Intramolecular Hydrogen Bonds Quench Photoluminescence and Enhance Photocatalytic Activity of Carbon Nanodots

Understanding the photoluminescence (PL) and photocatalytic properties of carbon nanodots (CNDs) induced by environmental factors such as pH through surface groups is significantly important to rationally tune the emission and photodriven catalysis of CNDs. Through adjusting the pH of an aqueous solution of CNDs, it was found that the PL of CNDs prepared by ultrasonic treatment of glucose is strongly quenched at pH 1 because of the formation of intramolecular hydrogen bonds among the oxygen‐containing surface groups. The position of the strongest PL peak and its corresponding excitation wavelength strongly depend on the surface groups. The origins of the blue and green emissions of CNDs are closely related to the carboxyl and hydroxyl groups, respectively. The deprotonated COO^? and CO^? groups weaken the PL peak of the CNDs and shift it to the red. CNDs alone exhibit photocatalytic activity towards degradation of Rhodamine B at different pH values under UV irradiation. The photocatalytic activity of the CNDs is the highest at pH 1 because of the strong intramolecular hydrogen bonds formed among the oxygen‐containing groups.     

Intramolecular Hydrogen Bonds in Low‐Molecular‐Weight Polyethylene Glycol

We used static DFT calculations to analyze, in detail, the intramolecular hydrogen bonds formed in low‐molecular‐weight polyethylene glycol (PEG) with two to five repeat subunits. Both red‐shifted O?H???O and blue‐shifting C?H???O hydrogen bonds, which control the structural flexibility of PEG, were detected. To estimate the strength of these hydrogen bonds, the quantum theory of atoms in molecules was used. Car–Parrinello molecular dynamics simulations were used to mimic the structural rearrangements and hydrogen‐bond breaking/formation in the PEG molecule at 300 K. The time evolution of the H???O bond length and valence angles of the formed hydrogen bonds were fully analyzed. The characteristic hydrogen‐bonding patterns of low‐molecular‐weight PEG were described with an estimation of their lifetime. The theoretical results obtained, in particular the presence of weak C?H???O hydrogen bonds, could serve as an explanation of the PEG structural stability in the experimental investigation.     

On the Nature of Blueshifting Hydrogen Bonds

The block‐localized wave function (BLW) method can derive the energetic, geometrical, and spectral changes with the deactivation of electron delocalization, and thus provide a unique way to elucidate the origin of improper, blueshifting hydrogen bonds versus proper, redshifting hydrogen bonds. A detailed analysis of the interactions of F₃CH with NH₃ and OH₂ shows that blueshifting is a long‐range phenomenon. Since among the various energy components contributing to hydrogen bonds, only the electrostatic interaction has long‐range characteristics, we conclude that the contraction and blueshifting of a hydrogen bond is largely caused by electrostatic interactions. On the other hand, lengthening and redshifting is primarily due to the short‐range n(Y)→σ*(X?H) hyperconjugation. The competition between these two opposing factors determines the final frequency change direction, for example, redshifting in F₃CH ??? NH₃ and blueshifting in F₃CH ??? OH₂. This mechanism works well in the series F_nCl_3?nCH ??? Y (n=0–3, Y=NH₃, OH₂, SH₂) and other systems. One exception is the complex of water and benzene. We observe the lengthening and redshifting of the O?H bond of water even with the electron transfer between benzene and water completely quenched. A distance‐dependent analysis for this system reveals that the long‐range electrostatic interaction is again responsible for the initial lengthening and redshifting.     

The Nature of Activated Non‐classical Hydrogen Bonds: A Case Study on Acetylcholinesterase–Ligand Complexes

Molecular recognition events in biological systems are driven by non‐covalent interactions between interacting species. Here, we have studied hydrogen bonds of the CH???Y type involving electron‐deficient CH donors using dispersion‐corrected density functional theory (DFT) calculations applied to acetylcholinesterase–ligand complexes. The strengths of CH???Y interactions activated by a proximal cation were considerably strong; comparable to or greater than those of classical hydrogen bonds. Significant differences in the energetic components compared to classical hydrogen bonds and non‐activated CH???Y interactions were observed. Comparison between DFT and molecular mechanics calculations showed that common force fields could not reproduce the interaction energy values of the studied hydrogen bonds. The presented results highlight the importance of considering CH???Y interactions when analysing protein–ligand complexes, call for a review of current force fields, and opens up possibilities for the development of improved design tools for drug discovery.     

