Rotational spectrum of thiophene⋯ClF and the role of thiophene as a π- or n-electron pair donor in weakly bound complexes

Rotational constants, centrifugal distortion constants and all five independent components χaa, χbb−χcc, χab, χac and χbc of the Cl nuclear quadrupole coupling tensor χαβ have been determined for thiophene⋯35ClF and thiophene⋯37ClF. A method for determining the geometry of this complex of C1 symmetry is presented which uses the direction cosines θaz, θbz and θcz (z is the ClF axis), determined by diagonalisation of the χαβ tensor, in combination with principal moments of inertia. Four geometries of thiophene⋯ClF are consistent with the observed quantities, but each has ClF interacting with the π-electron system on one face of the thiophene ring.