A microwave study of the ammonia-nitric acid complex

Microwave spectra of H{sub 3}{sup 14}N-H{sup 14}NO{sub 3}, H{sub 3}{sup 15}N-H{sup 14}NO{sub 3}, H{sub 3}{sup 14}N-H{sup 15}NO{sub 3}, and H{sub 3}{sup 14}N-D{sup 14}NO{sub 3} have been obtained by Fourier transform spectroscopy in a supersonic jet. The spectra are consistent with a hydrogen-bonded structure having a planar arrangement of the heavy atom frame and a linear or near-linear hydrogen bond involving the HNO{sub 3} proton and the lone pair on the ammonia. The hydrogen bond length obtained from the rotational constants is 1.736(63) {angstrom} and the nitrogen-nitrogen separation is 3.344(12) {angstrom}. The NO single bond of the nitric acid and the C{sub 3} axis of the NH{sub 3} form angles of 54(4){degree} and 21(3){degree}, respectively, with the line joining the centers of mass of the monomers. The hydrogen bond length is significantly shorter than that observed in related complexes such as H{sub 3}N-HCl and H{sub 3}N-HBr, indicating a particularly strong interaction between ammonia and nitric acid.