Water clusters: Untangling the mysteries of the liquid, one molecule at a time

Материалы собраны Мосиным Олегом.

Frank N. Keutsch* and Richard J. Saykally† 

Department of Chemistry, University of California, Berkeley, CA 94720-1460 

This contribution is part of the special series of Inaugural Articles by members of the National Academy of Sciences elected on April 27, 1999. 

Contributed by Richard J. Saykally, May 29, 2001 

Extensive terahertz laser vibration-rotation-tunneling spectra and
mid-IR laser spectra have been compiled for several isotopomers of
small (dimer through hexamer) water clusters. These data, in
conjunction with new theoretical advances, quantify the struc-
tures, force fields, dipole moments, and hydrogen bond rearrange-
ment dynamics in these clusters. This new information permits us
to systematically untangle the intricacies associated with cooper-
ative hydrogen bonding and promises to lead to a more complete
molecular description of the liquid and solid phases of water,
including an accurate universal force field. 

The quest to achieve an accurate description of liquid water
has produced major advances in the last t wo decades (1), yet
despite the constr uction of hundreds of model force fields for use
in simulations, the great advances in comput ational technolog y,
and the development of power ful ab initio molecular dynamics
methods, we remain unable to accurately calculate the properties
of liquid water (e.g., heat capacit y, densit y, dielectric const ant,
compressibilit y) over significant ranges in conditions (2). We do
not yet have a satisfactor y molecular description of how a proton
moves in the liquid, we do not fully underst and the molecular
nature of the sur faces of either ice or liquid water (3), nor do we
underst and the origin of the intriguing anomalies and singulari-
ties found in the deeply supercooled region (4). Although it is
clear that the hydrogen bond net work and its f luctuations and
rearrangement dynamics deter mine the properties of the liquid,
no experiment al studies ex ist that reveal det ailed infor mation on
a molecular level without considerable interpret ation (5). More-
over, the reliabilit y of water models for simulating solvation
phenomena and biological processes remains relatively untested.
A principal obst acle to resolv ing these issues is that of
correctly describing the many-body, or cooperative nature of the
hydrogen bonding interactions among a collection of water
molecules. Theoretical work has shown that the H-bond is
dominated by electrost atic interactions, balanced by the repul-
sive electron exchange, but that dispersion makes an appreciable
contribution, whereas induction (polarization) is the dominant
many-body ef fect (6, 7). It has proven notoriously dif ficult to
accurately parameterize these interactions f rom ab initio calcu-
lations. Moreover, the ab initio molecular dynamics methods are
based on densit y functional methods that explicitly omit the
dispersion, and its expense mandates rather small sample sizes
(e.g., 64 molecules) in simulations (8). But perhaps the central
obst acle to developing quantit atively accurate and general meth-
ods has simply been the lack of a suit ably precise dat a set with
which to test and calibrate theoretical approaches. 

The central goal of the research rev iewed below is to advance
the cause for accurately describing water in all its phases over
arbitrarily large ranges of conditions, and the central contribu-
tion of our group has been to develop and apply novel methods
of laser spectroscopy for the highly det ailed study of water
clusters to prov ide such a dat a set. Recently, we also have
initiated studies of the hydrogen bond breaking dynamics in
water clusters and comparison of them with mechanisms pro-
posed to prevail in liquid water. 

Terahertz Laser Vibration-Rotation-Tunneling (VRT) Spectroscopy of Clusters 

The first far-IR (FIR) spectra of gaseous water clusters were
measured near 22 cm􏰃1 (455 􏰇m) by Busarow et al . in 1989 (9).
The spectra consisted of 56 Ka 􏰅 2 4 1 rot ation-tunneling
transitions of (H2O)2, which complemented the microwave dat a
(10, 11) obt ained by the pioneering work of Dyke et al . (10), in
obt aining an accurate description of the dimer ground st ate.
Zwart et al . (12) subsequently extended these dat a to other
quantum st ates. Af ter some import ant technical developments
that extended the operating range of the spectrometer to higher
f requencies, Pugliano and Saykally (13) first measured an inter-
molecular VRT spectr um of a water cluster in 1992, with the
detection of a torsional v ibration of the D2O trimer near 89.5
cm􏰃1 (112 􏰇m) (Fig. 1) (14 –16). This striking spectr um exhibited
an exact symmetric rotor pattern, and ever y rot ational line was
split into a distinctive quartet pattern that we now know results
f rom quantum tunneling v ia t wo dif ferent hydrogen bond path-
ways connecting 48 degenerate minima on the 12-dimensional
inter molecular potential sur face. Pugliano et al . (17) quickly
followed with the first obser vation of a dimer inter molecular
v ibration (acceptor t wist), near 83 cm􏰃1 (120 􏰇m). 

