Electrophilicity of Oxalic Acid Monomer Is Enhanced in the Dimer by Intermolecular Proton Transfer

Excess electron induces proton transfer in the dimer of oxalic acid and leads to formation of very stable anions.</p

Dipole-bound anions: formed by Rydberg electron transfer (RET) and studied by velocity map imaging–anion photoelectron spectroscopy (VMI–aPES)

Here, we demonstrate the capabilities of the unique combination of Rydberg electron transfer (RET) and velocity map imaging–anion photoelectron spectroscopy (VMI–aPES) to form dipole bound anions and to measure their photoelectron spectra. For these purposes, we have chosen the dipole bound anions of acetonitrile, ammonia–water dimer, water dimer, dimethyl sulfoxide and thymine as examples. All of these had been previously formed and/or studied but by other methodologies

Enormous Hydrogen Bond Strength Enhancement through π‑Conjugation Gain: Implications for Enzyme Catalysis

Surprisingly large resonance-assistance effects may explain how some enzymes form extremely short, strong hydrogen bonds to stabilize reactive oxyanion intermediates and facilitate catalysis. Computational models for several enzymic residue–substrate interactions reveal that when a π-conjugated, hydrogen bond donor (XH) forms a hydrogen bond to a charged substrate (Y^–), XH can become significantly more π-electron delocalized, and this “extra” stabilization may boost the [XH···Y^–] hydrogen bond strength by ≥15 kcal/mol. This reciprocal relationship departs from the widespread p<i>K</i>_a concept (i.e., the idea that short, strong hydrogen bonds form when the interacting moieties have matching p<i>K</i>_a values), which has been the rationale for enzymic acid–base reactions. The findings presented here provide new insight into how short, strong hydrogen bonds could form in enzymes