OH-stretching Frequencies Research Articles

Kinetic Energy Density as a Predictor of Hydrogen-Bonded OH-Stretching Frequencies.

This work considers the nature of the intermolecular hydrogen bond in a series of 15 different complexes with OH donor groups and N, O, P, or S acceptor atoms. To complement the existing literature, room-temperature gas-phase vibrational spectra of the methanol-pyridine, ethanol-pyridine, and 2,2,2-trifluoroethanol-pyridine complexes were recorded. These complexes were chosen, as they exhibit hydrogen bonds of intermediate strength as compared to previous investigations that involved strong or weak hydrogen bonds. Non Covalent Interactions (NCI) theory was used to calculate various properties of the intermolecular hydrogen bonds, which were compared to the experimental OH-stretching vibrational red shifts. We find that the experimental OH-stretching red shifts correlate strongly with the kinetic energy density integrated within the reduced density gradient volume that describes a hydrogen bond [G(s0.5)]. Given that vibrational red shifts are commonly used as a metric of the strength of a hydrogen bond, this suggests that G(s0.5) could be used as a predictor of hydrogen bonding strength.

Intramolecular hydrogen-bonding effects on O[sbnd]H stretch overtone excitation for fluorinated hydroperoxides

Applying computational methods for predicting gas-phase OH stretch overtone spectra to the series of fluorinated hydroperoxides CF3OOH, CHF2OOH, and CH2FOOH reveals how intramolecular hydrogen bonding changes their overtone photochemistry, relative to their un-fluorinated analog CH3OOH. Intramolecular hydrogen bonding between fluorine and the hydroxyl hydrogen atom decreases OH stretch frequencies and raises the barrier for torsion about the OO bond. Whereas OO torsion accompanies OH stretch overtone excitation for CH3OOH, this is not the case for the fluorinated analogs that have intramolecular hydrogen bonding. The comparison highlights the molecular features that contribute to the feasibility of atmospheric overtone-induced OO bond dissociation for CH3OOH, but not for its fluorinated analogs.

Microsolvation of the 5-hydroxyindole cation (5HI+) with nonpolar and quadrupolar ligands: Infrared photodissociation spectra of 5HI+-Ln clusters with L = Ar and N2 (n ≤ 3)

Solvation of biomolecules and their building blocks has a strong influence on their structure and function. Herein we characterize the initial microsolvation of the 5-hydroxyindole cation (5HI+) in its 2A″ ground electronic state with nonpolar and quadrupolar ligands (L=Ar, N2) by infrared photodissociation (IRPD) spectroscopy of cold and mass-selected 5HI+-Ln (n≤3) clusters in a molecular beam and dispersion-corrected density functional theory calculations (B3LYP-D3/aug-cc-pVTZ). The isomer-selective OH and NH stretch frequency shifts (ΔνOH/NH) disentangle the competition between H-bonding to the acidic OH and NH groups and π-stacking to the conjugated bicyclic aromatic π-electron system, the intermolecular interaction strengths, and the cluster growth sequence. For 5HI+-Arn, H-bonding and π-stacking strongly compete, indicating that dispersion forces are important for the interaction of 5HI+ with nonpolar ligands. In contrast, for 5HI+-(N2)n clusters, the H-bonds are much stronger than the π-bonds and largely determine the initial solvation process. In all clusters, the OH…L bonds are stronger than the NH…L bonds followed by the π-bonds. The interaction of 5HI+ with N2 is roughly twice stronger than with Ar, mainly due to the additional quadrupole moment of N2. The nature and strength of the individual interactions are quantified by the noncovalent interaction approach. Comparison of 5HI+-L with the corresponding neutral clusters reveals the strong impact of ionization on the total and relative interaction strengths of the H-bonds and π-bonds. In addition, comparison of 5HI+-L with corresponding clusters of the phenol, indole, and pyrrole radical cations illustrates the effects of substitution of functional groups and the addition of aromatic rings to the various subunits of 5HI on the intermolecular potential.

Energy decomposition analysis in an adiabatic picture.

