Structure and tunneling dynamics in a model system of peptide co-solvents: Rotational spectroscopy of the 2,2,2-trifluoroethanol⋯water complex

The first HyDRA challenge for computational vibrational spectroscopy

Vibrational spectroscopy in supersonic jet expansions is a powerful tool to assess molecular aggregates in close to ideal conditions for the benchmarking of quantum chemical approaches. The low temperatures achieved as well as the absence of environment effects allow for a direct comparison between computed and experimental spectra. This provides potential benchmarking data which can be revisited to hone different computational techniques, and it allows for the critical analysis of procedures under the setting of a blind challenge. In the latter case, the final result is unknown to modellers, providing an unbiased testing opportunity for quantum chemical models. In this work, we present the spectroscopic and computational results for the first HyDRA blind challenge. The latter deals with the prediction of water donor stretching vibrations in monohydrates of organic molecules. This edition features a test set of 10 systems. Experimental water donor OH vibrational wavenumbers for the vacuum-isolated monohydrates of formaldehyde, tetrahydrofuran, pyridine, tetrahydrothiophene, trifluoroethanol, methyl lactate, dimethylimidazolidinone, cyclooctanone, trifluoroacetophenone and 1-phenylcyclohexane-cis-1,2-diol are provided. The results of the challenge show promising predictive properties in both purely quantum mechanical approaches as well as regression and other machine learning strategies.

How Water Interacts with the NOH Group: The Rotational Spectrum of the 1:1 N,N-diethylhydroxylamine·Water Complex

The rotational spectrum of the 1:1 N,N-diethylhydroxylamine-water complex has been investigated using pulsed jet Fourier transform microwave spectroscopy in the 6.5-18.5 GHz frequency region. The most stable conformer has been detected as well as the 13C monosubstituted isotopologues in natural abundance and the 18O enriched water species, allowing to determine the nitrogen nuclear quadrupole coupling constants and the molecular structure in the vibrational ground state. The molecule has a Cs symmetry and the water lies in the bc symmetry plane forming two hydrogen bonds with the NOH frame with length: dHOH·NOH = 1.974 Å and dH2O·HON = 2.096 Å. From symmetry-adapted perturbation theory calculations coupled to atoms in molecule approach, the corresponding interaction energy values are estimated to be 24 and 13 kJ·mol-1, respectively. The great strength of the intermolecular interaction involving the nitrogen atom is in agreement with the high reactivity of hydroxylamine compounds at the nitrogen site.

Alternating 1-Phenyl-2,2,2-Trifluoroethanol Conformational Landscape With the Addition of One Water: Conformations and Large Amplitude Motions

The 1:1 adduct of 1-phenyl-2,2,2-trifluoroethanol (PhTFE), a chiral fluoroalcohol, with water was investigated using chirped pulse Fourier transform microwave spectroscopy and computational methods. While PhTFE itself was predicted to have three minima, I (gauche+), II (trans), and III (gauche-), only I and II were stable and only I was observed experimentally. A systematic search of the PhTFE···H₂O conformational landscape identified 110 stable minima, 14 of which are within a 15 kJ mol^-1 energy window. Rotational spectra of the two PhTFE···H₂O conformers along with several deuterium and ¹⁸O isotopologues were assigned, and the isotopic data were used to verify the corresponding structures. In the two observed monohydrate conformers, one contains PhTFE I where the water subunit is inserted into the existing intramolecular OH···F contact of I, and the binary adduct is stabilized by two intermolecular contacts: OH···O_W and H_W···F, whereas the other contains PhTFE II where the water subunit interacts with both the alcohol hydrogen and phenyl ring of II, demonstrating that interaction with water sufficiently stabilizes II for its observation in a jet expansion. Interestingly, the predicted electric dipole moment components at the identified minima deviate considerably from the experimental ones. Such deviations were analyzed in terms of dynamic effects associated with the large amplitude motions of the unbound H_W. In addition, tunnelling effects associated with the exchange of the bonded and nonbonded H_W were also discussed.

