Hydrogen bond and σ‐hole interaction in M2C=S···HCN (M = H,F, Cl,Br, HO,H3C,H2N) complex: Dual roles of C=S group and substitution effect

An ab initio computational study of the dual functions of C?S group in the M₂C?S ··· HCN (M = H, F, Cl, Br, HO, H₃C, H₂N) complex has been performed at the MP2(Full)/aug‐cc‐pVTZ level. The C?S group can act as both the electron donor and acceptor, thus two minima complexes were found for each molecular pairs. The interaction energy of hydrogen bond in the F, Cl, or Br substituted complexes is less negative than that in the corresponding H₂CS one, while the interaction energy of the σ‐hole interaction is more negative. The OH substitution weakens the hydrogen bond, whereas the H₃C and H₂N substitution strengthens it. The σ‐hole interaction in the HO, H₃C, and H₂N complexes is very weak. The substitution effect has been understood with electrostatic induction and conjugation effects. The energy decomposition analysis has been performed for the halogen‐substituted complexes. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012.     

The enhancing effects of molecule X (X = PH2Cl,SHCl, ClCl) on chalcogen–chalcogen interactions in cyclic trimers Y···Y···X (Y = SHCl,SeHCl)

The enhancing effects of molecule X (X = PH₂Cl, SHCl, ClCl) on S···S and Se···Se chalcogen–chalcogen bonds in the cyclic trimers SHCl···SHCl···X and SeHCl···SeHCl···X were investigated by calculations at the MP2/aug‐cc‐pVTZ level. When molecule X is added to the dimer SHCl···SHCl (SeHCl···SeHCl), cyclic trimers are formed. Compared with the dimer, all the cyclic trimers have shorter S···S (Se···Se) lengths, greater electron densities, negative three‐body interaction energies, and larger second‐order perturbation energies. These results indicate that the addition of molecule X strengthens the original S···S (Se···Se) bond. For the SHCl···SHCl···X cyclic trimers, the S···S bond is strongest in SHCl···SHCl···PH₂Cl, weaker in SHCl···SHCl···SHCl, and weakest in SHCl···SHCl···ClCl. This same trend is observed for the Se···Se bond in SeHCl···SeHCl···X. This means that PH₂Cl provides the greatest enhancement to the S···S (Se···Se) interaction.     

Computational Study on the Unconventional Hydrogen‐Bonded F–H···C Systems

The F–H···YZ₂ (Y = C, Si, BH, A1H;Z = H, PH₃) systems were examined using density functional theory calculations. The main focus of this work is to demonstrate that the chemistry of Y(PH₃)₂ exhibits a novel feature which is a central Y atom with unexpected high basicity. Further, the hydrogen bond strength can be adjusted by the substitution of H atoms of YH₂ by PH₃ groups. The FH···C(PH₃)₂ system has the strongest hydrogen bond interaction, which is larger than a conventional hydrogen bond. In addition to electrostatic interaction, donor‐acceptor interaction also plays an important role in determining the hydrogen bond strength. Therefore, a carbon atom can not only be the hydrogen bond acceptor but also can create an unusual stabilized hydrogen bond complex. Also, X₃B–YZ₂ (X = H, F; Y = C, Si, BH, A1H;Z = PH₃, NH₃) systems were examined, and it was found that the bond strength is controlled predominately by the HOMO‐LUMO gap (ΔIP). The smaller the ΔIP, the larger the bond dissociation energy of the B–Y bond. In addition, NH₃ is a better electron‐donating group than PH₃, and thus forms the strongest donor‐acceptor interaction between X₃B and Y(NH₃)₂.     

N2O…HOCl复合物结构和性质的理论研究

分别在DFT-B3LYP和MP2/6-311++G**水平上求得HOCl + N2O复合物势能面上的六种(S1, S2, S3, S4, S5和S6)和四种(S1, S2, S4和S5)构型. 频率分析表明,其中的S1和S3为过渡态,其它为稳定构型. 在复合物S3, S5 和S6中,HOCl 单体的σ*(5O-6H)作为质子供体,与N2O单体中作为质子受体的3O原子相互作用,形成氢键结构,而在氢键复合物S2中, 质子受体为N2O单体中的端1N原子;复合物S1中,HOCl分子的σ*(5O-4Cl)作为质子供体与N2O分子中的端1N原子(质子受体)相互作用,形成卤键结构,而复合物S4中的卤键结构的质子受体为N2O分子中的端3O原子. 经B3LYP/6-311++G**水平上的计算,考虑了基组重叠误差(BSSE)校正的单体间相互作用能在-1.56 ~ -8.73 kJ·mol-1之间. 采用自然键轨道理论(NBO)对两种单体间相互作用的本质进行了考查,并通过分子中原子理论(AIM)分析了复合物中氢键和卤键键鞍点处的电子密度拓扑性质.     

