Increasing Hydrogen Bond Strength Research Articles

Theoretical study on the cascaded double proton transfer mechanism involving two adjacent hydrogen bonds in conjugated organic molecules

The cascaded double proton transfer reaction has attracted the interest of many researchers due to its effective six-level system cycle. Herein, the double proton transfer mechanism of naphthalene-based analog NDH and anthracene-based analog ADH in different solvents is investigated using density functional theory (DFT) and time-dependent density functional theory (TDDFT). The results indicate that the first occurrence of excited-state intramolecular proton transfer (ESIPT) in NDH and ADH is an ultrafast, barrierless process. Additionally, ADH undergoes a second ESIPT reaction, radiating a wider range of fluorescence spectrum. Theoretical studies reveal that this is due to the significantly increased hydrogen bond strength of ADH compared to NDH in the first excited (S1) state, creating favorable conditions for the second ESIPT. Analysis of electronic properties indicates that ADH exhibits enhanced chemical and kinetic activity, with a more pronounced intramolecular charge transfer (ICT), leading to increased stability of the keto structure (KB2*) after the second ESIPT. Finally, we identify the factors influencing the cascaded double proton transfer reaction. The second ESIPT process is promoted by extending the π-conjugated skeleton of the core molecule horizontally and leveraging the driving force provided by the H3 atom near the proton donor O3–H2. It is expected that these studies will provide theoretical guidance and new insights for the design and synthesis of near infrared (NIR) organic materials.

Intermolecular Hydrogen Bonding Tunes Vibronic Coupling in Heptazine Complexes.

To better understand how hydrogen bonding influences the excited-state landscapes of aza-aromatic materials, we studied hydrogen-bonded complexes of 2,5,8-tris (4-methoxyphenyl)-1,3,4,6,7,9,9b-heptaazaphenalene (TAHz), a molecular photocatalyst related to graphitic carbon nitride, with a variety of phenol derivatives (R-PhOH). By varying the electron-withdrawing character of the para-substituent on the phenol, we can modulate the strength of the hydrogen bond. Using time-resolved photoluminescence, we extract a spectral component associated with the R-PhOH-TAHz hydrogen-bonded complex. Surprisingly, we noticed a striking change in the relative amplitude of vibronic peaks in the TAHz-centered emission as a function of R-group on phenol. To gain a physical understanding of these spectral changes, we employed a displaced-oscillator model of molecular emission to fit these spectra. This fit assumes that two vibrational modes are dominantly coupled to the emissive electronic transition and extracts their frequencies and relative nuclear displacements (related to the Huang-Rhys factor). With the aid of quantum chemical calculations, we found that heptazine ring-breathing and ring-puckering modes are likely responsible for the observed vibronic progression, and both modes indicate decreasing molecular distortion in the excited state with increasing hydrogen bond strength. This finding offers new insights into intermolecular excited-state hydrogen bonding, which is a crucial step toward controlling excited-state proton-coupled electron transfer and proton transfer reactions.

Backbone amide 15N chemical shift tensors report on hydrogen bonding interactions in proteins: A magic angle spinning NMR study

Chemical shift tensors (CSTs) are an exquisite probe of local geometric and electronic structure. 15N CST are very sensitive to hydrogen bonding, yet they have been reported for very few proteins to date. Here we present experimental results and statistical analysis of backbone amide 15N CSTs for 100 residues of four proteins, two E. coli thioredoxin reassemblies (1-73-(U-13C,15N)/74-108-(U-15N) and 1-73-(U-15N)/74-108-(U-13C,15N)), dynein light chain 8 LC8, and CAP-Gly domain of the mammalian dynactin. The 15N CSTs were measured by a symmetry-based CSA recoupling method, ROCSA. Our results show that the principal component δ11 is very sensitive to the presence of hydrogen bonding interactions due to its unique orientation in the molecular frame. The downfield chemical shift change of backbone amide nitrogen nuclei with increasing hydrogen bond strength is manifested in the negative correlation of the principal components with hydrogen bond distance for both α-helical and β-sheet secondary structure elements. Our findings highlight the potential for the use of 15N CSTs in protein structure refinement.

