Why oil and water do not mix: It is well known that oil forms stable droplets that carry a negative electrophoretic mobility (and negative charge) upon dispersing in water. However, the underlying mechanism is a long-debated topic. Using vibrational sum-frequency scattering spectroscopy, Pullanchery et al . recorded the interfacial vibrational spectrum in the oxygen–deuterium and carbon–hydrogen stretching regions of a hexadecane–water interface. Their spectral analysis accompanied by molecular dynamics simulations showed that water molecules form “improper” interfacial hydrogen bonds with alkyl hydrogens, resulting in the water-to-oil charge transfer that stabilizes oil droplets. This work demonstrates that sum-frequency scattering spectroscopy is a powerful technique that can improve our understanding of hydrophobicity in water-mediated chemical and biological systems. —YS

The molecular structure of water is dynamic, with intermolecular hydrogen (H) bond interactions being modified by both electronic charge transfer and nuclear quantum effects (NQEs). Electronic charge transfer and NQEs potentially change under acidic or basic conditions, but such details have not been measured. In this work, we developed correlated vibrational spectroscopy, a symmetry-based method that separates interacting from noninteracting molecules in self- and cross-correlation spectra, giving access to previously inaccessible information. We found that hydroxide (OH ⁻ ) donated ~8% more negative charge to the H bond network of water, and hydronium (H ₃ O ⁺ ) accepted ~4% less negative charge from the H bond network of water. Deuterium oxide (D ₂ O) had ~9% more H bonds compared with water (H ₂ O), and acidic solutions displayed more dominant NQEs than basic ones.

The scientific community is allocating more and more resources to space missions and astronomical observations dedicated to the search for life beyond Earth. This experimental endeavor needs to be backed by a theoretical framework aimed at defining universal criteria for the existence of life. With this aim in mind, we have explored which chemical and physical properties should be expected for life possibly different from the terrestrial one, but similarly sustained by genetic and catalytic molecules. We show that functional molecules performing genetic and catalytic tasks must feature a hierarchy of chemical interactions operating in distinct energy bands. Of all known chemical bonds and forces, only hydrogen bonds are able to mediate the directional interactions of lower energy that are needed for the operation of genetic and catalytic tasks. For this reason and because of the unique quantum properties of hydrogen bonding, the functional molecules involved in life processes are predicted to have extensive hydrogen-bonding capabilities. A molecular medium generating a hydrogen-bond network is probably essential to support the activity of the functional molecules. These hydrogen-bond requirements constrain the viability of hypothetical biochemistries alternative to the terrestrial one, provide thermal limits to life molecular processes, and offer a conceptual framework to define a transition from a “covalent-bond stage” to a “hydrogen-bond stage” in prebiotic chemistry.