Subsequent work at Berkeley by Liu et al . (18) produced much
more extensive trimer spectra and the first det ailed assignment
of the transitions. Cr uzan et al . (19) discovered VRT spectra of
the tetramer shortly af ter ward, and Liu et al . followed with the
detection of the pent amer (20) and hexamer (21). Recent ef forts
have produced highly det ailed characterizations of both the
dimer and trimer, as well as greatly expanded dat a for the other
clusters (22–27). We describe the current underst anding of the
dimer through hexamer clusters that has been achieved f rom
these dat a, and through the ef forts of many concurrent theo-
retical studies, in a later section. 

IR Cavity Ringdown Spectroscopy 

While mid-IR spectra of water clusters had been obser ved by the
Pimentel group in matrix studies in 1957 (28), the OH stretching
v ibrations of gaseous water clusters were first studied indirectly
in 1982 by Vernon et al . (29) in IR predissociation experiments
in supersonic beams, and shortly af ter that by Page et al . (30). 

Vernon et al . assigned the spectra to (H2O)n, n 􏰅 1–5, and
recorded a narrow transitions (15 cm􏰃1) at 3,715 cm􏰃1, which
they attributed to the f ree OH stretch in cyclic water clusters, and
a much broader feature (200 cm􏰃1) at lower f requency that they
attributed to the bound OH stretch. Page et al . concentrated on
the water dimer, finding four peaks, including the bound OH
stretch, a broad transition at 3,545 cm􏰃1, red-shif ted f rom the
f ree monomer OH stretches. Coker et al . (31) found four dimer
OH stretch f requencies identical to those deter mined by Page
Abbreviations: VRT, vibration-rotation-tunneling; FIR, far-IR; ASP, anisotropic site poten-
tial; IPS, intermolecular potential surface. 

*Present address: Department of Chemistry and Chemical Biology, Harvard University,
Cambridge, MA 02138. 

To whom reprint requests should be addressed. E-mail: saykally@uclink4.berkeley.edu. 

 and also identified larger clusters in supersonic expansions carr ying increasing concentrations of water. Huang and Miller
(32, 33) reported the first rot ationally resolved spectr um of
(H2O)2 and obser ved the four OH stretch v ibrations, and
recently Frochtenicht et al . (34) used a size selection technique
in which a He beam is used to eject clusters f rom a molecular
beam as a function of their size. They were able to measure the
f ree and bound OH stretching f requencies for clusters up to the
pent amer. 

The wide tuning range of our IR cav it y ringdown laser
absorption spectrometer recently per mitted the first det ailed
studies of both the covalent bending v ibrations of H2O clusters
(35), which occur near 1,600 cm􏰃1, and the stretching v ibrations
of D2O clusters (36, 37), which fall near 2,700 cm􏰃1 (Fig. 2). All
of the obser ved clusters except the dimer exhibit strong v ibra-
tional predissociation broadening of their OD stretch spectra
that obscures rot ation-tunneling features. For the D2O dimer,
however, the ac ceptor antisy mmetric stretch exhibits well-
resolved acceptor switching doublets for each rot ational line,
whereas the donor stretch exhibits rot ational lines that are
broadened, but by about 30 times less than found for the H2O
isotopomer (36, 37). All bands obser ved for the cluster HOH
bending v ibrations are severely broadened, implying a stronger
coupling with the dissociation coordinate (35). The sharp
rot ation-tunneling str ucture measured for (D2O)2 (Fig. 2b) was
import ant for the deter mination of the dimer potential sur face
(38, 39), because the acceptor switching splittings cannot be
deter mined directly in the FIR experiments because of prohib-
itive selection r ules. With the use of theoretical integrated band
intensities, these cav it y ringdown measurements per mitted the
first deter mination of the absolute water cluster concentrations
in a supersonic beam (40). Interestingly, the trimer dominates
the cluster distribution for both H2O and D2O. This domination
is probably caused by the discontinuous increase in the per-
monomer binding energ y (D0), which jumps f rom 1􏰋2 D0 to D0
f rom dimer to trimer, while increasing much more slowly for
larger clusters.