The role of hydroxyl groups in interchain interactions in cellulose Iα and Iβ

Complex networks of hydrogen bonds within the cellulose I_α and I_β contribute greatly to cellulose's anisotropic physical properties such as material stiffness. The interchain hydrogen bonding interactions through hydroxyl groups are isolated in each of the three lattice planes of the adjacent chains within the unit cell of two allomorphs of natural cellulose. In our density function theory study with dispersion corrected Perdew–Burke–Ernzerhof (PBE‐D2) functional, these hydroxyl groups participate in strong hydrogen bonding interactions (?24.8 and ?24.8 kcal/mol for cellulose I_α and I_β, respectively) in the side‐to‐side lattice plane. Unexpectedly, the hydroxyl groups also participate significantly in hydrogen bonding interactions (?11.0 and ?12.4 kcal/mol for cellulose I_α and I_β, respectively) in one of the diagonal lattice planes in both cellulose I_α and I_β. Both PM7 and PBE‐D2 method predict that the overall interaction is asymmetric and stronger in the right diagonal lattice plane. While hydrogen bonding interactions are strongest in side‐to‐side lattice plane as expected, the role of hydrogen bonding interactions for keeping the sheet together is more significant than previously thought.     

CO2‐Cross‐Linked Frustrated Lewis Networks as Gas‐Regulated Dynamic Covalent Materials

The design of structurally dynamic molecular networks can offer strategies for fabricating stimuli‐responsive adaptive materials. Herein we first report a gas‐responsive dynamic gel system based on frustrated Lewis pair (FLP) chemistry. Two trefoil‐like molecules with bulky triphenylborane and triphenylphosphine groups are synthesized as complementary Lewis acid and base with trivalent sites. They can together bind CO₂ gas molecules and further form a cross‐linked network via the bonding interactions between FLPs and CO₂. Such CO₂‐bridged dative linkages are shown to be dynamic covalent bonds, which endow the frustrated Lewis network with adaptable behaviors and unprecedented gas‐regulated viscoelastic, mechanical, and self‐healing performance. This study is an initial attempt to apply the FLP concept in materials chemistry, but we believe that this strategy will open a promising future for gas‐sensitive smart materials.     

Investigation of substituent effects in aerogen‐bonding interaction between ZO3 (Z=Kr,Xe) and nitrogen bases

The substituent effects in aerogen bond interactions between ZO₃ (Z = Kr, Xe) and different nitrogen bases are studied at the MP2/aug‐cc‐pVTZ level of theory. The nitrogen bases include the sp bases NCH, NCF, NCCl, NCBr, NCCN, NCOH, NCCH₃ and the sp³ bases NH₃, NH₂F, NH₂Cl, NH₂Br, NH₂CN, NH₂OH, and NH₂CH₃. The nature of aerogen bonds in these complexes is analyzed by means of molecular electrostatic potential, electron localization function, quantum theory atoms in molecules, noncovalent interaction index, and natural bond orbital analyses. The interaction energy (E_int) ranges from ?4.59 to ?9.65 kcal/mol in the O₃Z···NCX complexes and from ?5.30 to ?13.57 kcal/mol in the O₃Z···NH₂X ones. The dominant charge‐transfer interaction in these complexes occurs across the aerogen bond from the nitrogen lone‐pair (n_N) of the Lewis base to the σ*_Z‐O antibonding orbital of the ZO₃. Besides, the formation of aerogen bond tends to decrease the ⁸³Kr or ¹³¹Xe chemical shielding values in these complexes. © 2016 Wiley Periodicals, Inc.     

A benchmark quantum chemical study of the stacking interaction between larger polycondensed aromatic hydrocarbons

