15min:

N. R. WALKER, S. L. STEPHENS AND A. C. LEGON, School of Chemistry, University of Bristol, Bristol, BS8 1TS, U.K..

The rotational spectra of three isotopologues of H₂S ICF₃ and four isotopologues of H₂O ICF₃ have been measured between 7 and 18.5 GHz by chirped-pulse Fourier transform microwave spectroscopy. The rotational constant, B ₀, the centrifugal distortion constants, D _ J and D _ JK , and the nuclear quadrupole coupling constant of ¹²⁷I, _aa(I), are precisely determined for H₂S ICF₃ and H₂O ICF₃ by fitting observed transitions to the Hamiltonians appropriate to symmetric tops. The measured rotational constants allow determination of the molecular geometries. The C ₂ axis of H₂O / H₂S intersects the C ₃ axis of the CF₃I sub-unit at the oxygen atom. The r ₀ lengths of halogen bonds identified between iodine and sulphur, r (S I), and iodine and oxygen, r (O I), are determined to be 3.5589(2) \rA and 3.0517(18) \rA respectively. The angle, , between the local C ₂ axis of the H₂S / H₂O sub-unit and the C ₃ axis of CF₃I is found to be 93.7(2)^\circ in H₂S ICF₃ and 34.4(20)^\circ in H₂O ICF₃. The observed symmetric top spectra imply nearly free internal precession of the C ₂ axis of the hydrogen sulphide/water unit about the C ₃ axis of CF₃I in each of these complexes. Additional transitions of H₂¹⁶O ICF₃, D₂¹⁶O ICF₃ and H₂¹⁸O ICF₃ 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 H₂O ICF₃.