Uncovering Intramolecular π‐Type Hydrogen Bonds in Solution by NMR Spectroscopy and DFT Calculations

Reaction between the phosphinito bridged diplatinum species [(PHCy₂)Pt(μ‐PCy₂){κ²P,O‐μ‐P(O)Cy₂}Pt(PHCy₂)](Pt–Pt) ( 1 ), and (trimethylsilyl)acetylene at 273 K affords the σ‐acetylide complex [(PHCy₂)(η¹‐Me₃SiC≡C)Pt(μ‐PCy₂)Pt(PHCy₂){κP‐P(OH)Cy₂}](Pt–Pt) ( 2 ) featuring an intramolecular π‐type hydrogen bond. Scalar and dipolar couplings involving the POH proton were detected by 2D NMR experiments. Relativistic DFT calculations of the geometry, relative energy, and NMR properties of model systems of 2 confirmed the structural assignment and allowed the energy of the π‐type hydrogen bond to be estimated (ca. 22 kJ mol^?1).     

Theoretical Study of Structure,Energetic, Hydrogen Bonds Strength and Intramolecular Proton Transfer of 2‐(2‐R (ROH,NH2, SH) Phenyl (or Pyridyl)) Benzoxazoles (or Benzothiazoles)

The ground‐ and excited‐state intramolecular proton transfer processes of 2‐(2‐R (R?OH, NH₂, SH) phenyl (or pyridyl)) benzoxazoles (or benzothiazoles) are investigated by the DFT methods. The calculated results indicate that in the ground state there is a high correlation (R=0.9950) between the proton transfer barrier and the intramolecular hydrogen bonds (IMHB) strength. The increase of the strength of IMHB in the proton transfer processes leads to a larger barrier contributions. Intramolecular proton transfer process pathway is along with the minimal difference of change value in the IMHB angle. In the excited‐state, there is a similar relationship between the IMHB and the barrier.     

Computational Design of Iron Diphosphine Complexes with Pendant Amines for Hydrogenation of CO2 to Methanol: A Mimic of [NiFe] Hydrogenase

Inspired by the active‐site structure of the [NiFe] hydrogenase, we have computationally designed the iron complex [P^tBu₂N^tBu₂)Fe(CN)₂CO] by using an experimentally ready‐made diphosphine ligand with pendant amines for the hydrogenation of CO₂ to methanol. Density functional theory calculations indicate that the rate‐determining step in the whole catalytic reaction is the direct hydride transfer from the Fe center to the carbon atom in the formic acid with a total free energy barrier of 28.4 kcal mol^?1 in aqueous solution. Such a barrier indicates that the designed iron complex is a promising low‐cost catalyst for the formation of methanol from CO₂ and H₂ under mild conditions. The key role of the diphosphine ligand with pendent amine groups in the reaction is the assistance of the cleavage of H₂ by forming a Fe?H^δ????H^δ+?N dihydrogen bond in a fashion of frustrated Lewis pairs.     

Influence of Solvent Polarity and Hydrogen Bonding on the Electronic Transition of Coumarin 120: A TDDFT Study

The characteristics of the electronic transition energy of Coumarin 120 (C120) and its H‐bonded complexes in various solvents have been examined by time‐dependent density functional theory (TDDFT) in combination with a polarizable continuum solvent model (PCM). Molecular structures of C120 and its H‐bonded complexes are optimized with the B3LYP method in PCM solution, and the dihedral angle H14? N13? C7? H15 is dependent on solvent polarity and the type of H‐bond. A linear correlation of the absorption maximum of C120 with the solvent polarity function is revealed with the PCM model for all solvents except DMSO. The experimental absorption maximum of C120 in nine solvents is well described by a PCM–TDDFT scheme augmented with explicit inclusion of a few H‐bonded solvent molecules, and quantitative agreement between our calculated results and experimental measurements is obtained with an average error of less than 2 nm. H‐bonding at three different sites shifts the absorption wavelength of C120 either to the blue or to the red, that is, a significant role is played by solvent molecules in the first solvation shell in determining the electronic transition energy of C120. The dependence on the H‐bonding site and solvent polarity is examined by using the Kamlet–Taft equation for solvatochromism.     

