Electromagnetic Spectrum and Spectroscopy Techniques

Introduction

  • Discussion begins with the electromagnetic (EM) spectrum.
  • The EM spectrum consists of different photon energies encountered in various contexts, highlighting the types of light sources utilized in scientific techniques.

Electromagnetic Spectrum

Overview of the EM Spectrum

  • Divided into categories based on energy:
    • High Energy: Located on the left side of the spectrum (X-rays, Gamma rays)
    • Low Energy: Located on the right side
  • Visible Light: Sits in the middle of the spectrum.

Photon Energies and Their Effects

  • X-Rays and Gamma Rays: High-energy photons that can ionize molecules, potentially leading to cellular damage in biological systems.
    • Techniques involving these photon energies are usually avoided due to their possible destructive consequences.
  • Techniques Covered:
    • X-ray diffraction (uses X-rays despite their potential danger).
    • UV-Vis Spectroscopy: Utilizes light at visible wavelengths and slightly higher energies to interact with double bonds and conjugated systems.
    • IR Spectroscopy: Causes bond vibrations, discussed in detail as one of the primary analysis techniques in chemistry.
    • NMR (Nuclear Magnetic Resonance): Uses low-energy radio waves, another important technique in structural analysis.

Relationship Between Frequency and Wavelength

Key Relationships

  • Inversely Correlated:
    • As frequency (A1C) increases, wavelength (A1D) decreases.
    • Conversely, as wavelength increases, frequency decreases.
  • Energy Consideration:
    • Longer wavelengths correspond to lower energy; shorter wavelengths correspond to higher energy.
    • Important for understanding IR spectroscopy.

Techniques Overview

Mass Spectrometry (Mass Spec)

  • Process Overview:
    • Ionization: Heating a sample to turn it into a gaseous state, followed by bombarding it with electrons to create charged ions.
    • The ionization can also occur through other means but primarily involves electrons.
    • Analysis Process:
    • Charged species are analyzed as they pass through a magnetic field, where their path deflection is assessed.
    • Heavier ions deflect less than lighter ones.
  • Output Data:
    • Spectrum produced shows peaks, where the x-axis represents mass/charge (m/z).
    • The highest peak corresponds to the original molecular weight of the compound analyzed.
    • Fragment peaks (like m - 15 or m - 29) indicate the loss of specific groups during ionization.
    • Isotope peaks can occur, appearing as smaller adjacent peaks indicating variations in atomic mass.

Infrared Spectroscopy (IR)

  • Nondestructive Technique: Samples can be reused post-analysis.
  • Percent Transmittance Graph:
    • Light is passed through a sample and the amount of light absorbed is plotted, with peaks indicating IR light absorbed by the sample.
    • Scale is represented in wave numbers (inverse centimeters), correlating to energy levels.
    • Higher wave numbers correspond to higher energy and vice versa.
  • Diagnostic vs Fingerprint Regions:
    • Diagnostic Region (above 1500 cm^-1): Easier for identifying functional groups.
    • Fingerprint Region (below 1500 cm^-1): Contains complex patterns unique to specific compounds, making it more challenging to analyze.
  • Functional Group Identification: IR spectrum can reveal presence of specific functional groups through their characteristic absorption peaks, detailed on a reference chart provided in analysis.

Hooke's Law and Vibrational Frequencies

Application to Molecular Bonds

  • Vibration Characterization: Bonds act like springs; their frequency of vibration can change based on bond strength and mass.
  • Bond Strength and Mass Relationship:
    • Increased mass leads to decreased frequency.
    • Increased spring constant corresponds to increased bond strength leading to increased frequency.
  • Bending vs. Stretching Vibrations:
    • Stretching vibrations are more straightforward and easy to identify in IR.
    • Bending vibrations occur primarily in the fingerprint region.
  • Characteristic Frequencies for Different Bonds:
    • C-H bonds differ based on hybridization (sp3, sp2, sp) affecting their vibrational frequency.
    • For example, C-H alkane is just below 3000 cm^-1 while alkenes and alkynes appear just above.

Summary: Application of Techniques in Organic Chemistry

Importance of Mass Spec and IR in Structure Determination

  • Mass spec provides the molecular weight of compounds through spectroscopic peaks, focusing on the highest mass peak.
  • IR spectroscopy aids in identifying functional groups based on known absorption frequencies; however, it does not provide complete structural determination due to limitations in analyzing the complex fingerprint region.

Future Techniques Discussed

  • NMR (Nuclear Magnetic Resonance): This technique will be discussed next, serving as a powerful tool for complete structure determination.
  • Practical application through practice with IR spectrums and reference charts to identify functional groups and analyze data effectively.