First kinetic evidence for the CH/π and π/π solute-solvent interaction of C60 in the Diels-Alder reaction with cyclohexadiene

The first CH/π solute-solvent interaction of C(60) was evidenced by the kinetic solvent effects in the Diels-Alder reaction with 1,3-cyclohexadiene based on the evaluation of linear free energy relationship of log k(2) with empirical solvent polarity and basicity parameters, E(T)(30) and D(π), respectively.     

Observation of a double C-H···π interaction in the CH2ClF···HCCH weakly bound complex

The structure of the CH(2)ClF···HCCH dimer has been determined using both chirped-pulse and resonant cavity Fourier-transform microwave spectroscopy. The complex has C(s) symmetry and contains both a double C-H···π interaction, in which one π-bond acts as acceptor to two hydrogen atoms from the CH(2)ClF donor, and a weak C-H···Cl interaction, with acetylene as the donor. Analysis of the rotational spectra of four isotopologues (CH(2)(35)ClF···H(12)C(12)CH, CH(2)(37)ClF···H(12)C(12)CH, CH(2)(35)ClF···H(13)C(13)CH, and CH(2)(37)ClF-H(13)C(13)CH) has led to a structure with C-H···π distances of 3.236(6) ? and a C-H···Cl distance of 3.207(22) ?, in good agreement with ab initio calculations at the MP2/6-311++G(2d,2p) level. Both weak contacts are longer than those observed in similar complexes containing a single C-H···π interaction that lies in the C(s) plane; however, this appears to be the first double C-H···π contact to be studied by microwave spectroscopy, so there is little data for direct comparison. The rotational and chlorine nuclear quadrupole coupling constants for the most abundant isotopologue are: A = 5262.899(14) MHz, B = 1546.8074(10) MHz, C = 1205.4349(7) MHz, χ(aa) = 28.497(5) MHz, χ(bb) = -65.618(13) MHz, and χ(cc) = 37.121(8) MHz.     

Direct measurements of electric fields in weak OH···π hydrogen bonds

Hydrogen bonds and aromatic interactions are of widespread importance in chemistry, biology, and materials science. Electrostatics play a fundamental role in these interactions, but the magnitude of the electric fields that support them has not been quantified experimentally. Phenol forms a weak hydrogen bond complex with the π-cloud of benzene, and we used this as a model system to study the role of electric fields in weak OH···π hydrogen bonds. The effects of complex formation on the vibrational frequency of the phenol OH or OD stretches were measured in a series of benzene-based aromatic solvents. Large shifts are observed and these can be converted into electric fields via the measured vibrational Stark effect. A comparison of the measured fields with quantum chemical calculations demonstrates that calculations performed in the gas phase are surprisingly effective at capturing the electrostatics observed in solution. The results provide quantitative measurements of the magnitude of electric fields and electrostatic binding energies in these interactions and suggest that electrostatics dominate them. The combination of vibrational Stark effect (VSE) measurements of electric fields and high-level quantum chemistry calculations is a general strategy for quantifying and characterizing the origins of intermolecular interactions.     

Induction-driven stabilization of the anion-π interaction in electron-rich aromatics as the key to fluoride inclusion in imidazolium-cage receptors

Intermolecular interactions that involve aromatic rings are key processes in both chemical and biological recognition. It is common knowledge that the existence of anion-π interactions between anions and electron-deficient (π-acidic) aromatics indicates that electron-rich (π-basic) aromatics are expected to be repulsive to anions due to their electron-donating character. Here we report the first concrete theoretical and experimental evidence of the anion-π interaction between electron-rich alkylbenzene rings and a fluoride ion in CH(3)CN. The cyclophane cavity bridged with three naphthoimidazolium groups selectively complexes a fluoride ion by means of a combination of anion-π interactions and (C-H)(+)···F(-)-type ionic hydrogen bonds. (1)H NMR, (19)F NMR, and fluorescence spectra of 1 and 2 with fluoride ions are examined to show that only 2 can host a fluoride ion in the cavity between two alkylbenzene rings to form a sandwich complex. In addition, the cage compounds can serve as highly selective and ratiometric fluorescent sensors for a fluoride ion. With the addition of 1 equiv of F(-), a strongly increased fluorescence emission centered at 385 nm appears at the expense of the fluorescence emission of 2 centered at 474 nm. Finally, isothermal titration calorimetry (ITC) experiments were performed to obtain the binding constants of the compounds 1 and 2 with F(-) as well as Gibbs free energy. The 2-F(-) complex is more stable than the 1-F(-) complex by 1.87 kcal mol(-1), which is attributable to the stronger anion-π interaction between F(-) and triethylbenzene.     

