C-H···π interactions in the CHBrF2···HCCH weakly bound dimer

The microwave spectra of four isotopologues of the CHBrF(2)···HCCH weakly bound dimer have been measured in the 6-18 GHz region using chirped-pulse and Balle-Flygare Fourier-transform microwave spectroscopy. Spectra of (13)CH(79)BrF(2) and (13)CH(81)BrF(2) monomers have also been measured, and spectroscopic constants are reported. Measurement of spectra for the (79)Br and (81)Br isotopologues of CHBrF(2) complexed with both (12)C(2)H(2) and (13)C(2)H(2) have allowed the determination of a structure with C(s) symmetry for this complex. CHBrF(2) interacts with the triple bond of acetylene via a C-H···π contact (R(H···π) = 2.670(8) ?) with the Br atom lying in the ab plane, located 3.293(40) ? from a hydrogen atom of the HCCH molecule. The structure of CHBrF(2)···HCCH has been compared with recently studied related acetylene complexes, including a comparison with (and further structural analysis of) the CHClF(2)···HCCH complex.     

Noncovalent functionalization of graphene via π-hole···π and σ-hole···π interactions

Physisorption of bromopentafluorobenzene (C₆F₅Br) on graphene can occur due to the unique σ-hole and π-hole characters of C₆F₅Br and the rich π-electrons region of graphene, leading to the formation of three types of π-hole···π and σ-hole···π interactions. The π-hole···π interactions are even stronger than the σ-hole···π interactions. The property of graphene was significantly affected by such physisorption.

     

On the weakly C-H···π hydrogen bonded complexes of sevoflurane and benzene

A vibrational assignment of the anaesthetic sevoflurane, (CF(3))(2)CHOCH(2)F, is proposed and its interaction with the aromatic model compound benzene is studied using vibrational spectroscopy of supersonic jet expansions and of cryosolutions in liquid xenon. Ab initio calculations, at the MP2/cc-pVDZ and MP2/aug-cc-pVDZ levels, predict two isomers for the 1?:?1 complex, one in which the near-cis, gauche conformer of sevoflurane is hydrogen bonded through its isopropyl-hydrogen atom, the other in which the same conformer is bonded through a bifurcated hydrogen bond with the fluoromethyl hydrogen atoms. From the experiments it is shown that the two isomers are formed, however with a strong population dominance of the isopropyl-bonded species, both in the jet and liquid phase spectra. The experimental complexation enthalpy in liquid xenon, ΔH(o)(LXe), of this species equals -10.9(2) kJ mol(-1), as derived from the temperature dependent behaviour of the cryosolution spectra. Theoretical complexation enthalpies in liquid xenon were obtained by combining the complete basis set extrapolated complexation energies at the MP2/aug-cc-pVXZ (X = D,T) level with corrections derived from statistical thermodynamics and Monte Carlo Free Energy Perturbation calculations, resulting in a complexation enthalpy of -11.2(3) kJ mol(-1) for the isopropyl-bonded complex, in very good agreement with the experimental value, and of -11.4(4) kJ mol(-1), for the fluoromethyl-bonded complex. The Monte Carlo calculations show that the solvation entropy of the isopropyl-bonded species is considerably higher than that of the fluoromethyl-bonded complex, which assists in explaining its dominance in the liquid phase spectra.     

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.     

Intramolecular OH···π interactions in alkenols and alkynols

The vibrational overtone spectra of propargyl alcohol (prop-2-yn-1-ol, PA), allyl alcohol (prop-2-en-1-ol, AA), propargyl carbinol (but-3-yn-1-ol, PC) and allyl carbinol (but-3-en-1-ol, AC) were recorded with intracavity laser photoacoustic spectroscopy (ICL-PAS) in the Δv(OH) = 3, 4 and 5 regions for propargyl alcohol and allyl alcohol and in the Δv(OH) = 4 and 5 regions for propargyl carbinol and allyl carbinol. Local mode anharmonic oscillator calculations were performed with explicitly correlated coupled cluster methods to guide spectral assignment. Atoms in molecules (AIM) and non-covalent interactions (NCI) calculations were carried out to analyze the interactions between the OH-group and the π-electrons of the carbon-carbon multiple bonds. We ascertain the effect of the carbon chain length and saturation on the conformation and spectroscopy of the four alcohols in relation to intramolecular hydrogen bonding interactions.     