Unconventional H‐bonds: SH···N interaction

Quantum calculations at the MP2/aug‐cc‐pVDZ level are used to analyze the SH···N H‐bond in complexes pairing H₂S and SH radical with NH₃, N(CH₃)₃, NH₂NH₂, and NH₂N(CH₃)₂. Complexes form nearly linear H‐bonds in which the S? H covalent bond elongates and shifts its stretching frequency to the red. Binding energies vary from 14 kJ/mol for acceptor NH₃ to a maximum of 22 kJ/mol for N(CH₃)₃ and N(CH₃)₂NH₂. Analysis of geometric, vibrational, and electronic data indicate that the SH···N interaction involving SH is slightly stronger than that in which the closed‐shell H₂S serves as donor. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

An insight into CH···N hydrogen bond and stability of the complexes formed by trihalomethanes with ammonia and its monohalogenated derivatives

A theoretical study of the C?H···N hydrogen bond in the interactions of trihalomethanes CHX₃ (X = F, Cl, Br) with ammonia and its halogen derivatives NH₂Y (Y = F, Cl, Br) has been carried out thoroughly. The complexes are quite stable, and their stability increases in going from CHF₃ to CHCl₃ then to CHBr₃ when Y keeps unchanged. With the same CHX₃ proton donor, enhancement of the gas phase basicity of NH₂Y strengthens stability of the CHX₃···NH₂Y complex. The C?H···N hydrogen bond strength is directly proportional to the increase of proton affinity (PA) at N site of NH₂Y and the decrease of deprotonation enthalpy (DPE) of C?H bond in CHX₃. The CHF₃ primarily appears to favor blue shift while the red‐shift is referred to the CHBr₃. The blue‐ or red‐shift of CHCl₃ strongly depends on PA at N site of NH₂Y. We suggest the ratio of DPE/PA as a factor to predict which type of hydrogen bond is observed upon complexation. The SAPT2+ results show that all C?H···N interactions in the complexes are electrostatically driven regardless of the type of hydrogen bond, between 48% and 61% of the total attractive energy, and partly contributed by both induction and dispersion energies.     

Blue‐shifted and red‐shifted hydrogen bonds: Theoretical study of the CH3CHO· · ·HNO complexes

The blue‐shifted and red‐shifted H‐bonds have been studied in complexes CH₃CHO…HNO. At the MP2/6‐31G(d), MP2/6‐31+G(d,p) MP2/6‐311++G(d,p), B3LYP/6‐31G(d), B3LYP/6‐31+G(d,p) and B3LYP/6‐311++G(d,p) levels, the geometric structures and vibrational frequencies of complexes CH₃CHO…HNO are calculated by both standard and CP‐corrected methods, respectively. Complex A exhibits simultaneously red‐shifted C? H…O and blue‐shifted N? H…O H‐bonds. Complex B possesses simultaneously two blue‐shifted H‐bonds: C? H…O and N? H…O. From NBO analysis, it becomes evident that the red‐shifted C? H…O H‐bond can be explained on the basis of the two opposite effects: hyperconjugation and rehybridization. The blue‐shifted C? H…O H‐bond is a result of conjunct C? H bond strengthening effects of the hyperconjugation and the rehybridization due to existence of the significant electron density redistribution effect. For the blue‐shifted N? H…O H‐bonds, the hyperconjugation is inhibited due to existence of the electron density redistribution effect. The large blue shift of the N? H stretching frequency is observed because the rehybridization dominates the hyperconjugation. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

The dual role of halogen,chalcogen, and pnictogen atoms as Lewis acid and base: Triangular XBr:SHX:PH2X complexes (X = F,Cl, Br,CN, NC,OH, NH2, and OCH3)

Using ab initio calculations, we have investigated the possibility of formation of triangular XBr:SHX:PH₂X complexes, where X = F, Cl, Br, CN, NC, OH, NH₂, and OCH₃. These complexes are formed through the interaction of a positive electrostatic potential region (σ‐hole) on a molecule with the negative region in another one. The results show that the combined halogen, chalcogen, and pnictogen interactions can give rise to stable cyclic structures. The interaction energies of these complexes span over a wide range, from ?3.55 to ?24.93 kcal/mol. Nice quadratic correlations are found between the interaction energies and binding distances in the trimers. To understand the nature of the interactions in these complexes, molecular electrostatic potential and quantum theory of atoms in molecule analyses are performed. © 2015 Wiley Periodicals, Inc.     

