Noncovalent interactions: a challenge for experiment and theory

Water is of fundamental importance for human life and plays an important role in many biological and chemical systems. Although water is the most abundant compound on earth, it is definitely not a simple liquid. It possesses strongly polar hydrogen bonds which are responsible for a striking set of anomalous physical and chemical properties. For more than a century the combined importance and peculiarity of water inspired scientists to construct conceptual models, which in themselves reproduce the observed behavior of the liquid. The exploration of structural and binding properties of small water complexes provides a key for understanding bulk water in its liquid and solid phase and for understanding solvation phenomena. Modern ab initio quantum chemistry methods and high-resolution spectroscopy methods have been extremely successful in describing such structures. Cluster models for liquid water try to mimic the transition from these clusters to bulk water. The important question is: What cluster properties are required to describe liquid-phase behavior?

Water: From Clusters to the Bulk

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 structures, force fields, dipole moments, and hydrogen bond rearrangement dynamics in these clusters This new information permits us to systematically untangle the intricacies associated with cooperative 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

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

The optimal structures and harmonic vibrational frequencies of cyclic water clusters, (H2O)n, have been determined at the Hartree–Fock (for n=2–6) and second order perturbation theory (for n=2–4) levels of theory with an augmented correlation consistent double zeta basis set At the MP2 level this basis set yields very accurate results for the structure, dipole moment, and polarizability of the water monomer as well as results of comparable accuracy for the structure, binding energy, and harmonic vibrational frequencies of the water dimer The optimal structure of (H2O)4 and the harmonic frequencies of (H2O)3,4 are the first ones reported at a correlated level for these species Analysis of the structural trends reveals that the separation between neighboring oxygen atoms decreases exponentially with increasing cluster size The predicted R0(O–O) for the ring hexamer is less than 002 A shorter than the interoxygen separation in ice Ih Furthermore, the trends in the harmonic vibrational frequencies sugge

Ab initio studies of cyclic water clusters (H2O)n, n=1–6. I. Optimal structures and vibrational spectra

The average dipole moment of a water molecule in the condensed phase is enhanced by around 40 percent relative to that of an isolated monomer. This enhancement results from the large polarization caused by the electric field induced by surrounding monomers. A quantitative molecular description of this polarization is essential for modeling aqueous solvation phenomena. This combined theoretical and experimental study of dipole moments in small water clusters provides such a description and also gives insights into the structure of liquid water.

The Water Dipole Moment in Water Clusters

Measurement of the far-infrared vibration-rotation tunneling spectrum of the perdeuterated water tetramer is described. Precisely determined rotational constants and relative intensity measurements indicate a cyclic quasi-planar minimum energy structure, which is in agreement with recent ab initio calculations. The O-O separation deduced from the data indicates a rapid exponential convergence to the ordered bulk value with increasing cluster size. Observed quantum tunneling splittings are interpreted in terms of hydrogen bond rearrangements connecting two degenerate structures.

https://science.sciencemag.org/content/sci/271/5245/59.full.pdf

Quantifying hydrogen bond cooperativity in water: VRT spectroscopy of the water tetramer.

3. Ab Initio Calculations 2543 C. Dynamics 2550 1. Group Theory 2551 2. H-Bond Network Rearrangement (HBNR) 2552 3. Intramolecular Vibrations 2557 4. Intermolecular Vibrations 2558 III. Experimental Data 2558 A. Condensed-Phase Environments 2560 1. Matrix-Isolation Spectroscopy 2560 2. Inorganic Host Complexes 2561 3. Water Trimer in Liquid Helium Droplets 2561 B. Gas-Phase Spectroscopy of the Free Water Trimer 2561

The water trimer.

Far-infrared laser vibration-rotation tunneling spectroscopy was used to measure an intermolecular vibration (81.19198 wave numbers) of the isolated water (D 2 O) pentamer. Rotational analysis supports the chiral, slightly puckered ring structure predicted by theory. The experimentally deduced interoxygen separations for the water clusters up to the pentamer showed exponential convergence toward the corresponding distance in bulk phase water.

https://science.sciencemag.org/content/sci/271/5245/62.full.pdf

Vibration-Rotation Tunneling Spectra of the Water Pentamer: Structure and Dynamics

Using tunable far infrared laser absorption spectroscopy, 12 vibration–rotation‐tunneling (VRT) subbands, consisting of approximately 230 transitions have been measured and analyzed for an 82.6 cm−1 intermolecular vibration of the water dimer‐d4. Each of the VRT subbands originate from K‘a = 0 and terminate in either K’a = 0 or 1. These data provide a complete characterization of the tunneling dynamics in the vibrationally excited state as well as definitive symmetry labels for all VRT energy levels. Furthermore, an accurate value for the A’ rotational constant of 122.9 GHz is found to agree well with its corresponding ground state value. All other excited state rotational constants are fitted, and discussed in terms of the corresponding ground state constants. In this vibration, the quantum tunneling motions are determined to exhibit large dependencies with both the K’a quantum number and the vibrational coordinate, as is evidenced by the measured tunneling splittings. The generalized internal‐axis‐metho...

Vibrational and K’a dependencies of the multidimensional tunneling dynamics in the 82.6 cm−1 intermolecular vibration of the water dimer‐d4

The detailed analysis of a parallel vibration−rotation-tunneling (VRT) band of the isolated cyclic perdeuterated (d10) water pentamer measured near 2.4 THz (81.2 cm-1) is presented. The vibrational...

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J. D. Cruzan

Papers

Quantifying hydrogen bond cooperativity in water: VRT spectroscopy of the water tetramer.

The water trimer.

Vibration-Rotation Tunneling Spectra of the Water Pentamer: Structure and Dynamics

Vibrational and K’a dependencies of the multidimensional tunneling dynamics in the 82.6 cm−1 intermolecular vibration of the water dimer‐d4

Terahertz Laser Spectroscopy of the Water Pentamer: Structure and Hydrogen Bond Rearrangement Dynamics