Transcript Why is water special? Studies on small water clusters

40 years of quantum chemistry
from polywater to the real stuff
Ad van der Avoird
Nijmegen, 25 April 2008
Water
• Most abundant compound on the Earth’s surface
• Principal constituent of all living organisms
• Subject of longer and more vigorous study than
any other substance
From: L.C. Allen, The rise and fall of polywater
New Scientist, 16 August 1973.
Anomalies of water
•
•
•
•
•
•
•
•
•
High boiling point
Large heat of vaporization
Solid (ice) floats on liquid (water)
Ice has many different phases
Liquid density maximum at 4°C
Compressibility decreases with increasing T up to 46.5°C
Large specific heat Cp with minimum at 36°C
Large dielectric constant
High surface tension (highest except metals)
Polywater, also called “anomalous water”
•
•
•
•
Freezing point −40◦C
Boiling point >150◦C
Density 1.4 g/cm3
High viscosity
1962
1966
1966-1972
Discovered by N. Fedyakin, Sovjet Union
Then studied by B. Deryagin et al., Moscow
Discussion of the Faraday Society
Intensive studies in UK, USA
Cat’s cradle (Kurt Vonnegut, 1963):
“There are several ways in which certain liquids can crystallize—can freeze—
several ways in which their atoms can stack and lock in an orderly, rigid way.
Suppose that the sort of ice we skate upon and put into highballs—what we call
ice-l—is only one of several types of ice. Suppose water always froze as ice-l
on Earth because it never had a seed to teach it how to form ice-two, ice-three,
ice-four…? And suppose that there were one form, which we will call ice-nine,
a crystal as hard as this desk with a melting point of 130°F. And suppose that
one Marine had with him a tiny capsule containing a seed of ice-nine, a new
way for the atoms of water to stack and lock, to freeze. If that Marine threw that
seed into the nearest puddle…?”
“The puddle would freeze?” I guessed.
“And all the muck around the puddle?”
“It would freeze.”
“And all the puddles in the frozen muck? They would freeze?”
“You bet they would, and the United States Marines would rise from the swamp
and march on.”
“There is such stuff?”
“No, no, no, no ... if you'd been listening to what I have been trying to tell you
about pure research men, you wouldn't ask such a question. Pure research
men work on what fascinates them, not on what fascinates other people.”
“If the streams flowing through the swamp froze as ice-nine, what about the
rivers and lakes the streams fed?”
“They'd freeze.”
“And the oceans the frozen rivers fed?"
“They'd freeze, of course.”
“And the springs feeding the frozen lakes and streams, and all the water
underground feeding the springs?”
“They'd freeze, damn it!”
“And the rain?”
“When it fell, it would freeze into hard little hobnails of ice-nine and that would
be the end of the world.”
Tetrahedral hydrogen bonded network
in normal water
Hexagonal ice (ice Ih) structure
Structure of polywater (ice-nine)
Each H atom is shared by two O atoms,
no distinction between OH bonds and hydrogen bonds
E.R. Lippincott et al., Science 164, p. 1482 (1969)
Infrared and Raman spectra completely different from
normal water
K. Morokuma, Chem. Phys. Lett. 4, p. 358 (1969)
L.C. Allen, P.A. Kollman, Science 167, p.1443 (March 1970)
“Explained” properties by quantum chemistry
(semi-empirical CNDO calculations)
L.C. Allen, P.A. Kollman, J. Am. Chem. Soc. 92, p. 4108 (July 1970)
Presented evidence that polywater does not exist
(ab initio calculations)
End of polywater story: 1973
Special properties caused by solution of various
impurities from capillary walls
What could be learned?
• Role of media: popular journals, newspapers
(fear of ice-nine)
• What is “New York Times physics”?
• Sociology of scientist world
• How efficient the scientific method can be (L.C. Allen)
Structure, motion, and properties
of water
are determined by energy landscape
(potential surface, force field,
forces between the molecules)
Can be determined by quantum chemistry
ab initio calculations required
First ab initio calculated water potentials:
• Popkie, Kistenmacher, Clementi (IBM)
J. Chem. Phys. 59, 1325 (1973)
Hartree-Fock calculations, 190 points
• Matsuoka, Clementi, Yoshimine (IBM)
J. Chem. Phys. 64,1351 (1976)
Configuration-Interaction calculations, 66 points
2007 / 2008
New ab initio water potential: CC-pol
• Based on > 2500 CCSD(T) points
extrapolated to CBS limit at MP2 level
• Analytic representation, correct long-range behavior
• Polarizable, includes many-body polarization effects
• Supplemented with 3-body exchange terms
• Tested by spectroscopy on dimer and trimer
• Used in MD simulations for liquid water
Water cluster spectra (far-infrared, high resolution)
from R.J. Saykally group (UC Berkeley)
and from W.L. Meerts et al. (Nijmegen)
How to use of water cluster spectra
to test potential surfaces?
Pair
potential
Pair + 3-body
potential

Dimer
quantum levels

Dimer
spectrum

Trimer
quantum levels

Trimer
spectrum
Compare with experimental spectra
Quantum tunneling
Double
well
potential
Energy
levels
Wave
functions
Water dimer
Hydrogen-bonded equilibrium structure
Water
dimer
tunneling
pathways
Tunneling
splitting of
water dimer
levels
(J=K=0)
acceptor
interchange
donor
tunneling + tunneling + tunneling
(H2O)2 levels from various older potentials (J=K=0)
(H2O)2 levels
from
CC-pol potential
and
experiment
(in cm-1)
(H2O)2
intermolecular
vibrations
(D2O)2
intermolecular
vibrational
levels
from CC-pol
and experiment
MD simulations of liquid water, T = 298 K
Atom-atom radial distribution functions
Conclusion
CC-pol, with the accompanying many-body interaction
model, is the first universal water force field: good for the
dimer and trimer, as well as for liquid water
R. Bukowski, K. Szalewicz, G.C. Groenenboom, A.van der Avoird
Science, 315, 1249 (2007)
J. Chem. Phys. 128, 094313, 094314 (2008) + unpublished
After 40 years
Complete set of maps of the water energy landscape
is now available
Quantum chemistry can be trusted to make
accurate predictions, not only for water
(in the hands of experts)
Cat’s cradle (Kurt Vonnegut, 1963)
2007 †
“See the cat? See the cradle?”
A cat's cradle is nothing but a bunch of X's between somebody's
hands, and little kids look and look and look at all those X's, but
there's no damn cat, and no damn cradle.
“It’s just an illusion!”