Nature and physical origin of CH/pi interaction: significant difference from conventional hydrogen bonds

Recently reported high-level ab initio calculations and gas phase spectroscopic measurements show that the nature of CH/pi interactions is considerably different from conventional hydrogen bonds, although the CH/pi interactions were often regarded as the weakest class of hydrogen bonds. The major source of attraction in the CH/pi interaction is the dispersion interaction and the electrostatic contribution is small, while the electrostatic interaction is mainly responsible for the attraction in the conventional hydrogen bonds. The nature of the "typical" CH/pi interactions is similar to that of van der Waals interactions, if some exceptional "activated" CH/pi interactions of highly acidic C-H bonds are excluded. Shifts of C-H vibrational frequencies and electronic spectra also support the similarity. The hydrogen bond is important in controlling structures of molecular assemblies, since the hydrogen bond is sufficiently strong and directional due to the large electrostatic contribution. On the other hand, the directionality of the "typical" CH/pi interaction is very weak. Although the "typical" CH/pi interaction is often regarded as an important interaction in controlling the structures of molecular assemblies as in the cases of conventional hydrogen bonds, the importance of the "typical" CH/pi interactions is questionable.     

Study of the interaction in clusters formed by phenol and CH3X (X=CN,F,Cl) molecules

The characteristics of the interaction between phenol and acetonitrile, methyl fluoride and methyl chloride were studied. The most stable structures for clusters containing one or two CH3X molecules and one phenol moiety were located by means of ab initio and density functional theory calculations. Phenol-acetonitrile dimer presents two almost equally stable structures; one of them is a typical linearly hydrogen bonded minimum, whereas in the other one, a C-H...pi contact is established accompanied by a distorted O-H...N hydrogen bond. Although the latter minimum presents the larger interaction energy, deformation effects favor the formation of the linear hydrogen bonded one. In complexes with methyl fluoride and methyl chloride, this arrangement is the most stable structure and no linear hydrogen bonded structures were located. Our best estimates for the interaction energies amount to -27.8, -21.6, and -19.7 kJ/mol for clusters of phenol with acetonitrile, methyl fluoride, and methyl chloride, respectively. The main contribution to the stabilization of these clusters is of electrostatic nature, although in structures where a C-H...pi contact is present, the dispersion contribution is also significant. In clusters formed by phenol and two CH3X units, the most stable arrangement corresponds to a head to tail disposal with O-H...X, C-H...X, and C-H...pi contacts forming a cycle. Only for this type of arrangement, three body effects are non-negligible even though they constitute a minor effect. The results also indicate that interactions with methyl fluoride and methyl chloride are of similar intensity, although weaker than with acetonitrile. Significant frequency shifts are predicted for the O-H stretching, which increase when increasing the number of CH3X molecules.     

Ab initio study of the interaction of CHX3 (X = H, F, Cl, or Br) with benzene and hexafluorobenzene

In this paper, the results of a study of the interaction of methane, fluoroform, chloroform, and bromoform with benzene and hexafluorobenzene are presented. The benzene complexes were studied at the MP2/6-31G(d) and MP2/6-311++G(2d,p) levels, and the hexafluorobenzene complexes were only studied at the MP2/6-31G(d) level. The optimized geometries, stabilization energies, potential energy surfaces, harmonic frequencies, and vibrational intensities are reported. A net attraction is predicted for all four benzene complexes, whereas for the CHX3.C6F6 complexes, it was found that MP2/6-31G(d) predicts a net attraction for the CH4, CHCl3, and CHBr3 complexes and does not predict a stable complex for CHF3.C6F6. The three complexes with net attractions all have blue-shifts of the CHX3 CH stretching wavenumber and a slight contraction (0.001-0.003 A) of the CH bond in CHX3. The MP2/6-31G(d) level predicts that the intensity of the CHX3 CH stretch will vary widely. For CH4.C6H6 and CHF3.C6H6, it is predicted that the intensity will be smaller for the complexes than the free molecules, whereas for the other complexes, anywhere from a 30% increase to an increase of 87 times is predicted. The atoms in molecules analysis showed that only three of the eight criteria for normal hydrogen bonding are satisfied for all eight complexes studied. Criterion 3 (value of the Laplacian at the bond critical point) is not satisfied for any of the eight complexes.     