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.     

Nature of MoH···I bonds in Cp2Mo(L)H···I‐C≡C‐R Complexes (L=H,CN, PPh2, C(CH3)3; R=NO2, Cl,Br, H,OH, CH3, NH2)

The nature of the MoH···I bond in Cp₂Mo(L)H···I‐C≡C‐R (L= H, CN, PPh₂, C(CH₃)₃; R=NO₂, Cl, Br, H, OH, CH₃, NH₂) was investigated using electrostatic potential analysis, topological analysis of the electron density, energy decomposition analysis and natural bond orbital analysis. The calculated results show that MoH···I interactions in the title complexes belong to halogen‐hydride bond, which is similar to halogen bonds, not hydrogen bonds. Different to the classical halogen bonds, the directionality of MoH···I bond is low; Although electrostatic interaction is dorminant, the orbital interactions also play important roles in this kind of halogen bond, and steric interactions are weak; the strength of H···I bond can tuned by the most positive electrostatic potential of the I atom. As the electron‐withdrawing ability of the R substituent in the alkyne increases, the electrostatic potential maximum of the I atom increases, which enhances the strength of the H···I halogen bond, as well as the electron transfer.     

Influence of substitution on the strength and nature of CH···N hydrogen bond in XCCH···NH3 complexes

The effect of substitution on the strength and nature of CH···N hydrogen bond in XCCH···NH₃ (X = F, Cl, Br, OH, H, Me) and NCH···NH₃ complexes were investigated by quantum chemical calculations. Ab initio calculations were performed using MP2 method with a wide range of basis sets. With tacking into account the BSSE and ZPVE, the values of BEs decrease. Replacement of the nonparticipatory hydrogen atom of HCCH by the electronegative atoms (F, Cl, and Br), lead to the BEs increases. The BE corresponding to the replacement of the nonparticipatory hydrogen atom of HCCH by the OH and CH₃ groups decreases. A far greater enhancement of the interaction energy arises from replacement of HCCH by the more acidic HCN. The natural bond orbital analysis and the Bader's quantum theory of atoms in molecules were also used to elucidate the interaction characteristics of these complexes. The electrostatic nature of H‐bond interactions is predicted from QTAIM analysis. In addition, the relationship between the isotropic and anisotropic chemical shifts of the bridging hydrogen and binding energy of complexes as well as electron density at N···H BCPs were investigated. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Ability of Lewis Acids with Shallow σ-Holes to Engage in Chalcogen Bonds in Different Environments

Molecules of the type XYT = Ch (T = C, Si, Ge; Ch = S, Se; X,Y = H, CH₃, Cl, Br, I) contain a σ-hole along the T = Ch bond extension. This hole can engage with the N lone pair of NCH and NCCH₃ so as to form a chalcogen bond. In the case of T = C, these bonds are rather weak, less than 3 kcal/mol, and are slightly weakened in acetone or water. They owe their stability to attractive electrostatic energy, supplemented by dispersion, and a much smaller polarization term. Immersion in solvent reverses the electrostatic interaction to repulsive, while amplifying the polarization energy. The σ-holes are smaller for T = Si and Ge, even negative in many cases. These Lewis acids can nonetheless engage in a weak chalcogen bond. This bond owes its stability to dispersion in the gas phase, but it is polarization that dominates in solution.     

The competition of Y⋯o and X⋯n halogen bonds to enhance the group V σ‐hole interaction in the NCY⋯oPH3⋯NCX and OPH3⋯NCX⋯NCY (X,YF,Cl, and Br) complexes

The positive electrostatic potentials (ESP) outside the σ‐hole along the extension of O? P bond in O?PH₃ and the negative ESP outside the nitrogen atom along the extension of the C? N bond in NCX could form the Group V σ‐hole interaction O?PH₃?NCX. In this work, the complexes NCY?O?PH₃?NCX and O?PH₃?NCX?NCY (X, Y?F, Cl, Br) were designed to investigate the enhancing effects of Y?O and X?N halogen bonds on the P?N Group V σ‐hole interaction. With the addition of Y?O halogen bond, the V _{S, max} values outside the σ‐hole region of O?PH₃ becomes increasingly positive resulting in a stronger and more polarizable P?N interaction. With the addition of X?N halogen bond, the V _{S, min} values outside the nitrogen atom of NCX becomes increasingly negative, also resulting in a stronger and more polarizable P?N interaction. The Y?O halogen bonds affect the σ‐hole region (decreased density region) outside the phosphorus atom more than the P?N internuclear region (increased density region outside the nitrogen atom), while it is contrary for the X?N halogen bonds. © 2015 Wiley Periodicals, Inc.     

