Cooperativity between the Halogen Bond and the Hydrogen Bond in H3N⋅⋅⋅XY⋅⋅⋅HF Complexes (X,Y=F,Cl, Br)

Ab initio calculations are used to provide information on H₃N???XY???HF triads (X, Y=F, Cl, Br) each having a halogen bond and a hydrogen bond. The investigated triads include H₃N???Br₂‐HF, H₃N???Cl₂???HF, H₃N???BrCI???HF, H₃N???BrF???HF, and H₃N???ClF???HF. To understand the properties of the systems better, the corresponding dyads are also investigated. Molecular geometries, binding energies, and infrared spectra of monomers, dyads, and triads are studied at the MP2 level of theory with the 6‐311++G(d,p) basis set. Because the primary aim of this study is to examine cooperative effects, particular attention is given to parameters such as cooperative energies, many‐body interaction energies, and cooperativity factors. The cooperative energy ranges from ?1.45 to ?4.64 kcal mol^?1, the three‐body interaction energy from ?2.17 to ?6.71 kcal mol^?1, and the cooperativity factor from 1.27 to 4.35. These results indicate significant cooperativity between the halogen and hydrogen bonds in these complexes. This cooperativity is much greater than that between hydrogen bonds. The effect of a halogen bond on a hydrogen bond is more pronounced than that of a hydrogen bond on a halogen bond.     

Influence of the Li⋅⋅⋅π Interaction on the H/X⋅⋅⋅π Interactions in HOLi⋅⋅⋅C6H6⋅⋅⋅HOX/XOH (X=F,Cl, Br,I) Complexes

The influences of the Li???π interaction of C₆H₆???LiOH on the H???π interaction of C₆H₆???HOX (X=F, Cl, Br, I) and the X???π interaction of C₆H₆???XOH (X=Cl, Br, I) are investigated by means of full electronic second‐order Møller–Plesset perturbation theory calculations and “quantum theory of atoms in molecules” (QTAIM) studies. The binding energies, binding distances, infrared vibrational frequencies, and electron densities at the bond critical points (BCPs) of the hydrogen bonds and halogen bonds prove that the addition of the Li???π interaction to benzene weakens the H???π and X???π interactions. The influences of the Li???π interaction on H???π interactions are greater than those on X???π interactions; the influences of the H???π interactions on the Li???π interaction are greater than X???π interactions on Li???π interaction. The greater the influence of Li???π interaction on H/X???π interactions, the greater the influences of H/X???π interactions on Li???π interaction. QTAIM studies show that the intermolecular interactions of C₆H₆???HOX and C₆H₆???XOH are mainly of the π type. The electron densities at the BCPs of hydrogen bonds and halogen bonds decrease on going from bimolecular complexes to termolecular complexes, and the π‐electron densities at the BCPs show the same pattern. Natural bond orbital analyses show that the Li???π interaction reduces electron transfer from C₆H₆ to HOX and XOH.     

Influence of the Homopolar Dihydrogen Bonding CH⋅⋅⋅HC on Coordination Geometry: Experimental and Theoretical Studies

The reaction of the N‐thiophosphorylated thiourea (HOCH₂)(Me)₂CNHC(S)NHP(S)(OiPr)₂ (HL), deprotonated by the thiophosphorylamide group, with NiCl₂ leads to green needles of the pseudotetrahedral complex [Ni(L‐1,5‐S,S′)₂] ? 0.5 (n‐C₆H₁₄) or pale green blocks of the trans square‐planar complex trans‐[Ni(L‐1,5‐S,S′)₂]. The former complex is stabilized by homopolar dihydrogen C?H???H?C interactions formed by n‐hexane solvent molecules with the [Ni(L‐1,5‐S,S′)₂] unit. Furthermore, the dispersion‐dominated C?H??? H?C interactions are, together with other noncovalent interactions (C?H???N, C?H???Ni, C?H???S), responsible for pseudotetrahedral coordination around the Ni^II center in [Ni(L ‐1,5‐S,S′)₂] ? 0.5 (n‐C₆H₁₄).     

The Cage Structure of IndanCHF3 is Based on the Cooperative Effects of CH⋅⋅⋅π and CH⋅⋅⋅F Weak Hydrogen Bonds

The structural and energetic features of the C?H???π interaction and the internal dynamics of the CHF₃ group change drastically in going from benzene?CHF₃ to indan?CHF₃, according to the analysis of the rotational spectrum of the latter complex generated in a supersonic expansion.     