Magnitude and nature of interactions in benzene-X (X=ethylene and acetylene) in the gas phase: significantly different CH/pi interaction of acetylene as compared with those of ethylene and methane

The accurate interaction energies of the CH/pi interaction in the benzene-X clusters (X = ethylene and acetylene) were experimentally and theoretically determined. Two-color multiphoton ionization spectroscopy was applied, and the binding energies in the neutral ground state of the clusters were evaluated from the dissociation threshold measurements of the cluster cations. The experimental binding energies of the clusters (D0) were 1.4+/-0.2 and 2.7+/-0.2 kcal/mol, respectively. Estimated CCSD(T) interaction energies for the clusters at the basis set limit (De) were 2.2 and 2.8 kcal/mol, respectively. Calculated D0 values (1.7 and 2.4 kcal/mol, respectively) are close to the experimental values. Large electron correlation contributions (Ecorr=-3.6 and -2.8 kcal/mol, respectively) show that dispersion is the major source of the attraction in both clusters. The electrostatic interaction in the ethylene cluster is very small (-0.38 kcal/mol), as in the case of the benzene-methane cluster, whereas the electrostatic interaction in the acetylene cluster is large (-1.70 kcal/mol). The shifts of the S1-S0 transition also suggest that the ethylene cluster is a van der Waals-type cluster, but the acetylene cluster is a pi-hydrogen-bonded cluster. The nature of the CH/pi interaction of the "activated" alkyne C-H bond is significantly different from that of the "nonactivated" (or typical) alkane and alkene C-H bonds.     

Experimental and theoretical determination of the accurate interaction energies in benzene-halomethane: the unique nature of the activated CH/pi interaction of haloalkanes

The CH/pi interaction energies between benzene and halomethanes (CH(2)Cl(2) and CHCl(3)) were accurately determined. Two-color ionization spectroscopy was applied to the benzene-CH(2)Cl(2) and -CHCl(3) clusters, and the binding energies in the neutral ground state, i.e. the CH/pi interaction energies in these model cluster systems, were precisely evaluated on the basis of the dissociation threshold measurements of the clusters in the cationic state and the ionization potential value of the bare molecule. The experimentally determined interaction energies were 3.8 +/- 0.2 and 5.2 +/- 0.2 kcal mol(-1) for benzene-CH(2)Cl(2) and -CHCl(3) respectively, and the remarkable enhancement of the CH/pi interaction energy with chlorine-substitution was quantitatively confirmed. The experimental interaction energies were well reproduced by the high-level ab initio calculations. The theoretical calculations clarified the unique nature of the activation of the CH/pi interaction by the chlorine-substitution.     

On the physical origin of blue-shifted hydrogen bonds

For blue-shifted hydrogen-bonded systems, the hydrogen stretching frequency increases rather than decreases on complexation. In computations at various levels of theory, the blue-shift in the archetypical system, F(3)C-H.FH, is reproduced at the Hartree-Fock level, indicating that electron correlation is not the primary cause. Calculations also demonstrate that a blue-shift does not require either a carbon center or the absence of a lone pair on the proton donor, because F(3)Si-H.OH(2), F(2)NH.FH, F(2)PH.NH(3), and F(2)PH.OH(2) have substantial blue-shifts. Orbital interactions are shown to lengthen the X-H bond and lower its vibrational frequency, and thus cannot be the source of the blue-shift. In the F(3)CH.FH system, the charge redistribution in F(3)CH can be reproduced very well by replacing the FH with a simple dipole, which suggests that the interactions are predominantly electrostatic. When modeled with a point charge for the proton acceptor, attractive electrostatic interactions elongate the F(3)C-H, while repulsive interactions shorten it. At the equilibrium geometry of a hydrogen-bonded complex, the electrostatic attraction between the dipole moments of the proton donor and proton acceptor must be balanced by the Pauli repulsion between the two fragments. In the absence of orbital interactions that cause bond elongation, this repulsive interaction leads to compression of the X-H bond and a blue-shift in its vibrational frequency.     