Energy decomposition analysis (EDA) of electronic structure calculations has facilitated quantitative understanding of diverse intermolecular interactions. Nevertheless, such analyses are usually performed at a single geometry and thus decompose a "single-point" interaction energy. As a result, the influence of the physically meaningful EDA components on the molecular structure and other properties are not directly obtained. To address this gap, the absolutely localized molecular orbital (ALMO)-EDA is reformulated in an adiabatic picture, where the frozen, polarization, and charge transfer energy contributions are defined as energy differences between the stationary points on different potential energy surfaces (PESs), which are accessed by geometry optimizations at the frozen, polarized and fully relaxed levels of density functional theory (DFT). Other molecular properties such as vibrational frequencies can thus be obtained at the stationary points on each PES. We apply the adiabatic ALMO-EDA to different configurations of the water dimer, the water-Cl- and water-Mg2+/Ca2+ complexes, metallocenes (Fe2+, Ni2+, Cu2+, Zn2+), and the ammonia-borane complex. This method appears to be very useful for unraveling how physical effects such as polarization and charge transfer modulate changes in molecular properties induced by intermolecular interactions. As an example of the insight obtained, we find that a linear hydrogen bond geometry for the water dimer is preferred even without the presence of polarization and charge transfer, while the red shift in the OH stretch frequency is primarily a charge transfer effect; by contrast, a near-linear geometry for the water-chloride hydrogen bond is achieved only when charge transfer is allowed.

Intramolecular Hydrogen Bonding in Substituted Aminoalcohols.

The qualifying features of a hydrogen bond can be contentious, particularly where the hydrogen bond is due to a constrained intramolecular interaction. Indeed there is disagreement within the literature whether it is even possible for an intramolecular hydrogen bond to form between functional groups on adjacent carbon atoms. This work considers the nature of the intramolecular interaction between the OH (donor) and NH2 (acceptor) groups of 2-aminoethanol, with varying substitution at the OH carbon. Gas-phase vibrational spectra of 1-amino-2-methyl-2-propanol (BMAE) and 1-amino-2,2-bis(trifluoromethyl)-2-ethanol (BFMAE) were recorded using Fourier transform infrared spectroscopy and compared to literature spectra of 2-aminoethanol (AE). Based on the experimental OH-stretching frequencies, the strength of the intramolecular hydrogen bond appears to increase from AE < BMAE ≪ BFMAE. Non-covalent interaction analysis shows evidence of an intramolecular hydrogen bond in all three molecules, with the order of the strength of interaction matching that of experiment. The experimental OH-stretching vibrational frequencies were found to correlate well with the calculated kinetic energy density, suggesting that this approach can be used to estimate the strength of an intramolecular hydrogen bond.

Hydrogen bonding in the carboxylic acid–aldehyde complexes

Carboxylic acids have been recognized as important precursor species in atmospheric new particle formation. To clearly understand how aerosols form at the molecular level we investigated the hydrogen bonding interactions in the complexes between carboxylic acids and aldehydes using density functional theory methods. The geometric optimization shows that the Z-type carboxylic acid and aldehyde subunits are linked together through a classical OH⋯O and a weak CH⋯O hydrogen bond, which forms ring structures, and the E-type carboxylic acid and aldehyde subunits are linked through an OH⋯O hydrogen bond with no ring structure. The binding energies and OH-stretching frequency calculations show that the carboxylic acid (Z)–aldehyde complexes are bonding stronger than the carboxylic acid (E)–aldehyde complexes. In addition, atoms in molecules (AIM) theory and natural bond orbital (NBO) analyses were performed to further interpret the intermolecular properties of the carboxylic acid–aldehyde complexes. This study helps to elucidate the nature of the hydrogen bond in these complexes and reveal their roles in atmospheric new particle formation.

Origin of the Hadži ABC structure: An ab initio study.