Large-scale electronic structure calculations were performed for the interaction energy between coronene, C₂₄H₁₂ with circumcoronene, C₅₄H₁₈, and between two circumcoronene molecules, in order to get a picture of the interaction between larger graphene sheets. Most calculations were performed at the SCS-MP2 level but we have corrected them for higher-order correlation effects using a calculation on the coronene-circumcoronene system at the quadratic CI, QCISD(T) level. Our best estimate for the interaction energy between coronene and circumcoronene is 32.1?kcal/mol. We estimate the binding of coronene on a graphite surface to be 37.4 or 1.56?kcal/mol per carbon atom (67.5?meV/C atom). This is also our estimate for the exfoliation energy of graphite. It is higher than most previous theoretical estimates. The SCS-MP2 method which reproduces the CCSD(T) and QCISD(T) values very well for smaller aromatic hydrocarbons, e.g., for the benzene dimer, increasingly overestimates dispersion as the bandgap (the HOMO-LUMO separation) decreases. The barrier to the sliding motion of coronene on circumcoronene is 0.45?kcal/mol, and for two circumcoronene molecules 1.85?kcal/mol (0.018 and 0.034?kcal/mol per C atom, respectively). This means that larger graphenes cannot easily glide over each other.     

Strong interactions through the XAu–Y bridge: the Au bond?

The interaction between AuOH and the lone-pair donors (HF, H₂O) is shown to result in well-bound complexes whose structure resembles that of the corresponding H-bonded systems with the gold atom replacing hydrogen. The dissociation energies are estimated to be 10.7 and 27.4 kcal/mol for HFAu–OH and H₂OAu–OH, respectively. However, the interaction between AuOH and the lone pair donors is found to involve significant charge transfer. Furthermore, the Au–O stretching frequency increases upon the complex formation. It is concluded that, in spite of certain similarity to the H-bonded species, the Au-bonded complexes should be considered as Lewis acid–base pairs.     

The π-hole tetrel bond between X2TO and CO2: Substituent effects and its potential adsorptivity for CO2

Quantum chemical calculations are applied to study the complexes between X₂TO (X = H, F, Cl, Br, CH₃; T = C, Si, Ge, Sn) and CO₂. The carbon atom of CO₂ as a Lewis acid participates in the C···O carbon bond, whereas its oxygen atom as a base engages in the O···T tetrel bond with X₂TO. Most of complexes are stabilized by a combination of both C···O and O···T interactions. The interaction energy increases in the T = C < Ge < Sn < Si sequence for most complexes. Both the electron-withdrawing halogen group and the electron-donating methyl group increase the interaction energy, up to 51 kJ/mol in F₂SiO···CO₂. One F₂SiO molecule can bind with different numbers of CO₂ molecules (1–4); as the number of CO₂ molecules increases, the average interaction energy for each CO₂ decreases and each CO₂ molecule can contribute with at least 27 kJ/mol. Therefore, silicon-containing molecules are good absorbents for CO₂.     

FT-IR study on interactions between solutes and entrainers in supercritical carbon dioxide

Fourier transform infrared (FT-IR) spectroscopy has been used to measure the molarities of hydrogen bonding species between carboxylic acids (acetic acid and palmitic acid) and water in supercritical CO₂. The equilibrium constants of dimerization for the carboxylic acids were determined in supercritical CO₂ with octane. Further, the interactions of propanol-d (1- and 2-propanol-d) or xylenol (2,5-, 2,6- and 3,4-xylenol) isomers with acetone in supercritical CO₂ were studied. Experiments were carried out at 308.2–313.2 K and 7.0–20.0 MPa. The molarities of hydrogen bonding species between the carboxylic acids and water in supercritical CO₂ increase with the increasing molarity of water. The carboxylic acids interact more easily with ethanol than water in supercritical CO₂. For supercritical CO₂+carboxylic acid+octane systems, the equilibrium constants between the carboxylic acid monomer and dimer increase with the increasing molarity of octane. The equilibrium constants of the carboxylic acids seem to approach to those in liquid paraffin according to addition of octane in supercritical CO₂. The amount of the interaction species between 1-propanol-d and acetone is larger than that between 2-propanol-d and acetone. The amount of acetone interacting with OH group for 3,4-xylenol is the largest among those for xylenol isomers. These differences among the isomers may be caused by the screen effects of methyl groups around hydroxyl group for the isomers.     

