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.     

Nature of the water/aromatic parallel alignment interactions

The water/aromatic parallel alignment interactions are interactions where the water molecule or one of its O? H bonds is parallel to the aromatic ring plane. The calculated energies of the interactions are significant, up to ΔE_{CCSD(T)(limit)} = ?2.45 kcal mol^?1 at large horizontal displacement, out of benzene ring and CH bond region. These interactions are stronger than CH···O water/benzene interactions, but weaker than OH···π interactions. To investigate the nature of water/aromatic parallel alignment interactions, energy decomposition methods, symmetry‐adapted perturbation theory, and extended transition state‐natural orbitals for chemical valence (NOCV), were used. The calculations have shown that, for the complexes at large horizontal displacements, major contribution to interaction energy comes from electrostatic interactions between monomers, and for the complexes at small horizontal displacements, dispersion interactions are dominant binding force. The NOCV‐based analysis has shown that in structures with strong interaction energies charge transfer of the type π → σ*(O? H) between the monomers also exists. © 2014 Wiley Periodicals, Inc.     

Enhancement of Iodine–Hydride Interaction by Substitution and Cooperative Effects in NCX–NCI–HMY Complexes

The NCX‐NCI‐HMY (X=H, Cl, Br, I, Li; M=Be, Mg; Y=H, Li, Na) trimers are investigated to find ways to enhance the iodine–hydride interaction. The interaction energy in the NCI–HMH dimer is ?2.87 and ?5.87 kcal mol^?1 for M=Be and Mg, respectively. When the free H atom in the NCI–HMH dimer is replaced with an alkali atom, the interaction energy is enhanced greatly. When NCX is added into this dimer, the interaction energy of the iodine–hydride interaction is increased by 9–45 % and its increased percentage follows the order X=Cl<Br<H<I<Li and M=Be<Mg. The combination of the alkali substitution and the cooperativity results in a more prominent enhancing effect. The largest interaction energy is found for the NCLi–NCI–HMgLi trimer (?7.03 kcal mol^?1). The influence of the I???H interaction on the X???N interaction is also studied in the trimers. Both types of interactions are analyzed with NBO, AIM, and MEP. The interaction energy in the trimer is also unveiled by a many‐body analysis.     

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.     

The ability of Ex2Box4+ to interact with guests containing π‐electron‐rich and π‐electron‐poor moieties

The ability of Ex 2 Box⁴⁺ as a host, able to trap guests containing both π‐electron rich (polycyclic aromatic hydrocarbons‐PAHs) and π‐electron poor (quinoid‐ and nitro‐PAHs) moieties was investigated to shed light on the main factors that control the host–guest (HG) interaction. The nature of the HG interactions was elucidated by energy decomposition (EDA‐NOCV), noncovalent interaction (NCI), and magnetic response analyses. EDA‐NOCV reveals that dispersion contributions are the most significant to sustain the HG interaction, while electrostatic and orbital contributions are very tiny. In fact, no significant covalent character in the HG interactions was observed. The obtained results point strictly to NCIs, modulated by dispersion contributions. Regardless of whether the guests contain π‐electron‐rich or π‐electron‐poor moieties, and no significant charge‐transfer was observed. All in all, HG interactions between guests 3‐14 and host 2 are predominantly modulated by π‐π stacking.     

Nature of the interaction between ammonia derivatives and carbon disulfide. A theoretical investigation

The interactions between NH₃, its methylated and chlorinated derivatives and CS₂ are investigated by ab initio CCSD(T) and density functional BLYP‐D3 methods. The CCSD(T)/aug‐cc‐pVTZ calculated interaction energies of complexes characterized by the S···N chalcogen bonds range between ?1.71 and ?2.78 kcal mol^?1. The S···N bonds are studied by atoms in molecules, natural bond orbital, and noncovalent interaction methods. The lack of correlation between the interaction energies of methylated amines complexes and the electrostatic potential results from the lone pair effect in aliphatic amines. Different structures of CS₂ complexed with ammonia derivatives, stabilized by other than the S···N chalcogen bonds, are also predicted. These structures are characterized by interaction energies ranging between 1.15 and 3.46 kcal mol^?1. The results show that the complexing ability of CS₂ is not very high but this molecule is able to attack the electrophilic or nucleophilic sites of a guest molecule.     

