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 (H3N)-N-14...(ClF)-Cl-35, (H3N)-N-14...(ClF)-Cl-37, (H3N)-N-15...(ClF)-Cl-35 and (H3N)-N-15...(ClF)-Cl-37 of the ammonia-chlorine monofluoride complex were observed with a fast-mixing nozzle/F-T microwave spectrometer combination. The determined spectroscopic constants B-O, D-J, D-JK, chi(aa)(Cl), M(aa)(Cl) and Tr M(Cl) allow the conclusion that the complex has a C-3v geometry with nuclei in the order H3N...ClF and a relatively strong intermolecular bond (k(sigma)= 34.3(6) N m(-1)). Comparison of both chi(aa)(Cl) and k(sigma) for the series B...Cl-2 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.