Fully rotationally resolved spectra of three isotopic species of 1:1 clusters of benzene with water (H(2)O, D(2)O, and HDO) were fit to yield moments of inertia that demonstrate unambiguously that water is positioned above the benzene plane in nearly free internal rotation with both hydrogen atoms pointing toward the pi cloud. Ab initio calculations (MP2 level of electron correlation and 6-31 G(**) basis set with basis set superposition error corrections) predict a binding energy D(e) greater, similar 1.78 kilocalories per mole. In both the experimental and theoretical structures, water is situated nearly 1 angstrom within the van der Waals contacts of the monomers, a clear manifestation of hydrogen bond formation in this simple model of aqueous-pi electron interactions.

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Microwave and far-infrared spectra of the H, N-HOH dimer have been recorded from 36 to 86 GHz and 520 to 800 GHz with a planar supersonic jet/tunable laser sideband spectrometer. The a-type pure rotational microwave data extend the previous m = 0, K = 0 A symmetry manifold measurements of Herbine and Dyke [J. Chem. Phys. 83, 3768 ( 1980) ] to higher frequency and also provide an additional set of microwave transitions in the mK = + 1 E symmetry manifold. Two sets of five b-type rotation-tunneling bands, one set shifted from the other by an approximately constant 113 MHz, have been observed in the far infrared. The splitting into two sets arises from water tunneling, while the overall band structure is due to internal rotation of the ammonia top. Nonlinear least-squares fits to an internal rotor Hamiltonian provided rotational constants, and an estimation of V, = 10.5 f 5.0 cm-' for the barrier height to internal rotation for the NH, monomer. A nonlinear equilibrium hydrogen bond is most consistent with the vibrationally averaged rotational constants; with the angle cos-'[(A,)] d e t ermined from (A,), the projection of the ammonia's angular momentum onto the framework; and with the nitrogen quadrupole coupling constants of Herbine and Dyke. The water tunneling splitting and observed selection rules place constraints on the barrier height for proton exchange of the water as well as the most feasible water tunneling path along the intermolecular potential energy surface. An estimated barrier of -700 cm -' is derived for the water tunneling motion about its c axis.

A series of experiments were conducted to examine potential vulnerabilities of wood decks to continuous, wind-driven firebrand showers. Sections of wood decks (1.2 m by 1.2 m) were constructed and attached to a reentrant corner assembly. The deck/reentrant corner assembly was then exposed to continuous, winddriven firebrand bombardment generated by the newly developed NIST full-scale Continuous Feed Firebrand Generator (NIST full-scale Continuous Feed Dragon) installed in the Building Research Institute's (BRI) Fire Research Wind Tunnel Facility (FRWTF). Three different wood deck types were exposed to wind-driven firebrand showers at wind speed of 6 m/s; Western Red Cedar, Douglas-Fir, and Redwood. For each wood deck tested (exposed to a total firebrand mass flux of 17.1 g/m 2 s), firebrands accumulated on the deck surface, and each wood deck type was observed to ignite by flaming ignition. The average time to flaming ignition was 437 s for Cedar, 934 s for Douglas-Fir, and 756 s for Redwood. Therefore, wood decks were observed to be vulnerable to ignition from continuous, wind-driven firebrand showers. Results of these experiments are discussed in detail.

Large outdoor fires are an increasing danger to the built environment. Wildfires that spread into communities, labeled as Wildland-Urban Interface (WUI) fires, are an example of large outdoor fires. Other examples of large outdoor fires are urban fires including those that may occur after earthquakes as well as in informal settlements. When vegetation and structures burn in large outdoor fires, pieces of burning material, known as firebrands, are generated, become lofted, and may be carried by the wind. This results in showers of wind-driven firebrands that may land ahead of the fire front, igniting vegetation and structures, and spreading the fire very fast. Post-fire disaster studies indicate that firebrand showers are a significant factor in the fire spread of multiple large outdoor fires. The present paper provides a comprehensive literature summary on the role of firebrand mechanisms on large outdoor fire spread. Experiments, models, and simulations related to firebrand generation, lofting, burning, transport, deposition, and ignition of materials are reviewed. Japan, a country that has been greatly influenced by ignition induced by firebrands that have resulted in severe large outdoor fires, is also highlighted here as most of this knowledge remains not available in the English language literature. The paper closes with a summary of the key research needs on this globally important problem.