Molecular geometries of H2S⋯ICF3 and H2O⋯ICF3 characterised by broadband rotational spectroscopy

Abstract

The rotational spectra of three isotopologues of H₂S⋯ICF₃ and four isotopologues of H₂O⋯ICF₃ are measured from 7–18 GHz by chirped-pulse Fourier transform microwave spectroscopy. The rotational constant, B₀, centrifugal distortion constants, D_J and D_JK, and nuclear quadrupole coupling constant of ¹²⁷I, χ_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 C₂ axis of H₂O/H₂S intersects the C₃ axis of the CF₃I sub-unit at the oxygen atom. The 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) Å and 3.0517(18) Å respectively. The angle, ϕ, between the local C₂ axis of the H₂S/H₂O sub-unit and the C₃ axis of CF₃I is found to be 93.7(2)° in H₂S⋯ICF₃ and 34.4(20)° in H₂O⋯ICF₃. The observed symmetric top spectra imply nearly free internal rotation of the C₂ axis of the hydrogen sulphide/water unit about the C₃ axis of CF₃I in each of these complexes. Additional transitions of H₂¹⁶O⋯ICF₃, D₂¹⁶O⋯ICF₃ and H₂¹⁸O⋯ICF₃ can be assigned only using asymmetric top Hamiltonians, suggesting that the effective rigid-rotor fits employed do not completely represent the internal dynamics of H₂O⋯ICF₃.