Noncovalent Interactions between Aromatic Heterocycles and Carboxylic Acids: Rotational Spectroscopy of the Furan-Formic Acid and Thiophene-Formic Acid Complexes

The binary molecular complexes formed between the aromatic heterocycles furan and thiophene with formic acid were investigated using pulsed-jet Fourier transform microwave spectroscopy and quantum chemical computations. For both of the complexes, rotational spectra of the lowest energy isomer were detected and assigned. Rotational spectroscopic results and density functional theory calculations support that the preferred conformation of the furan-formic acid complex is characterized by a relatively strong O-H center dot center dot center dot O and a weak C-H center dot center dot center dot O hydrogen bonds while the O-H center dot center dot center dot pi and C-H center dot center dot center dot O hydrogen bonds stabilize the thiophene-formic acid complex. Natural bond orbital analysis further proves the experimental observation, suggesting that the strength of the O-H center dot center dot center dot O(furan) interaction is about two times stronger than that of O-H center dot center dot center dot pi(thiophene). The symmetry adapted perturbation theory analysis reveals that electrostatic interaction is dominant in stabilizing the two complexes and that dispersion becomes significant in the thiophene-formic acid complex compared to furan-formic acid.