Charge Spreading in Deep Eutectic Solvents.

Ab initio molecular dynamic simulations reveal significantly reduced ion charges in several choline-based deep eutectic solvents, which are cheap and eco-friendly alternatives to ionic liquids. Increasing hydrogen bond strength between the anion and the organic compound enhances charge spreading from the anion to the organic compound while the positive charge is stronger located at the cation. Nonetheless, the negative charge transferred from chloride to urea in choline chloride urea mixtures is negligible. Thus, it seems questionable if charge delocalization occurring through hydrogen bonding between the halide anion and the organic compound is responsible for the deep eutectic melting point.

Equilibrium configuration of (H2O)n, for n = 1–3

The equilibrium configurations of water molecule (H2O)n=1, its dimer (H2O)n=2 and trimer (H2O)n=3 have been studied in this paper. The ionic character of the OHO hydrogen bond, formed between the electronegative oxygen atoms, in (H2O)n=2, appears as a change in bond lengths in one of the water molecules while the bond lengths in the other molecule remain same as in (H2O)n=1. The increased hydrogen bond strength, caused by the cooperative effect of three-body forces (H2O)n=3, as compared to (H2O)n=2, results in the equilibrium structure of the water trimer to be much more rigid than that of the dimer. An attempt has been made, from the energy standpoint, to understand the controversy on chemical formula of water as H2O or H1·5O by studying the equilibrium configuration of (H2O) OH, which corresponds to 2H1·5O. The ground-state energy of (H2O) OH is found to be lower than the sum of the ground-state energies of OH and H2O at infinite separation. Calculations also show the ground-state energy of (H2O) OH to be lower than the sum of the energies of the constituent atoms at infinite separation. This shows that the binding energy of (H2O) OH is positive, which indicates that (H2O) OH is relatively stable.

Structure and vibrational assignment of 3-nitro-2,4-pentanedione: A density functional theoretical study

The molecular structure of 3-nitro-2,4-pentanedione, known as α-nitro-acetylacetone (NO 2AA), has been investigated by means of Density Functional Theory (DFT) calculations and the results were compared with those of acetylacetone (AA). The harmonic and anharmonic vibrational frequencies of the most stable cis-enol form were calculated using the B3LYP functional and 6-31G∗∗ and 6-311G∗∗ basis sets. The calculated frequencies and the Raman and IR intensities were compared with the experimental results. The calculated geometrical parameters for NO 2AA show a very strong hydrogen bond, compared with its unsubstituted parent (AA). The calculated O⋯O distance of 2.416–2.457 Å is about 0.09–0.1 Å shorter than in AA. According to the theoretical calculations, NO 2AA has an asymmetric structure with a hydrogen bond strength of about 74.0 kJ/mol (calculated with 6-311G∗∗ basis set), which is 4.7 kJ/mol stronger than the hydrogen bond of AA. This increase in the hydrogen bond strength is also consistent with the experimental results. Natural Bond Orbital (NBO) analyses indicate that both electron delocalization and steric effects are responsible for the increased hydrogen bond strength in NO 2AA compared with that in AA.

Influence of Intramolecular Hydrogen Bond Strength on OH-Stretching Overtones

Vapor-phase OH-stretching overtone spectra of 1,3-propanediol and 1,4-butanediol were recorded and compared to the spectra of ethylene glycol to investigate the effect of increased intramolecular hydrogen bond strength on OH-stretching overtone transitions. The spectra were recorded with laser photoacoustic spectroscopy in the second and third OH-stretching overtone regions. The room-temperature spectra of each molecule are dominated by two conformers that show intramolecular hydrogen bonding. Anharmonic oscillator local-mode calculations of the OH-stretching transitions have been performed to aid assignment of the different conformers in the spectra and to illustrate the effect of the intramolecular hydrogen bonding. The hydrogen bond strength increases in the order ethylene glycol, 1,3-propanediol, and 1,4-butanediol. The overtone transitions of the hydrogen-bonded hydroxyl groups are more difficult to observe with increasing intramolecular hydrogen bond strength. We suggest that the bandwidth of these transitions increases with increasing hydrogen bond strength and with increasing overtone and furthermore that these changes are in part responsible for the lack of observed overtone spectra for complexes.

