Characterization of intramolecular hydrogen bonds by atomic charges and charge fluxes

The electronic charge redistribution and the infrared intensities of the two types of intramolecular hydrogen bonds, O-H···O and O-H···π, of o-hydroxy- and o-ethynylphenol, respectively, together with a set of related intermolecular hydrogen bond complexes are described in terms of atomic charges and charge fluxes derived from atomic polar tensors calculated at the B3LYP/cc-pVTZ level of theory. The polarizable continuum model shows that both the atomic charges and charge fluxes are strongly dependent on solvent. It is shown that their values for the OH bond in an intramolecular hydrogen bond are not much different from those for the "free" OH bond, but the changes are toward the values found for an intermolecular hydrogen bond. The intermolecular hydrogen bond is characterized not only by the decreased atomic charge but also by the enlarged charge flux term of the same sign producing thus an enormous increase in IR intensity. The overall behavior of the charges and fluxes of the hydrogen atom in OH and ≡CH bonds agree well with the observed spectroscopic characteristics of inter- and intramolecular hydrogen bonding. The main reason for the differences between the two types of the hydrogen bond lies in the molecular structure because favorable linear proton donor-acceptor arrangement is not possible to achieve within a small molecule. The calculated intensities (in vacuo and in polarizable continuum) are only in qualitative agreement with the measured data.     

气相水杨酸激发态分子内质子转移特性分析

采用密度泛函理论(DFT)和含时密度泛函理论(TD-DFT)研究了气相水杨酸(SA)分子的激发态氢键动力学过程。通过对水杨酸分子基态和激发态结构的优化,以及对其稳态吸收和发射光谱特性、前线分子轨道、红外振动光谱和势能曲线的计算分析,阐明水杨酸分子内质子转移可在激发态下自发地发生,导致其激发态可存在烯醇式和酮式两种异构体结构,并揭示了这种质子转移源于分子内电荷转移的激发态氢键的加强机制。     

Neutral and ionic hydrogen bonding in Schiff bases

Low-temperature, high-resolution X-ray studies of charge distributions in the three Schiff bases, the dianil of 2-hydroxy-5-methylisophthaldehyde, 3,5-dinitro-N-salicylidenoethylamine and 3-nitro-N-salicylidenocyclohexylamine, have been carried out. These structures exhibit interesting weak interactions, including two extreme cases of intramolecular hydrogen bonds that are ionic N(+)-H...O- and neutral O-H...N in nature. These two types of hydrogen bond reflect differences in geometrical parameters and electron density distribution. At the level of geometry, the neutral O-H...N hydrogen bond is accompanied by an increase in the length of the C(1)-O(1) bond, opening of the ipso-C(1) angle, elongation of the aromatic C-C bonds, shortening of the C(7)-N(2) bond and increased length of the C(1)-C(7) bond, relative to the ionic hydrogen bond type. According to the geometrical and critical point parameters, the neutral O-H...N hydrogen bond seems to be stronger than the ionic ones. There are also differences between charge density parameters of the aromatic rings consistent with the neutral hydrogen bond being stronger than the ionic ones, with a concomitant reduction in the aromaticity of the ring. Compounds with the ionic hydrogen bonds show a larger double-bond character in the C-O bond than appears in the compound containing a neutral hydrogen bond; this suggests that the electronic structure of the former pair of compounds includes a contribution from a zwitterionic canonical form. Furthermore, in the case of ionic hydrogen bonds, the corresponding interaction lines appear to be curved in the vicinity of the hydrogen atoms. In the 3-nitro-N-salicylidenocyclohexylamine crystal there exists, in addition to the intramolecular hydrogen bond, a pair of intermolecular O...H interactions in a centrosymmetric dimer unit.     

Direct observation of the substitution effects on the hydrogen bridge dynamics in selected Schiff bases--a comparative molecular dynamics study

We have studied substituent effects on the properties of the intramolecular hydrogen bond of some ortho-hydroxy Schiff bases using density functional theory (DFT) based first-principle molecular dynamics (FPMD) and path integral molecular dynamics. The studied compounds possess a strong intramolecular hydrogen bond (r((O???N)) ≤ 2.6 A?), which can be tuned by substitution to either (i) enhance the basicity of the acceptor moiety by induction effects or (ii) decrease the hydrogen bond length through steric repulsion. DFT calculations and FPMD were employed to investigate structural and dynamical properties of the selected molecules, while quantum effects on the structural properties were assessed using path integral FPMD. The simulations were performed in vacuo and in the solid state to study the influence of the environment on the hydrogen bond and spectroscopic properties. We give computational support to the suggestion that induction effects are less effective to tune the intramolecular hydrogen bond properties of the discussed ortho-hydroxy Schiff bases than the steric or the environmental effects.     