Medium and strong hydrogen bonds are well known to give rise to broad features in the vibrational spectrum often spanning several hundred wavenumbers. In some cases, these features can span over 1000 cm(-1) and even contain multiple broad peaks. One class of strongly hydrogen-bonded dimers that includes many different phosphinic, phosphoric, sulfinic, and selenic acid homodimers exhibits a three-peaked structure over 1500 cm(-1) broad. This unusual feature is often referred to as the Hadži ABC structure. The origin of this feature has been debated since its discovery in the 1950s. Only a couple of theoretical studies have attempted to interpret the origin of this feature; however, no previous study has been able to reproduce this feature from first principles. Here, we present the first ab initio calculation of the Hadži ABC structure. Using a reduced dimensionality calculation that includes four vibrational modes, we are able to reproduce the three-peak structure and much of the broadness of the feature. Our results indicate that Fermi resonances of the in-plane bend, out-of-plane bend, and combination of these bends play significant roles in explaining this feature. Much of the broadness of the feature and the ability of the OH stretch mode to couple with many overtone bending modes are captured by including an adiabatically separated dimer stretch mode in the model. This mode modulates the distance between the monomer units and accordingly the strength of the hydrogen-bonds causing the OH stretch frequency to shift from 2000 to 3000 cm(-1). Using this model, we were also able to reproduce the vibrational spectrum of the deuterated isotopologue which consists of a single 500 cm(-1) broad feature. Whereas previous empirical studies have asserted that Fermi resonances contribute very little to this feature, our study indicates that while not appearing as a separate peak, a Fermi resonance of the in-plane bend contributes substantially to the feature.

CH-Stretching Vibrational Trends in Natural Gas Hydrates Studied by Quantum-Chemical Computations

Vibrational Raman spectroscopy of hydrocarbon CH-stretching vibrations is often used to study natural gas hydrates. In this work, CH-stretching vibrational Raman spectra of hydrocarbon molecules (CH4, C2H6, C3H6, C3H8, C4H8, i-C4H10, and n-C4H10) encapsulated in the water cages (D, ID, T, P, H, and I) of the sI, sII, sH, and sK crystal phases are derived from quantum-chemical computations at the ωB97X-D/6-311++G(2d,2p) level of theory. The trends of CH-stretching vibrational frequencies of hydrocarbon molecules in natural gas hydrates are found to follow the prediction by the “loose cage–tight cage” model: as the size of water cavity increases, the CH frequencies will first decrease and then increase until equal to that in the gas phase. In the “tight cage” situation, the frequency will be greater than in the gas phase; in the “loose cage” situation, the frequency will be smaller or asymptotic to that in the gas phase. Furthermore, the OH-stretching frequencies are sensitive to the H-bond configuration, a...

Infrared and Near-Infrared Spectroscopy of Acetylacetone and Hexafluoroacetylacetone.

The infrared and near-infrared spectra of acetylacetone, acetylacetone-d8, and hexafluoroacetylacetone are characterized from experiment and computations at different levels. In the fundamental region, the intramolecular hydrogen bonded OH-stretching transition is clearly observed as a very broad band with substantial structure and located at significantly lower frequency compared to common OH-stretching frequencies. There is no clear evidence for OH-stretching overtone transitions in the near-infrared region, which is dominated by the CH-stretching overtones of the methine and methyl CH bonds. From molecular dynamics (MD) simulations, with a potential energy surface previously validated for tunneling splittings, the infrared spectra are determined and used in assigning the experimentally measured ones. It is found that the simulated spectrum in the region associated with the proton transfer mode is exquisitely sensitive to the height of the barrier for proton transfer. Comparison of the experimental and the MD simulated spectra establishes that the barrier height is around 2.5 kcal/mol, which favorably compares with 3.2 kcal/mol obtained from high-level electronic structure calculations.

Probing the Hydrogen-Bonded Water Network at the Active Site of a Water Oxidation Catalyst: [Ru(bpy)(tpy)(H2O)](2+)·(H2O)(0-4).