Three-dimensional potential energy surface of selected carbohydrates' CH/π dispersion interactions calculated by high-level quantum mechanical methods

In this study we present the first systematic computational three‐dimensional scan of carbohydrate hydrophobic patches for the ability to interact through CH/π dispersion interactions. The carbohydrates β‐d‐ glucopyranose, β‐d‐ mannopyranose and α‐l‐ fucopyranose were studied in a complex with a benzene molecule, which served as a model of the CH/π interaction in carbohydrate/protein complexes. The 3D relaxed scans were performed at the SCC‐DFTB‐D level with 3 757 grid points for both carbohydrate hydrophobic sides. The interaction energy of all grid points was recalculated at the DFT‐D BP/def2‐TZVPP level. The results obtained clearly show highly delimited and separated areas around each CH group, with an interaction energy up to ?5.40 kcal mol^?1. The results also show that with increasing H???π distance these delimited areas merge and form one larger region, which covers all hydrogen atoms on that specific carbohydrate side. Simultaneously, the interaction becomes weaker with an energy of ?2.5 kcal mol^?1. All local energy minima were optimized at the DFT‐D BP/def2‐TZVPP level and the interaction energies of these complexes were refined by use of the high‐level ab initio computation at the CCSD(T)/CBS level. Results obtained from the optimization suggest that the CH group hydrogen atoms are not equivalent and the interaction energy at the CCSD(T)/CBS level range from ?3.54 to ?5.40 kcal mol^?1. These results also reveal that the optimal H???π distance for the CH/π dispersion interaction is approximately (2.310±0.030) Å, and the angle defined as carbon‐hydrogen‐benzene geometrical centre is (180±30)°. These results reveal that whereas the dispersion interactions with the lowest interaction energies are quite strictly located in space, the slightly higher interaction energy regions adopt a much larger space.     

Is Aerogen–π Interaction Capable of Initiating the Noncovalent Chemistry of Group 18?

The interactions between atoms of noble gases and π systems are generally considered as van der Waals interaction, which have not attracted attention yet. Herein, we present high‐level ab initio calculations to show the unexpected noncovalent interaction between a covalently bonded noble gas atom and a delocalized aromatic π electron using XeO₃?benzene as the prototype. The CCSD(T)/CBS reference data show its strength amounting to ?10.2 kcal mol^?1, comparable to a typical H‐bond or an anion–π interaction. The energy decomposition analysis reveals that the aerogen–π interaction is favored by the electrostatic interaction (27.7 %), the induction (13.4 %), and the dispersion (21.6 %). This interaction may prompt us to consider the noncovalent chemistry of aerogen derivatives in the near future.     

Strong N−X⋅⋅⋅O−N Halogen Bonds: A Comprehensive Study on N‐Halosaccharin Pyridine N‐Oxide Complexes

A study of the strong N?X???^?O?N⁺ (X=I, Br) halogen bonding interactions reports 2×27 donor×acceptor complexes of N‐halosaccharins and pyridine N‐oxides (PyNO). DFT calculations were used to investigate the X???O halogen bond (XB) interaction energies in 54 complexes. A simplified computationally fast electrostatic model was developed for predicting the X???O XBs. The XB interaction energies vary from ?47.5 to ?120.3 kJ mol^?1; the strongest N?I???^?O?N⁺ XBs approaching those of 3‐center‐4‐electron [N?I?N]⁺ halogen‐bonded systems (ca. 160 kJ mol^?1). ¹H NMR association constants (K_XB) determined in CDCl₃ and [D₆]acetone vary from 2.0×10⁰ to >10⁸ m ^?1 and correlate well with the calculated donor×acceptor complexation enthalpies found between ?38.4 and ?77.5 kJ mol^?1. In X‐ray crystal structures, the N‐iodosaccharin‐PyNO complexes manifest short interaction ratios (R_XB) between 0.65–0.67 for the N?I???^?O?N⁺ halogen bond.     