Halogen Bonding versus Hydrogen Bonding: A Molecular Orbital Perspective

We have carried out extensive computational analyses of the structure and bonding mechanism in trihalides DX⋅⋅⋅A⁻ and the analogous hydrogen-bonded complexes DH⋅⋅⋅A⁻ (D, X, A=F, Cl, Br, I) using relativistic density functional theory (DFT) at zeroth-order regular approximation ZORA-BP86/TZ2P. One purpose was to obtain a set of consistent data from which reliable trends in structure and stability can be inferred over a large range of systems. The main objective was to achieve a detailed understanding of the nature of halogen bonds, how they resemble, and also how they differ from, the better understood hydrogen bonds. Thus, we present an accurate physical model of the halogen bond based on quantitative Kohn–Sham molecular orbital (MO) theory, energy decomposition analyses (EDA) and Voronoi deformation density (VDD) analyses of the charge distribution. It appears that the halogen bond in DX⋅⋅⋅A⁻ arises not only from classical electrostatic attraction but also receives substantial stabilization from HOMO–LUMO interactions between the lone pair of A⁻ and the σ* orbital of D–X.     

A Theoretical Study on the Structure,Intramolecular Interactions,and Detonation Performance of Hydrazinium Dinitramide

The structures of hydrazinium dinitramide (HDN) in the gas phase and in aqueous solution have been studied at different levels of theory by using quantum chemistry. The intramolecular hydrogen‐bond interactions in HDN were studied by employing the quantum theory of atoms in molecules (QTAIM), as well as those in ammonium dinitramide (ADN), hydrazinium nitroformate (HNF), and ammonium nitroformate (ANF) for comparison. The results showed that HDN possessed the strongest hydrogen bonds, with the largest hydrogen‐bond energy (?47.95 kJ mol^?1) and the largest total hydrogen‐bond energy (?60.29 kJ mol^?1). In addition, the charge transfer between the cation and the anion, the binding energy, the energy difference between the frontier orbitals, and the second‐order perturbation energy of HDN were all the largest among the investigated compounds. These strongest intramolecular interactions accounted for the highest decomposition temperature of HDN among all four compounds. The IR spectra in the gas phase and in aqueous solution were very different and showed the significant influence of the solvent. The UV spectrum showed the strongest absorption at about 253 nm. An orbital‐interaction diagram demonstrated that the transition of electrons mainly happened inside the anion of HDN. The detonation velocity (D=8.34 km s^?1) and detonation pressure (P=30.18 GPa) of HDN were both higher than those of ADN (D=7.55 km s^?1 and P=24.83 GPa). The composite explosive HDN/CL‐20 with the weight ratio w_CL?20/w_HDN=0.388:0.612 showed the best performance (D=9.36 km s^?1, P=39.82 GPa), which was close to that of CL‐20 (D=9.73 km s^?1, P=45.19 GPa) and slightly better than that of the composite explosive ADN/CL‐20 (w_CL?20/w_ADN=0.298:0.702, D=9.34 km s^?1, P=39.63 GPa).     

Ab initio Study of the Interactions between CO2 and N‐Containing Organic Heterocycles

In the garden of dispersion: High‐accuracy ab initio calculations are performed to determine the nature of the interactions and the most favorable geometries between CO₂ and heteroaromatic molecules containing nitrogen (see figure). Dispersion forces play a key role in the stabilization of the dimer, because correlation effects represent about 50 % of the total interaction energy.

