Is the gas-phase complex of ammonia and chlorine monofluoride H3N…ClF or [H3NCl]+…F−? Evidence from rotational spectroscopy

Ground-state rotational spectra of the symmetric-topic isotopomers H314N…35ClF, H314N…37ClF, H315N…35ClF and H315N…37ClF of the ammonia-chlorine monoffluoride complex were observed with a fast-mixing nozzle/F-T microwave spectrometer combination. The determied spectroscopic constants B0, DJ, DJK, χaa(Cl), Maa(Cl) and Tr M(Cl) allow the conclusion that the complex has a C3v geometry with nuclei in the order H3N…ClF and a relatively strong intermolecular bond (kσ = 34.3(6) N m−1). Comparison of both χaa(Cl) and kσ for the series B…Cl2 and B…ClF (B = CO, HCN and NH3) suggests a small contribution of the ionic structure [H3NCl]+…F− in the valence-bond description of H3N…ClF.