σ‐Hole Bond Versus Hydrogen Bond: From Tetravalent to Pentavalent N,P, and As Atoms

Ab initio calculations were performed on complexes of ZH₄⁺ (Z=N, P, As) and their fluoro derivatives, ZFH₃⁺ and ZF₄⁺, with a HCN (or LiCN) molecule acting as the Lewis base through the nitrogen electronegative center. It was found that the complexes are linked by the Z? H???N hydrogen bond or another type of noncovalent interaction in which the tetravalent heavy atom of the cation acts as the Lewis acid center, that is, when the Z???N link exists, which may be classified as the σ‐hole bond. The formation of the latter interaction is usually preferable to the formation of the corresponding hydrogen bond. The Z???N interaction may be also considered as the preliminary stage of the S_N2 reaction. This is supported by the observation that for a short Z???N contact, the corresponding complex geometry coincides with the trigonal‐bipyramidal geometry typical for the transition state of the S_N2 reaction. The Z???N interaction for some of complexes analyzed here possesses characteristics typical for covalent bonds. Numerous interrelations between geometrical, topological and energetic parameters are discussed. The natural bond orbital method as well as the Quantum Theory of “Atoms in Molecules” is applied to characterize interactions in the analyzed complexes. The experimental evidences of the existence of these interactions, based on the Cambridge Structure Database search, are also presented. In addition, it is justified that mechanisms of the formation of the Z???N interactions are similar to the processes occurring for the other noncovalent links. The formation of Z???N interaction as well as of other interactions may be explained with the use of the σ‐hole concept.     

Gas‐Phase Energetics of Reductive Elimination from a Palladium(II) N‐Heterocyclic Carbene Complex

Energy‐resolved collision‐induced dissociation experiments using tandem mass spectrometry are reported for an phenylpalladium N‐heterocyclic carbene (NHC) complex. Reductive elimination of an NHC ligand as a phenylimidazolium ion involves a barrier of 30.9(14) kcal mol^?1, whereas competitive ligand dissociation requires 47.1(17) kcal mol^?1. The resulting three‐coordinate palladium complex readily undergoes reductive C? C coupling to give the phenylimidazolium π complex, for which the binding energy was determined to be 38.9(10) kcal mol^?1. Density functional calculations at the M06‐L//BP86/TZP level of theory are in very good agreement with experiment. In combination with RRKM modeling, these results suggest that the rate‐determining step for the direct reductive elimination process switches from the C? C coupling step to the fragmentation of the resulting σ complex at low activation energy.     

CH⋅⋅⋅N Hydrogen‐Bonding Interaction in 7‐Azaindole:CHX3 (X=F,Cl) Complexes

The C?H???Y (Y=hydrogen‐bond acceptor) interactions are somewhat unconventional in the context of hydrogen‐bonding interactions. Typical C?H stretching frequency shifts in the hydrogen‐bond donor C?H group are not only small, that is, of the order of a few tens of cm^?1, but also bidirectional, that is, they can be red or blue shifted depending on the hydrogen‐bond acceptor. In this work we examine the C?H???N interaction in complexes of 7‐azaindole with CHCl₃ and CHF₃ that are prepared in the gas phase through supersonic jet expansion using the fluorescence depletion by infra‐red (FDIR) method. Although the hydrogen‐bond acceptor, 7‐azaindole, has multiple sites of interaction, it is found that the C?H???N hydrogen‐bonding interaction prevails over the others. The electronic excitation spectra suggest that both complexes are more stabilized in the S₁ state than in the S₀ state. The C?H stretching frequency is found to be red shifted by 82 cm^?1 in the CHCl₃ complex, which is the largest redshift reported so far in gas‐phase investigations of 1:1 haloform complexes with various substrates. In the CHF₃ complex the observed C?H frequency is blue shifted by 4 cm^?1. This is at variance with the frequency shifts that are predicted using several computational methods; these predict at best a redshift of 8.5 cm^?1. This discrepancy is analogous to that reported for the pyridine‐CHF₃ complex [W. A. Herrebout, S. M. Melikova, S. N. Delanoye, K. S. Rutkowski, D. N. Shchepkin, B. J. van der Veken, J. Phys. Chem. A­ 2005 , 109, 3038], in which the blueshift is termed a pseudo blueshift and is shown to be due to the shifting of levels caused by Fermi resonance between the overtones of the C?H bending and stretching modes. The dissociation energies, (D₀), of the CHCl₃ and CHF₃ complexes are computed (MP2/aug‐cc‐pVDZ level) as 6.46 and 5.06 kcal mol^?1, respectively.     

