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

The rotational spectra of CF3I center dot center dot center dot NH3 and CF3I center dot center dot center dot 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, B-0, centrifugal distortion constants, D-J, D-Jm, D-JKm, nuclear quadrupole coupling constants of the N-14 and I-127 atoms, chi(N)(aa) and chi(I)(aa), are determined for each complex. D-JK is additionally determined for CF3I center dot center dot center dot NH3. Results are presented for both N-14 and 15 N-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 center dot center dot center dot(NH3)-N-14 and CF3I center dot center dot center dot N-14(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 center dot center dot center dot I). Values of r(N center dot center dot center dot I) are thus determined to lie in the ranges 3.054 angstrom > r(N center dot center dot center dot I) > 3.034 angstrom and 2.790 angstrom > (rN center dot center dot center dot I) > 2.769 angstrom for CF3I center dot center dot center dot NH3 and CF3I center dot center dot center dot N(CH3)(3) respectively.