CH/pi hydrogen bonds determine the selectivity of the Src homology 2 domain to tyrosine phosphotyrosyl peptides: an ab initio fragment molecular orbital study

The CH/pi hydrogen bond is a weak molecular force occurring between CH groups (soft acids) and pi-systems (soft bases), and has been recognized to be important in the interaction of proteins with their specific ligands. For instance, it is well known that Src homology-2 protein (SH2) recognizes its specific pTyr peptide in two key regions, pTyr-binding region and specificity-determining region, by the use of attractive molecular forces, including the CH/pi hydrogen bond. We hypothesized that the CH/pi hydrogen bond plays a key role in determining the selectivity of SH2 proteins, and studied this issue by the ab initio fragment molecular orbital (FMO) method. The FMO calculations were carried out, at the HF/6-31G* and MP2/6-31G* level, for SH2 domains of Src, Grb2, P85alpha(N), Syk, and SAP, in complex with corresponding pTyr peptides. CH/pi hydrogen bonds have in fact been found to be important in stabilizing the structure of the complexes. We conclude that the CH/pi hydrogen bond plays an indispensable role in the recognition of SH2 domains with their specific pTyr peptides, thus playing a vital role in the signal transduction system.     

Luminescent mu-ethynediyl and mu-butadiynediyl binuclear gold(I) complexes: observation of (3)(pi pi*) emissions from bridging C(n)(2-) units.

The synthesis and X-ray structural and spectroscopic characterization for LAuC triple bond CAuL x 4CHCl(3) and LAuC triple bond C--C triple bond CAuL x 2CH(2)Cl(2) (1 x 4CHCl(3) and 2 x 2CH(2)Cl(2), respectively; L = PCy(3), tricyclohexylphosphine) are reported. The bridging C(n)(2-) units are structurally characterized as acetylene or diacetylene units, with C triple bond C distances of 1.19(1) and 1.199(8) A for 1 x 4CHCl(3) and 2 x 2CH(2)Cl(2), respectively. An important consequence of bonding to Au(I) for the C(n)(2-) moieties is that the lowest-energy electronic excited states, which are essentially acetylenic (3)(pi pi*) in nature, acquire sufficient allowedness via Au spin-orbit coupling to appear prominently in both electronic absorption and emission spectra. The origin lines for both complexes are well-defined and are observed at 331 and 413 nm for 1 and 2, respectively. Sharp vibronic progressions corresponding to v(C triple bond C) are observed in both emission and absorption spectra. The acetylenic (3)(pi pi) excited state of 2 has a long lifetime (tau(0) = 10.8 mus) in dichloromethane at room temperature and is a powerful reductant (E degrees [Au(2)(+)/Au(2)] < or = -1.85 V vs SSCE).     

Structures, photoluminescence, and reversible vapoluminescence properties of neutral platinum(II) complexes containing extended pi-conjugated cyclometalated ligands

Reacting K2PtCl4 with the tridentate R-C(wedge)N(wedge)C-H2 ligands 2,6-di-(2'-naphthyl)-4-R-pyridine (R = H, 1a; Ph, 1b; 4-BrC6H4, 1c; 3,5-F2C6H3, 1d) in glacial acetic acid, followed by heating in dimethyl sulfoxide (DMSO), gave complexes [(R-C(wedge)N(wedge)C)Pt(DMSO)] (2a-d). In the crystal structures of 2a-c, the molecules are paired in a head-to-tail orientation with Pt...Pt separations >6.3 A, and there are extensive close C-H...pi (d = 2.656-2.891 A), pi...pi (d = 3.322-3.399 A), and C-H...O=S (d = 2.265-2.643 A) contacts. [(Ph-C(wedge)N(wedge)C)Pt(PPh3)] (3) was prepared by reacting 2b with PPh3. Reactions of 2a-d with bis(diphenylphosphino)methane (dppm) gave [(R-C(wedge)N(wedge)C)2Pt2(mu-dppm)] (4a-d). Both head-to-head (syn) and head-to-tail (anti) conformations were found for 4a.6CHCl3.C5H12, whereas only one conformation was observed for 4b.2CHCl3 (syn), 4c.3CH2Cl2 (syn), and 4d.2CHCl3 (anti). In the crystal structures of 4a-d, there are close intramolecular Pt...Pt contacts of 3.272-3.441 A in the syn conformers, and long intramolecular Pt...Pt separations of 5.681-5.714 A in the anti conformers. There are weak C-H...X (d = 2.497-3.134 A) and X...X (X = Cl or Br; d = 2.973-3.655 A) interactions between molecules 4a-d and occluded CHCl3/CH2Cl2 molecules, and their solvent channels are of varying diameters (approximately 9-28 A). Complexes 2a-d, 3, and 4a-d are photoluminescent in the solid state, with emission maxima at 602-643 nm. Upon exposure to volatile organic compounds, 4a shows a fast and reversible vapoluminescent response, which is most intense with volatile halogenated solvents (except CCl4). Powder X-ray diffraction analysis of desolvated 4a revealed a more condensed molecular packing of syn and anti complexes than crystal 4a.6CHCl3.C5H12.     