Self‐Assembly and (Hydro)gelation Triggered by Cooperative π–π and Unconventional CH⋅⋅⋅X Hydrogen Bonding Interactions

Weak C? H???X hydrogen bonds are important stabilizing forces in crystal engineering and anion recognition in solution. In contrast, their quantitative influence on the stabilization of supramolecular polymers or gels has thus far remained unexplored. Herein, we report an oligophenyleneethynylene (OPE)‐based amphiphilic Pt^II complex that forms supramolecular polymeric structures in aqueous and polar media driven by π–π and different weak C‐H???X (X=Cl, O) interactions involving chlorine atoms attached to the Pt^II centers as well as oxygen atoms and polarized methylene groups belonging to the peripheral glycol chains. A collection of experimental techniques (UV/Vis, 1D and 2D NMR, DLS, AFM, SEM, and X‐Ray diffraction) demonstrate that the interplay between different weak noncovalent interactions leads to the cooperative formation of self‐assembled structures of high aspect ratio and gels in which the molecular arrangement is maintained in the crystalline state.     

Probing the CH⋅⋅⋅π Weak Hydrogen Bond in Anesthetic Binding: The Sevoflurane–Benzene Cluster

Cooperativity between weak hydrogen bonds can be revealed in molecular clusters isolated in the gas phase. Here we examine the structure, internal dynamics, and origin of the weak intermolecular forces between sevoflurane and a benzene molecule, using multi‐isotopic broadband rotational spectra. This heterodimer is held together by a primary C? H???π hydrogen bond, assisted by multiple weak C? H???F interactions. The multiple nonbonding forces hinder the internal rotation of benzene around the isopropyl C? H bond in sevoflurane, producing detectable quantum tunneling effects in the rotational spectrum.     

Cooperativity between the Dihydrogen Bond and the N⋅⋅⋅HC Hydrogen Bond in LiH–(HCN)n Complexes

The cooperativity between the dihydrogen bond and the N???HC hydrogen bond in LiH–(HCN)_n (n=2 and 3) complexes is investigated at the MP2 level of theory. The bond lengths, dipole moments, and energies are analyzed. It is demonstrated that synergetic effects are present in the complexes. The cooperativity contribution of the dihydrogen bond is smaller than that of the N???HC hydrogen bond. The three‐body energy in systems involving different types of hydrogen bonds is larger than that in the same hydrogen‐bonded systems. NBO analyses indicate that orbital interaction, charge transfer, and bond polarization are mainly responsible for the cooperativity between the two types of hydrogen bonds.     

Cation–Cation Pairing by NCH⋅⋅⋅O Hydrogen Bonds

The pairing of ions of opposite charge is a fundamental principle in chemistry, and is widely applied in synthesis and catalysis. In contrast, cation–cation association remains an elusive concept, lacking in supporting experimental evidence. While studying the structure and properties of 4‐oxopiperidinium salts [OC₅H₈NH₂]X for a series of anions X^? of decreasing basicity, we observed a gradual self‐association of the cations, concluding in the formation of an isolated dicationic pair. In 4‐oxopiperidinium bis(trifluoromethylsulfonyl)amide, the cations are linked by N? H???O?C hydrogen bonds to form chains, flanked by hydrogen bonds to the anions. In the tetra(perfluoro‐tert‐butoxy)aluminate salt, the anions are fully separated from the cations, and the cations associate pairwise by N? C? H???O?C hydrogen bonds. The compounds represent the first genuine examples of self‐association of simple organic cations based merely on hydrogen bonding as evidenced by X‐ray structure analysis, and provide a paradigm for an extension of this class of compounds.     

Preparing Gold(I) for Interactions with Proton Donors: The Elusive [Au]⋅⋅⋅HO Hydrogen Bond

MP2 and DFT calculations with correlation consistent basis sets indicate that isolated linear anionic dialkylgold(I) complexes form moderately strong (ca. 10 kcal mol^?1) Au???H hydrogen bonds with single H₂O molecules as donors in the absence of sterically demanding substituents. Relativistic effects are critically important in the attraction. Such bonds are significantly weaker in neutral, strong σ‐donor N‐heterocyclic carbene (NHC) complexes (ca. 5 kcal mol^?1). The overall association (>11 kcal mol^?1), however, is strengthened by co‐operative, synergistic classical hydrogen bonding when the NHC ligands bear NH units. Further manipulation of the interaction by ligands positioned trans to the carbene, is possible.     