Cooperativity between S···π and Rg···π in the OCS···C6H6···Rg (Rg = He, Ne, Ar, and Kr) van der Waals complexes

The complexes OCS···C(6)H(6), C(6)H(6)···Rg, and OCS···C(6)H(6)···Rg (Rg = He, Ne, Ar, and Kr) have been studied by means of MP2 calculations and QTAIM analyses. The optimized geometries of the title complexes have C(6v) symmetry. The intermolecular interactions in the OCS···C(6)H(6)···Rg complexes are comparatively stronger than that in the OCS···C(6)H(6) complex, which prove that the He, Ne, Ar, and Kr atoms have the ability to form weak bonds with the benzene molecule. In QTAIM studies, the π-electron density of benzene was separated from the total electron density. The molecular graphs and topological parameters of the OCS···πC(6)H(6), πC(6)H(6)···Rg, and OCS···πC(6)H(6)···Rg complexes indicate that the interactions are mainly attributed to the electron density provided by the π-bonding electrons of benzene and the top regions of the S and Rg atoms. Charge transfer is observed from the benzene molecule to SCO/Rg in the formation of the OCS···C(6)H(6), C(6)H(6)···Rg, and OCS···C(6)H(6)···Rg complexes. Molecular electrostatic potential (MEP) analyses suggest that the electrostatic energy plays a pivotal role in these intermolecular interactions.     

A theoretical exploration of unexpected amine···π interactions

Counterintuitive amine lone pair···π interactions are computationally revealed by MP2 and CCSD(T) methods, attractive lone pair···π interactions are observed when the lone pair of nitrogen points toward the π system. Symmetry adapted perturbation theory (SAPT) calculations and atoms in molecules (AIM) analyses were performed and the origin of the calculated attractive interaction between nitrogen lone pairs and π rings is discussed. Dispersion effects were revealed to play a crucial role in the attractive lone pair···π interaction.     

On the directionality of anion-π interactions

The directionality of two important noncovalent interactions involving aromatic rings (namely anion-π and cation-π) is investigated. It has been recently published that the anion-π interactions observed in X-ray structures where the anion is located exactly over the center of the ring are scarce compared to cation-π interactions. To explain this behavior, we have analyzed how the interaction energy (RI-MP2/aug-cc-pVDZ level of theory) is affected by moving the anion from the center of the ring to several directions in anion-π complexes of chloride with either hexafluorobenzene or trifluoro-s-triazine. We have compared the results with the directionality of the cation-π interaction in the sodium-benzene complex. The results are useful to explain the experimental differences between both ion-π interactions. We have also computed the van der Waals radii of several halide anions and we have compared them to the neutral halogen atoms.     

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.     

Competition between H···π and H···O interactions in furan heterodimers

Here the interactions of furan with HZ (Z = CCH, CCF, CN, Cl, and F) are studied using a variety of electron correlation methods (MP2, CCSD(T), DFT-SAPT) and correlation-consistent triple- and quadruple-ζ basis sets including complete basis set (CBS) extrapolation. For Fu-HF all methods agree that a n-type structure with a hydrogen bridge between the oxygen lone-pair of furan and the hydrogen atom of HF is the global minimum structure. It is found to be significantly more stable than a π-type structure where the hydrogen atom of HF points toward the π system of furan. For the other four dimers MP2 and DFT-SAPT predict the π-type structure to be somewhat more stable, while CCSD(T) favors the n-type structure as the global minimum for Fu-HCl and predicts both structures as nearly isoenergetic for Fu-HCCH and Fu-HCCF. From a geometrical point of view, the Fu-HCN dimer structures are more related to those of the Fu-HCl complex than to Fu-HCCH. The different behavior of HCCF and HF upon complexation with furan evidence the effect of the presence of a π system in the aggregation of fluorine derivatives. It is shown that aggregates of furan cannot be understood by means of dipole-dipole and electrostatic analysis only. Yet, through a combined and detailed analysis of DFT-SAPT energy contributions and resonance effects on the molecular charge distributions a consistent explanation of the aggregation of furan with both π electron rich molecules and halogen hydrides is provided.     