Characterization of <Emphasis Type="Italic">σ</Emphasis>-hole interactions in 1:1 and 1:2 complexes of YOF<Subscript>2</Subscript>X (X = F,Cl, Br,I; Y = P,As) with ammonia: competition between halogen and pnicogen bonds

Ab initio calculations at the MP2/aug-cc-pVTZ level of theory are performed to examine 1:1 and 1:2 complexes of YOF₂X (X = F, Cl, Br, I; Y = P, As) with ammonia. The YOF₂X:NH₃ complexes are formed through the interaction of the lone pair of the ammonia with the σ-hole region associated with the X or Y atom of YOF₂X molecule. The calculated interaction energies of halogen-bonded complexes are between ?1.06 kcal/mol in the POF₃···NH₃ and ?6.21 kcal/mol in the AsOF₂I···NH₃ one. For a given Y atom, the largest pnicogen bond interaction energy is found for the YOF₃, while the smallest for the YOF₂I one. Almost a strong linear relationship is evident between the interaction energies and the magnitudes of the positive electrostatic potentials on the X and Y atoms. The results indicate that the interaction energies of halogen and pnicogen bonds in the ternary H₃N:YOF₂X:NH₃ systems are less negative relative to the respective binary systems. The interaction energy of Y···N bond is decreased by 1–22 %, whereas that of X···N bond by about 5–61 %. That is, both Y···N and X···N interactions exhibit anticooperativity or diminutive effects in the ternary complexes.     

Comparative analysis of blue‐shifted hydrogen bond versus conventional hydrogen bond in methyl radical complexes

Methyl radical complexes H₃C…HCN and H₃C…HNC have been investigated at the UMP2(full)/aug‐cc‐pVTZ level to elucidate the nature of hydrogen bonds. To better understand the intermolecular H‐bond interactions, topological analysis of electron density at bond critical points (BCP) is executed using Bader's atoms‐in‐molecules (AIM) theory. Natural bond orbital (NBO) analysis has also been performed to study the orbital interactions and change of hybridization. Theoretical calculations show that there is no essential difference between the blue‐shift H‐bond and the conventional one. In H₃C…HNC complex, rehybridization is responsible for shortening of the N? H bond. The hyperconjugative interaction between the single electron of the methyl radical and N? H antibonding orbital is up to 7.0 kcal/mol, exceeding 3.0 kcal/mol, the upper limit of hyperconjugative n(Y)→σ*(X–H) interaction to form the blue‐shifted H‐bond according to Alabugin's theory. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Smart Transformation of a Polyhedral Oligomeric Silsesquioxane Shell Controlled by Thiolate Silver(I) Nanocluster Core in Cluster@Clusters Dendrimers

Using polyhedral oligomeric silsesquioxane (POSS) modified by a thiol group as a protected ligand, atom‐precise multi‐heteorocluster‐based dendrimers Ag₁₂@POSS₆ ( 1 a and 1 b ) were assembled. Through the reactive ?SH groups, six POSS shell ligands stabilize the central 12‐core silver(I) cluster by diverse Ag?S interactions. When such Ag₁₂@POSS₆ complex was stimulated by different solvents (acetone or tetrahydrofuran), the core Ag₁₂ silver(I) cluster underwent reversible structural transformation between flattened cubo‐octahedral (in 1 a ) and normal cubo‐octahedral (in 1 b ); concomitantly shell POSS clusters rearranged from pseudo‐octahedral to quasi‐octahedral. Furthermore, the film matrix modified by 1 a or 1 b showed different hydrophobicity.     

Adsorption and dissociation of carbon trioxide on Ag(100)

Using density functional theory methods, we have studied carbon trioxide, its adsorption and dissociation on Ag(100). In the gas phase, two isomers are found, D_3h and C_2v, with the latter of 2.0 kcal mol^?1 lower in energy at the PW91PW91/6?31G(d) level. For CO₃ on Ag(100), the calculated adsorption energy is 91.2 and 89.1 kcal mol^?1 for the bi‐coord perpendicular and tri‐coord parallel structures, respectively. Upon the adsorption, 0.50 ～ 0.56 electron is transferred from silver to CO₃, indicative of significant ionic characters of the adsorbate‐surface bonding. In addition, the geometry of CO₃ is largely changed by its strong interaction with silver. For CO_3(ad) → O_(ad) + CO_2(gas), the energy barrier is calculated to be 19.8 kcal mol^?1 through the bi‐coord path. The process is endothermic with an enthalpy change of +17.3 ～ +26.7 kcal mol^?1 and the weakly chemisorbed CO₂ is identified as an intermediate on the potential energy surface. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Spectroscopic Measurement of a Halogen Bond Energy