Investigating pressure effects on structural and dynamical properties of liquid methanol with many-body interactions

Molecular-dynamics simulations utilizing a many-body potential was used to study the pressure dependence of structural and dynamical properties for liquid methanol. The liquid density as a function of pressure agreed well with experiment, and a combination of radial and angular distribution functions were used to analyze molecular structure. From these distribution functions, it was observed that hydrogen bond strength increased with increasing pressure. This observation coincided with an increase in the molecular dipole as a function of pressure, having a significant effect on the observed increased hydrogen bond strength. Also, methanols were found to more strongly favor exactly two hydrogen bonds, with fewer methanols of zero, one, or three hydrogen bonds present at higher pressures. Furthermore, a majority of the compression with increased pressure was found to occur in regions perpendicular to the methanol hydrogen-oxygen bond vector. This was the case despite hydrogen-oxygen nonbonded distances between hydrogen bonding species being shorter, but their stiffer oxygen-hydrogen-(nonbonded) oxygen angle offsets this, resulting in their oxygen-oxygen distances being relatively unaffected. The methanol translational diffusion decreased significantly with increased pressure, while the rotational diffusion decreased at a similar magnitude around the oxygen-hydrogen and oxygen-carbon bond vectors, despite having very different overall diffusion. Finally, the hydrogen bond lifetime increased significantly with pressure, owing to the increased hydrogen bond strength, and the slower translational and rotational dynamics.

Pulsed field ionization-ZEKE spectroscopy of 4-aminobenzonitrile–H2O. Hydrogen-bonding interaction in the amino site

Pulsed field ionization (PFI)-zero-kinetic-energy (ZEKE) photoelectron spectra are measured for 4-aminobenzonitrile (4ABN) and “NH2-donor isomer” of 4ABN–H2O 1 : 1 complex in which the oxygen atom of water is bonded to the amino hydrogen (K. Sakota, N. Yamamoto, K. Ohashi, H. Sekiya, M. Saeki, S. Ishiuchi, M. Sakai and M. Fujii, Chem. Phys. Lett., 2001, 341, 70.). We have investigated changes in intermolecular interaction and structure of the NH2-donor isomer upon photoionization. The adiabatic ionization potentials of 4ABN and NH2-donor isomer are determined to be 66 493 and 62 843 cm−1, respectively. The oxygen atom of water is hydrogen bonded to an amino hydrogen atom in the D0 state of NH2 donor isomer as well as in the S1 and S0 states. However, the hydrogen bond is suggested to be much stronger in the D0 state than in the S1 state from a substantial decrease (3650 cm−1) in the adiabatic potential due to the formation of a hydrogen bond. Normal mode analysis has been made for the intermolecular vibrations in the D0 state with the aid of density functional theory calculations at the B3LYP/aug-cc-pVDZ level. The observation of a prominent progression of the intermolecular stretch mode in the PFI-ZEKE spectrum via the electronic origin of the S1–S0 transition is consistent with the increased hydrogen bond strength in the D0 state.

Orientational relaxation of liquid water molecules as an activated process

Femtosecond mid-infrared pump–probe spectroscopy is used to study the orientational relaxation of HDO molecules dissolved in liquid D2O. In this technique, the excitation of the O–H stretch vibration is used as a label in order to follow the orientational motion of the HDO molecules. The decay of the anisotropy is nonexponential with a typical time scale of 1 ps and can be described with a model in which the reorientation time depends on frequency and in which the previously observed spectral diffusion is incorporated. From the frequency and temperature dependence of the anisotropy decay, the activation energy for reorientation can be derived. This activation energy is found to increase with increasing hydrogen bond strength.

Requirement for Hydrogen-Bonding Cooperativity in Small Polyamides: A Combined VT-NMR and VT-IR Investigation.