N-H···π hydrogen-bonding and large-amplitude tipping vibrations in jet-cooled pyrrole-benzene

The N-H···π hydrogen bond is an important intermolecular interaction in many biological systems. We have investigated the infrared (IR) and ultraviolet (UV) spectra of the supersonic-jet cooled complex of pyrrole with benzene and benzene-d(6) (Pyr·Bz, Pyr·Bz-d(6)). DFT-D density functional, SCS-MP2 and SCS-CC2 calculations predict a T-shaped and (almost) C(s) symmetric structure with an N-H···π hydrogen bond to the benzene ring. The pyrrole is tipped by ω(S(0)) = ±13° relative to the surface normal of Bz. The N···ring distance is 3.13 ?. In the S(1) excited state, SCS-CC2 calculations predict an increased tipping angle ω(S(1)) = ±21°. The IR depletion spectra support the T-shaped geometry: The NH stretch is redshifted by -59 cm(-1), relative to the "free" NH stretch of pyrrole at 3531 cm(-1), indicating a moderately strong N-H···π interaction. The interaction is weaker than in the (Pyr)(2) dimer, where the NH donor shift is -87 cm(-1) [Dauster et al., Phys. Chem. Chem. Phys., 2008, 10, 2827]. The IR C-H stretch frequencies and intensities of the Bz subunit are very similar to those of the acceptor in the (Bz)(2) dimer, confirming that Bz acts as the acceptor. While the S(1)←S(0) electronic origin of Bz is forbidden and is not observable in the gas-phase, the UV spectrum of Pyr·Bz in the same region exhibits a weak 0 band that is red-shifted by 58 cm(-1) relative to that of Bz (38?086 cm(-1)). The origin appears due to symmetry-breaking of the π-electron system of Bz by the asymmetric pyrrole NH···π hydrogen bond. This contrasts with (Bz)(2), which does not exhibit a 0 band. The Bz moiety in Pyr·Bz exhibits a 6a band at 0 + 518 cm(-1) that is about 20× more intense than the origin band. The symmetry breaking by the NH···π hydrogen bond splits the degeneracy of the ν(6)(e(2g)) vibration, giving rise to 6a' and 6b' sub-bands that are spaced by ～6 cm(-1). Both the 0 and 6 bands of Pyr·Bz carry a progression in the low-frequency (10 cm(-1)) excited-state tipping vibration ω', in agreement with the change of the ω tipping angle predicted by SCS-MP2 and SCS-CC2 calculations.     

First principle studies toward the design of a new class of carbene superbases involving intramolecular H···π interactions

The higher basicity of carbenes has been exploited with H···π non-bonding interactions to design a new class of organic superbases. This simple molecular architecture gains a basicity comparable to some of the known functionalized nitrogen superbases and phosphazenes.     

Quantification of binding affinities of essential sugars with a tryptophan analogue and the ubiquitous role of C-H···π interactions

The role of noncovalent interactions in carbohydrate recognition by aromatic amino acids has long been reported. To develop a molecular understanding of noncovalent interactions in the recognition process, we have examined a series of binary complexes between 3-methylindole (3-MeIn) and sugars. In particular, the geometries and binding affinities of 3-MeIn with α/β-D-glucose, β-D-galactose, α-D-mannose and α/β-L-fucose are obtained using the MP2(full)/6-31G(d,p) and the M06/TZV2D//MP2/6-31G(d,p) level of theories. The conventional hydrogen bonding such as N-H···O and C-H···O as well as nonconventional O-H···π and C-H···π type of interactions is, in general, identified as responsible for the moderately strong interaction energies. Large variations in the position-orientations of 3-MeIn with respect to saccharide are noticed, within the same sugar family, as well as across different sugar series. Furthermore, complexes with large differences in their geometries are recognized as capable of exhibiting very similar interaction energies, underscoring the significance of exhaustive conformation sampling, as carried out in the present study. These observations are readily attributed to the differences in the efficiency of the type of interactions enlisted above. The highest and lowest interaction energies, upon inclusion of 50% BSSE correction, are found to be -16.02 and -6.22 kcal mol(-1), respectively, for α-D-glucose (1a) and α-L-fucose (5j). While more number of prominent conventional hydrogen bonding contacts remains as a characteristic feature of the strongly bound complexes, the lower end of the interaction energy spectrum is dominated by multiple C-H···π interactions. The complexes exhibiting as many as four C-H···π contacts are identified in the case of α/β-D-glucose, β-D-galactose, and α/β-L-fucose with an interaction energy hovering around -8 kcal mol(-1). The presence of effective C-H···π interactions is found to be dependent on the saccharide configuration as well as the area of the apolar patch constituted by the C-H groups. The study offers a comprehensive set of binary complexes, across different saccharides, which serves as an illustration of the significance and ubiquitous nature of C-H···π interactions in carbohydrate binding in saccharide-protein complexes.     