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.     

A DFT study of Lp···π/halogen bond competition in complexes of perhalogenated alkenes with oxygen/nitrogen containing simple molecules

The possible noncovalent lone pair‐π/halogen bond (lp···π/HaB) complexes of perhalogenated unsaturated C₂Cl_nF_4?n (n = 0–4) molecules with four simple molecules containing oxygen or nitrogen as electron donor, formaldehyde (H₂CO), dimethyl ether (DME), NH₃, and trimethylamine (TMA), have been systematically examined at the M062X/aug‐cc‐pVTZ level. Natural bond orbital (NBO) analysis at the same level is used for understanding the electron density distributions of these complexes. The progressive introduction of Cl atom on C₂Cl_nF_4?n influences more on the lp···π complexes over the corresponding HaB ones. Within the scope of this study, gem‐C₂Cl₂F₂ is the best partner molecule for lp···π interaction with the simple molecules, coupled with the greatest interaction energy (IE) and second‐order orbital interaction [E(2) value], whereas C₂F₄ is the poorest one. The C₂Cl₃F·H₂CO and C₂Cl₄·H₂CO complexes exhibit reverse lp···π bonding, while the Z/E‐C₂Cl₂F₂·NH₃, C₂Cl₃F·NH₃ and C₂Cl₄·NH₃ complexes perform half‐lp···π bonding according to the NBO analysis. The lp···π interaction involving the oxygen/nitrogen and the π‐hole of C₂Cl_nF_4?n overwhelms the HaB involving the oxygen/nitrogen and the σ‐hole of the Cl atom. The electron‐donating methyl groups contribute significantly to the two competitive interactions, therefore, DME and TMA engage stronger in the partner molecules than H₂CO and NH₃. Our theoretical study would be useful for future experimental investigation on noncovalent complexes. © 2016 Wiley Periodicals, Inc.     

Prominent enhancing effects of substituents on the strength of π···σ‐hole tetrel bond

The potential applications of tetrel bonds involving π‐molecules in crystal materials and biological systems have prompted a theoretical investigation of the strength of π···σ‐hole tetrel bond in the systems with acetylene and its derivatives of CH₃, AuPH₃, Li, and Na as well as benzene as the π electron donors. A weak tetrel bond (ΔE < 15 kJ/mol) is found between acetylene and tetrel donor molecule TH₃F (T = C, Si, Ge, Sn, and Pb). All substituents strengthen the π tetrel bond, but the electron‐donating sodium atoms have the largest enhancing effect and the interaction energy is up to about 24 kJ/mol in C₂Na₂‐CH₃F. The electron‐donating ability of the AuPH₃ fragment is intermediate between the methyl group and alkali metal atom. The origin of the stability of the π tetrel‐bonded complex is dependent on the nature of the tetrel donor and acceptor molecules and can be regulated by the substituents.     

Comparison of σ-/π-Hole Tetrel Bonds between TH3F/F2TO and H2CX (X=O,S, Se)

Several σ-hole and π-hole tetrel-bonded complexes with a base H₂CX (X=O, S, Se) have been studied, in which TH₃F (T=C−Pb) and F₂TO (T=C and Si) act as the σ-hole and π-hole donors, respectively. Generally, these complexes are combined with a primary tetrel bond and a weak H-bond. Only one minimum tetrel-bonded structure is found for TH₃F, whereas two minima tetrel-bonded complexes for some F₂TO. H₂CX is favorable to engage in the π-hole complex with F₂TO relative to TH₃F in most cases, and this preference further expands for the Si complex. Particularly, the double π-hole complex between F₂SiO and H₂CX (X=S and Se) has an interaction energy exceeding 500 kJ/mol, corresponding to a covalent-bonded complex with the huge orbital interaction and polarization energy. Both the σ-hole interaction and the π-hole interaction are weaker for the heavier chalcogen atom, while the π-hole interaction involving F₂TO (T=Ge, Sn, and Pb) has an opposite change. Both types of interactions are electrostatic in nature although comparable contributions from dispersion and polarization are respectively important for the weaker and stronger interactions.     

