Precise Equilibrium Structure Determination Of Hydrazoic Acid (Hn3) By Millimeter-Wave Spectroscopy

The millimeter-wave spectrum of hydrazoic acid (HN3) was analyzed in the frequency region of 235-450 GHz. Transitions from a total of 14 isotopologues were observed and fit using the A-reduced or S-reduced Hamiltonian. Coupled-cluster calculations were performed to obtain a theoretical geometry, as well as rotation-vibration interaction corrections. These calculated vibration-rotation correction terms were applied to the experimental rotational constants to obtain mixed theoretical/experimental equilibrium rotational constants (A(e), B-e, and C-e). These equilibrium rotational constants were then used to obtain an equilibrium (R-e) structure using a least-squares fitting routine. The R-e structural parameters are consistent with a previously published R-s structure, largely falling within the uncertainty limits of that R-s structure. The present R-e geometric parameters of HN3 are determined with exceptionally high accuracy, as a consequence of the large number of isotopologues measured experimentally and the sophisticated (coupled-cluster theoretical treatment (CCSD(T))/ANO(2)) of the vibration-rotation interactions. The R-e structure exhibits remarkable agreement with the CCSD(T)/cc-pCV5Z predicted structure, validating both the accuracy of the ab initio method and the claimed uncertainties of the theoretical/experimental structure determination. (C) 2015 AIP Publishing LLC.