Life Hinges on Water’s Competing Quantum Properties

Life Hinges on Water’s Competing Quantum Properties

Ice floats. Clouds bring rain (most of the time). Water vapor warms the Earth. Water absorbs heat efficiently. Most of Earth’s water contains noticeable amounts of salt.

None of these statements seems remarkable, but life requires water to possess all these properties—and many more.

Most of water’s unusual, life-friendly properties stem from its attractive interaction between atoms, called hydrogen bonding. Or at least it would seem so from a “classical” (as opposed to a “quantum”) understanding of how atoms and molecules behave. According to the classical perspective, the atoms in a molecule of water always have the same, fixed spacing and the molecules move in a continuous, well-specified way. Starting with this classical picture of water, modeling the atomic and molecular dynamics readily reproduces the hydrogen bonding scientists observe from all sorts of measurements.

When chemists began modeling the interactions of water molecules and they included the quantum effects, the models predicted that the effects of hydrogen bonding would be noticeably diminished.1 The Heisenberg uncertainty principle dictates that water molecules cannot occupy a definite position relative to one another. Similarly, the atomic positions in the molecule suffer the same consequence. In practical terms, this means that the atomic and molecular positions continually fluctuate, with the net effect of reducing the strength of hydrogen bonding and, more importantly of eliminating many of water’s life-friendly properties.

More recent modeling of water’s interactions, which includes the quantum effects in a more-detailed and accurate way, shows how two competing quantum processes balance one another to give water the properties so vital for life.2 Quantum fluctuating distances between molecules weaken the strength of hydrogen bonding. The fluctuating distances between the atoms (and the fluctuations in the bond angle, which averages to 104.5º) in turn, lead to a greater separation between the slight negative and positive charges on the water molecule. (Scientists call two charges separated by some distance a dipole.) Such increased distance in the negative and positive charge seems larger to other molecules and, consequently, causes a greater dipole attraction (opposite charges attract) between water molecules. The more-accurate models showed that the increase in the dipole attraction effectively offsets the decrease in hydrogen bonding.


Absolute and Unchanging

Before the quantum revolution in the late 1800s and early 1900s, scientists envisioned the world in a classical way. Specifically, time and space were absolute and unchanging. Matter and energy behaved in a continuous fashion such that one could conceivably divide them into arbitrarily small amounts (at least until you reached the last atom) without changing the fundamental properties of the matter or energy. Additionally, one could determine, to an arbitrary precision, any and all characteristics of the matter and energy. However, according to quantum mechanics, all things (space, time, matter, and energy) come in discrete (stepwise) bundles. Furthermore, fundamental limits exist on how precisely scientists can measure certain quantities (like the position and momentum of a particle) at the same time.

Experiments by another team of physicists validated this picture where two competing quantum effects produce water’s properties. Water forms from two different isotopes of hydrogen: (1) hydrogen with only one proton, and (2) deuterium with one proton and one neutron. The extra mass of deuterium makes it less susceptible—compared to hydrogen—to the quantum uncertainties in bond length. This means that the deuterium-oxygen bond should be shorter than the hydrogen-oxygen bond. The researchers measured the bond lengths in both kinds of water by shooting a beam of neutrons through the water and seeing how they scattered. Their tests confirmed a shorter bond length in the deuterium-oxygen bonds in agreement with the “competing-quantum-effects” model.3

Water’s properties, uniquely suited for life, arise from the strength of interactions within the molecule and with other molecules. Quantum mechanics reduces the molecular interactions in one way—by changing the probability that hydrogen will tunnel from one molecule to another. And yet quantum mechanics increases the molecular interactions in a completely different way—by affecting the distribution of charges within the molecule to strengthen electromagnetic attractions between molecules. The fact that this set of properties relies on two competing quantum effects argues that water appears to be purposely designed.

  1. Robert A. Kuharski and Peter J. Rossky, “A Quantum Mechanical Study of Structure in Liquid H2O and D20,” Journal of Chemical Physics 82 (February 1985): 5164–77.
  2. Scott Habershon, Thomas E. Markland, and David E. Manolopoulos, “Competing Quantum Effects in the Dynamics of a Flexible Water Molecule,” Journal of Chemical Physics 131 (July 2009): 024501.
  3. Anita Zeidler et al., “Oxygen as a Site Specific Probe of the Structure of Water and Oxide Materials,” Physical Review Letters 107 (September 30, 2011): 145501.