Saturated hydrocarbon-benzene complexes: theoretical study of cooperative CH/pi interactions

High-level ab initio calculations at the CCSD(T)/aug-cc-pVTZ//MP2/aug(d,p)-6-311G(d,p) level were employed to investigate the cooperative CH/pi effects between the pi face of benzene and several modeled saturated hydrocarbons, propane, isobutane, cyclopropane, cyclobutane, cyclopentane, cyclohexane, cyclopentane, cyclooctane, and bicyclo[2.2.2]octane. In all cases, multiple C-H groups (2-4) are found to interact with the pi face of benzene, with one C-H group pointing close to the center of the benzene ring. The geometries of these complexes are governed predominantly by electrostatic interaction between the interacting systems. The calculated interaction energies (10-14 kJ mol(-1)) are 2-3 times larger than that of the prototypical methane-benzene complex. The trends of geometries, interaction energies, binding properties, as well as electron-density topological properties were analyzed. The calculated interaction energies correlate well with the polarizabilities of the hydrocarbons. AIM analysis confirms the hydrogen-bonded nature of the CH/pi interactions. Significant changes in proton chemical shift and stretching frequency (blue shift) are predicted for the ring C-H bond in these complexes.     

Periodic ab initio approach for the cooperative effect of CH/pi interaction in crystals: relative energy of CH/pi and hydrogen-bonding interactions

The bonding property of the CH/pi interaction in organic crystals has been investigated by the means of a periodic ab initio method. The energy of the CH(sp(2))/pi interaction in crystals, estimated with periodic RHF/6-21G*, showed a reasonable attractive CH(sp(2))/pi interaction owing to a cooperative effect, whereas the results calculated with RHF/cc-pVDZ indicate a negligibly small or repulsive interaction. The relative contribution of the CH(sp(2))/pi interaction to the column packing energy was found to be roughly half of the energy of a conventional hydrogen bond. The calculation of the charge distributions on the aromatic rings participating in the CH(sp(2))/pi interaction in crystals revealed that the atoms were more ionic than those in the gas phase. These theoretical calculations suggest a hydrogen-bonding characteristic for the CH(sp(2))/pi interaction in crystals, which does not occur in solution nor gas phase. We present computational evidence of the existence of the cooperative effect of CH(sp(2))/pi interaction in crystals.     

Magnitude and directionality of interaction in ion pairs of ionic liquids: relationship with ionic conductivity

The intermolecular interaction energies of nine ion pairs of room temperature ionic liquids were studied by MP2/6-311G level ab initio calculations. The magnitude of the interaction energies of 1-ethyl-3-methylimidazolium (emim) complexes follows the trend CF(3)CO(2)(-) > BF(4)(-) > CF(3)SO(3)(-) > (CF(3)SO(2))(2)N(-) approximately PF(6)(-) (-89.8, -85.2, -82.6, -78.8, and -78.4 kcal/mol, respectively). The interaction energies of BF(4)(-) complexes with emim, ethylpyridinium (epy), N-ethyl-N,N,N-trimethylammonium ((C(2)H(5))(CH(3))(3)N), and N-ethyl-N-methylpyrrolidinium (empro) are not very different (-85.2, -82.8, -84.6, and -84.4 kcal/mol, respectively), while the size of the orientation dependence of the interaction energies follows the trend emim > epy approximately (C(2)H(5))(CH(3))(3)N > empro. Comparison with the experimental ionic conductivities shows that the magnitude and directionality of the interaction energy of the ion pairs play a crucial role in determining the ionic dissociation/association dynamics in the ionic liquids. The electrostatic interaction is the major source of attraction between ions. The induction contribution is small but not negligible. The hydrogen bonding with the C(2)-H of imidazolium is not essential for the attraction in the ion pair. The interaction energy of the BF(4)(-) complex with 1-ethyl-2,3-dimethylimidazolium (em2im) (-81.8 kcal/mol) is only 4% smaller than that of the emim complex.     

