MICROWAVE SPECTRA, MOLECULAR STRUCTURES AND INTERNAL DYNAMICS OF H$_{2}$S$\cdots$ICF$_{3}$ and H$_{2}$O$\cdots$ICF$_{3}$ REVEALED BY BROADBAND ROTATIONAL SPECTROSCOPY

Abstract

The rotational spectra of three isotopologues of H$_{2}$S$\cdots$ICF$_{3}$ and four isotopologues of H$_{2}$O$\cdots$ICF$_{3}$ have been measured between 7 and 18.5 GHz by chirped-pulse Fourier transform microwave spectroscopy. The rotational constant, {\it{B}}$_{0}$, the centrifugal distortion constants, {\it{D}}$_{\it{J}}$ and {\it{D}}$_{\it{JK}}$, and the nuclear quadrupole coupling constant of $^{127}$I, ${\chi}_{aa}$(I), are precisely determined for H$_{2}$S$\cdots$ICF$_{3}$ and H$_{2}$O$\cdots$ICF$_{3}$ 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}}$_{2}$ axis of H$_{2}$O / H$_{2}$S intersects the {\it{C}}$_{3}$ axis of the CF$_{3}$I sub-unit at the oxygen atom. The {\it{r}}$_{0}$ 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, $\phi$, between the local {\it{C}}$_{2}$ axis of the H$_{2}$S / H$_{2}$O sub-unit and the {\it{C}}$_{3}$ axis of CF$_{3}$I is found to be 93.7(2)$^irc}$ in H$_{2}$S$\cdots$ICF$_{3}$ and 34.4(20)$^irc}$ in H$_{2}$O$\cdots$ICF$_{3}$. The observed symmetric top spectra imply nearly free internal precession of the {\it{C}}$_{2}$ axis of the hydrogen sulphide/water unit about the {\it{C}}$_{3}$ axis of CF$_{3}$I in each of these complexes. Additional transitions of H$_{2}$$^{16}$O$\cdots$ICF$_{3}$, D$_{2}$$^{16}$O$\cdots$ICF$_{3}$ and H$_{2}$$^{18}$O$\cdots$ICF$_{3}$ 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$_{2}$O$\cdots$ICF$_{3}$.