The infrared spectra of gas-phase mass-selected [Ru(bpy)(tpy)(H2O)](2+)·(H2O)(0-4) clusters (bpy = 2,2'-bipyridine; tpy = 2,2':6,2″-terpyridine) in the OH stretching region are reported. These species are formed by bringing the homogeneous water oxidation catalyst [Ru(bpy)(tpy)(H2O](2+) from solution into the gas phase via electrospray ionization (ESI) and reconstructing the water network at the active site by condensing additional water onto the complex in a cryogenic ion trap. Infrared predissociation spectroscopy is used to probe the structure of these clusters via their distinctive OH stretch frequencies, which are sensitive to the shape and strength of the local hydrogen-bonding network. The analysis of the spectra, aided by electronic structure calculations, highlights the formation of strong hydrogen bonds between the aqua ligand and the solvating water molecules in the first solvation shell. These interactions are found to propagate through the subsequent solvation shells and lead to the stabilization of asymmetric solvation motifs. Electronic structure calculations show that these strong hydrogen bonds are promoted by charge transfer from the H atom of the aqua ligand to the Ru-OH2 bond.

Positively Charged Phosphorus as a Hydrogen Bond Acceptor.

Phosphorus (P) is an element that is essential to the life of all organisms, and the atmospheric detection of phosphine suggests the existence of a volatile biogeochemical P cycle. Here, we investigate the ability of P to participate in the formation of OH···P hydrogen bonds. Three bimolecular alcohol-trimethylphosphine complexes have been detected. Initially, the complexes were detected using matrix isolation spectroscopy, which favors complex formation. Subsequently, the fundamental OH-stretching vibration was observed in room-temperature gas-phase spectra. On the basis of our measured OH-stretching frequency red shifts and quantum chemical calculations, we find that P is an acceptor atom similar in strength to O and S and that all three P, O, and S atoms are weaker acceptors than N. The quantum chemical calculations show that both H and P in the OH···P hydrogen bond have partial positive charges, as expected from their electronegativities. However, the electrostatic potentials show a negative potential area on the electron density surface around P that facilitates formation of hydrogen bonds.

Interfacial water in the vicinity of a positively charged interface studied by steady-state and time-resolved heterodyne-detected vibrational sum frequency generation.

To investigate the properties of water in the close vicinity of a positively charged surfactant/water interface, steady-state and femtosecond time-resolved interfacial vibrational spectra were measured in the presence of excess alkali halide salts. The steady-state Imχ((2)) spectra show a drastic intensity decrease with excess salts, indicating that the thickness of the probed water layer is substantially reduced. Fluoride salts do not noticeably affect spectral features in the OH stretch region whereas the chloride and bromide salts induce significant blue shifts of the OH stretch frequency. Femtosecond time-resolved ΔImχ((2)) spectra obtained with fluoride salts exhibit a very broad bleach even at 0 fs as observed without excess salts, while chloride and bromide salts give rise to a narrow spectral hole burning. These results indicate that the excess chloride and bromide ions strongly interact with interfacial water in the vicinity of the charged interface and it suppresses intramolecular coupling (i.e., Fermi resonance) that causes spectral broadening.

Periodic, vdW-corrected density functional theory investigation of the effect of Al siting in H-ZSM-5 on chemisorption properties and site-specific acidity

Twelve crystallographically distinct Al substitution sites of H-ZSM-5 are modeled by periodic density functional theory using the vdW-DF functional. While the stability of Al substitution and Brønsted acid OH bond length at different active site positions are similar, the OH stretch frequency, SiOAl bond angle, and adsorption energies of various probe molecules differ notably without observable correlations between these properties. Comparison of adsorption energy values with and without van der Waals corrections demonstrates the significance of the inclusion of dispersion interactions. Our data indicate that theoretical investigations of H-ZSM-5 require a careful selection of the location of the active site.

