Lecture Notes: Infrared Spectroscopy and the Color of Water

Introduction to Infrared Spectroscopy

• Understanding infrared spectroscopy helps to answer fundamental questions about substances, including the color of water.

Perception of Water's Color

• Small amounts of water (e.g., a glass) appear clear.
• Larger quantities (e.g., bathtubs, swimming pools, oceans) show a bluish tint.
• Water is blue in large quantities due to absorption characteristics.

Absorption Characteristics of Water

• Water absorbs light at various frequencies in the infrared spectrum:
  • 3650 cm⁻¹ (symmetric stretch)
  • 1595 cm⁻¹ (bending mode)
  • 3756 cm⁻¹ (asymmetric stretch)
• Additional absorption occurs at:
  • 5000 cm⁻¹ and slightly above 7000 cm⁻¹ (overtones)

Vibrational Modes of Water

• The vibrational modes are:
  • Symmetric Stretch (nu1)
  • Bending Mode (nu2)
  • Asymmetric Stretch (nu3)
• Overtones arise from non-harmonic behavior of vibrations.

Overtones Explained

• Overtones involve simultaneous excitation of multiple vibrational modes:
  • Combine nu1 and nu2: e.g., nu1 + nu2 = ~5000 cm⁻¹
  • Acceptable but weaker than fundamental modes.
• Higher order overtones (up to 4 times the fundamental) show a significant decrease in intensity.

Log Scale Representation

• The spectrum can be plotted on a log scale to highlight differences in intensity.
  • Fundamental vibrational peaks are stronger than overtone peaks.
  • Weaker peaks are less than 1/100 in intensity compared to fundamental modes.
  • Notable peaks:
    • 2x nu1 or nu3: ~7000 cm⁻¹ (weaker)
    • Higher order overtones show much weaker absorption.

Absorption in Visible Spectrum

• Water has weak absorption in the visible spectrum, particularly in:
  • Red, orange, yellow, and parts of green due to higher overtones.
• When light passes through a significant amount of water, red/orange/yellow/green are absorbed, allowing blue light to be transmitted.

Conclusion

• Water appears blue because of its absorption characteristics in the infrared and visible spectrums.
• The complexity of water's spectroscopy is foundational for understanding larger, more complex molecules.

Next Steps

• Upcoming discussions will involve analyzing larger polyatomic molecules and their spectroscopy.