Advanced #12

Warm water vibrates longer than cold water

The OH stretching vibration in warm water decays more slowly than in cold water.

Scientific Explanation

Every water molecule possesses characteristic vibrational modes — including the OH stretching vibration, in which the hydrogen atoms periodically move away from and back toward the oxygen atom. When this vibration is excited by an ultrashort laser pulse, researchers can measure how quickly the vibrational energy is transferred to the surroundings. The lifetime of this vibration in liquid water is typically just a few hundred femtoseconds (10 to the power of minus 15 seconds).

The surprising finding: in warm water, this vibration decays more slowly than in cold water. For most liquids, the opposite is true — higher temperatures lead to stronger collisions between molecules and thus faster energy dissipation. Water behaves counterintuitively.

The explanation lies in the structure of the hydrogen bond network. At low temperatures, hydrogen bonds are stronger and the network is more ordered. These strong bonds enable efficient energy transfer between neighboring molecules, so vibrational energy quickly “drains away.” At higher temperatures, the bonds become weaker and more disordered — the coupling between molecules decreases, and energy remains stored in the excited molecule for longer.

The lifetime of the OH stretching vibration increases from about 260 femtoseconds at 0 degrees Celsius to approximately 320 femtoseconds at 65 degrees Celsius — an increase of about 23 percent. This effect has been precisely measured using ultrafast infrared spectroscopy and provides valuable information about the dynamics of the hydrogen bond network.

OH Stretching Vibration Lifetime in Warm vs Cold Water Schematic showing the vibrational energy decay of the OH stretching mode over time. Warm water (red-cyan curve) shows a slower decay (longer lifetime) than cold water (blue curve), which is counterintuitive since higher temperature usually increases energy dissipation. Time (fs) Vibrational Energy 0 200 400 600 Warm (50 °C) Slower decay Cold (5 °C) Faster decay O H H OH Stretch Vibration Lifetime
Lifetime of the OH stretching vibration: warm water (cyan) shows a slower energy decay than cold water (blue).

Everyday Relevance

While this anomaly may seem abstract at first glance, it has fundamental significance for understanding water at the molecular level. The dynamics of vibrations determine how quickly water can absorb and redistribute thermal energy — a process that plays a role in every chemical reaction in aqueous solution.

For biology, this is particularly relevant: enzymes and proteins function in an aqueous environment, and the speed at which water redistributes energy directly influences the rate of biochemical reactions. The temperature-dependent change in vibrational dynamics may be another piece of the puzzle explaining why biological processes have an optimal temperature — and why that optimum so often falls in the range where water’s anomalies are most pronounced.