     

Hydrogen Bonding between Metal‐Ion Complexes and Noncoordinated Water: Electrostatic Potentials and Interaction Energies

The hydrogen bonding of noncoordinated water molecules to each other and to water molecules that are coordinated to metal‐ion complexes has been investigated by means of a search of the Cambridge Structural Database (CSD) and through quantum chemical calculations. Tetrahedral and octahedral complexes that were both charged and neutral were studied. A general conclusion is that hydrogen bonds between noncoordinated water and coordinated water are much stronger than those between noncoordinated waters, whereas hydrogen bonds of water molecule in tetrahedral complexes are stronger than in octahedral complexes. We examined the possibility of correlating the computed interaction energies with the most positive electrostatic potentials on the interacting hydrogen atoms prior to interaction and obtained very good correlation. This study illustrates the fact that electrostatic potentials computed for ground‐state molecules, prior to interaction, can provide considerable insight into the interactions.     

Carbon Dioxide Capture and Release by Anions with Solvent‐Dependent Behaviour: A Theoretical Study

Recently, Clyburne and co‐workers [Science, 2014 , 344, 75–78] reported the novel synthesis of the elusive cyanoformate anion, NCCO₂^?. The stability of this anion is dependent on the dielectric constant of the local environment (polarity‐switchable solvent): it is stable in low‐polarity media and unstable in high‐polarity solvents; hence, capturing and then releasing CO₂. The possibility of extending such behaviour to other anions is explored herein. Specifically, the CO₂ capture process is studied for 26 anions in the gas phase and 3 distinct solvents (water, tetrahydrofuran, and toluene) by using the polarisable continuum model. Calculations are performed with the M06‐2X and B3LYP‐D3 density functionals and the aug‐cc‐pVTZ basis set. The design of new CO₂ complexes with the anion, which can be formed or destroyed on demand by changing the solvent, is possible; the results for the alkoxylate and thiolate anions are especially promising. The nature of the substituents connected to the atom that bonds to CO₂ in the anion is crucial in modulating the relative stability of the products—a key point for reversibility in the CO₂ capture process. A moderate interaction for the anion–CO₂ adduct—about 10 kcal mol^?1 relative free energy with respect to the isolated reactants in the gas phase—and a relevant effect in the dielectric constant of the local environment are also key ingredients to achieve solvent dependency.     

Mechanistic Insight into the CO2 Capture by Amidophosphoranes: Interplay of the Ring Strain and the trans Influence Determines the Reactivity of the Frustrated Lewis Pairs

CO₂ capture has attracted increasing attention owing to its contribution to global warming and climate change as a greenhouse gas. As an alternative strategy to transition‐metal‐based chemistry and catalysis, frustrated Lewis pairs have been developed to sequester CO₂ efficiently under mild conditions. However, the mechanism of CO₂ sequestration with amidophosphoranes remains unclear. Herein, we present a thorough density functional theory study on a series of amidophosphoranes. Our results reveal that the interplay of the ring strain and the trans influence determines the reactivities, thus opening a new avenue to the design of frustrated Lewis pairs for CO₂ capture.     

Directionality of Dihydrogen Bonds: The Role of Transition Metal Atoms

A theoretical study on two series of electron‐rich group 8 hydrides is carried out to evaluate involvement of the transition metal in dihydrogen bonding. To this end, the structural and electronic parameters are computed at the DFT/B3PW91 level for hydrogen‐bonded adducts of [(PP₃)MH₂] and [Cp*MH(dppe)] (M=Fe, Ru, Os; PP₃=κ⁴‐P(CH₂CH₂PPh₂)₃, dppe= κ²‐Ph₂PCH₂CH₂PPh₂) with CF₃CH₂OH (TFE) as proton donor. The results are compared with those of adduct [Cp₂NbH₃] ? TFE featuring a “pure” dihydrogen bond, and classical hydrogen bonds in pyridine ? TFE and Me₃N ? TFE. Deviation of the H ??? H? A fragment from linearity is shown to originate from the metal participation in dihydrogen bonding. The latter is confirmed by the electronic parameters obtained by NBO and AIM analysis. Considered together, orbital interaction energies and hydrogen bond ellipticity are salient indicators of this effect and allow the MH ??? HA interaction to be described as a bifurcate hydrogen bond. The impact of the M ??? HA interaction is shown to increase on descending the group, and this explains the experimental trends in mechanisms of proton‐transfer reactions via MH ??? HA intermediates. Strengthening of the M ??? H interaction in the case of electron‐rich 5d metal hydrides leads to direct proton transfer to the metal atom.