Bridged aromatic alkenes for the study of carbocation-π interaction

Toward the goal of gaining further insight into carbocation-π interactions, bridged-ring aromatic alkene model systems are being investigated in which one isomer will permit π complexation of an intramolecular tertiary carbocation with a benzene ring, but the other isomer will not. The syntheses of three sets of such isomers, having, respectively, benzobicyclo[3.2.1]octene, benzobicyclo[2.2.1]heptene, and benzobicyclo[4.2.1]nonene structures, are described.     

Cooperativity of H-bonding and anion-π interaction in the binding of anions with neutral π-acceptors

A rare anion-π complex between bromide and a neutral receptor is reported and related receptor systems are studied with a series of anions. The interaction is observed in the solid state and in solution, and further evidence for it is obtained by a computational study.     

Experimental and theoretical determination of the accurate CH/π interaction energies in benzene-alkane clusters: correlation between interaction energy and polarizability

The CH/π interaction energies in benzene-alkane model clusters were precisely determined by laser spectroscopy and theoretical calculations. Two-color resonant two-photon ionization spectroscopy was employed to experimentally determine the interaction energies with isomer selectivity. High precision ab initio calculations were also performed to evaluate the CCSD(T) level interaction energies of various isomers at the basis set limit. Binary clusters of benzene with ethane, propane, n-butane, iso-butane, and cyclohexane were studied. The experimental interaction energies were well reproduced by the theoretical evaluations. The magnitude of the interaction energy clearly correlates with the average polarizability of the alkane moiety, demonstrating that the CH/π interaction energy is dominated by the dispersion interaction. On the other hand, the number of C-H groups in contact with the phenyl ring has no relation to the magnitude of the interaction energy, and it indicates that the role of the hydrogen atom in the CH/π interaction is essentially different from that in hydrogen bonds.     

Lone pair···π interactions on the stabilization of intra and intermolecular arrangements of coordination compounds with 2-methyl imidazole and benzimidazole derivatives

Abstract

Spectroscopic and single crystal X-ray diffraction studies of coordination compounds of Co^II, Ni^II, Zn^II, and Pd^II with phenylsulfonyl imidazole and benzimidazole derivatives (2-mfsiz, 2-mfsbz) were performed. The relevance of non-covalent interactions on the stabilization of intra and intermolecular arrangements in the ligands and their coordination compounds was investigated. The imidazole 2-mfsiz ligand presents two enantiomeric conformers, where the ethylphenylsulfone moiety stabilizes intermolecular lone pair···π (S–O···π_(phe)) and H···π contacts, while its tetrahedral coordination compounds, [M(2-mfsiz)₂X₂] (M²⁺?=?Co, Ni, Zn; X?=?Cl, Br) showed intramolecular lone pair···π interactions (S–O···π_(iz)). On the other hand, compounds [Cu₂(2-mfsiz)₂(µ₂-AcO)₄] and trans-[Pd(2-mfsiz)₂Cl₂] do not present lone pair···π interactions due to the metal ion geometry (square base pyramidal or square planar), which leads to formation of π_(iz)···π_(phe) interactions. For the benzimidazole ligand 2-mfsbz, an intramolecular, H_(phe)···π_(bz) contact was observed, remaining in its tetrahedral and octahedral coordination compounds, [M(2-mfsbz)₂X₂] (M²⁺?=?Co, Zn; X?=?Cl, Br, NO₃). This interaction limits the free rotation of the ethylphenylsulfone moiety for stabilization of an intermolecular lone pair···π interaction (S–O···π_(iz)). The dimeric [Zn₂(2-mfsiz)₂(µ²-AcO)₄] compound has a π_(bz)···π_(phe) contact. Theoretical calculations confirmed the non-covalent interactions in the ligands and their coordination compounds.     