Synthesis,Spectroscopy and Crystal Structures of Chiral Organic Ammonium Tetrathiometalates Showing N‐H···S and C‐H···S Interactions

The reaction of ammonium tetrathiometalate (NH₄)₂[MS₄] (M = W or Mo) with the R(+) or S(?) forms of the organic amine α‐methylbenzylamine [PhCH(CH₃)NH₂] results in the formation of the corresponding non‐centrosymmetric bis(α‐methylbenzylammonium) tetrathiometalate complexes [PhCH(CH₃)NH₃]₂[MS₄] (R‐ammonium M = W 1 ; R‐ammonium M = Mo 2 ; S‐ammonium M = W 3 , S‐ammonium M = Mo 4 ) which were characterized by elemental analysis, IR, Raman, UV‐Vis and CD spectra, X‐ray powder diffractometry and single crystal X‐ray crystallography. Compounds 1 ‐ 4 crystallize in the chiral space group P2₁ and constitute the first examples of structurally characterized chiral organic ammonium group VI tetrathiometalates. The structures of 1 ‐ 4 consist of two crystallographically independent chiral organic ammonium cations and a tetrahedral tetrathiometalate dianion. The N‐H···S and C‐H···S interactions between the anions and cations organise them such that the organic ammonium ions always point towards the S atoms of [MS₄]^2?.     

Hydrogen bonding characterization of XH2NH2···HNO(X = B,Al, Ga) complexes: A theoretical investigation

The complexes of XH₂NH₂···HNO(X = B, Al, Ga) are characterized as head to tail with hydrogen bonding interactions. The structural characteristics can be confirmed by atoms in molecules (AIM) analysis, which also provide comparisons of hydrogen bonds strengths. The calculated interaction energies at G2MP2 level show that stability of complexes decrease as BH₂NH₂···HNO > AlH₂NH₂···HNO > GaH₂NH₂···HNO. On the basis of the vibrational frequencies calculations, there are red‐shifts for ν(X1? H) and blue‐shifts for ν(N? H) in the complexes on dihydrogen bonding formations (X1? H···H? N). On hydrogen bonding formations (N? H···O), there are red‐shifts for ν(N? H) compared to the monomers. Natural bond orbital (NBO) analysis is used to discuss the reasons for the ν(X1? H) and ν(N? H) stretching vibrational shifts by hyperconjugation, electron density redistribution, and rehybridization. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Assessment of intermolecular interactions at three sites of the arylalkyne in phenylacetylene‐containing lithium‐bonded complexes: Ab initio and QTAIM studies

The intermolecular interactions existing at three different sites between phenylacetylene and LiX (X = OH, NH₂, F, Cl, Br, CN, NC) have been investigated by means of second‐order Møller?Plesset perturbation theory (MP2) calculations and quantum theory of “atoms in molecules” (QTAIM) studies. At each site, the lithium‐bonding interactions with electron‐withdrawing groups (? F, ? Cl, ? Br, ? CN, ? NC) were found to be stronger than those with electron‐donating groups (? OH and ? NH₂). Molecular graphs of C₆H₅C?CH···LiF and πC₆H₅C?CH···LiF show the same connectional positions, and the electron densities at the lithium bond critical points (BCPs) of the πC₆H₅C?CH···LiF complexes are distinctly higher than those of the σC₆H₅C?CH···LiF complexes, indicating that the intermolecular interactions in the C₆H₅C?CH···LiX complexes can be mainly attributed to the π‐type interaction. QTAIM studies have shown that these lithium‐bond interactions display the characteristics of “closed‐shell” noncovalent interactions, and the molecular formation density difference indicates that electron transfer plays an important role in the formation of the lithium bond. For each site, linear relationships have been found between the topological properties at the BCP (the electron density ρ_b, its Laplacian ?²ρ_b, and the eigenvalue λ₃ of the Hessian matrix) and the lithium bond length d(Li‐bond). The shorter the lithium bond length d(Li‐bond), the larger ρ_b, and the stronger the π···Li bond. The shorter d(Li‐bond), the larger ?²ρ_b, and the greater the electrostatic character of the π···Li bond. © 2012 Wiley Periodicals, Inc.