Methyl formate and its mono and difluoro derivatives: conformational manifolds, basicity, and interaction with HF theoretical investigation

The conformational manifolds, scenarios of protonation, and hydrogen bond propensity of methyl formate and its mono and difluoro derivatives, which possess two oxygen atoms with different basicities, are studied at the B3LYP/6-311++G(3df,3pd) computational level. The optimized geometries of the title molecules, their energetics, and relevant harmonic vibrational frequencies, mainly of the ν(CH) mode of the H-C═O group, are of a primary focus. The Natural Bond Orbital analysis is invoked to obtain the second-order intra- or intermolecular hyperconjugation energies, occupations of antibonding orbitals, and hybridization of the carbon atoms. It is demonstrated that the Z conformers (and their rotamers) of the three title molecules are characterized by a higher stability compared to the E ones. The stabilities depend on the intramolecular hyperconjugative interaction and on the attraction or repulsion nonbonded interaction. The proton affinity of the carbonyl oxygen exceeds, by 15-20 kcal·mol(-1), that of the methoxy oxygen. Fluorine substitution causes a moderate lowering of the proton affinity of the oxygens. Protonation on the oxygen atoms yields a contraction of the C-H bond and large concomitant blue shift of the ν(CH) vibration. These changes are mainly determined by a lowering of the occupation of the corresponding σ*(CH) orbitals. The esters under consideration are probed on the interaction with the HF molecule. The complexes that are formed under this interaction on the oxygen of the H-C═O group are stronger than those formed on the oxygen belonging to the methoxy one. It is deduced that the hydrogen bond energies show a linear dependence on the proton affinities of the corresponding oxygen atoms. Hydrogen-bonded complexes of moderate strength are also formed, while HF interacts with the fluorine atoms of the fluorinated esters.     

Anomeric effects in the symmetrical and asymmetrical structures of triethylamine. Blue-shifts of the C-h stretching vibrations in complexed and protonated triethylamine

Quantum mechanical calculations using density functional theory with the hybrid B3LYP functional and the 6-31++G(d,p) basis set are performed on isolated triethylamine (TEA), its hydrogen-bond complex with phenol, and protonated TEA. The calculations include the optimized geometries and the results of a natural bond orbital (NBO) analysis (occupation of sigma* orbitals, hyperconjugative energies, and atomic charges). The harmonic frequencies of the C-H stretching vibrations of TEA are predicted at the same level of theory. Two stable structures are found for isolated TEA. In the most stable symmetrical structure (TEA-S), the three C-C bond lengths are equal and one of the C-H bond of each of the three CH2 groups is more elongated than the three other ones. In the asymmetrical structure (TEA-AS), one of the C-C bonds and two C-H bonds of two different CH2 groups are more elongated than the other ones. These structures result from the hyperconjugation of the N lone pair to the considered sigma*(C-H) orbitals (TEA-S) or to the sigma*(C-C) and sigma*(C-H) orbitals of the CH2 groups (TEA-AS). The formation of a OH...N hydrogen bond with phenol results in a decrease of the hyperconjugation, a contraction of the C-H bonds, and blue-shifts of 28-33 cm-1 (TEA-S) or 40-48 cm-1 (TEA-AS) of the nus(CH2) vibrations. The nu(CH3) vibrations are found to shift to a lesser extent. Cancellation of the lone pair reorganization in protonated TEA-S and TEA-AS results in large blue-shifts of the nu(CH2) vibrations, between 170 and 190 cm-1. Most importantly, in contrast with the blue-shifting hydrogen bonds involving C-H groups, the blue-shifts occurring at C-H groups not participating in hydrogen bond formation is mainly due to a reduction of the hyperconjugation and the resulting decrease in the occupation of the corresponding sigma*(C-H) orbitals. A linear correlation is established between the C-H distances and the occupation of the corresponding sigma*(C-H) orbitals in the CH2 groups.     