Solvent‐Dependent Structure of the I3− Ion Derived from Photoelectron Spectroscopy and Ab Initio Molecular Dynamics Simulations

Ab initio molecular dynamics (MD) simulations of the solvation of LiI₃ in four different solvents (water, methanol, ethanol, and acetonitrile) are employed to investigate the molecular and electronic structure of the I₃^? ion in relation to X‐ray photoelectron spectroscopy (XPS). Simulations show that hydrogen‐bond rearrangement in the solvation shell is coupled to intramolecular bond‐length asymmetry in the I₃^? ion. By a combination of charge analysis and I 4 d core‐level XPS measurements, the mechanism of the solvent‐induced distortions has been studied, and it has been concluded that charge localization mediates intermolecular interactions and intramolecular distortion. The approach involving a synergistic combination of theory and experiment probes the solvent‐dependent structure of the I₃^? ion, and the geometric structure has been correlated with the electronic structure.     

A theoretical investigation on the excited state intramolecular single or double proton transfer mechanism of a salicyladazine system

Given the tremendous potential applications of excited state intramolecular proton transfer (ESIPT) systems, ESIPT molecules have received widespread attention. In this work, based on density functional theory (DFT) and time‐dependent DFT (TDDFT) methods, we theoretically study the excited state dynamical behaviors of salicyladazine (SA) molecules. Our simulated results show that the double intramolecular hydrogen bonds of SA are strengthened in the S₁ state via exploring bond distances, bond angles, and infrared (IR) vibrational spectra. Exploring the frontier molecular orbitals (MOs), we confirm that charge redistributions indeed have effects on excited state dynamical behaviors. The increased electronic densities on N atoms and the decreased electronic densities on O atoms imply that charge redistribution may trigger the ESPT process. Analyzing the constructed S₀‐state and S₁‐state potential energy surfaces (PESs), we confirm that only the excited state single proton transfer reaction can occur although SA possesses two intramolecular hydrogen bonds. In this work, we clarify the specific ESIPT mechanism, which may facilitate developing novel applications based on the SA system in future.     

Intramolecular hydrogen exchanges during the electron impact-induced fragmentation of complex alicyclic amines

The three-step mechanism of formation of the important fragment ion of mass 84 from 3-dimethylaminosteroids under electron impact is accompanied by intramolecular hydrogen exchanges, as shown by the mass spectra of deuterium labelled derivatives. The hydrogen exchanges are shown to occur in the ring A opened molecular ion, i.e. after the initial C-3–-C-4 bond rupture, and to compete with the final C-1–-C-10 bond rupture. (The intensity of hydrogen-deuterium exchange reactions depends upon the activation energy of the final rupture.) The electron impact-induced-fragmentation of large alicyclic molecules bearing a strong charge stabilizing functional group, such as amines, appears to be the result of a series of displacements of the radical over different sites of the molecular ion through hydrogen (or carbon) rearrangements, with some of the radical sites being in the appropriate position to induce a final rupture.     

A DFT Study on Intramolecular Hydrogen Bond in Substituted Catechols and Their Radicals

Density functional theory(DFT)at B3LYP/6-31G(d,p) level was employed to calculate intramolecular hydrogen bond enthalpies (H1HB),O-H charge differencces,O-H bond lengths and bond orders for various substituted catechols and their radicals generated after H-abstraction.It was found that although the charge difference between hydrogen-bonded H and O played a role in determining H1HB,H1HB was mainly governed by the hydrogen bond length.As the oxygen-centedred radiocal has great tendency to form a chemical bond with the H atom,hydrogen bond lengths in catechloic radicals are systematically shorter than those in catechlos,hence,the H1HB for the former are higher than those for the latter.     

Ultrafast Relaxation Dynamics of the Excited States of 1‐Amino‐ and 1‐(N,N‐Dimethylamino)‐fluoren‐9‐ones

The dynamics of the excited states of 1‐aminofluoren‐9‐one (1AF) and 1‐(N,N‐dimethylamino)‐fluoren‐9‐one (1DMAF) are investigated by using steady‐state absorption and fluorescence as well as subpicosecond time‐resolved absorption spectroscopic techniques. Following photoexcitation of 1AF, which exists in the intramolecular hydrogen‐bonded form in aprotic solvents, the excited‐state intramolecular proton‐transfer reaction is the only relaxation process observed in the excited singlet (S₁) state. However, in protic solvents, the intramolecular hydrogen bond is disrupted in the excited state and an intermolecular hydrogen bond is formed with the solvent leading to reorganization of the hydrogen‐bond network structure of the solvent. The latter takes place in the timescale of the process of solvation dynamics. In the case of 1DMAF, the main relaxation pathway for the locally excited singlet, S₁(LE), or S₁(ICT) state is the configurational relaxation, via nearly barrierless twisting of the dimethylamino group to form the twisted intramolecular charge‐transfer, S₁(TICT), state. A crossing between the excited‐state and ground‐state potential energy curves is responsible for the fast, radiationless deactivation and nonemissive character of the S₁(TICT) state in polar solvents, both aprotic and protic. However, in viscous but strong hydrogen‐bond‐donating solvents, such as ethylene glycol and glycerol, crossing between the potential energy surfaces for the ground electronic state and the hydrogen‐bonded complex formed between the S₁(TICT) state and the solvent is possibly avoided and the hydrogen‐bonded complex is weakly emissive.     

