Knowledge Bank

The rotational spectra of three isotopologues of H$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$S$\cdots$ICF$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$ and four isotopologues of H$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$O$\cdots$ICF$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$ have been measured between 7 and 18.5 GHz by chirped-pulse Fourier transform microwave spectroscopy. The rotational constant, {\it{B}}$\mathrm{<mrow\; data-mjx-texclass="ORD">0</mrow>}$, the centrifugal distortion constants, {\it{D}} and {\it{D}}, and the nuclear quadrupole coupling constant of $\mathrm{<mrow\; data-mjx-texclass="ORD">127</mrow>}$I, $\chi aa$(I), are precisely determined for H$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$S$\cdots$ICF$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$ and H$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$O$\cdots$ICF$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$ by fitting observed transitions to the Hamiltonians appropriate to {\it{symmetric}} tops. The measured rotational constants allow determination of the molecular geometries. The {\it{C}}$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ axis of H$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$O / H$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$S intersects the {\it{C}}$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$ axis of the CF$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$I sub-unit at the oxygen atom. The {\it{r}}$\mathrm{<mrow\; data-mjx-texclass="ORD">0</mrow>}$ lengths of halogen bonds identified between iodine and sulphur, {\it{r}}{(S$\cdots$I)}, and iodine and oxygen, {\it{r}}(O$\cdots$I), are determined to be 3.5589(2) {\r{A}} and 3.0517(18) {\r{A}} respectively. The angle, $\varphi$, between the local {\it{C}}$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ axis of the H$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$S / H$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$O sub-unit and the {\it{C}}$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$ axis of CF$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$I is found to be 93.7(2)$^irc\}$ in H$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$S$\cdots$ICF$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$ and 34.4(20)$^irc\}$ in H$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$O$\cdots$ICF$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$. The observed symmetric top spectra imply nearly free internal precession of the {\it{C}}$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ axis of the hydrogen sulphide/water unit about the {\it{C}}$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$ axis of CF$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$I in each of these complexes. Additional transitions of H$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$$\mathrm{<mrow\; data-mjx-texclass="ORD">16</mrow>}$O$\cdots$ICF$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$, D$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$$\mathrm{<mrow\; data-mjx-texclass="ORD">16</mrow>}$O$\cdots$ICF$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$ and H$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$$\mathrm{<mrow\; data-mjx-texclass="ORD">18</mrow>}$O$\cdots$ICF$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$ can only be assigned using Hamiltonians appropriate to asymmetric tops, suggesting that the effective rigid-rotor fits employed do not completely represent the internal dynamics of H$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$O$\cdots$ICF$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$.