Existence of both blue-shifting hydrogen bond and Lewis acid-base interaction in the complexes of carbonyls and thiocarbonyls with carbon dioxide

In this study, 16 gas phase complexes of the pairs of XCHZ and CO(2) (X = F, Cl, Br; Z = O, S) have been identified. Interaction energies calculated at the CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVTZ level including both BSSE and ZPE corrections range from -5.6 to -10.5 kJ mol(-1) for XCHOCO(2) and from -5.7 to -9.1 kJ mol(-1) for XCHS···CO(2). Substitution of one H atom by one halogen in formaldehyde and thioformaldehyde reduces the interaction energy of XCHZ···CO(2), while a CH(3) substitution increases the interaction energy of both CH(3)CHO···CO(2) and CH(3)CHS···CO(2). NBO and AIM analyses also point out that the strength of Lewis acid-base interactions decreases going from >C1=S3···C6 to >C1=O3C6 and to >C1-X4···C6. This result suggests the higher capacity of solubility of thiocarbonyl compounds in scCO(2), providing an enormous potential application for designing CO(2)-philic materials based on the >C=S functional group in competition with >C=O. The Lewis acid-base interaction of the types >C=S···C, >C-Cl···C and >C-Br···C is demonstrated for the first time. The contribution of the hydrogen bonding interaction to the total interaction energy is larger for XCHS···CO(2) than for XCHO···CO(2). Upon complexation, a contraction of the C1-H2 bond length and a blue shift of its stretching frequency have been observed, as compared to the isolated monomer, indicating the existence of a blue-shifting hydrogen bond in all complexes examined. Calculated results also lend further support for the viewpoint that when acting as proton donor, a C-H bond having a weaker polarization will induce a stronger distance contraction and frequency blue shift upon complexation, and vice versa.     

Theoretical study of (CH...C)- hydrogen bonds in CH(4-n)X(n) (X = F, Cl; n = 0, 1, 2) systems complexed with their homoconjugate and heteroconjugate carbanions

This work deals with a theoretical study of the (CH...C)- hydrogen bonds in CH4, CH3X, and CH2X2 (X = F, Cl) complexed with their homoconjugate and heteroconjugate carbanions. The properties of the complexes are calculated with the B3LYP method using the 6-311++G(d,p) or 6-311++G(2df,2p) basis sets. The deprotonation enthalpies (DPE) of the CH bond or the proton affinities of the carbanions (PA(C-) are calculated as well. All the systems with the exception of the CH4...CHCl2(-) one are characterized by a double minimum potential. In some of the complexes, the (CH(b)...C)- hydrogen bond is linear. In other systems, such as CH3F...CH2F- and CH3F...CHF2(-), there is a large departure from linearity, the systems being stabilized by electrostatic interactions between the nonbonded H of the neutral molecule and the F atom of the carbanion. In the transition state, the (CH(b)...C)- bond is linear, and there is a large contraction of the intermolecular C...C distance. The binding energies vary within a large range, from -1.4 to -11.1 kcal mol(-1) for the stable complexes and -8.6 to -44.1 kcal mol(-1) for the metastable complexes. The energy barriers to proton transfer are between 5 and 20 kcal mol(-1) for the heteroconjugate systems and between 3.8 and 8.3 kcal mol(-1) for the homoconjugate systems. The binding energies of the linear complexes depend exponentially on 1.5DPE - PA(C-), showing that the proton donor is more important than the proton acceptor in determining hydrogen bond strength. The NBO analysis indicates an important electronic reorganization in the two partners. The elongations of the CH bond resulting from the interaction with the carbanion depend on the occupation of the sigma*(CH(b)) antibonding orbitals and on the hybridization of the C bonded to H(b). The frequency shifts of the nu(CH)(A1) stretching vibration range between 15 and 1150 cm(-1). They are linearly correlated to the elongation of the CH(b) bond.     

