Showing papers by "Richard J. Saykally published in 1996"

Characterization of a cage form of the water hexamer

Abstract: WATER has been studied more extensively than any other liquid, yet its microscopic properties remain poorly understood. The difficulty in obtaining a rigorous molecular-scale description of water structure is largely a consequence of the extended, dynamic hydrogen-bonded network that exists throughout the liquid1. Studies of the structure and dynamics of isolated small clusters of water molecules2–6 provide a means of quantifying the intermolecular forces and hydrogen-bond rearrangements that occur in condensed phases. Experiments2–7 and theory8 strongly suggest that the water trimer, tetramer and pentamer have cyclic minimum energy structures. Larger water clusters are expected8 to have three-dimensional geometries, with the hexamer representing the transition from cyclic to such three-dimensional structures. Here we report investigations by terahertz laser vibration–rotation tunnelling spectroscopy3 of the structure of the water hexamer. A comparison of our results with quantum Monte Carlo simulations of this species suggests that the most stable form of (H2O)6 is indeed a cage-like structure, held together by eight hydrogen bonds (Fig. 1).

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

Abstract: 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

Abstract: 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.

Infrared emission spectra of candidate interstellar aromatic molecules

Abstract: Interstellar dust is responsible, through surface reactions, for the creation of molecular hydrogen, the main component of the interstellar clouds in which new stars form. Intermediate between small, gas-phase molecules and dust are the polycyclic aromatic hydrocarbons (PAHs). Such molecules could account for 2-30% of the carbon in the Galaxy, and may provide nucleation sites for the formation of carbonaceous dust. Although PAHs have been proposed as the sources of the unidentified infrared emission bands that are observed in the spectra of a variety of interstellar sources, the emission characteristics of such molecules are still poorly understood. Here we report laboratory emission spectra of several representative PAHs, obtained in conditions approximating those of the interstellar medium, and measured over the entire spectral region spanned by the unidentified infrared bands. We find that neutral PAHs of small and moderate size can at best make only a minor contribution to these emission bands. Cations of these molecules, as well as much larger PAHs and their cations, remain viable candidates for the sources of these bands.

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

Abstract: 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.

Infrared cavity ringdown laser absorption spectroscopy (ir-crlas) of jet-cooled water clusters

Abstract: $^{1}$1 J.J. Scherer, J.B. Paul, C.P. Collier, and R.J. Saykally, J. Chem. Phys. 102, 5190 (1995). $^{2}$ J.B. Paul, J.J. Scherer, C.P. Collier, and R.J. Saykally, J. Chem. Phys. 104, 2782 (1996). $^{3}$ J.J. Scherer, D. Voelkel, D.J. Rakestraw, J.B. Paul, C.P. Collier, R.J. Saykally, and A. O'Keefe, Chem. Phys. Lett. 245, 273 (1995).

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

Abstract: 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 ...

Tunneling dynamics, symmetry, and far-infrared spectrum of the rotating water trimer. II. Calculations and experiments

Abstract: With the Hamiltonian derived in the preceding paper and the ab initio potentials of T. Burgi, S. Graf, S. Leutwyler, and W. Klopper [J. Chem. Phys. 103, 1077 (1995)] and of J. G. C. M. van Duijneveldt‐van de Rijdt and F. B. van Duijneveldt [Chem. Phys. Lett. 237, 560 (1995)], we calculate the pseudo‐rotation tunneling levels in a rotating water trimer. The internal motions are treated by a three‐dimensional discrete variable representation and the Coriolis coupling with the overall rotation is included. Also the effects of donor tunneling are included, by introducing semi‐empirical coupling matrix elements. New experimental data are presented for the c‐type band at 87.1 cm−1 in (H2O)3, which show that specific levels in the donor tunneling quartets of this band are further split into doublets. With the results of our quantitative calculations and the model of the preceding paper we can understand the mechanisms of all the splittings observed in the earlier high‐resolution spectra of (H2O)3 and (D2O)3, as well as these new splittings, in terms of pseudo‐rotation tunneling, donor tunneling and Coriolis coupling. An unambiguous assignment is given of all the bands observed and analyzed. The ab initio potential of the Van Duijneveldts yields accurate energies of the lower pseudo‐rotation levels, the potential of Burgi et al. performs better for the higher levels. With our analysis we can deduce from the spectra that donor tunneling involves inversion of the trimer.

Far-infrared VRT spectroscopy of two water trimer isotopomers: vibrationally averaged structures and rearrangement dynamics.

Abstract: We report the measurement of far-infrared vibration–rotation tunnelling parallel bands of two partially deuterated water trimer isotopomers: (D2O)2DOH and (H2O)2DOH at 97•2607 cm-1 and ∼ 86 cm-1, respectively. The hydrogen bond rearrangement dynamics of the two mixed trimers can be described by the simplified molecular symmetry G8, which accounts for both the flipping and bifurcation tunnelling motions previously established for (H2O)3 and (D2O)3. The observed donor tunnelling quartet, rather than triplet, splitting indicates that the two homogeneous monomers (D2O or H2O) in each mixed trimer experience slightly different environments. Vibrationally averaged structures of (H2O)3, (D2O)3, and (D2O)2DOH were examined in a Monte Carlo simulation of the out-of-plane flipping motions of the free atoms. The simulation addresses both the symmetric top behaviour and the negative zero-point inertial defect for (H2O)3 and (D2O)3, which were insufficiently counted in all previous structure models. The average ground...

Infrared laser spectroscopy of jet-cooled carbon clusters: the nu 5 band of linear C9

Abstract: The nu 5 antisymmetric stretching vibration of 1 sigma+g C9 has been observed using direct infrared diode laser absorption spectroscopy of a pulsed supersonic cluster beam. Twenty-eight rovibrational transitions measured in the region of 2079-2081 cm-1 were assigned to this band. A combined least squares fit of these transitions with previously reported nu 6 transitions yielded the following molecular constants for the nu 5 band: nu 0 = 2 079.673 58(17) cm-1, B"= 0.014 321 4(10) cm-1, and B'=0.014 288 9(10) cm-1. The IR intensity of the nu 5 band relative to nu 6 was found to be 0.108 +/- 0.006. Theoretical predictions for the relative intensities vary widely depending upon the level of theory employed, and the experimental value reported here is in reasonable agreement only with the result obtained from the most sophisticated ab initio calculation considered (CCSD).

Cavity ringdown laser spectroscopy

Abstract: Summary form only given. CRLAS was developed in 1988 for spectroscopic applications by O'Keefe and Deacon. We have utilized this new approach to measure electronic spectra of laser-generated metal clusters, metal silicides, and infrared spectra of stable molecules and water clusters. A scan of a vibrational band of the water pentamer is shown. Theoretical considerations and some applications are discussed.

Space Carbon--Neutral Pathways?

Abstract: Complex chemistry takes place in interstellar matter, aided by the energetic photons of ultraviolet radiation that ionize molecules and induce reactions. In their Perspective, Heath and Saykally discuss results published in the same issue by Kaiser et al. (p. 1508) that reveal the importance of neutral species in interstellar chemistry. Such reactions explain the reaction-product branching ratios in such different objects as the TMC-1 molecular cloud and the carbon star IRC+10216.