Computational study of formamide–water complexes using the SAPT and AIM methods

Effects of methylation in acceptors on the hydrogen bond complexes between 2,2,2-trifluoroethanol and cyclic ethers

In order to explore the effect of methylation on the stability and spectral shift of hydrogen bond complexes, the complexes of 2,2,2-trifluoroethanol (TFE) with propylene oxide (PO) and isobutylene oxide (IBO) were investigated by Fourier transform infrared (FTIR) spectroscopy and ab initio computations. The comparable OH-stretching red shifts were observed upon complexation, and an enhancement of the OH-stretching band is shown with the partial pressure of monomers increasing. The OH-stretching frequency of TFE is red shifted by 180 and 201 cm^-1 with PO and IBO, respectively. By using quantum chemical calculations, we predicted the geometric parameters, binding energies, and spectral shifts of TFEPO/IBO hydrogen bond complexes. The calculated and observed spectral shifts follow the same trends. Compared with the TFEethylene oxide (EO) complex, the strength of the hydrogen bond in complex increases with the addition of methyl group, which likely results from the increase in basicity of the hydrogen bond acceptor. By combining the experimental integrated absorbance and the calculated IR intensity of the OH-stretching vibrational transition, the equilibrium constant for the complex formation was determined. In addition, atoms-in-molecules (AIM) and natural bond orbital (NBO) analyses were carried out to explain the red shift and the nature of the interaction in these complexes.

Ring-Size Effects on the Stability and Spectral Shifts of Hydrogen Bonded Cyclic Ethers Complexes

In order to investigate ring-size effects on the stability and spectral shifts of hydrogen bonded cyclic ethers complexes, the strength of hydrogen bonds in gas phase complexes formed between 2,2,2-trifluoroethanol (TFE) and selected cyclic ethers were examined using FTIR spectroscopy. TFE was chosen as hydrogen bond donor in these complexes, while trimethylene oxide (TMO), tetrahydrofuran (THF) and tetrahydropyran (THP) were selected as hydrogen bond acceptors. Comparable OH-stretching red shifts were observed in the three kinds of complexes. The difference of red shifts is so small (<7 cm⁻¹) for TFE−TMO/THF/THP complexes that one can conclude that their stabilities and the strength of the hydrogen bonds are nearly similar and do not show any marked dependence with the ring size of the hydrogen bond acceptor. The equilibrium constants for the complexation were determined, and atoms-in-molecules (AIM) and natural bond orbital (NBO) analyses were performed to further investigate the intermolecular interactions. Regardless of the ring size, hydrogen bonds in the complexes showed similar strength, in agreement with the observed OH-stretching red shifts.

The Influence of the Position of the Double Bond and Ring Size on the Stability of Hydrogen Bonded Complexes

To study the influence of the position of the double bond and ring size on the stability of hydrogen bonded complexes, the 1:1 complexes formed between 2,2,2-trifluoroethanol (TFE) and three heterocyclic compounds including 2,3-dihydrofuran (2,3-DHF), 2,5-dihydrofuran (2,5-DHF) and 3,4-dihydropyran (3,4-DHP) were investigated systematically. The formation of hydrogen bonded TFE−2,3-DHF, TFE−2,5-DHF and TFE−3,4-DHP complexes were identified by gas phase FTIR spectroscopy at room temperature, and the OH-stretching fundamental transition of TFE was red shifted upon complexation. The competition between the O atom and π-electrons bonding sites within the complexes was studied, and the O−H···π type hydrogen bond was found to be less stable than the O−H···O in all three cases. The observed red shifts of the OH-stretching fundamental transitions in the complexes were attributed to the formation of O−H···O hydrogen bond. Equilibrium constants of the complexation reactions were determined from measured and calculated OH-stretching fundamental intensities. Both theoretical calculations and experimental results reveal that the hydrogen bond strengths in the complexes follow the sequence: TFE−2,5-DHF > TFE−2,3-DHF ≈ TFE−3,4-DHP, thus the position of the double bond exerts significantly larger influence than ring size on the stability of the selected hydrogen bonded complexes.

Hydrogen bond docking site competition in methyl esters

The OH⋯O hydrogen bonds in the 2,2,2-trifluoroethanol (TFE)-methyl ester complexes in the gas phase have been investigated by FTIR spectroscopy and DFT calculations. Methyl formate (MF), methyl acetate (MA), and methyl trifluoroacetate (MTFA) were chosen as the hydrogen bond acceptors. A dominant inter-molecular hydrogen bond was formed between the OH group of TFE and different docking sites in the methyl esters (carbonyl oxygen or ester oxygen). The competition of the two docking sites decides the structure and spectral properties of the complexes. On the basis of the observed red shifts of the OH-stretching transition with respect to the TFE monomer, the order of the hydrogen bond strength can be sorted as TFE-MA (119cm^-1)>TFE-MF (93cm^-1)>TFE-MTFA (44cm^-1). Combining the experimental infrared spectra with the DFT calculations, the Gibbs free energies of formation were determined to be 1.5, 4.5 and 8.6kJmol^-1 for TFE-MA, TFE-MF and TFE-MTFA, respectively. The hydrogen bonding in the MTFA complex is much weaker than those of the TFE-MA and TFE-MF complexes due to the effect of the CF₃ substitution on MTFA, while the replacement of an H atom with a CH₃ group in methyl ester only slightly increases the hydrogen bond strength. Topological analysis and localized molecular orbital energy decomposition analysis was also applied to compare the interactions in the complexes.

Hydrogen bonding in alcohol–ethylene oxide and alcohol–ethylene sulfide complexes

The O–H⋯O and O–H⋯S hydrogen bonds are of similar strength in the corresponding alcohol–EO and alcohol–ES complexes.

Hydrogen bonding in cyclic complexes of carboxylic acid–sulfuric acid and their atmospheric implications

Carboxylic acids form cyclic ring structures with sulfuric acid and they could potentially be important in new particle formation.