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.     

Characteristics of X-H···π interactions: ab initio and QTAIM studies

The MP2/6-311++G(d,p) calculations were performed on several hydrogen-bonded systems. Different complexes were taken into account to analyze various types of hydrogen bonds, possessing different types of proton donors and proton acceptors as well as characterized by the broad range of the interaction energy. The Quantum Theory of Atoms in Molecules is applied. The results of the hybrid variational-perturbational approach are discussed. The unique properties of hydrogen bonds, where π-electrons act as the proton acceptor (X-H···π), are analyzed, and these interactions are compared with the other types of hydrogen bonds, mainly with C-H···Y interactions. It is shown that for X-H···π systems the ellipticity at the bond critical point of the proton···acceptor interaction is much greater than for the other types of hydrogen bonds. However, both X-H···π and C-H···Y interactions are characterized by the dominance of the dispersive energy.     

N-Cα bond cleavage of the peptide backbone via hydrogen abstraction

The specific cleavage of N-C_α bonds on the peptide backbone to form the so-called ‘c’ and ‘z + 2’ products, which can be used for the rapid determination of protein amino-acid sequences, has been examined to clarify the mechanism(s) that occur during hydrogen abstraction induced by bombardment with 337-nm laser photons in matrix-assisted laser desorption/ionization (MALDI) method. Intramolecular hydrogen abstraction, which results from the hydrogen(s) on the C_α or C_β carbon, did not occur with a deuterium-labeled dodecapeptide. To confirm a proposition that intermolecular hydrogen abstraction occurs between the peptide and the MALDI matrix, a deuterium dodecapeptide embedded in a deuterium 2,5-dihydroxybenzoic acid matrix at a molar ratio of 1:7000 was analyzed. The resulting deuterium c product ions suggested that c ions form via intermolecular hydrogen abstraction, although the results obtained did not deny any other possibilities such as intramolecular transfer of labile hydrogen. A mechanism for the N-C_α bond cleavage has been proposed that the formation of hypervalent radical species and subsequent prompt bond cleavages occur. The proposed mechanism successfully rationalizes the formation of both the z + 2 and the c product ions.     

Microwave spectroscopic and theoretical studies on the phenylacetylene···H2O complex: C-H···O and O-H···π hydrogen bonds as equal partners

Rotational spectra of five isotopologues of the title complex, C(6)H(5)CCH···H(2)O, C(6)H(5)CCH···HOD, C(6)H(5)CCH···D(2)O, C(6)H(5)CCH···H(2)(18)O and C(6)H(5)CCD···H(2)O, were measured and analyzed. The parent isotopologue is an asymmetric top with κ = -0.73. The complex is effectively planar (ab inertial plane) and both 'a' and 'b' dipole transitions have been observed but no c dipole transition could be seen. All the transitions of the parent complex are split into two resulting from an internal motion interchanging the two H atoms in H(2)O. This is confirmed by the absence of such doubling for the C(6)H(5)CCH···HOD complex and a significant reduction in the splitting for the D(2)O analog. The rotational spectra, unambiguously, reveal a structure in which H(2)O has both O-H···π (π cloud of acetylene moiety) and C-H···O (ortho C-H group of phenylacetylene) interactions. This is in agreement with the structure deduced by IR-UV double resonance studies (Singh et al., J. Phys. Chem. A, 2008, 112, 3360) and also with the global minimum predicted by advanced electronic structure theory calculations (Sedlack et al., J. Phys. Chem. A, 2009, 113, 6620). Atoms in Molecule (AIM) theoretical analysis of the complex reveals the presence of both O-H···π and C-H···O hydrogen bonds. More interestingly, based on the electron densities at the bond critical points, this analysis suggests that both these interactions are equally strong. Moreover, the presence of both these interactions leads to significant deviation from linearity of both hydrogen bonds.     