The Energetic Significance of Metallophilic Interactions

Metallophilic interactions are increasingly recognized as playing an important role in molecular assembly, catalysis, and bio‐imaging. However, present knowledge of these interactions is largely derived from solid‐state structures and gas‐phase computational studies rather than quantitative experimental measurements. Here, we have experimentally quantified the role of aurophilic (Au^I???Au^I), platinophilic (Pt^II???Pt^II), palladophilic (Pd^II???Pd^II), and nickelophilic (Ni^II???Ni^II) interactions in self‐association and ligand‐exchange processes. All of these metallophilic interactions were found to be too weak to be well‐expressed in several solvents. Computational energy decomposition analyses supported the experimental finding that metallophilic interactions are overall weak, meaning that favorable dispersion and orbital hybridization contributions from M???M binding are largely outcompeted by electrostatic or dispersion interactions involving ligand or solvent molecules. This combined experimental and computational study provides a general understanding of metallophilic interactions and indicates that great care must be taken to avoid over‐attributing the energetic significance of metallophilic interactions.     

Homonuclear chalcogen–chalcogen bond interactions in complexes pairing YO3 and YHX molecules (Y = S,Se; X = H,Cl, Br,CCH, NC,OH, OCH3): Influence of substitution and cooperativity

MP2/aug‐cc‐pVTZ calculations are performed on complexes of YO₃ (Y = S, Se) with a series of electron‐donating chalcogen bases YHX (X = H, Cl, Br, CCH, NC, OH, OCH₃). These complexes are formed through the interaction of a positive electrostatic potential region (π‐hole) on the YO₃ molecule with the negative region in YHX. Interaction energies of the binary O₃Y???YHX complexes are in the range of ?4.37 to ?12.09 kcal/mol. The quantum theory of atoms in molecules and the natural bond orbital analysis were applied to characterize the nature of interactions. It was found that the formation and stability of these binary complexes are ruled mainly by electrostatic effects, although the electron charge transfer from YHX to YO₃ unit also seems to play an important role. In addition, mutual influence between the Y???N and Y???Y interactions is studied in the ternary HCN???O₃Y???YHX complexes. The results indicate that the formation of a Y???N interaction tends to weaken Y???Y bond in the ternary systems. Although the Y???Y interaction is weaker than the Y???N one, however, both types of interactions seem to compete with each other in the HCN???O₃Y???YHX complexes. © 2016 Wiley Periodicals, Inc.     

Hydrogen bonding in the hydroxysulfinyl radical‐formic acid‐water system: A theoretical study

Quantum chemical methods have been employed to evaluate the possible configurations of the 1:1 and 1:2 HOSO‐formic acid complexes and 1:1:1 HOSO‐formic acid‐water complexes. The first type of complex involves two H bonds, while the other two types comprise three H bonds in a ring. The complexes are relatively stable, with CBS‐QB3 computed binding energies of 14.3 kcal mol^?1, 23.4 kcal mol^?1, and 21.1 kcal mol^?1 for the lowest‐energy structures of the 1:1, 1:2, and 1:1:1 complexes, respectively. Complex formations induce a large spectral red‐shift and an enhancement of the IR intensity for the H‐bonded OH stretching modes relative to those in the parent monomers. TDDFT calculations of the low‐lying electronic excited states demonstrate that the complexes are photochemically quite stable in the troposphere. Small spectral shifts in comparison to the free HOSO radical suggest that the radical and the complexes would not be easily distinguishable using standard UV/vis absorption spectroscopy. © 2016 Wiley Periodicals, Inc.     

Anionic CH⋅⋅⋅X− Hydrogen Bonds: Origin of Their Strength,Geometry, and Other Properties

CF₃H as a proton donor was paired with a variety of anions, and its properties were assessed by MP2/aug‐cc‐pVDZ calculations. The binding energy of monoanions halide, NO₃^?, formate, acetate, HSO₄^?, and H₂PO₄^? lie in the 12–17 kcal mol^?1 range, although F^? is more strongly bound, by 26 kcal mol^?1. Dianions SO₄^2? and HPO₄^2? are bound by 27 kcal mol^?1, and trianion PO₄^3? by 45 kcal mol^?1. When two O atoms are available on the anion, the CH???O^? H‐bond (HB) is usually bifurcated, although asymmetrically. The CH bond is elongated and its stretching frequency redshifted in these ionic HBs, but the shift is reduced in the bifurcated structures. Slightly more than half of the binding energy is attributed to Coulombic attraction, with smaller contributions from induction and dispersion. The amount of charge transfer from the anions to the σ*(CH) orbital correlates with many of the other indicators of bond strength, such as binding energy, CH bond stretch, CH redshift, downfield NMR spectroscopic chemical shift of the bridging proton, and density at bond critical points.     

