| Summary: | <jats:title>Significance</jats:title> <jats:p>Since water vapor exists everywhere around us and is crucial to life, the stable complexes that water molecules form with each other and with various environmental constituents have been studied extensively. Transient, metastable complexes are more elusive. A recently developed method, two-dimensional rotational spectroscopy, directly measures correlations between the rotational transitions in a conventional spectrum. Measurements of water vapor showed that rotations of one water molecule can change the rotational frequencies of another. Distinct spectral peaks provide direct experimental signatures of previously unseen complexes between the water molecules involved. The sensitivity of the method to intermolecular interactions has directly identified metastable cooperative behavior in one of the most extensively studied molecular species and promises new insights about many others.</jats:p>
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