Cage, Book, and Prism: The array of hydrogen bonds governing the extended structure of liquid water is so intricate that chemists have often sought to understand it by studying simpler clusters. Even so, it has been challenging to get a handle on the preferred arrangement adopted by just six water molecules. Interdependent theoretical and spectroscopic studies have narrowed down the lowest-energy hexamer structures to three isomers—respectively designated the cage, the book, and the prism—but their relative energies remain uncertain. Now, Pérez et al. (p. 897 ; see the Perspective by Saykally and Wales ) have observed all three isomers in a single experiment, using Fourier transform microwave spectroscopy, and were able to establish definitively their energy ordering.

Gear-like rotation by a wobbly water duo: The molecules in liquid water move about constantly, but on average they cling to each other through hydrogen bonds, like dancers who keep switching partners. Richardson et al. uncovered a fresh twist in this molecular dance (see the Perspective by Clary). Studying clusters of six molecules each—essentially the smallest three-dimensional water droplets—they observed coupled motion of two different molecules in the cluster. The process breaks two different hydrogen bonds concurrently in a pattern akin to rotating gears. Science , this issue p. 1310 ; see also p. 1267

Chirped-pulse Fourier transform spectroscopy has recently been extended to millimeter wave spectroscopy as a technique for the characterization of room-temperature gas samples. Here we present a variation of this technique that significantly reduces the technical requirements on high-speed digital electronics and the data throughput, with no reduction in the broadband spectral coverage and no increase in the time required to reach a given sensitivity level. This method takes advantage of the frequency agility of arbitrary waveform generators by utilizing a series of low-bandwidth chirped excitation pulses paired in time with a series of offset single frequency local oscillators, which are used to detect the molecular free induction decay signals in a heterodyne receiver. A demonstration of this technique is presented in which a 67 GHz bandwidth spectrum of methanol (spanning from 792 to 859 GHz) is acquired in 58 μs.

The structures of the phenol dimer and phenol trimer complexes in the gas phase have been determined using chirped-pulse Fourier transform microwave spectroscopy in the 2–8 GHz band. All fourteen ¹³C and ¹⁸O phenol dimer isotopologues were assigned in natural abundance. A full heavy atom experimental substitution structure was determined, and a least-squares fit ground state r₀ structure was determined by proper constraint of the M06-2X/6-311++g(d,p) ab initio structure. The structure of phenol dimer features a water dimer-like hydrogen bond, as well as a cooperative contribution from inter-ring dispersion. Comparisons between the experimental structure and previously determined experimental structures, as well as ab initio structures from various levels of theory, are discussed. For phenol trimer, a C₃ symmetric barrel-like structure is found, and an experimental substitution structure was determined via measurement of the six unique ¹³C isotopologues. The least-squares fit r_m ⁽¹⁾ structure reveals a similar interplay between hydrogen bonding and dispersion in the trimer, with water trimer-like hydrogen bonding and C–H⋯π interactions.