Halogen bond and internal dynamics in the σ-complex of pyridine-chlorotrifluoromethane: A rotational study

The rotational spectrum of the 1:1 complex pyridine-chlorotrifluoromethane shows that the two moieties are held together through a C-Cl center dot center dot center dot N halogen bond (R-Cl center dot center dot center dot N = 2.909(5) angstrom). The quadrupolar effects of the Cl-35 (or Cl-37) and N-14 nuclei combined with the high density of the rotational transitions give rise to a very congested spectrum. The multidimensional large amplitude internal dynamics is reflected in a two times larger rotational constant A, on an unexpected positive values of the inertial defect of the ground state, and on a negative value of the D-J centrifugal distortion constant. This comes from a combination of a free internal rotation and of a very low energy in plane oscillation of CF3Cl with respect to pyridine. However, a coupled Hamiltonian including Coriolis interactions with an hypothetical (unobserved) v = 1 state solves the initial result of an unphysical negative value for the centrifugal distortion constant. (C) 2020 Elsevier Inc. All rights reserved.