Structure and Spectroscopy of Furan:H₂O Complexes

Abstract

An analysis of the 1:1 complex of furan and water is presented. In this study, computation and matrix isolation FTIR were used to determine stable complexes of furan:water. Density functional theory and Møller-Plesset second-order, perturbation theory calculations found four, unique geometries for the complex. Two complexes were characterized by C-H···O interactions, one complex was characterized by O-H···O, and the fourth complex was characterized by O-H···π. Optimizations completed using B3LYP, B3LYP-GD3BJ, M05-2X, and MP2 showed the most stable species to be bound by O-H···O interactions. Matrix isolation experiments of mixtures of furan and water held in nitrogen at 15 K showed evidence of stable complexes when probed by FTIR. These signatures grew in intensity when matrices were annealed at 30 K. These vibrational features were predominately associated with perturbation of the water monomer. Additionally, the spectra of complexes containing water isotopologues were recorded. Analysis of spectral features pointed to the presence of a single geometry formed in the matrix, which is best described as a 1:1 complex stabilized by a O-H···O interaction.

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

“…Based on the quantum mechanical calculations from Table , conformer I is predicted to be the global minimum geometry for the complex and appears to be stabilized by O–H···π interactions in which one hydrogen of water binds to the π-orbital of a CC bond, while the other hydrogen atom is directed above the middle of the ring (Figure ). This is contrary to the most stable conformer reported for the monohydrates of furan (O···H–O), pyrrole (N–H···O), thiazole (N···H–O), and pyridine (N···H–O) where the center of mass of water lies in the plane of the ring such that water binds to the most electronegative heteroatom or with the hydrogen attached to it in the case of pyrrole. The structure reported to be the most abundant based on the Ar matrix IR spectrum, forming a C–H···O HB (conformer II in this work), is predicted to be at least 2.4 kJ mol –1 (Table ) less stable than conformer I at the B2PLYP-D3­(BJ)/def2-TZVP level.…”

“…Although the water complexes of the oxygen and sulfur counterparts of pyrrole, namely, furan and thiophene, have not been investigated by rotational spectroscopy, their vibrational spectra were reported by Ar matrix FTIR (Fourier transform infrared) spectroscopy. 10 While for furan−w, the IR spectrum was consistent with a single geometry with the oxygen atom of water in the plane of the ring and was stabilized by an O−H•••O HB, in agreement with computational results, 7 for thiophene−w, the dominant species in the matrix is governed by a weak C−H•••O interaction with all heavy atoms in one plane. This was rather unexpected as the lowest energy conformer is predicted to have the water molecule above the plane of the thiophene ring and to be stabilized by an O−H•••π interaction.…”

“…Overall, from the SAPT results in Table , the nature of the interactions binding thiophene to water are more similar to those of benzene–w than to those of heterocycles, such as furan and pyrrole. The preference of water to interact primarily with the π-system in thiophene rather than to the heteroatom as in furan, pyrrole, , and thiazole is explained by key differences in the electron density distribution in the heterocycles. While in furan and pyrrole, the negative charges are concentrated around O and N due to their high electronegativity, in thiophene, the negative charge from the heteroatom is more dispersed into the ring, making the π-systems the preferred binding site for the water molecule. , …”

“…6 The scenario is more complicated for aromatic heterocycles as additional contacts may be established with the heteroatom. [3][4][5]7,8 The presence of multiple potential binding sites with interaction energies that are often sensitive to the computational levels chosen affirms the importance of experimental studies of isolated solvent− solute complexes.…”

“…Noncovalent interactions involving aromatic rings play a key role in many chemical and biological processes, such as molecular recognition, aggregation, and protein folding. , Despite their importance, accurate experimental and computational descriptions of the molecular-level forces involved in solvation of an aromatic molecule remain challenging even for small prototypical systems, such as 1:1 solvent–solute molecular aggregates. − When a solvent molecule approaches an aromatic ring, interactions may be formed with the π-electron clouds of the ring, either at its center or on a CC bond, or with the hydrogens in which the solvent may serve as a proton donor and/or acceptor . The scenario is more complicated for aromatic heterocycles as additional contacts may be established with the heteroatom. − ,, The presence of multiple potential binding sites with interaction energies that are often sensitive to the computational levels chosen affirms the importance of experimental studies of isolated solvent–solute complexes.…”