Internal rotation and halogen bonds in CF3I⋯NH3 and CF3I⋯N(CH3)3 probed by broadband rotational spectroscopy

Abstract

The rotational spectra of CF₃I⋯NH₃ and CF₃I⋯N(CH₃)₃ are measured between 6.7 and 18 GHz using a chirped-pulse Fourier transform microwave spectrometer. Transitions in each spectrum are assigned to A and E species associated with ground and excited internal rotor states respectively. Rotational constants, B₀, centrifugal distortion constants, D_J, D_Jm, D_JKm, nuclear quadrupole coupling constants of the ¹⁴N and ¹²⁷I atoms, χ^N_aa and χ^I_aa, are determined for each complex. D_JK is additionally determined for CF₃I⋯NH₃. Results are presented for both ¹⁴N and ¹⁵N-substituted isotopologues. All data are consistent with C_3v symmetric top structures for both complexes. The nuclear quadrupole coupling constants of iodine are determined to be −2230.030(83) MHz and −2241.61(17) MHz in CF₃I⋯¹⁴NH₃ and CF₃I⋯¹⁴N(CH₃)₃ respectively. The data are interpreted through a model that accounts for the internal dynamics of the complexes in order to determine the length of the halogen bond between the iodine and nitrogen atoms, r_N⋯I. Values of r_N⋯I are thus determined to lie in the ranges 3.054 Å > r_N⋯I > 3.034 Å and 2.790 Å > r_N⋯I > 2.769 Å for CF₃I⋯NH₃ and CF₃I⋯N(CH₃)₃ respectively.