Anionic Dimers of Fluorinated Alcohols

Negative-ion mode electrospray ionization of solutions of ethanol (R^F0OH), 2-fluoroethanol (R^F1OH), 2,2-difluoroethanol (R^F2OH), and/or 2,2,2-trifluoroethanol (R^F3OH) produces anionic dimers of the types (R^FnO)₂H^- and (R^FnO)(R^Fn+1O)H^-. The exchange reactions of these anionic dimers with the neutral alcohols are examined in a quadrupole-ion trap to extract kinetic data, from which the reaction Gibbs energies are obtained. In all cases, the formation of anionic dimers containing the more highly fluorinated alcohols is favored. Quantum chemical calculations confirm this trend and, besides affording structural data, also determine the dissociation energies of the anionic dimers. These dissociation energies are much higher than those of the corresponding neutral dimers and increase further for the more highly fluorinated alcohols due to the stronger hydrogen-bond donor ability of the latter. The present results on the interaction of individual alkoxide anions and neutral alcohol molecules contribute to a better understanding of the association of the fluorinated alcohols in solution.

Rotational Spectroscopy of the 2,2,3,3,3-Pentafluoropropanol⋅⋅⋅Water Complex: Conformations and Large Amplitude Motions

The 2,2,3,3,3-pentafluoropropanol (PFP) monomer can exist in five conformations defined by the CCCO and CCOH dihedral angles: four mirror-imaged pairs (G+g+/G-g-, G+g-/G-g+, G+t/G-t, Tg+/Tg-) and an achiral Tt form. We examined the conformational landscape of the PFP⋅⋅⋅water complex using chirped pulsed Fourier transform microwave spectroscopy and theoretical calculations. Rotational spectra of two PFP⋅⋅⋅water conformers, PFPG+g+⋅⋅⋅W_H and PFPTg+⋅⋅⋅W_H , and seven deuterated isotopologues of each, were assigned. Tunneling splittings were observed for both conformers and are attributed to the exchange of the bonded and non-bonded hydrogen atoms of water. On the other hand, the tunneling splitting associated with the OH flipping motion in PFPTg+/Tg- appears to be quenched upon hydrogen bonding with water. The large amplitude motions associated with the water subunits were examined in detail to explain the very different magnitudes of the experimental and theoretical permanent electric dipole moment components. The study highlights the challenge in correctly identifying the conformers observed when large amplitude motions are involved. Quantum theory of atoms in molecules (QTAIM) and non-covalent interaction (NCI) analyses, as well as electrostatic potential (ESP) calculations were carried out to explore the nature of the non-covalent interactions and to appreciate the effects of fluorination.

2,2,3,3,3-pentafluoro-1-propanol and its dimer: structural diversity, conformational conversion, and tunnelling motion

Rotational spectra of the 2,2,3,3,3-pentafluoro-1-propanol (PFP) were measured using cavity and chirped pulse Fourier transform microwave spectrometers. Of the nine possible PFP configurations which include four mirror-imaged pairs and an...

Higher-Energy Hexafluoroisopropanol···Water Isomer and Its Large Amplitude Motions: Rotational Spectra and DFT Calculations

Rotational spectra of the 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP)···water complex were measured using a chirped pulse Fourier-transform microwave spectrometer. The spectral analyses, aided by density functional theory calculations, reveal two HFIP···water isomers: one previously reported, trans HFIP (HFIP_t)···water (Phys. Chem. Chem. Phys. 2015, 119, 5650-5657), and a new isomer, gauche HFIP (HFIP_g)···water. To confirm the identity of the new isomer, rotational spectra of seven of its deuterated species were also measured and analyzed. Both the experimental and theoretical pieces of evidence indicate that the intermolecular interaction with water preferentially stabilizes the HFIP_g monomer configuration over the global minimum configuration, HFIP_t. The relative energy difference between these monomeric forms is 4.1 kJ mol^-1 and decreases to 2.5 kJ mol^-1 in the respective monohydrates at the B3LYP-D3(BJ)/def2-QZVP level of theory. Both rigid and relaxed potential energy surface scans were carried out to gain insights into the large-amplitude water motions in HFIP_g···water. The nonobservation of a water tunneling splitting in HFIP_t···water has been explained to be a result of a barrier-less (after zero-point-energy correction) pathway for the water motion, whereas in HFIP_g···water, a relatively large water tunneling barrier was identified as the cause of barely resolved water tunneling splittings. Noncovalent interaction and quantum theory of atoms and molecule analyses were used to evaluate the changes in HFIP_g···water when going from the minimum to the transition state in terms of attractive interactions such as the OH···H and OH···F contacts. The effect of fluorination is discussed by comparing the vastly different binding topologies of isopropanol···water and HFIP···water.

