The
rotational fingerprint of the thiophene–water complex
was investigated for the first time using Fourier transform microwave
spectroscopy (7–20 GHz) aided by quantum mechanical calculations.
Transitions for a single species were observed, and the rotational
constants for the parent and 18O isotopomers are consistent
with a geometry that is highly averaged over a barrierless large-amplitude
motion of water that interconverts two equivalent forms corresponding
to the global minimum (B2PLYP-D3(BJ)/def2-TZVP). In this effective
geometry, the water lies above the thiophene ring close to its σv plane of symmetry. The observed transitions are split by
a second water-centered tunneling motion that exchanges its two protons
by internal rotation about its C2 axis with a calculated
barrier of ∼2.7 kJ mol–1 (B2PLYP-D3(BJ)/def2-TZVP).
Based on quantum theory of atoms in molecules, noncovalent interaction,
and symmetry-adapted perturbation theory analyses, the observed geometry
enables two intermolecular interactions (O–H···π
and O–H···S) whose electrostatic and dispersive
contributions favor formation of the thiophene–water complex.