every molecule has a unique fingerprint region (1400-600 cm⁻¹) due to complex bending vibrations.
- Enantiomers are the only exception, as they have identical fingerprint regions.
IR for Different Functional Groups
- O-H and N-H:
- 3600-3200 cm⁻¹
- Alcohol O-H: Broad with rounded tip.
- Secondary amine: One sharp peak.
- Primary amine: Forked peak.
- No signal for tertiary amines.
- =C-H (alkynes): 3300 cm⁻¹
- =C-H (alkenes and benzene rings): 3100-3000 cm⁻¹
- C-H (alkanes): 3000-2900 cm⁻¹
- O-H (carboxylic acid): 3500-2500 cm⁻¹ due to strong hydrogen bonding.
- C=C and C=N: 2400-2100 cm⁻¹
- C=O: 1800-1650 cm⁻¹
- C=N and C=C: 1650-1550 cm⁻¹
- Substituted benzene rings:
- Mono-substituted or meta-disubstituted: Two peaks between 800-700 cm⁻¹
- Ortho- or para-disubstituted: One peak between 800-700 cm⁻¹
Proton Nuclear Magnetic Resonance Spectroscopy (¹H-NMR)
- Identifies the carbon-hydrogen framework of a molecule.
- Provides information about the number and type of hydrogen atoms in a molecule.
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