A Microwave Study of the Ammonia - Nitric Acid Complex

Microwave spectra of H₃¹⁴N-H¹⁴NO₃, H₃¹⁵N-H¹⁴NO₃, H₃¹⁴N-H¹⁵NO₃, and H₃¹⁴N-D¹⁴NO₃ 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 HNO₃ proton and the lone pair on the ammonia. The hydrogen bond length obtained from the rotational constants is 1.736(63) Å and the nitrogen-nitrogen separation is 3.344(12) Å. The NO single bond of the nitric acid and the C₃ axis of the NH₃ form angles of 54(4)° and 21(3)°, 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 H₃N-HCI and H₃N-HBr, indicating a particularly strong interaction between ammonia and nitric acid.