The C-H...O and C-H...N intramolecular hydrogen bonds in betainealdehydes of azoles containing the pyridinium cation

The comparison of the values of the chemical shifts of the pyridinium protons in the ylides of -dicarbonyl compounds and in betainealdehydes of thiazole and imidazole established the presence of the intramolecular C-H...O and C-H...N hydrogen bond between the -protons of the pyridinium and the oxygen atoms of the formyl group and the nitrogen of the amide fragment in the anionoid part of the betaine. The conclusion was confirmed by the varying influence of the effects of protonation on the character of the deshielding of the - and -protons.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 626–628, May, 1989.     

Experimental evidence for intramolecular blue-shifting C-H...O hydrogen bonding by matrix-isolation infrared spectroscopy

Experimental evidence for intramolecular blue-shifting C-H...O hydrogen bonding is presented. Argon matrix-isolation infrared spectra of 1-methoxy-2-(dimethylamino)ethane exhibit a band at 3016.5 cm-1. Spectral behavior with annealing indicates that this band is assigned to the most stable conformer, trans-gauche-(trans|gauche'), with an intramolecular C-H...O hydrogen bond. Density functional calculations show that this band arises from the stretching vibration of the C-H bond participating in the formation of the C-H...O hydrogen bond. The C-H bond is shortened by 0.004 A, and the C-H stretching band is blue-shifted by at least 35 cm-1 on the formation of the hydrogen bond. The (C)H...O distance is calculated as 2.38 A, which is shorter than the corresponding van der Waals separation by 0.3 A.     

Weak C-H...O and C-H...F-C hydrogen bonds in the oxirane-trifluoromethane dimer

The oxirane-trifluoromethane dimer generated in a supersonic expansion has been characterized by Fourier transform microwave spectroscopy. The rotational spectra of the parent species and of its two (13)C isotopomers in combination with ab initio calculations have been used to establish a C(s)() geometry for the dimer with the two monomers bound by one C-H.O and two C-H.F-C hydrogen bonds. An overall bonding energy of about 6.7 kJ/mol has been derived from the centrifugal distortion analysis. The lengths of the C-H.O and C-H.F hydrogen bonds, r(O.H) and r(F.H), are 2.37 and 2.68 A, respectively. The C-H.F-C interactions give rise to the HCF(3) internal rotation motion barrier of 0.55(1) kJ/mol, which causes the A-E splittings observed in the rotational spectra. The analysis of the structural and energetic features of the C-H.O and C-H.F-C interactions allows us to classify them as weak hydrogen bonds. Ab initio calculations predict these weak interactions to produce blue shifts in the C-H vibrational frequencies and shortenings of the C-H lengths.     

Raman spectroscopic evidence for cooperative C-H...O interactions in the acetaldehyde-CO2 complex

In carbonyl compounds having a hydrogen atom attached to the alpha-carbon or directly to the carbonyl carbon, our recent ab initio and solvation studies have predicted the presence of a weak C-H...O interaction with CO2. This intermolecular interaction acts cooperatively with the Lewis acid-Lewis base interaction that exists between CO2 and the carbonyl group. This communication provides the first experimental evidence for this cooperative interaction in the acetaldehyde-CO2 complex using Raman spectroscopy at 25.0 degrees C in the gas phase. The implications of these interactions in the solvation of carbonyl-based CO2-philes are also discussed.     

Steric effect on construction of extended architectures of Ni(II) complexes directed by intermolecular C-H...F and C-H...O interactions

Five new Ni(II) complexes with pyridine carboxamide ligands have been synthesized and the crystal structures of three of the complexes were determined. Strong distortion effects of 6-methyl substitution were observed in the complexes with 6-methyl-substituted pyridyl bpb ligands. The C-H...F and C-H...O hydrogen bond interactions build extended architectures in the crystals studied. This result suggests that the steric effect of 6-methyl substitution plays an important role in the distortion of the structure, and the 6-methyl substitution can facilitate hydrogen bond interactions between methyl hydrogen atoms and O(carbonyl) or F atoms. Twelve Ni(II) complexes, including seven complexes reported previously, show reversible redox behavior, implying that the reduced Ni(I) state of each complex is stable in the time scale of CV measurement. The steric effect of R₁ substituent and the electronic effects of X₁ and X₂ groups were found to be the main factors contributing to the shift of the redox potential of the Ni(II) complexes     

Perturbing the structure of the 2-norbornyl cation through C-H...N and C-H...pi interactions

Quantum chemical studies on complexes of the 2-norbornyl cation with ammonia and benzene are described. These calculations demonstrate that the delocalized, bridged, nonclassical geometry that is usually favored for this carbocation can be perturbed significantly toward the normally less favorable classical geometry through appropriately oriented noncovalent interactions. Since such cations have been proposed as intermediates in enzyme and antibody catalyzed reactions, these results have implications for the nature of the cationic species that may be generated in the presence of electron-rich amino acid side chains of the sort that may be present in the active sites of biocatalysts.     

