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Two isomers of a complex formed between thiazole and two water molecules, thi···(H2O)2, have been identified through Fourier transform microwave spectroscopy between 7.0 and 18.5 GHz. The complex was generated by co-expansion of a gas sample containing trace amounts of thiazole and water in an inert buffer gas. For each isomer, rotational constants, A0, B0 and C0, centrifugal distortion constants, DJ, DJK, d1, d2, and nuclear quadrupole coupling constants, and , have been determined through fitting of a rotational Hamiltonian to the frequencies of observed transitions. The molecular geometry, energy, and components of the dipole moment of each isomer have been calculated using Density Functional Theory (DFT). The experimental results for four isotopologues of isomer I allow accurate determinations of the atomic coordinates of the oxygen atoms by r0 and rs methods. Isomer II has been assigned as the carrier of an observed spectrum on the basis of very good agreement between DFT-calculated results and spectroscopic parameters (including A0, B0 and C0 rotational constants) determined by fitting to measured transition frequencies. Non-covalent interaction (NCI) and natural bond orbital (NBO) analyses reveal that two strong hydrogen bonding interactions are present within each of the identified isomers of thi···(H2O)2. The first of these binds an H2O to the nitrogen of thiazole (OH···N) and the second binds the two water molecules (OH···O). A third, weaker interaction binds the H2O sub-unit to the hydrogen atom that is attached to C2 (for isomer I) or C4 (for isomer II) of the thiazole ring (CH···O).

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.

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The binary intermolecular complexes of amides and formaldehyde can be taken as suitable models to investigate the non-covalent interactions of the peptide with carbonyl group. We herein studied the rotational...