The X<Superscript>−</Superscript>···benzohydrazide complexes: the interplay between anion-π and H-bond interactions

The compounds containing the benzohydrazide (BH) nucleus have a variety of biological activities because of various noncovalent intermolecular interactions. The interplay between anion-π and H-bond interactions, which can affect the activity of compounds, has been investigated in ten substituted BH exposed to the chloride ion using the quantum mechanical calculations. The total interaction energy is separated into the anion-π (ΔE _Aπ) and H-bond (ΔE _HB) contributions where both interactions are presented in the complexes. The electron-withdrawing substituents (EWSs) increase |ΔE _Aπ| and decrease |ΔE _HB|, while reversed changes are observed with the electron-donating substituents (EDSs). In addition, the total binding energy (ΔE) becomes more/less negative in the presence of EWSs/EDSs. The synergetic effects of mentioned interactions and substituent effects have also been investigated using the atoms in molecules (AIM), natural bond orbital (NBO) and molecular electrostatic potential (MEP) analyses. A good correlation is found between the energy data and the Hammett constants, the minimum of electrostatic potential (V _min) and the results of population analyses.     

New hyperbranched carbosiloxane–carbosilane polymers with aromatic units in the backbone

New hyperbranched polymers based on a carbosiloxane–carbosilane skeleton with aromatic units in the backbone have been prepared via one-pot hydrosilylation reaction using HSi(Me)₂–O–CH₂–C₆H₄–OSiMe–(CH₂)₄(C₃H₅)₂ as a novel AB₂ monomer. These polymers are easy to prepare, have narrow polydispersity values and present allyl groups on the surface which can be used as synthetic platforms for the introduction of different terminal groups like amine groups through hydrosilylation reactions, opening the door to functionalized polymers. The polymerization process was monitored using real-time ¹H NMR spectroscopy and the resulting hyperbranched polymers were characterized using ¹H NMR, ¹³C NMR, ²⁹Si NMR and SEC/MALLS. The degree of branching in these polymers was determined by quantitative ²⁹Si NMR spectroscopy and found to be very close to the theoretical value of 0.50 for AB₂ systems. The hydrolytic degradation of these polymers in protic solvents has been studied by ²⁹Si NMR.     

C-X···π halogen and C-H···π hydrogen bonding: interactions of CF3X (X = Cl, Br, I or H) with ethene and propene

Using FTIR and Raman spectroscopy, the formation of halogen bonded complexes of the trifluorohalomethanes CF(3)Cl, CF(3)Br and CF(3)I with ethene and propene dissolved in liquid argon has been investigated. For CF(3)Br and CF(3)I, evidence was found for the formation of C-X···π halogen bonded 1:1 complexes. At a higher ratio of CF(3)I/propene, weak absorptions due to a 2:1 complex were also observed. Using spectra recorded at different temperatures, the complexation enthalpies for the complexes were determined to be -5.3(2) kJ mol(-1) for CF(3)Br·ethene, -7.5(2) kJ mol(-1) for CF(3)I·ethene, -5.6(1) kJ mol(-1) for CF(3)Br·propene, -8.8(1) kJ mol(-1) for CF(3)I·propene and -16.5(6) kJ mol(-1) for (CF(3)I·)(2)propene. The complexation enthalpies of the hydrogen bonded counterparts, with CF(3)H as the Lewis acid, were determined to be -4.6(4) kJ mol(-1) for CF(3)H·ethene and -5.1(2) kJ mol(-1) for CF(3)H·propene. For both hydrogen bonded complexes, a blue shift, by +4.8 and +4.0 cm(-1), respectively, was observed for the C-H stretching mode. The results from the cryospectroscopic study are compared with ab initio calculations at the MP2/aug-cc-pVDZ(-PP) level.