Organic Chemistry Ch. 14: Infrared Spectroscopy and Mass Spectroscopy Notes

Organic Chemistry Ch. 14: Infrared Spectroscopy and Mass Spectroscopy

Electromagnetic Radiation: Understanding the nature of light and its interaction with matter is crucial in spectroscopy.
Infrared Spectroscopy Basics: A significant method for determining molecular structure.
Wavenumber vs Frequency: Essential concepts for interpreting spectra.

Objective: By the end of this chapter, students should be able to interpret an Infrared spectrum of an organic molecule, including:
- Identifying the presence of various functional groups.
- Understanding bonding patterns, such as:
- Double Carbon-Carbon Bonds.
- Triple Carbon-Carbon Bonds.
- Recognizing how structural factors like conjugation affect the spectrum.

Spectrum Interaction: Spectroscopy involves the interaction between matter and light or electromagnetic radiation.
Spectroscopic Methods: Various techniques are based on the wavelength of light used, including:
- Nuclear Magnetic Resonance (NMR)
- Infrared Spectroscopy (IR)
- UV-Visible Spectroscopy
- X-Ray Spectroscopy
Properties of Light Waves: Important properties include:
- Wavelength
- Frequency

Electromagnetic Spectrum: The complete range of frequencies of light.
- Wavelength Illustrations: A scale of wavelengths from 10^3 m to 10^{-12} m, including the following regions:
  - Longer Wavelengths: Radio Waves, Microwaves, Infrared
  - Visible Light: Part of the spectrum that is perceived by human eyes.
  - Shorter Wavelengths: Ultraviolet, X-Rays (both Hard and Soft), Gamma Rays
Examples of Sources for Different Light Types:
- AM and FM radio for radio waves.
- Microwave ovens for microwaves.
- Light bulbs emitting visible light.
- X-ray machines for X-rays.

Frequency: Expressed in waves per second (Hz), covering values from 10^6 to 10^{20} Hz.
Energy of a Photon: Ranges from low energy for longer wavelengths to high energy for short wavelengths, measured in electron volts (eV).

Electromagnetic Spectrum Regions: Different regions correspond to varying molecular structural information:
- Type of Spectroscopy | Region of Electromagnetic Spectrum | Information Obtained
- NMR Spectroscopy | Radio Waves | Arrangement of Carbon and Hydrogen atoms
- IR Spectroscopy | Infrared | Presence of functional groups
- UV-Vis Spectroscopy | Visible and Ultraviolet | Conjugated π systems present in the compound

Functionality of IR Spectrophotometer:
- An IR spectrophotometer exposes a sample to all frequencies of IR light; the absorbed frequencies provide insight into the types of bonds and functional groups present.
- Common methods of sample preparation include:
- Depositing samples on a sodium chloride (NaCl) plate.
- Dissolving compounds in solvents.
- Embedding in potassium bromide (KBr) pellets.

Absorption Spectrum:
- An IR spectrum plots % transmittance against frequency.
- Peaks in the spectrum are referred to as absorption bands, which indicate specific bond characteristics.

Definition of Wavenumbers:
- Wavenumbers are units of frequency in IR spectroscopy, traditionally expressed in cm^{-1}.
- Typical ranges from 400 to 4000 cm^{-1}.

Characteristics of a Peak in IR Spectrum:
- Each signal (or peak) has three critical characteristics:
1. Wavenumber: Indicates the frequency of the vibrating bond.
2. Intensity: Relates to the amount of light absorbed, corresponding to the concentration of bonds present.
3. Shape: Refers to the contour of the peak, which can suggest specific electronic environments or chemical shifts.