Knowledge Bank

We report a microwave spectroscopic study of the $NaCl-(H2O)n(n=1,2,3)$ complexes. In the frequency region from 5 to 25 GHz, rotational spectra of $NaCl-(H2O)n(n=1,2,3)$ have been observed for the first time by using a Fourier-transform microwave spectrometer coupled to a laser ablation source. Molecular constants including the nuclear quadrupole coupling constants of Na and Cl nuclei were precisely determined for each complex. We observed various isotopic species of each complex to determine the molecular structures. $NaCl-H2O$ has a nearly planar and cyclic (Na-Cl-H-O) structure, $NaCl-(H2O)2$ has a $C2$ structure with two equivalent $H2O$, and $NaCl-(H2O)3$ is a symmetric top molecule with three equivalent $H2O$. The determined structures clearly show that the bond length $r(NaCl)$ increases monotonically with the number of $H2O$. The determined nuclear quadrupole coupling constants of the Na and Cl nuclei show considerable changes of charge distribution around these atoms.