A study of intramolecular hydrogen bonding in chloroform for a small combinatorial library of nine triamides with varying connecting chain length has been completed. The starting materials for the triamides are three diacids (succinic, glutaric, and adipic acid) and three amino acids (glycine, beta-alanine, and gamma-aminobutyric acid). The preferences for the head-to-tail type of folding pattern are identified for the smaller triamides (1 and 4). The preference for the head-to-tail folding pattern can be explained by the energetic superiority of an optimal hydrogen bond geometry in which the NH---O bond angle is near linearity. The beta-alanine containing triamides 2, 5, and 8 are resistant to intramolecular hydrogen bonding, especially to nearest neighbor hydrogen bonding. At lower temperatures, triamides 2 and 5 exhibit a small population of head-to-tail type of folding, while triamide 8 shows a significant population of bifurcated conformation. Triamide 6, 7, and 9 prefer bicyclic structures involving nearest neighbor hydrogen bonding. A nine-membered ring is large enough to accommodate a near linear N-H--O bond angle. Entropic effects are probably responsible for the preference of the nine-membered ring over a 12- or a 14-membered ring. The enhancement of hydrogen bonding in triamide 9 is enormous, and both NHs have a very large temperature dependence of chemical shifts (-15 ppb/K and -13.3 ppb/K for the terminal and the internal NH protons, respectively). Using appropriate temperature-dependent lower and upper limits of chemical shifts, a van't Hoff analysis gives the hydrogen bond strength for the terminal NH (DeltaH = -3.1 +/- 0.5 kcal/mol) and for the internal NH (DeltaH = -2.8 +/- 0.5 kcal/mol). The increased hydrogen bond strength is taken as evidence for hydrogen-bonding cooperativity from the two mutually enhanced individual hydrogen bonds. A near linear NH--O bond angle is required for this effect.

Observation of site-dependent relaxation of the OH vibrational stretch mode in fused silica

We have investigated the vibrational relaxation of the OH impurity stretch mode in amorphous SiO 2. At low temperatures, a short relaxation time of ≈10 −10 s is found that varies strongly across the inhomogeneously broadened absorption line. The inhomogeneous broadenining is caused by a site-dependent frequency shift proportional to the strength of the hydrogen bond of the OH to neighboring atoms. Thus the observed lifetime decrease across the absorption line can be attributed to increasing hydrogen bond strength. A simple relation between the site-dependent vibrational lifetime and the frequency shift is presented.

Deuterium isotope effects on 13C chemical shifts of intramolecularly hydrogen‐bonded olefins

AbstractA series of intramolecularly hydrogen‐bonded enamines, enols and enethiols with ester carbonylic, ketonic carbonylic, thioester carbonylic, nitro and sulphoxide acceptors were investigated to obtain 13C chemical shifts and deuterium isotope effects. Results from 33 new compounds and six remeasurements are compared with already existing data. An important aim was to show that isotope effects on chemical shifts are useful descriptors of hydrogen‐bonded systems and not only a parameter proportional to the 13C chemical shifts. Substituent effects were studied and the donors and acceptors ranked according to their abilities to support hydrogen bonding. Steric effects strengthen the hydrogen bonding in cyclic five‐membered β‐diketones. Plots of two‐bond [2ΔC(OD)] vs. four‐bond isotope effects [4ΔC(OD)] show that 4ΔC(OD) increases with increasing hydrogen bond strength and that large deviations from this relationship can be an indicator of tautomerism.

Hydrophobic interactions affect hydrogen bond strengths in complexes between peptides and vancomycin or ristocetin.

A study on complexes of the glycopeptide antibiotics vancomycin and ristocetin with various dipeptides and tripeptides shows that the intermolecular hydrogen bond strengths of the complexes are reasonably well correlated with their free energy of formation. Such correlation is not anticipated on the basis of a purely hydrophobic interaction of the peptide side-chains with the antibiotics. It is also shown that the free energy changes observed are very different from those expected as a result of hydrophobic forces. These facts suggest that addition of a hydrophobic group not only allows hydrophobic bonding but also strengthens existing hydrogen bonds. The increased hydrogen bond strength can be an important factor in determining the overall binding energy.