Oxa-bicyclocalixarenes: a new cage for anions via C-H...anion hydrogen bonds and anion...pi interactions

Density functional theory calculations indicate that the cage molecule 4 can trap F- in the gas phase (-80.5 kcal/mol) as well as in CH2Cl2 (-14.7 kcal/mol) via strong C-H...F- hydrogen bonds and pi...F- interaction.     

Matrix-isolation infrared studies of 1:1 molecular complexes containing chloroform (CHCl3) and Lewis bases: seamless transition from blue-shifted to red-shifted hydrogen bonds

The infrared spectra of molecular complexes containing chloroform (CHCl(3)) and Lewis bases (N(2), CO, H(2)O, and CH(3)CN) have been observed in an Ar matrix, and vibrational peaks for the 1:1 complexes have been assigned. The C-H stretching band of chloroform in the complexes showed a seamless transition from a blue shift (for N(2) and CO) to a red shift (H(2)O and CH(3)CN), in accord with the proton affinity of the base molecules. Density functional calculations predicted that the C-H··(σ-type lone pair) isomer is the most stable, which is consistent with the observed vibrational peak shift upon complex formation. The underlying mechanisms of the C-H hydrogen bond were explored using the topological properties of the electronic charge density and natural orbital analyses.     

C(Ar)-H···O hydrogen bonds in substituted isobenzofuranone derivatives: geometric, topological, and NMR characterization

Substituted isobenzofuranone derivatives 1a-3a and bindone 4 are characterized by the presence of an intramolecular C(Ar)-H···O hydrogen bond in the crystal (X-ray), solution ((1)H NMR and specific and nonspecific IEF-PCM solvation model combined with MP2 and B3LYP methods), and gas (MP2 and B3LYP) phases. According to geometric and AIM criteria, the C(Ar)-H···O interaction weakens in 1a-3a (independent of substituent nature) and in 4 with the change in media in the following order: gas phase > CHCl(3) solution > DMSO solution > crystal. The maximum value of hydrogen bond energy is 4.6 kcal/mol for 1a-3a and 5.6 kcal/mol for 4. Both in crystals and in solutions, hydrogen bond strength increases in the order 1a < 2a < 3a with the rising electronegativity of the ring substituents (H < OMe < Cl). The best method for calculating (1)H NMR chemical shifts (δ(calcd) - δ(expl) < 0.7 ppm) of hydrogen bonded and nonbonded protons in 1a-3a and 1b-3b (isomers without hydrogen bonds) is the GIAO method at the B3LYP level with the 6-31G** and 6-311G** basis sets. For the C-H moiety involved in the hydrogen bond, the increase of the spin-spin coupling constant (1)J((13)C-(1)H) by about 7.5 Hz is in good agreement with calculations for C-H bond shortening and for blue shifts of C-H stretching vibrations (by 55-75 cm(-1)).     

Hydrogen/chlorine exchange reactions of gaseous carbanions

Gas-phase reactions of three typical carbanions CH(2)NO(2)(-), CH(2)CN(-), and CH(2)S(O)CH(3)(-) with the chloromethanes CH(2)Cl(2), CHCl(3), and CCl(4), examined by tandem mass spectrometry, show a novel hydrogen/chlorine exchange reaction. For example, reaction between the nitromethyl anion CH(2)NO(2)(-) and carbon tetrachloride CCl(4) forms the ion CHClNO(2)(-). The suggested reaction mechanism involves nucleophilic attack by CH(2)NO(2)(-) at the chlorine of CCl(4) followed by proton transfer within the resulting complex [CH(2)ClNO(2) + CCl(3)(-)] to form CHClNO(2)(-) and CHCl(3). Two other carbanions CH(2)CN(-) and CH(2)S(O)CH(3)(-) also undergo the novel hydrogen/chlorine exchange reactions with CCl(4) but to a much smaller extent, their higher nucleophilicities favoring competitive nucleophilic attack reactions. Proton abstraction is the exclusive pathway in the reactions of these carbanions with CHCl(3). While CH(2)CN(-) and CH(2)S(O)CH(3)(-) promote mainly proton abstraction and nucleophilic displacement in reactions with CH(2)Cl(2), CH(2)NO(2)(-) does not react.