Strong CH⋅⋅⋅Halide Hydrogen Bonds from 1,2,3‐Triazoles Quantified Using Pre‐Organized and Shape‐Persistent Triazolophanes

The structures associated with halide (F^?, Cl^?, Br^?) complexation inside CH hydrogen‐bonding macrocyclic receptors, called triazolophanes, are characterized using density functional theory (DFT). The associated binding energies in the gas and solution phases are evaluated. The ruffles in the empty triazolophane become smoothed‐out upon Cl^?‐ and Br^?‐ion binding directly into the middle of the cavity. The largely pre‐organized cavity morphs into an elliptical shape to facilitate shorter hydrogen bonds in the north and south regions and longer ones west and east. The smaller F^? ion sits in, and flattens‐out, only the north (or south) region. The 1,2,3‐triazoles show shorter CH???Cl^? contacts than for the phenylenes. Both Cl^? and Br^? show the same binding geometries but Cl^? has a larger binding energy consistent with its stronger Lewis basicity. Model triads were used to decompose the overall binding energy into those of its components. In the course of this triad analysis, anion polarization was identified and its contribution to the triad???Cl^? binding energy estimated. Consequently, the binding energies for the individual aryl units within the comparatively non‐polarized triazolophanes were estimated. The 1,2,3‐triazoles are twice as strong as the phenylenes thus contributing most of the interaction energy to Cl^?‐ion binding. Therefore, the 1,2,3‐triazoles appear to approach the hydrogen bond strengths of the NH donors of pyrrole units.     

Small Cycloalkane (CN)2CC(CN)2 Structures Are Highly Directional Non‐covalent Carbon‐Bond Donors

High‐level calculations (RI‐MP2/def2‐TZVP) disclosed that the σ‐hole in between two C atoms of cycloalkane X₂C?CX₂ structures (X=F, CN) is increasingly exposed with decreasing ring size. The interacting energy of complexes of F^?, HO^?, N≡C^?, and H₂CO with cyclopropane and cyclobutane X₂C?CX₂ derivatives was calculated. For X=F, these energies are small to positive, while for X=CN they are all negative, ranging from ?6.8 to ?42.3 kcal mol^?1. These finding are corroborated by a thorough statistical survey of the Cambridge Structural Database (CSD). No clear evidence could be found in support of non‐covalent carbon bonding between electron‐rich atoms (El.R.) and F₂C?CF₂ structures. In marked contrast, El.R.???(CN)₂C?C(CN)₂ interactions are abundant and highly directional. Based on these findings, the hydrophobic electrophilic bowl formed by 1,1′,2,2′‐tetracyano cyclopropane or cyclobutane derivatives is proposed as a new and synthetically accessible supramolecular synthon.     

OH⋅⋅⋅N and CH⋅⋅⋅O Hydrogen Bonds Control Hydration of Pivotal Tropane Alkaloids: Tropinone⋅⋅⋅H2O Complex

The effect of monohydration in equatorial/axial isomerism of the common motif of tropane alkaloids is investigated in a supersonic expansion by using Fourier‐transform microwave spectroscopy. The rotational spectrum reveals the equatorial isomer as the dominant species in the tropinone???H₂O complex. The monohydrated complex is stabilized primarily by a moderate O?H???N hydrogen bond. In addition, two C?H???O weak hydrogen bonds also support this structure, blocking the water molecule and avoiding any molecular dynamics in the complex. The water molecule acts as proton donor and chooses the ternary amine group over the carbonyl group as a proton acceptor. The experimental work is supported by theoretical calculations; the accuracy of the B3LYP, M06‐2X, and MP2 methods is also discussed.     

Behavior of Halogen Bonds of the Y−X⋅⋅⋅π Type (X,Y=F,Cl, Br,I) in the Benzene π System,Elucidated by Using a Quantum Theory of Atoms in Molecules Dual‐Functional Analysis

The nature of halogen bonds of the Y?X‐?‐π(C₆H₆) type (X, Y=F, Cl, Br, and I) have been elucidated by using the quantum theory of atoms in molecules (QTAIM) dual‐functional analysis (QTAIM‐DFA), which we proposed recently. Asterisks (?) emphasize the presence of bond‐critical points (BCPs) in the interactions in question. Total electron energy densities, H_b( r _c), are plotted versus H_b( r _c)?V_b( r _c)/2 [=(?²/8m)?²ρ_b( r _c)] for the interactions in QTAIM‐DFA, in which V_b( r _c) are potential energy densities at the BCPs. Data for perturbed structures around fully optimized structures were used for the plots, in addition to those of the fully optimized ones. The plots were analyzed by using the polar (R, θ) coordinate for the data of fully optimized structures with (θ_p, κ_p) for those that contained the perturbed structures; θ_p corresponds to the tangent line of the plot and κ_p is the curvature. Whereas (R, θ) corresponds to the static nature, (θ_p, κ_p) represents the dynamic nature of the interactions. All interactions in Y?X‐?‐π(C₆H₆) are classified by pure closed‐shell interactions and characterized to have vdW nature, except for Y?I‐?‐π(C₆H₆) (Y=F, Cl, Br) and F?Br‐?‐π(C₆H₆), which have typical hydrogen‐bond nature without covalency. I?I‐?‐π(C₆H₆) has a borderline nature between the two. Y?F‐?‐π(C₆H₆) (Y=Br, I) were optimized as bent forms, in which Y‐?‐π interactions were detected. The Y‐?‐π interactions in the bent forms are predicted to be substantially weaker than those in the linear F?Y‐?‐π(C₆H₆) forms.     

