Single-Water Microsolvation Promotes Proton Sharing in a Neutral Formic Acid-Ammonia Complex

Can single-water microsolvation promote proton sharing in a neutral acid-base complex? We address this question for formic acid-ammonia by isolating its monohydrated complex with broadband rotational spectroscopy and interpreting the resolved 14N hyperfine structure with calibrated high-level quantum chemistry. The observed complex is a single dominant HCOOH-NH3-H2O isomer with effective Cs symmetry, in which NH3 remains the primary proton acceptor, while water closes a cooperative hydrogen-bond cycle by accepting from NH3 and donating to the carbonyl oxygen. Rather than simply stabilizing the binary acid-base contact, this water molecule measurably increases its proton-sharing character. A resolved A/E tunneling pattern from NH3 internal rotation supplies the missing orientation needed to rotate the experimental 14N quadrupole tensor into the local ammonia frame, yielding an experimental χzz component and a quadrupole-derived Townes-Dailey ionicity of approximately 26%. High-level structures show the same directional response: O-H elongation, H···N contraction, and O···N contraction relative to HCOOH-NH3. Together, these results show that one water molecule can promote proton sharing in a neutral formic acid-ammonia complex without producing a separated ion pair.