In-situ infrared spectra of hydroxyl in wadsleyite and ringwoodite at high pressure and high temperature

The infrared spectra of hydroxyl in synthetic hydrous wadsleyite (β-Mg 2 SiO 4 ) and ringwoodite (γ-Mg 2 SiO 4 ) were measured at room temperature up to ~18.8 GPa for wadsleyite and up to ~21.5 GPa for ringwoodite. High-temperature spectra were measured in an externally heated diamond-anvil cell up to 650 °C at ~14.2 GPa for wadsleyite and up to 900 °C at ~18.4 GPa for ringwoodite. The synthetic samples reproduce nearly all the important OH bands previously observed at ambient conditions. Only subtle changes were observed in the infrared spectra of both minerals, both upon compression at room temperature and upon heating at high pressure. For wadsleyite, upon compression to ~18.8 GPa, the frequencies of the bands at ~3600 cm −1 remain almost unchanged, while the main bands at 3200–3400 cm −1 shift to lower frequencies. During heating at 14.2 GPa to 650 °C the bands at 3200–3400 cm −1 broaden and shift to slightly lower frequencies. For ringwoodite, upon compression to ~21.5 GPa, the main bands at 3115 cm −1 progressively shift to lower frequencies. During heating at 18.4 GPa to 900 °C, no frequency shift was observed for the band at ~3700 cm −1 , but the band initially at ~3115 cm −1 shifts very slightly to higher frequencies, which should yield almost the same band positions at 1300–1400 °C as those measured at ambient conditions. Our data suggest that water speciation in hydrous wadsleyite and ringwoodite at ambient conditions may be comparable to that under mantle conditions, except perhaps for subtle changes in hydrogen bonding. The low OH-stretching frequencies in wadsleyite and ringwoodite under transition zone conditions imply a large H/D fractionation during degassing of the deep mantle. This may explain the apparent disequilibrium between the hydrogen isotopic composition of the upper mantle and the ocean.

Intramolecular Interactions in 2-Aminoethanol and 3-Aminopropanol

Gas-phase vibrational spectra of 2-aminoethanol and 3-aminopropanol were recorded up to the third OH-stretching overtone using Fourier transform infrared spectroscopy, cavity ringdown spectroscopy, and intracavity laser photoacoustic spectroscopy. The experimental investigation was supplemented by local mode calculations, and the intramolecular interactions were investigated using atoms in molecules (AIM) and noncovalent interactions (NCI) theories. All calculations were performed at the CCSD(T)-F12a/VDZ-F12 level of theory. For both compounds the most abundant conformer has a structure that allows for hydrogen bond interaction from the OH group to the N atom of the amino group (OH-N). The spectra show signals from both hydrogen bonded and free OH stretches, implying the presence of several conformers. We observe hydrogen-bond-like interactions in both compounds. The red shift of the bonded OH-stretching frequency and intensity enhancement of the fundamental transition suggest that the hydrogen bond interaction is more pronounced in 3-aminopropanol. AIM analysis supports the presence of a hydrogen bond in 3-aminopropanol but not in 2-aminoethanol, whereas NCI analysis shows hydrogen bonding in both compounds with the stronger interaction found in 3-aminopropanol.

The Dissolution of Polyols in Salt Solutions and Ionic Liquids at Molecular Level: Ions, Counter Ions, and Hofmeister Effects

The dissolving process of polyols in salt solutions (TBAF, TBAC, TBAB, TBAI, TMAF) and imidazolium-based ionic liquids ([C2 mim][OAc], [C2 mim][Et2 PO4 ], [C2 mim][EtSO4 ], [C2 mim][SCN]) is exemplarily studied by IR spectroscopy. Vibrational bands and their shifts in the OH stretch region reveal crucial information for the dissolved polyol interacting with the anions of the salt solutions and ionic liquids. The well-chosen set of ionic solutions confirms the linear relation between the OH-stretch frequencies and the solubility capacity of the salt solutions. Likewise, it also provides an explanation of the dissolving process at molecular level. Notably, the solubility capacities of the anions in the salt solutions follow the well-known Hofmeister series. This phenomenon can be understood on the basis of the disruption power of the anions and the specific size ratio of the anion/cation combinations.

