Nature, geometry, and binding strength of the ammonia-hydrogen chloride dimer determined from the rotational spectrum of ammonium chloride vapor

The rotational spectra of the seven isotopic species (14NH3,H35Cl), (14NH3,H37Cl), (15NH3,H35Cl), (15NH3,H37Cl), (14NH3,D35Cl), (14NH2D,H35Cl), and (14NH2D,D35Cl) of a dimer formed by ammonia and hydrogen chloride have been observed in the vapor above heated ammonium chloride by using pulsed-nozzle, Fourier-transform microwave spectroscopy. The ground-state spectroscopic constants determined for the symmetric-top species (14NH3,H35Cl) are B0=4243.2593(16)MHz, DJ=12.8(2) kHz, DJK=371.5(8) kHz, χ(14N)=−3.248(14) MHz, and χ(35Cl)=−47.607(9) MHz. Analysis of the rotational constants for the isotopomers investigated establishes that the observed dimer has C3v symmetry with the nuclei in the order H3NHCl and with r(N⋅⋅⋅Cl)=3.136 Å. The contribution χp to the change in the 35Cl–nuclear quadrupole coupling constant of HCl caused by the electric field due to NH3 in its equilibrium position has been determined to be 10.8 MHz from the χ(35Cl) values of (14NH3,H35Cl) and (14NH3,D35Cl). Simultaneously, the HCl subunit oscillation amplitude is established to be θav =cos−1〈cos2 θ〉=13.4(1)°. A simple hydrogen-bonded model H3N⋅⋅⋅HCl of the dimer, in which only polarization of the HCl by the NH3 subunit and a small extension of the HCl bond are invoked, adequately accounts for the value of χ(35Cl). No substantial proton transfer from HCl to NH3 need be considered. The value of the hydrogen-bond stretching force constant kσ=17.6(3) Nm−1 determined from DJ is also shown to be consistent with this conclusion.