Identifying the 'Signatures' of Protons in Water

Jul 13, 2005

Free protons from acids associate with 1, 2 or 3 molecules of water and the structures can be identified by unique infrared laser spectrum signatures, according to a report in Science by Yale professor of chemistry Mark A. Johnson and his collaborators at Yale, the University of Pittsburgh and the University of Georgia.

Acids yielding free protons are common in biological and chemical systems and the measurement of pH to determine acidity of an aqueous solution is a simple, standard procedure. However, it has not been as easy to determine where the liberated protons are located and how they interact with water molecules.

The scientists tackled these questions using infra-red laser light, at much lower energies than were previously accessible, to monitor how the vibration profile changes when a proton is associated with two to eleven water molecules.

The researchers first established a spectral signature for the symmetrically hydrated Eigen cation, which has a minimum energy (H3O)+ ion core and three associated “dangling” water molecules. As they successively added or subtracted water molecules and compared the spectral signatures, they mimicked water fluctuations.

“Surprisingly large spectral shifts are driven by small changes in the hydration environment,” said Johnson. “Although previous work anticipated a change from Zundel to Eigen structures as you progress from 8 to 9 water molecules, the change in the low energy bands here is dramatic. The profile for the 9-membered cluster is much like bulk water, but then the 10-membered cluster is again simpler.”

The study shows that the proton associated with the Eigen cation undergoes vibrations highest in energy because it supports the greatest distribution of charge, that is, over three H atoms. As different numbers of water molecules surround the H3O+ core, the excess charge can become more localized onto two or even one of the H atoms, causing substantial, size-dependent shifts in the spectral signature of the excess proton. This extreme response to breaking symmetry is consistent with Zundel’s model of the excess proton being a highly polarizable species.

“The basic point is that the proton is a moving target, rapidly switching its character from one species to the next according to how many water molecules it is associated with,” said Johnson. “Now that the spectral signatures of various local environments in water are known, the big question left is how this all comes together as we continue to grow crystals toward bulk water (ice).”

Citation: Science 308: 1765-176917 (June 2005).

Source: Yale University

Explore further: Research clarifies the physics of water repelling surfaces

Related Stories

MIRO maps water in comet's coma

Jun 22, 2015

MIRO, the Microwave Instrument for the Rosetta Orbiter, first detected the emission from water molecules in the coma of Comet 67P/C-G on 6 June 2014, when Rosetta was 350,000 km from the comet, approximately ...

Exposing breast cancer using nanoscale polymers

May 13, 2015

Photoacoustic imaging is a ground-breaking technique for spotting tumors inside living cells with the help of light-absorbing compounds known as contrast agents. A*STAR researchers have now discovered a way ...

In the realm of eternal ice

Apr 23, 2015

On 6 November 2010, the light of the star known as NOMAD1 0856-0015072 in the Cetus constellation dimmed. What had happened? A dwarf planet at the edge of the solar system had moved in front of the distant ...

Recommended for you

To conduct, or to insulate? That is the question

19 hours ago

A new study has discovered mysterious behaviour of a material that acts like an insulator in certain measurements, but simultaneously acts like a conductor in others. In an insulator, electrons are largely stuck in one place, ...

Soundproofing with quantum physics

19 hours ago

Sebastian Huber and his colleagues show that the road from abstract theory to practical applications needn't always be very long. Their mechanical implementation of a quantum mechanical phenomenon could soon ...

Extreme lab at European X-ray laser XFEL is a go

20 hours ago

The Helmholtz Senate has given the green light for the Association's involvement in the Helmholtz International Beamline (HIB), a new kind of experimentation station at the X-ray laser European XFEL in Hamburg, ...

User comments : 0

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.