Characterization of Large-Amplitude Motions and Hydrogen Bonding Interactions in the Thiophene–Water Complex by Rotational Spectroscopy

Abstract

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, non covalent 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 favour formation of the thiophene-water complex.