Binding of an acetonitrile molecule inside the ethereal cavity of a hexaarylbenzene-based receptor via a synergy of C-H...O/C-H...pi interactions

A pair of hexaarylbenzene-based receptors, which contain a circular, as well as a partially-broken, ethereal fence around the central benzene ring, bind acetonitrile molecules via a synergy of C-H...O and C-H...pi interactions, as probed by X-ray crystallography.     

Characterization of persistent intramolecular C-H...X(N,O) bonds in solid state and solution

The formation of intramolecular CH...X(N,O) bonds and their persistence in solution were studied by X-ray crystallography and NMR techniques in two different rotamers of a molecule containing the ortho-carborane cage, an amide group and a quinoline ring. Experimental data were confirmed by theoretical ab initio calculations. From the resolved structure of the two forms of this potentially active drug for boron neutron capture therapy, accurate bonding and geometric parameters were extracted for this non-classic hydrogen interaction, and their strength was calculated. These findings provided new insight in the theory of CH...X bonds, which appear stronger and less rare than it was previously thought.     

Exploring the lower limit in hydrogen bonds: analysis of weak C-H...O and C-H...pi interactions in substituted coumarins from charge density analysis

Experimental electron densities in coumarin, 1-thiocoumarin, and 3-acetylcoumarin have been analyzed based on the X-ray diffraction data at 90 K. These compounds pack in the crystal lattice with weak C-H...O and C-H...pi interactions, and variations in charge density properties and derived local energy densities have been investigated in the regions of intermolecular interactions. Theoretical charge density calculations on crystals using the B3LYP/6-31G* method show remarkable agreement with the derived properties and energy densities from the experiment. The intermolecular interactions follow an exponential dependence of electron density and energy densities at the bond critical points. The Laplacian follows a "Morse-like" dependence on the length of the interaction line. Based on the set of criteria defined using the theory of "atoms in molecules", it has become possible to distinguish between a hydrogen bond (C-H...O) and a van der Waals interaction (C-H...pi). This has resulted in the identification of a "region of overlap" in terms of electron densities, energy densities, and mutual penetration of the hydrogen and acceptor atoms with respect to the interaction length. This approach suggests a possible tool to distinguish between the two types of interactions.     

The effect of the solvents on the specific intramolecular C-H...N interactions studied by1H NMR spectroscopy

The analysis of the effect of the solvents on the proton chemical shifts in¹H NMR spectra of 2-vinyloxypyridine indicates that the C—H...N interaction of weak intramolecular hydrogen bond type hinders the formation of intermolecular hydrogen C—H...X and C-H... bonds. The protonating solvents reduce the intramolecular C—H...N interaction due to association with the N atom of the pyridine cycle.Translated fromIzvestiya Akademii Nauk. Seriya Khimieheskaya, No. S, pp. 1202–1204, May, 1996.     

Very strong C-H...O, N-H...O, and O-H...O hydrogen bonds involving a cyclic phosphate.

Very short C-H...O, N-H...O, and O-H...O hydrogen bonds have been generated utilizing the cyclic phosphate [CH2(6-t-Bu-4-Me-C6H2O)2]P(O)OH (1). X-ray structures of (i) 1 (unsolvated, two polymorphs), 1...EtOH, and 1...MeOH, (ii) [imidazolium](+)[CH2(6-t-Bu-4-Me-C6H2O)2PO2](-)...MeOH [2], (iii) [HNC5H4-N=N-C5H4NH](2+)[(CH2(6-t-Bu-4-Me-C6H2O)2PO2)2](2-)...4CH3CN...H2O [3], (v) [K, 18-crown-6](+)[(CH2(6-t-Bu-4-Me-C6H2O)2P(O)OH)(CH2(6-t-Bu-4-Me-C6H2O)2PO2)](-)...2THF [4], (vi) 1...cytosine...MeOH [5], (vii) 1...adenine...1/2MeOH [6], and (viii) 1...S-(-)-proline [7] have been determined. The phosphate 1 in both its forms is a hydrogen-bonded dimer with a short O-H...O distance of 2.481(2) [triclinic form] or 2.507(3) A [monoclinic form]. Compound 2 has a helical structure with a very short C-H...O hydrogen bond involving an imidazolyl C-H and methanol in addition to N-H...O hydrogen bonds. A helical motif is also seen in 5. In 3, an extremely short N-H...O hydrogen bond [N...O 2.558(4) A] is observed. Compounds 6 and 7 also exhibit short N-H...O hydrogen bonds. In 1...EtOH, a 12-membered hydrogen-bonded ring motif, with one of the shortest known O-H...O hydrogen bonds [O...O 2.368(4) A], is present. 1...MeOH is a similar dimer with a very short O(-H)...O bond [2.429(3) A]. In 4, the deprotonated phosphate (anion) and the parent acid are held together by a hydrogen bond on one side and a coordinate/covalent bond to potassium on the other; the O-H...O bond is symmetrical and very strong [O...O 2.397(3) A].     