Reinvestigation of intramolecular hydrogen bond in malonaldehyde derivatives: An ab initio,AIM and NBO study

The RAHB systems in malonaldehyde and its derivatives at MP2/ 6‐311++G(d,p) level of theory were studied and their intramolecular hydrogen bond energies by using the related rotamers method was obtained. The topological properties of electron density distribution in O? H···O intramolecular hydrogen bond have been analyzed in term of quantum theory of atoms in molecules (QTAIM). Correlations between the H‐bond strength and topological parameters are probed. The results of QTAIM clearly showed that the linear correlation between the electron density distribution at HB critical point and RAHB ring critical point with the corresponding hydrogen bond energies was obtained. Moreover, it was found a linear correlation between the electronic potential energy density, V(r_cp), and hydrogen bond energy which can be used as a simple equation for evaluation of HB energy in complex RAHB systems. Finally, the similar linear treatment between the geometrical parameters, such as O···O or O? H distance, and Lp(O)→σ*_OH charge transfer energy with the intramolecular hydrogen bond energy is observed. © 2010 Wiley Periodicals, Inc., Int J Quantum Chem, 2011     

Atomistic molecular dynamics simulations on the interaction of TEMPO-oxidized cellulose nanofibrils in water

Atomistic molecular dynamics simulations were carried out to obtain information on the rheological, aggregation and disintegration properties of carboxylated (TEMPO-oxidized) cellulose nanofibrils with different functionalization levels. The magnitude of the inter-fibril interaction was quantified for parallel nanofibrils using the umbrella sampling method. The obtained potential of mean force was found highly sensitive to the charge configuration for intermediate functionalization levels. This feature was further studied with an electrostatic model for similar charge configurations and system periodicity as in the case of the molecular dynamics simulations. The electrostatic contribution of the charged surfaces varied from repulsive to attractive depending on the distribution of the carboxylate groups and nearby counter-ions, as well as the distance between the fibrils. The simulated deviations from average behavior for single fibrils in both models suggest heterogeneity in their aggregation and disintegration behavior. This was seen in disintegration experiments, where the differences in disintegration energy and in the structural variation qualitatively agreed with the model predictions. As to aggregation behavior, the studied case with parallel fibrils reflects the upper boundary of the repulsive interaction.     

Structural changes of 2-dimethylamino-benzoic acid in solution—a spectral deconvolution study

Fourier deconvolution has resolved the broad carboxylic bands in 2-dimethylamino-benzoic acid to a number of individual bands which helped to identify the various species in solution. While neutral species with or without an intramolecular hydrogen bond predominate in non-polar solvents, the Zwitterionic species with or without an intramolecular hydrogen bond are the main species in polar solvents. The strong solvating power of water was able to break the strong intramolecular charge transfer hydrogen bond present in 2-dimethylamino-benzoic acid.     

TDDFT Study on Excited-State Hydrogen Bonding of 2'-Deoxyguano-sine in H2O Solution

The mono and dihydrated complexes of 2'-deoxyguanosine have been used to elucidate the importance of the 2'-hydroxy group in the hydration. Density functional theory and time-dependent density functional theory methods were performed to investigate the ground-and excited-state hydrogen bonding properties of 2'-deoxyguanosine-water (2'-dG-W) and 2'-deoxyguanosine-2water (2'-dG-2W). Infrared spectra, geometric optimizations, frontier molecular orbitals and Mulliken charges have also been studied. The results demonstrated that the excited-state intramolecular hydrogen bonding dynamics of complexes 2'-dG-W and 2'-dG-2W behaves differently upon photoexcitation, while their intermolecular hydro-gen bonding dynamics behaves similarly. Moreover, the significant weakening of the inter-molecular hydrogen bond O4···H1?N1 and the formation of the new strong hydrogen bond O4···H3?N2 in the 2'-dG-2W upon photoexcitation were due to the geometric structure bending of guanine and the rigidity of related molecules. In addition, the charge transfer properties were theoretically investigated by analysis of molecular orbital.     