Halogen bonding (XB) has emerged as an important bonding motif in supramolecules and biological systems. Although regarded as a strong noncovalent interaction, benchmark measurements of the halogen bond energy are scarce. Here, a combined anion photoelectron spectroscopy and density functional theory (DFT) study of XB in solvated Br^? anions is reported. The XB strength between the positively‐charged σ‐hole on the Br atom of the bromotrichloromethane (CCl₃Br) molecule and the Br^? anion was found to be 0.63 eV (14.5 kcal mol^?1). In the neutral complexes, Br(CCl₃Br)_1,2, the attraction between the free Br atom and the negatively charged equatorial belt on the Br atom of CCl₃Br, which is a second type of halogen bonding, was estimated to have interaction strengths of 0.15 eV (3.5 kcal mol^?1) and 0.12 eV (2.8 kcal mol^?1).     

Molecular modeling simulation and experimental measurements to characterize chitosan and poly(vinyl pyrrolidone) blend interactions

Blends of chitosan and poly(vinyl pyrrolidone) (PVP) have a high potential for use in various biomedical applications and in advanced drug‐delivery systems. Recently, the physical and chemical properties of these blends have been extensively characterized. However, the molecular interaction between these two polymers is not fully understood. In this study, the intermolecular interaction between chitosan and PVP was experimentally investigated using ¹³C cross‐polarization magic angle‐spinning nuclear magnetic resonance (¹³C CP/MAS NMR) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). According to these experimental results, the interaction between the polymers takes place through the carbonyl group of PVP and either the OH? C₆, OH? C₃, or NH? C₂ of chitosan. In an attempt to identify the interacting groups of these polymers, molecular modeling simulation was performed. Molecular simulation was able to clarify that the hydrogen atom of OH? C₆ of chitosan was the most favorable site to form hydrogen bonding with the oxygen atom of C?O of PVP, followed by that of OH? C₃, whereas that of NH? C₂ was the weakest proton donor group. The nitrogen atom of PVP was not involved in the intermolecular interaction between these polymers. Furthermore, the interactions between these polymers are higher when PVP concentrations are lower, and interactions decrease with increasing amounts of PVP. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1258–1264, 2008     

The gas phase hydrogen‐bonded dimers of HOCl: A high‐level quantum chemical study

The O···H? O and Cl···H? O hydrogen bonding interactions were analyzed for HOCl dimers by using B3LYP, MP2, CCSD, and MP4(SDTQ) methods in conjunction with the various basis sets. Five isomers were found for the HOCl dimer. The ZPE and BSSE corrected binding energies were computed at the different levels of theory. At the optimized geometries obtained at CCSD/AUG‐cc‐pVDZ level, energies were re‐evaluated at MP4(SDTQ)/AUG‐cc‐pVTZ and CCSD(T)/cc‐pVTZ levels of theory. We found an average of ?20.9 and ?9.6 kJ/mol for the strength of the O···H and Cl···H hydrogen bonding interactions, respectively. Excitation and vertical ionization energies as well as rotational constants were computed at different levels of theory. The quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analysis were used to elucidate the nature of the interactions of HOCl dimers. The interaction energies were decomposed by Morokuma methodology. We have computed Δ_fH°(HOCl) and Δ_fH°(HOCl⁺) using the atomization reactions. The Δ_fH°₂₉₈(HOCl) values are ?17.85 and ?18.05 kcal/mol by using CBS‐Q and CBS‐QB3 extrapolation models, respectively, in good agreement with the results given in JANAF tables. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Structure and dynamic features of an intramolecular frustrated Lewis pair

Di(mesityl)cyclohexenylphosphine undergoes hydroboration with Piers' borane [HB(C₆F₅)₂] to yield the cyclohexylene‐anellated frustrated Lewis pair 5 . This P/B pair splits H₂ with the formation of the product 4 and adds to the C?O double bond of phenyl isocyanate to yield 6 . In the crystal, compound 5 features a puckered four‐membered heterocyclic core structure with a long P? B bond (av. 2.197(5) Å). The activation energy of the P? B cleavage of the frustrated Lewis pair 5 was determined by dynamic ¹⁹F NMR spectroscopy at ΔG^≠(298 K)=12.1±0.3 kcal mol^?1.     