Structure and Internal Motions of a Multifunctional Alcohol-Water Complex: Rotational Spectroscopy of the Propargyl Alcohol···H2O Dimer

We have studied the rotational spectra of the propargyl alcohol (PA)-water complex using a pulsed-nozzle Fourier transform microwave spectrometer. A hydrogen-bonded ring structure is observed. The propargyl alcohol acts as an H-bond donor to form a strong O-H···O bond with H₂O, and H₂O donates back an H-bond to the acetylenic moiety, forming a weak O-H···π bond. Splittings of the rotational transitions were observed, which are indicative of internal motions of the H₂O fragment. The two lowest-energy conformers differ only in the position of the nonbonded hydrogen of H₂O. Several isotopic substitutions were carried out to ascertain the position of the nonbonded hydrogen of H₂O. Rotational spectroscopy helps to assign the observed structure to one, though it would be vibrationally averaged with a shallow potential along some coordinates, which could interchange the two conformers. These results are compared with earlier results on several alcohol-water complexes to understand the donor-acceptor capabilities of the OH groups in alcohol-water complexes. An empirical correlation between pK_a and H-bond donor ability has been observed.

Correlating solvation with conformational pathways of proteins in alcohol-water mixtures: a THz spectroscopic insight

Water, being an active participant in most of the biophysical processes, is important to trace how protein solvation changes as its conformation evolves in the presence of solutes or co-solvents. In this study, we investigate how the secondary structures of two diverse proteins - lysozyme and β-lactoglobulin - change in the aqueous mixtures of two alcohols - ethanol and 2,2,2-trifluoroethanol (TFE) using circular dichroism measurements. We observe that these alcohols change the secondary structures of these proteins and the changes are protein-specific. Subsequently, we measure the collective solvation dynamics of these two proteins both in the absence and in the presence of alcohols by measuring the frequency-dependent absorption coefficient (α(ν)) in the THz (0.1-1.2 THz) frequency domain. The alcohol-water mixtures exhibit a non-ideal behaviour with the highest absorption difference (Δα) obtained at X_alcohol = 0.2. The protein solvation in the presence of the alcohols shows an oscillating behaviour in which Δα_protein changes with X_alcohol. Such an oscillatory behaviour of protein solvation results from a delicate interplay between the protein-water, protein-alcohol and water-alcohol associations. We attempt to correlate the various structural conformations of the proteins with the associated solvation.

Structural and dynamical features of the 2,2,2-trifluoroethanol⋯ammonia complex

The preferred conformation of trifluoroethanol⋯ammonia is established experimentally and the effects of large vibrational motions on its properties evaluated.

Rotational spectra and theoretical study of tetramers and trimers of 2-fluoroethanol: dramatic intermolecular compensation for intramolecular instability

Cooperativity and chirality dominate in stabilizing the 2-fluoroethanol tetramer, thanks to a fully bifurcated C–H⋯F H-bond network.

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.

A Direct Link from the Gas to the Condensed Phase: A Rotational Spectroscopic Study of 2,2,2-Trifluoroethanol Trimers

Rotational spectra of the three most stable conformers (I, II, III) of the ternary 2,2,2-trifluoroethanol (TFE) cluster were measured using Fourier transform microwave spectrometers, and unambiguously assigned with the aid of ab initio calculations. The most stable conformer, I, contains one trans-TFE subunit which is unstable in its isolated gas phase form. The study offers new insights into how the trans conformation is stabilized in TFE clusters of increasing size, and eventually becomes a dominant conformation in the liquid phase. A detailed analysis shows that while O-H⋅⋅⋅O-H and O-H⋅⋅⋅F-C hydrogen bonds are the most significant attractive interactions which stabilize all three conformers, the main driving force for the stability of I over III, which has all gauche-TFE subunits, is the attractive interaction of C-F⋅⋅⋅F-C contact pairs. A new type of three-point F⋅⋅⋅F⋅⋅⋅F attractive contact interaction is also identified.

Tunnelling and barrier-less motions in the 2-fluoroethanol–water complex: a rotational spectroscopic and ab initio study

Rotational spectrum of 2-fluoroethanol–water reveals interesting water and methyl internal rotation tunneling and barrier-less motions in the hydrogen-bonded complex.