Mac G. Brown

Quantifying Hydrogen Bond Cooperativity in Water: VRT Spectroscopy of the Water Tetramer

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.

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

Far-infrared laser vibration-rotation tunneling spectroscopy was used to measure an intermolecular vibration (81.19198 wave numbers) of the isolated water (D2O) 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.

Terahertz laser spectroscopy of the water dimer intermolecular vibrations. II. (H2O)2

Terahertz VRT laser spectra of four (H2O)2 intermolecular vibrations consisting of 362 transitions have been measured between 87 and 108 cm−1 with ca. 2 MHz precision. The results differ both qualitatively and quantitatively from the predictions of dimer potentials tested. The spectra also reveal an ordering of the intermolecular vibrations which differs dramatically from that predicted by normal mode analysis. Strong coupling is indicated between the low barrier tunneling motions and the intermolecular vibrations as well as among different vibrations. In particular the 102.1 cm−1 (H2O)2 vibration assigned as the acceptor wag (ν8) exhibits two types of perturbations. In one of these a component of Ka=1 coupling with a tunneling component of Ka=0 in the 108 cm−1 acceptor twist (ν11) vibration. There is also an indication that the 103.1 cm−1 (H2O)2 band assigned as the donor in-plane bend (ν6) is coupled to the acceptor wag resulting in a lower of the in-plane bend frequency and a higher acceptor wag freque...

Spectroscopic determination of the water dimer intermolecular potential-energy surface

Two polarizable six-dimensional water dimer intermolecular potential surfaces have been determined by fitting the distributed multipole ASP (anisotropic site potential) potential form to microwave, terahertz, and midinfrared cavity ringdown (D2O)2 spectra via a rigorous calculation of the water dimer eigenstates with the PSSH (pseudo-spectral split Hamiltonian) method. The fitted potentials accurately reproduce most ground-state vibration-rotation-tunneling spectra and yield excellent second virial coefficients for both H2O and D2O. The calculated dimer structure and dipole moment are close to those determined from microwave spectroscopy and high level ab initio calculations, except that the O–O distance (2.952 A) is significantly shorter than the currently accepted experimental value. The dimer binding energy (4.85 kcal/mol) is considerably smaller than the accepted experimental result, but in excellent agreement with recent theoretical results, as are the acceptor switching and donor–acceptor interchang...

A long path length pulsed slit valve appropriate for high temperature operation: Infrared spectroscopy of jet‐cooled large water clusters and nucleotide bases

We report the design and performance of a pulsed slit valve for generation of supersonically cooled species in a long path length planar expansion. Utilizing three commercial solenoids driven synchronously by an economic power transistor circuit, the valve produces pulses adjustable in width from 500 to 1000 μs with a repetition rate up to 80 Hz. The pulsed valve can be operated continuously for 12 h, and operation over one month is typical before major maintenance is required. The path length × density product attained by this pulsed source is sufficient for observing large cluster species, such as the water hexamer, on our far‐infrared spectrometer. With the addition of a detachable sample oven, it can also be heated up to 230 °C to inject nonvolatile molecules into planar supersonic expansions.

Water dimer hydrogen bond stretch, donor torsion overtone, and “in-plane bend” vibrations

We report the measurement and analysis of 64 new Ka=0←0,1 and Ka=1←0,1 transitions of (H2O)2 and 16 new Ka=0←0 transitions of (D2O)2 by terahertz laser vibration–rotation–tunneling spectroscopy of a planar supersonic expansion between 140.5 and 145.5 cm−1. The transitions in both isotopomers correspond to A′ vibrations assigned to the hydrogen bond stretch (translational) and donor torsion overtone vibrations. The interchange splitting is 56.3 GHz in Ka=0 of the excited state of (H2O)2, nearly 3 times the value of the ground state, and the bifurcation tunneling splitting is 1.8 GHz, over 2 times the value of the ground state. We compare the existing experimental spectra with calculations on state-of-the-art intermolecular potential energy surfaces and critically review the vibrational assignments reported in the literature. We show that the discrepancy between theory and experiment regarding the assignment of the feature near 103 cm−1 can be resolved by considering E2→E1 transitions, which had not been co...

The far‐infrared vibration–rotation–tunneling spectrum of the water tetramer‐d8

The far‐infrared vibration–rotation–tunneling spectrum of (D2O)4 has been measured in the spectral region near 2.04 THz. Observation of additional transition doublets with a constant 5.6 MHz spacing in a parallel (c‐type) spectrum extends the first detailed study of this cluster [Science 271, 59 (1996)]. Three possibilities are explored for the origin of this small splitting: tunneling between degenerate equilibrium structures via facile torsional motions analogous to those observed in the water trimer, tunneling between nondegenerate structural frameworks, and tunneling made feasible only through excitation of a specific vibrational coordinate. The degenerate tunneling scheme best accounts for the spectral features, although the precise dynamics responsible for the observed spectral features cannot be uniquely established from the present data. A further doubling of spectral features, observed only in the K=2 manifold of transitions for J≥3, is symmetric about the unperturbed symmetric top energy levels ...

QUANTITATIVE CHARACTERIZATION OF THE (D2O)3 TORSIONAL MANIFOLD BY TERAHERTZ LASER SPECTROSCOPY AND THEORETICAL ANALYSIS

We report the measurement of two new perpendicular (D2O)3 torsional bands by terahertz laser vibration–rotation–tunneling (VRT) spectroscopy of a planar pulsed supersonic expansion. The first (28.0 cm−1) band corresponds to the k=±2l←0 transition, and is the lowest frequency vibrational spectrum observed for a water cluster. The second (81.8 cm−1) band originates in the first excited torsional state, and has been assigned as k=3u←±1l. An effective three-dimensional Hamiltonian is derived to describe the rotational structure of each torsional state. Degenerate torsional levels with k=±1 and k=±2 exhibit a Coriolis splitting linear in K implying the presence of vibrational angular momentum, and a second-order splitting from off-diagonal coupling between degenerate sublevels with +|k| and −|k|. With this effective Hamiltonian we fit a total of 554 rovibrational transitions in five different bands connecting the lowest nine torsional states, with a rms residual of 1.36 MHz. The data set comprises the two new ...

Quantitative characterization of the water trimer torsional manifold by terahertz laser spectroscopy and theoretical analysis. II. (H2O)(3)

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