Binding Energies of the π‐Stacked Anisole Dimer: New Molecular Beam—Laser Spectroscopy Experiments and CCSD(T) Calculations

Among noncovalent interactions, π–π stacking is a very important binding motif governed mainly by London dispersion. Despite its importance, for instance, for the structure of bio‐macromolecules, the direct experimental measurement of binding energies in π–π stacked complexes has been elusive for a long time. Only recently, an experimental value for the binding energy of the anisole dimer was presented, determined by velocity mapping ion imaging in a two‐photon resonant ionisation molecular beam experiment. However, in that paper, a discrepancy was already noted between the obtained experimental value and a theoretical estimate. Here, we present an accurate recalculation of the binding energy based on the combination of the CCSD(T)/CBS interaction energy and a DFT‐D3 vibrational analysis. This proves unambiguously that the previously reported experimental value is too high and a new series of measurements with a different, more sensitive apparatus was performed. The new experimental value of 1800±100 cm^?1 (5.15±0.29 kcal mol^?1) is close to the present theoretical prediction of 5.04±0.40 kcal mol^?1. Additional calculations of the properties of the cationic and excited states involved in the photodissociation of the dimer were used to identify and rationalise the difficulties encountered in the experimental work.     

Experimental and Computational Probes of the Nature of Halogen Bonding: Complexes of Bromine‐Containing Molecules with Bromide Anions

The nature of halogen bonding is examined via experimental and computational characterizations of a series of associates between electrophilic bromocarbons R? Br (R? Br=CBr₃F, CBr₃NO₂, CBr₃COCBr₃, CBr₃CONH₂, CBr₃CN, etc.) and bromide anions. The [R? Br, Br^?] complexes show intense absorption bands in the 200–350 nm range which follow the same Mulliken correlation as those observed for the charge‐transfer associates of bromide anions with common organic π‐acceptors. For a wide range of the associates, intermolecular R? Br???Br^? separations decrease and intramolecular C? Br bond lengths increase proportionally to the Br^?→R? Br charge transfer; and the energies of R? Br???Br^? bonds are correlated with the linear combination of orbital (charge‐transfer) and electrostatic interactions. On the whole, spectral, structural and thermodynamic characteristics of the [R? Br, Br^?] complexes indicate that besides electrostatics, the orbital (charge‐transfer) interactions play a vital role in the R? Br???Br^? halogen bonding. This indicates that in addition to controlling the geometries of supramolecular assemblies, halogen bonding leads to electronic coupling between interacting species, and thus affects reactivity of halogenated molecules, as well as conducting and magnetic properties of their solid‐state materials.     

An Interplay of Cooperativity between Cation⋅⋅⋅π, Anion⋅⋅⋅π and C?H⋅⋅⋅Anion Interactions

Mixed cation (Li⁺, Na⁺ and K⁺) and anion (F^?, Cl^?, Br^?) complexes of the aromatic π‐surfaces (top and bottom) are studied by using dispersion‐corrected density functional theory. The selectivity of the aromatic surface to interact with a cation or an anion can be tuned and even reversed by the electron‐donating/electron‐accepting nature of the side groups. The presence of a methyl group in the ? OCH₃, ? SCH₃, ? OC₂H₅ in the side groups of the aromatic ring leads to further cooperative stabilization of the otherwise unstable/weakly stable anion???π complexes by bending of the side groups towards the anion to facilitate C? H???anion interactions. The cooperativity among the interactions is found to be as large as 100 kcal mol^?1 quantified by dissection of the three individual forces from the total interaction energy. The crystal structures of the fluoride binding tripodal and hexapodal ligands provide experimental evidence for such cooperative interactions.