Every so often, the surrounding water molecules, each with a small electric field, line up and create a large field that pulls the hydrogen ion farther from the parent molecule than usual - as much as five neighbors away. Most of the time, the electric field quickly dies away and the proton snaps back.
"The electric field vanishes rapidly - it only lives for tens of femtoseconds - and then the proton just comes sweeping back because there is this strong electrical attraction from the chemical bond," he said. A femtosecond is a millionth of a billionth of a second.
Once every billionth of a second or so, however, the pathway back to association - a wire of hydrogen bonds - breaks, and a free proton is created.
"The proton is pulled far away 100s and 100s of times without leading to auto-dissociation," Chandler said. "This is the part that is surprising and that people didn't know. What finally leads to dissociation is, somewhere along this wire of hydrogen bonds, when there just happens to be a big electric field that's driven the proton away, by accident the hydrogen bond wire breaks and this proton is stuck off in left field."
Once the proton is liberated, it wanders through the liquid, free to participate in many other chemical reactions until it recombines with an hydroxide ion to make water again. Concentrations of hydrogen ions also set up electric fields that move other ionized molecules around.
"A proton gradient across a membrane, and the electrical voltage associated with that, is what drives all life processes," Chandler said.
This detailed picture comes from the team's calculations on what happens in pure water, which can be considered a weak acid, Chandler said. Water, or H2O, is composed of two hydrogen atoms and one oxygen atom, but in a liquid about one in every billion water molecules has d
'"/>
Contact: Robert Sanders
rls@pa.urel.berkeley.edu
510-643-6998
University of California - Berkeley
18-Mar-2001