Impact of Bran Addition on Water Properties and Gluten Secondary Structure in Wheat Flour Doughs Studied by Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy

ABSTRACTThe aim of this work was to elucidate the underlying physical mechanism(s) by which bran influences whole grain dough properties by monitoring the state of water and gluten secondary structure in wheat flour and bran doughs containing 35–50% moisture and 0–10% added bran. The system was studied with attenuated total reflectance (ATR) FTIR spectroscopy. Comparison of the OH stretch band of water in flour dough with that in H2O‐D2O mixtures having the same water content revealed the formation of two distinct water populations in flour dough corresponding to IR absorption frequencies at 3,600 and 3,200 cm–1. The band intensity at 3,200 cm–1, which is related to water bound to the dough matrix, decreased and shifted to lower frequencies with increasing moisture content of the dough. Addition of bran to the dough caused redistribution of water in the flour and bran dough system, as evidenced by shifts in OH stretch frequency in the 3,200 cm–1 region to higher frequencies and a reduction in monomeric water (free water). This water redistribution affected the secondary structure of gluten in the dough, as evidenced by changes in the second‐derivative ATR‐FTIR difference spectra in the amide I region. Bran addition caused an increase in β‐sheet content and a decrease in β‐turn (β‐spiral) content. However, this bran‐induced transconformational change in gluten was more significant in the 2137 flour dough than in Overley flour dough. This study revealed that when bran is added to flour dough, water redistribution among dough components promotes partial dehydration of gluten and collapse of β‐spirals into β‐sheet structures. This transconformational change may be the physical basis for the poor quality of bread containing added bran.

Ultrafast OH-stretching frequency shifts of hydrogen- bonded 2-naphthol photoacid-base complexes in solution

We characterize the transient solvent-dependent OH-stretching frequency shifts of photoacid 2-naphthol hydrogen-bonded with CH3CN in the S0- and S1-states using a combined experimental and theoretical approach, and disentangle specific hydrogen-bonding contributions from nonspecific dielectric response.

Hydrogen Bonding and OH-Stretch Spectroscopy in Water: Hexamer (Cage), Liquid Surface, Liquid, and Ice.

We present a unified picture of how OH-stretch spectroscopy in water can be understood in terms of hydrogen bonding for the four systems listed in the title. To understand the strength, and hence OH-stretch frequency, of a hydrogen bond, it is crucial to consider the number of additional acceptor hydrogen bonds made by both the donor and acceptor molecules. This necessity for focusing on the hydrogen-bond environment of both donor and acceptor molecules follows from quantum chemical considerations and is related to the three-body interactions in water. Armed with this understanding we can make a detailed interpretation of the OH-stretch IR absorption spectrum of the cage conformer for HOD(D2O)5 and the imaginary part of the ssp OH-stretch sum-frequency spectrum of the surface of liquid D2O with dilute HOD.

Theoretical study of structural, mechanical and spectroscopic properties of boehmite (γ-AlOOH)

The structural, mechanical and spectroscopic properties of boehmite (AlOOH polymorph)were investigated by means of first-principle density functional theory (DFT) andsemiempirical density functional based tight binding (DFTB) methods. Apart from amarginal underestimation of interlayer hydrogen bond distances the DFT method wellreproduces the experimental equilibrium low-pressure structure. For the DFTB methodsimilar good agreement was obtained for lattice parameters, however bond lengths andangles showed a larger deviation from experiment in comparison to DFT results. Theexperimental spectrum of the OH stretching region was interpreted by means of thecalculated frequencies within the frame of the harmonic approximation and by calculatingthe power spectra of the hydroxyl groups obtained from molecular dynamicssimulations. Using the latter approach, the strong coupling between the individualOH modes was demonstrated. Isostatic structural compression of the boehmitestructure was performed in order to obtain the bulk modulus and the dependenceof the vibrational spectrum on the pressure. The DFT method gives a value of97 GPa in the athermal limit. Comparison with available bulk moduli for otherAlOOH polymorphs reveals that boehmite shows the highest compression, forwhich mainly a strong shortening mechanism of interlayer hydrogen bonds isresponsible. The DFT method also described correctly the dependence of the OHstretch frequencies upon compression resulting in a strong red shift. Althoughgood performance is observed for the low-pressure region, the DFTB methodis not found to be suitable for high-pressure studies in cases such as boehmite.