Dampened circumrotation by CH···π interactions in hydrogen bonded [2]rotaxanes

Establishment of CH···π interactions between the aliphatic axis and the benzylic amide macrocycle of hydrogen-bonded [2]rotaxanes causes a measurable interference in the pirouetting submolecular motion of these interlocked molecules.     

Cation···π interactions: QTAIM and NBO studies on the interaction of alkali metal cations with heteroaromatic rings

The cation···π interactions of alkali metal cations (Li⁺, Na⁺, and K⁺) with five-membered heteroaromatic rings [furan(C₄H₄O), thiophene(C₄H₄S), pyrrole(C₄H₅N)] were examined by high level ab initio calculations, to investigate the different roles of C₄H₄O, C₄H₄S, and C₄H₅N as the electron donor, the influential factors that affect these interactions, the nature of this kind of cation···π interaction, and to determine topological and energetical properties to characterize these interactions. The sulfur atom in C₄H₄S plays a certain role in the cation···π interactions except the C–C π bond, which is different from C₄H₄O and C₄H₅N. The size of cation and the character of heteroaromatic ring are two influential factors that affect the cation···π interactions. The studied cation···π interactions can be classified as “closed-shell” and noncovalent interactions. The electron density and its Laplacian at the bond critical points and ring critical points generated upon complexation are useful measurements for the strength of cation···π interactions.     

Preference of intra- and intermolecular cation-π interaction: cis-trans geometrical effects of amide bond on the interaction mode

An intermolecular cation-π interaction is observed in trans-amide 3, whereas an intramolecular interaction is observed in cis-amide 4, suggesting that cis-trans conformational difference plays a critical role in the preference of the interaction modes.     

Communication: Competition between π···π interaction and halogen bond in solution: a combined 13C NMR and density functional theory study

Competition between π···π interaction and halogen bond in solution has been investigated by using carbon nuclear magnetic resonance spectroscopy ((13)C NMR) combined with density functional theory calculation. Both experimental and theoretical results clearly show that there are no C-Cl···π or C-Br···π halogen bonds and only the π···π interactions exist in the binary liquid mixtures of C(6)D(6) with C(6)F(5)Cl and C(6)F(5)Br, respectively. The case is totally different for the binary liquid mixtures of C(6)D(6) with C(6)F(5)I in which the C-I···π halogen bonds not the π···π interactions are present. The important role of entropy in the competition between π···π interaction and halogen bond in solution was also discussed.     

Conformation of aromatic rings in isolable atropisomers of 2-arylindoline derivatives and kinetic evidences for π-π interaction

The equilibrium constants [K=anti/syn] of a pair of atropisomers due to restricted rotation about Csp³-Csp² bond for [2-(2-hydroxynaphthalen-1-yl)-3,3-dimethylindolin-1-yl](4-substituted phenyl)methanone were determined in some solvents. The presence of the effective π-π interaction was demonstrated by the correlation between the equilibrium constants (K) and the substituent effect of the phenyl groups (σ_p), suggesting that the ‘neutral-type’ interaction is operative.     

