Lecture Notes on Infrared Spectroscopy and the Color of Water

Introduction

Understanding infrared spectroscopy can answer fundamental questions about water.
Common misconception: Water appears clear in small quantities, but it is blue in larger volumes (e.g., bathtubs, swimming pools, oceans).

Small quantities: Water looks clear (like in a glass).
Large quantities: Water takes on a bluish tint (e.g., bathtubs, swimming pools, oceans).

Water absorbs light at specific frequencies:
- 3650 cm⁻¹: Symmetric stretch
- 3657 cm⁻¹: Another symmetric stretch
- 1595 cm⁻¹: Bending mode
- 3756 cm⁻¹: Asymmetric stretch
Additional overtones exist for water, which are weaker than fundamental vibrational frequencies.

Water molecules can absorb light at frequencies related to their vibrational modes:
- Combination of frequencies (e.g., symmetric stretch and bending mode) leads to absorption at about 5000 cm⁻¹.
- Overtones can be observed at approximately:
  - 7000 cm⁻¹: Twice the frequency of symmetric/asymmetric stretches.
  - Higher order overtones are progressively weaker (e.g., three times, four times fundamental frequencies).
Intensity of absorption: Peaks decrease as overtone order increases (e.g., one millionth intensity of fundamental peaks).

Water absorbs light less effectively as frequency increases towards the visible spectrum.
Visible spectrum absorption:
- Some weak absorption occurs in red, orange, yellow, and green wavelengths due to overtone frequencies.
- As photons pass through a large body of water, red/orange/yellow/green light can be absorbed, resulting in blue light being transmitted.

Water appears blue in large quantities due to its weak absorption of red, orange, yellow, and green light, influenced by anharmonicity in vibrational modes.
The spectroscopy of water is complex and serves as a basis for understanding larger, more polyatomic molecules in future studies.