Intramolecular Hydrogen Bonds Enhance Disparity in Reactivity between Isomers of Photoswitchable Sorbents and CO2: A Computational Study

Reducing the emission of greenhouse gases, such as CO₂, requires efficient and reusable capture materials. The energy for regenerating sorbents is critical to the cost of CO₂ capture. Here, we design a series of photoswitchable CO₂ capture molecules by grafting Lewis bases, which can covalently bond CO₂, to azo‐based backbones that can switch configurations upon light stimulation. The first‐principles calculations demonstrate that intramolecular hydrogen bonds are crucial for enlarging the difference of CO₂ binding strengths to the cis and trans isomers. As a result, the CO₂–sorbent interaction can be light‐adjusted from strong chemical bonding in one configuration to weak bonding in the other, which may lead to a great energy reduction in sorbent regeneration.     

Structural,Spectroscopic and Computational Examination of the Dative Interaction in Constrained Phosphine–Stibines and Phosphine–Stiboranes

A series of phosphine–stibine and phosphine–stiborane peri‐substituted acenaphthenes containing all permutations of pentavalent groups ?SbCl_nPh_4–n ( 5 – 9 ), as well as trivalent groups ?SbCl₂, ?Sb(R)Cl, and ?SbPh₂ ( 2 – 4 , R=Ph, Mes), were synthesised and fully characterised by single crystal diffraction and multinuclear NMR spectroscopy. In addition, the bonding in these species was studied by DFT computational methods. The P–Sb dative interactions in both series range from strongly bonding to non‐bonding as the Lewis acidity of the Sb acceptor is decreased. In the pentavalent antimony series, a significant change in the P–Sb distance is observed between ?SbClPh₃ and ?SbCl₂Ph₂ derivatives 6 and 7 , respectively, consistent with a change from a bonding to a non‐bonding interaction in response to relatively small modification in Lewis acidity of the acceptor. In the Sb^III series, two geometric forms are observed. The P–Sb bond length in the SbCl₂ derivative 2 is as expected for a normal (rather than a dative) bond. Rather unexpectedly, the phosphine–stiborane complexes 5 – 9 represent the first examples of the σ⁴P→σ⁶Sb structural motif.     

Amide–π interactions between formamide and benzene

High‐level ab initio calculations have been carried out using a formamide–benzene model system to evaluate amide–π interactions. The interaction energies were estimated as a sum of the CCSD(T) correlation contribution and the HF energy at the complete basis set limit, for the geometries of the model structures at the energy minimum obtained by potential energy surface (PES) scans. NH/π geometry in a face‐on configuration was found to be the most attractive among the various geometries considered, with interaction energy of ?3.75 kcal/mol. An interaction energy of ?2.08 kcal/mol was calculated for the stacked N/Center type geometry, where the nitrogen atom of formamide points directly toward the center of the aromatic ring. The weakest C?O/π geometry, where a carbonyl oxygen atom points toward the plane of the aromatic ring, was found to have energy minimum at an intermolecular distance of 3.67 Å from the PES, with a repulsive interaction energy less than 1 kcal/mol. However, if there are simultaneous attractive interactions with other parts of the molecule besides the amide group, the weak repulsion could be easily overcome, to give a C?O/π geometry interaction. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2009     

Hydrogenation of CO2 to Formate with H2: Transition Metal Free Catalyst Based on a Lewis Pair

Hydrogenation of CO₂ to formate with H₂ in the absence of transition metal is a long‐standing challenge in catalysis. The reactions between tris(pentafluorophenyl)borane (BCF) and K₂CO₃ (or KHCO₃) are found to form a Lewis pair (K₂[(BCF)₂?CO₃]) which can react with both H₂ and CO₂ to produce formate. Based on these stoichiometric reactions, the first catalytic hydrogenation process of CO₂ to formate using transition metal free catalyst (BCF/M₂CO₃, M=Na, K, and Cs) is reported. The highest TON value of this catalytic process is up to 3941. Further research revealed the reaction mechanism in which the Lewis pair enables the splitting of H₂ and the insertion of CO₂ into the B?H bond.