Enthalpy parameters for interaction of small peptides with 18-crown-6 and aza-18-crown-6 in water at 25°C

Enthalpies of dilution have been determined for binary aqueous solutions of 1-aza-18-crown-6 as well as for ternary aqueous solutions containing glycine, glycylglycine, glycyl-L--alanine, L--alanyl-glycine, L--alanyl-L--alanine, DL--alanyl-DL--alanine, trialanine and 18-crown-6 and/or 1-aza-18-crown-6 and or 1,10-diaza-18-crown-6 at 25°C. The results have been treated by the McMillan-Mayer approach in order to obtain enthalpic virial coefficients for homotactic and heterotactic interactions. A significant exo-effect is demonstrated by the enthalpically favorable interaction between peptides and the 18-crown-6. The additivity of the positive alanyl group contribution toh _xy has been confirmed on the basis of oligomeric data. The influence on the enthalpy of the 18-crown-6-peptide interaction of the methyl group position, in relation to the ammonium group in peptides, has been found to result in the exo-effect decreasing with a decrease of this distance. Some decrease in enthalpy of L--alanyl-L--alanine and DL--alanyl-DL--alanine by 18-crown-6 has been observed as well.Deceased.     

Hydrogen bond CH…O in a methanolic solution of hydrogen chloride

The existence of a hydrogen bond in which a methyl group of the (MeOH)₂H⁺ ion acts as a proton donor is examined. The fundamental vibration frequencies of this ion were calculated for different numbers and strengths of CH…O bonds. The atomic charges in neutral ((MeOH) _n ,n=1–4) and protonated ((MeOH) _m H⁺,m=2–6) associates of methanol molecules were also calculated. The experimentally observed decrease in the v(CH) vibration frequencies of the (MeOH)₂H⁺ ion to 2890 cm⁻¹ and 2760 cm⁻¹ is attributable to the fact that each methyl group of the ion is involved in formation of two CH…O bonds with strength of −12.5 kJ mol⁻¹. The proton-donating ability of the CH bond depends on the charge on its H atom; however, it does not correlate with the dipole moment of this bond. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 306–312, February, 1999.     

Observation of CH⋅⋅⋅π Interactions between Methyl and Carbonyl Groups in Proteins

Protein structure and function is dependent on myriad noncovalent interactions. Direct detection and characterization of these weak interactions in large biomolecules, such as proteins, is experimentally challenging. Herein, we report the first observation and measurement of long-range “through-space” scalar couplings between methyl and backbone carbonyl groups in proteins. These J couplings are indicative of the presence of noncovalent C−H⋅⋅⋅π hydrogen-bond-like interactions involving the amide π network. Experimentally detected scalar couplings were corroborated by a natural bond orbital analysis, which revealed the orbital nature of the interaction and the origins of the through-space J couplings. The experimental observation of this type of CH⋅⋅⋅π interaction adds a new dimension to the study of protein structure, function, and dynamics by NMR spectroscopy.     

Direct colorimetric study on the interaction of Escherichia coli with mannose in polydiacetylene Langmuir–Blodgett films

The membranes of polydiacetylene backbone decorated with mannose assembled by Langmuir–Blodgett technology can interact with Escherichia coli. The interactions lead to the color transition of the membranes which was readily visible to the naked eyes and could be quantified by visible absorption spectroscopy. To understand the mechanism of the chromatic transition, the affinochromism properties of polydiacetylene were examined by resonance Raman spectroscopy. The results demonstrated that the side chains of polymer backbone performed rearrangement, and the electronic structure in the polymer backbone changed from acetylene to butatriene form when the chromatic transformation from blue to red. The direct colorimetric detection by polydiacetylene membranes not only opens a new path for the use of these membranes in the area of biosensor development but also offers new possibilities for diagnostic applications and screening for binding ligand.     

Stabilization of β-hairpin structures via inter-strand π-π and hydrogen bond interactions in α-, β-, γ-hybrid peptides

Synthesis and conformational studies of α-, β-, γ-hybrid peptides containing a pyrrole amino acid (Paa, 1) and a furan amino acid (Faa, 2), namely Boc-β-Phe-Faa-d-Pro-Gly-Paa-β-HGly-Faa-OMe (3) and Boc-Paa-β-Phe-Faa-d-Pro-Gly-Paa-β-HGly-Faa-OMe (4), were carried out and they adopt β-hairpin structures stabilized via inter-strand π-π and hydrogen bonding interactions.