MICROWAVE SPECTRUM AND STUCTURE OF $HNO_{3}-H_{2}O$

Abstract

The microwave spectrum of $HNO_{3}-H_{2}O$ 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 $HNO_{3}$ and the oxygen of the $H_{2}O$. A longer (2.76 \AA) hydrogen bond between one of the water protons and an oxygen of the $HNO_{3}$ completes the ring. The dimer was produced by continuously evaporating liquid water into a pulsed expansion of $HNO_{3}$ in Ar, and evidence is presented for extensive H/D isotopic scrambling during experiments using $H_{2}O + DNO_{3}$ or $D_{2}O + HNO_{3}$. The results are compared with recent ab initio calculations.