The water dimer I: Experimental characterization

Abstract

a b s t r a c tAs the archetype of water hydrogen bonding, the water dimer has been studied extensively by both theory and experiment for nearly seven decades. In this article, we present a detailed chronological review of the experimental dimer studies and the insights into the complex nature of water and hydrogen bonding gained from them. A subsequent letter will review the corresponding theoretical advances.

Observation of the Low‐Frequency Spectrum of the Water Dimer as a Sensitive Test of the Water Dimer Potential and Dipole Moment Surfaces

“…If AS tunneling is feasible,t he applicable molecular symmetry group is G 4 ,e ach rovibrational energy level is split into two energy levels (labeled as A 1 /B 1 and A 2 /B 2 ), see Figure 2b.Previously,the AS tunneling splittings in the vibrational ground state was deduced experimentally to be 11.18 cm À1 and 2.73 cm À1 for K a '' = 0 and K a '' = 1s tates,respectively. [32] Theb ifurcational (BI) tunneling describes ap roton exchange of hydrogen bond donors.T he tunneling mode involves an in plane librational motion of the donor and aflip of the acceptor monomer.Inthe ground state the barrier is on the order of 400 cm À1 . [30] This tunneling causes an addition splitting of each state into three sublevels.T hese states can be labeled in the G 16 molecular symmetry group (see Figure 2b).…”

“…[9,[29][30][31][32] Thew ater dimer is produced in as upersonic planar expansion and probed by several FIR laser (mixed with at unable MW source) for frequencies < 150 cm À1 as well as lead salt diode lasers in the frequency range from 515 to 528 cm À1 .T his allowed to record spectra with as pectral resolution of 0.1 MHz, that is,3 10 À6 cm À1 and to assign single vibrational rotational tunneling (VRT) transitions with resolved hyperfine splitting.B ased on the spectra below 150 cm À1 ,i tw as possible to characterize the three feasible tunneling motions that connect the 8d egenerate minima on the 12-dim water potential (including 6 intramolecular and 3i ntermolecular modes). [9,[29][30][31][32] Thew ater dimer is produced in as upersonic planar expansion and probed by several FIR laser (mixed with at unable MW source) for frequencies < 150 cm À1 as well as lead salt diode lasers in the frequency range from 515 to 528 cm À1 .T his allowed to record spectra with as pectral resolution of 0.1 MHz, that is,3 10 À6 cm À1 and to assign single vibrational rotational tunneling (VRT) transitions with resolved hyperfine splitting.B ased on the spectra below 150 cm À1 ,i tw as possible to characterize the three feasible tunneling motions that connect the 8d egenerate minima on the 12-dim water potential (including 6 intramolecular and 3i ntermolecular modes).…”

Overtone spectroscopy of molecular complexes containing small polyatomic molecules

“…The analysis, performed in Brussels, accounted for the two internal conversion mechanisms producing the largest energy splittings in the water dimer, i.e. the so-called acceptor-switching (AS) and interchange tunnelings [40]. Simulation of the band contours using PGOPHER [220] played a crucial role in the tentative sub-band assignment, summarised in Figure 31, after [219].…”

A Terahertz VRT spectrometer employing quantum cascade lasers

“…Of particular interest to our group is the study of water clusters, as these are a route to a more detailed understanding of bulk water [1][2][3][4]. However, the paucity of Terahertz sources operating above 3 THz, and increasing constraints on laser safety have stifled progress in this field [5].…”