Resonance Character of Copper/Silver/Gold Bonding in Small Molecule⋅⋅⋅MX (X=F,Cl, Br,CH3, CF3) Complexes

The resonance character of Cu/Ag/Au bonding is investigated in B???M?X (M=Cu, Ag, Au; X=F, Cl, Br, CH₃, CF₃; B=CO, H₂O, H₂S, C₂H₂, C₂H₄) complexes. The natural bond orbital/natural resonance theory results strongly support the general resonance‐type three‐center/four‐electron (3c/4e) picture of Cu/Ag/Au bonding, B:M?X?B⁺?M:X^?, which mainly arises from hyperconjugation interactions. On the basis of such resonance‐type bonding mechanisms, the ligand effects in the more strongly bound OC???M?X series are analyzed, and distinct competition between CO and the axial ligand X is observed. This competitive bonding picture directly explains why CO in OC???Au?CF₃ can be readily replaced by a number of other ligands. Additionally, conservation of the bond order indicates that the idealized relationship b_B???M+b_MX=1 should be suitably generalized for intermolecular bonding, especially if there is additional partial multiple bonding at one end of the 3c/4e hyperbonded triad.     

Streptococcal Hyaluronate Lyase Reveals the Presence of a Structurally Significant CH⋅⋅⋅O Hydrogen Bond

Carbon‐donated hydrogen bonds (CDHBs) are weak forms of hydrogen bonding (0.5–1.0 kcal mol^?1) that are difficult to detect, and thus their roles in the structure and functionality of chemical systems often go unrecognized. Utilizing a computational approach, the existence of a structurally significant CDHB in the medically relevant protein Streptococcus pneumoniae hyaluronate lyase (SpnHL) is affirmed. The structure of a tetrapeptide fragment model containing the CDHB was optimized with second‐order perturbation theory. From this, a CDHB with bond distance and angle consistent with previously discovered CDHBs and comparable to neighboring traditional HBs in the fragment model was found. The CDHB competes with another donor T253 OH, whereby the two alternate in strength between protein conformations, imbuing αHelix 3 appreciable flexibility. The CDHB seems to exist in spite of torsional and steric strain on the donor methyl group. It is postulated that the CDHB could aid in either counteracting the macrodipole of αHelix 3 or protecting the A249 CO from destabilizing interactions with the adjacent solvent. Employing the energy gradients from the optimization, the torque generated by the fragment model was computed, which accurately predicts the direction of rotation of αHelix 3 observed from experiment. A strongly correlated motion between αHelix 3 and αHelices 2, 4, and 5 was noted, which the interactions of the fragment model help drive by generating a torque much larger than necessary to rotate just αHelix 3. Considering these results, we conclude that CDHBs should be considered as possible beneficial components of chemical and biological phenomena.     

Very Long‐Range Effects: Cooperativity between Anion–π and Hydrogen‐Bonding Interactions

The interplay between two important non‐covalent interactions involving aromatic rings (namely anion–π and hydrogen bonding) is investigated. Very interesting cooperativity effects are present in complexes where anion–π and hydrogen bonding interactions coexist. These effects are found in systems where the distance between the anion and the hydrogen‐bond donor/acceptor molecule is as long as ～11 Å. These effects are studied theoretically using the energetic and geometric features of the complexes, which were computed using ab initio calculations. We use and discuss several criteria to analyze the mutual influence of the non‐covalent interactions studied herein. In addition we use Bader’s theory of atoms‐in‐molecules to characterize the interactions and to analyze the strengthening or weakening of the interactions depending upon the variation of the charge density at the critical points.     

Exploring the (Very Flat) Potential Energy Landscape of R−Br⋅⋅⋅π Interactions with Accurate CCSD(T) and SAPT Techniques

Halogen bonds involving an aromatic moiety as an acceptor, otherwise known as R?X???π interactions, have increasingly been recognized as being important in materials and in protein–ligand complexes. These types of interactions have been the subject of many recent investigations, but little is known about the ways in which the strengths of R?X???π interactions vary as a function of the relative geometries of the interacting pairs. Here we use the accurate CCSD(T) and SAPT2+3δMP2 methods to investigate the potential energy landscapes for systems of HBr, HCCBr, and NCBr complexed with benzene. It is found that only the separation between the complexed molecules have a strong effect on interaction strength while other geometric parameters, such as tilting and shifting R?Br???π donor relative to the benzene plane, affect these interactions only mildly. Importantly, it is found that the C_6v (T‐shaped) configuration is not the global minimum for any of the dimers investigated.