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···O and a weak C-H···O hydrogen bonds while the O-H···π and C-H···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···O(furan) interaction is about two times stronger than that of O-H···π(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.