Knowledge Bank

The microwave spectrum of $HNO3-H2O$ and six of its isotopic derivatives have been investigated by pulsed-nozzle Fourier transform microwave spectroscopy. The complex has a cyclic, doubly hydrogen bonden structure with a coplanar arrangement of the heavy atoms. One of the water protons lies above the plane. A near-linear hydrogen bond, 1.76 \AA in length, is formed between the proton of the $HNO3$ and the oxygen of the $H2O$. A longer (2.76 \AA) hydrogen bond between one of the water protons and an oxygen of the $HNO3$ completes the ring. The dimer was produced by continuously evaporating liquid water into a pulsed expansion of $HNO3$ in Ar, and evidence is presented for extensive H/D isotopic scrambling during experiments using $H2O+DNO3$ or $D2O+HNO3$. The results are compared with recent ab initio calculations.