A Mixed Quantum-Classical Approach for Computing Effects of Intramolecular Motion on the Low-Barrier Hydrogen Bond

The fractionation factor is defined as the equilibrium constant for the reaction: R – H + DOH R – D + HOH. Of interest are values of fractionation factors for reactions where reactants and/or products form intramolecular low-barrier hydrogen bonds. Experimentally measured isotopic fractionation factors are usually interpreted via a one-dimensional potential energy surface along the intrinsic proton hydrogen bond coordinate. Such a one-dimensional picture cannot be completely correct. Intramolecular motions, such as vibrations and librations, can modulate the underlying potential energy surface along the hydrogen bond coordinate and thus affect the isotopic fractionation factor. We have recently generated a picture of the motion of the proton in a low-barrier hydrogen bond as taking place in an effective single-dimensional potential, which we term the potential of mean force (PMF). In this paper, we compute the PMF for a molecule with an intramolecular hydrogen bond in order to quantify the effect of intramolecular motions on the fractionation factor. The PMF and isotopic fractionation factor are computed with a combination of high-level density functional theory and molecular dynamics simulations.     

A theoretical study on the excited‐state intramolecular proton transfer mechanism of 4′‐dimethylaminoflavonol chemosensor

In this work, density functional theory (DFT) and time‐dependent density functional theory (TDDFT) methods are used to explore the excited‐state intramolecular proton transfer (ESIPT) mechanism of a novel system 4′‐dimethylaminoflavonol (DAF). By analyzing the molecular electrostatic potential (MEP) surface, we verify that the intramolecular hydrogen bond in DAF exists in both the S₀ and S₁ states. We calculate the absorption and emission spectra of DAF in two solvents, which reproduce the experimental results. By comparing the bond lengths, bond angles, and relative infrared (IR) vibrational spectra involved in the hydrogen bonding of DAF, we confirm the hydrogen‐bond strengthening in the S₁ state. For further exploring the photoexcitation, we use frontier molecular orbitals to analyze the charge redistribution properties, which indicate that the charge transfer in the hydrogen‐bond moiety may be facilitating the ESIPT process. The constructed potential energy curves in acetonitrile and methylcyclohexane solvents with shortened hydrogen bond distances demonstrate that proton transfer is more likely to occur in the S₁ state due to the lower potential barrier. Comparing the results in the two solvents, we find that aprotic polar and nonpolar solvents seem to play similar roles. This work not only clarifies the excited‐state behaviors of the DAF system but also successfully explains its spectral characteristics.     

The Double‐Faced Nature of Hydrogen Bonding in Hydroxy‐Functionalized Ionic Liquids Shown by Neutron Diffraction and Molecular Dynamics Simulations

We characterize the double‐faced nature of hydrogen bonding in hydroxy‐functionalized ionic liquids by means of neutron diffraction with isotopic substitution (NDIS), molecular dynamics (MD) simulations, and quantum chemical calculations. NDIS data are fit using the empirical potential structure refinement technique (EPSR) to elucidate the nearest neighbor H???O and O???O pair distribution functions for hydrogen bonds between ions of opposite charge and the same charge. Despite the presence of repulsive Coulomb forces, the cation–cation interaction is stronger than the cation–anion interaction. We compare the hydrogen‐bond geometries of both “doubly charged hydrogen bonds” with those reported for molecular liquids, such as water and alcohols. In combination, the NDIS measurements and MD simulations reveal the subtle balance between the two types of hydrogen bonds: The small transition enthalpy suggests that the elusive like‐charge attraction is almost competitive with conventional ion‐pair formation.     

氢键对水杨酸酸度影响的量子化学研究

分子内氢键的形成影响羧酸的酸性,实验测定表明,水杨酸的酸性大约比苯甲酸强18倍,而其异构体——对羟基苯甲酸的酸性仅为苯甲酸的一半,已有文献从氢键形成角度定性讨论这一问题,我们采用CNDO/2法对其影响实质给予进一步的探讨,从微观角度阐述了该过程的可能机理,得到了与实验一致的结果。     

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.     

Ultrafast 2D IR anisotropy of water reveals reorientation during hydrogen-bond switching

Rearrangements of the hydrogen bond network of liquid water are believed to involve rapid and concerted hydrogen bond switching events, during which a hydrogen bond donor molecule undergoes large angle molecular reorientation as it exchanges hydrogen bonding partners. To test this picture of hydrogen bond dynamics, we have performed ultrafast 2D IR spectral anisotropy measurements on the OH stretching vibration of HOD in D(2)O to directly track the reorientation of water molecules as they change hydrogen bonding environments. Interpretation of the experimental data is assisted by modeling drawn from molecular dynamics simulations, and we quantify the degree of molecular rotation on changing local hydrogen bonding environment using restricted rotation models. From the inertial 2D anisotropy decay, we find that water molecules initiating from a strained configuration and relaxing to a stable configuration are characterized by a distribution of angles, with an average reorientation half-angle of 10°, implying an average reorientation for a full switch of ≥20°. These results provide evidence that water hydrogen bond network connectivity switches through concerted motions involving large angle molecular reorientation.