## Abstract

The ground state rotational spectrum of a hydrogen-bonded heterodimer formed from water and hydrogen cyanide has been detected and measured by using the technique of pulsed-nozzle, Fourier-transform microwave spectroscopy. Rotational constants (B$_0$, C$_0$) centrifugal distortion constants ($\Delta_J$, $\Delta_{JK}$) and, where appropriate, $^{14}$N-, D- or $^{17}$O-nuclear quadrupole coupling constants have been determined for the following isotopic species; H$_2$ $^{16}$O$\cdots$HC $^{14}$N, H$_2$ $^{18}$O$\cdots$HC $^{14}$N, H$_2$ $^{16}$O$\cdots$HC $^{15}$N, HD $^{16}$O$\cdots$HC $^{15}$N, D$_2$ $^{16}$O$\cdots$HC $^{15}$N, H$_2$ $^{16}$O$\cdots$DC $^{15}$N, HD $^{16}$O$\cdots$DC $^{15}$N and H$_2$ $^{17}$O$\cdots$HC $^{15}$N. An analysis of these spectroscopic constants indicates that the heterodimer is effectively planar, with a pair of equivalent protons and the arrangement H$_2$O$\cdots$HCN. The intermolecular interaction is through a hydrogen bond between HCN and H$_2$O and the distance between the O and C nuclei r(O...C) is 3.157 $\overset\circ{\mathrm A}$ (1 $\overset\circ{\mathrm A} = 10^{-10}$ m). An interpretation of the nuclear quadrupole coupling constants leads to the conclusion that arccos $\langle \cos^2 \phi\rangle^\frac{1}{2} \approx 51^\circ$, where $phi$ is the angle between the local C$_2$ axis of H$_2$O and the $a$-axis of the complex; and that arccos $\langle \cos^2 \theta\rangle^{\frac{1}{2}} \approx$ 10$^\circ$, where $\theta$ is the angle between the HCN axis and the a-axis. The intermolecular stretching force constant $k_\sigma$ = 11 Nm$^{-1}$ has been determined from $\Delta_J$.

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