Specific intramolecular C-H...O interactions in 1-vinyl-2-formylimidazole and 1-vinyl-2-formylbenzimidazole studied by1H and13C NMR spectral data

According to the¹H and¹³C NMR spectral data, the vinyl group in 1-vinyl-2-formylimidazole and 1-vinyl-2-formylbenzimidazole istrans-oriented with respect to the formyl fragment, while the carbonyl group occupies theanti-position with respect to the N atom of pyridine cycle. A specific intramolecular C—H...O interaction of a weak hydrogen bond type is realized between the -H atom of the vinyl group and O atom of the carbonyl group.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1197–1201, May, 1996.     

Highly stereoselective asymmetric Pummerer reactions that incorporate intermolecular and intramolecular nonbonded S...O interactions

New chiral sulfoxides (R(S),S)-3, (S(S),S)-3, (R(S),S)-4, and (S(S),S)-4 and known chiral sulfoxides (R(S))-5, (R(S))-6, and (R(S))-7 were synthesized, and the stereochemistry of the new sulfoxides (R(S),S)-3 and (R(S),S)-4 was determined by X-ray crystallographic analysis. In their crystallographic structures, the intramolecular nonbonded S...O close contacts were recognized. Analyses of several sulfoxide complexes including rac-11 with N,N-dimethylacetamide (DMAC) or N-methyl-2-pyrrolidone (NMP) in a MeOH solution utilizing cold-spray ionization mass spectrometry provided, for the first time, direct information for intermolecular nonbonded S...O interactions between sulfoxides and amide (or lactam) in a solution. Highly diastereoselective and enantioselective Pummerer reactions based on the concept of intermolecular and intramolecular nonbonded S...O interactions were performed by treatment of several chiral sulfoxides (R(S), S)-3, (S(S), S)-3, (R(S), S)-4, (S(S), S)-4, (R(S))-5, (R(S))-6, and (R(S))-7 with acetic anhydride and trimethylsilyl triflate (TMSOTf) in DMAC, NMP, N,N-dimethylformamide, and N-formylpiperidine. Mechanistic studies on these facile stereoselective Pummerer reactions revealed the necessity for the amide/TMSOTf complex, such as 26 or 27, to be an efficient activation reagent for Ac(2)O and a trapping reagent for the released acetate ion, and that DMAC and NMP had a positive effect on this highly stereoselective chiral transfer reaction.     

Coordination molecular hats binding acetonitrile via C-H...pi interactions

The [M(3)L(3)](3+) metallo-macrocycles with syn, syn, syn conformation, self-assembled from crown ether functionalized 1,10-phenanthroline Pd(II) nitrates with naphthanoimidazole, bind a solvent CH(3)CN within the naphthanoimidazolate-built cavity through C-H...pi interactions in the crystalline state.     

Geometric isotope effect of various intermolecular and intramolecular C-H...O hydrogen bonds, using the multicomponent molecular orbital method

The geometric isotope effect (GIE) of sp- (acetylene-water), sp(2)- (ethylene-water), and sp(3)- (methane-water) hybridized intermolecular C-H...O and C-D...O hydrogen bonds has been analyzed at the HF/6-31++G level by using the multicomponent molecular orbital method, which directly takes account of the quantum effect of proton/deuteron. In the acetylene-water case, the elongation of C-H length due to the formation of the hydrogen bond is found to be greater than that of C-D. In contrast to sp-type, the contraction of C-H length in methane-water is smaller than that of C-D. After the formation of hydrogen bonds, the C-H length itself in all complexes is longer than C-D and the H...O distance is shorter than D...O, similar to the GIE of conventional hydrogen bonds. Furthermore, the exponent (alpha) value is decreased with the formation of the hydrogen bond, which indicates the stabilization of intermolecular C-H...O hydrogen bonds as well as conventional hydrogen bonds. In addition, the geometric difference induced by the H/D isotope effect of the intramolecular C-H...O hydrogen bond shows the same tendency as that of intermolecular C-H...O. Our study clearly demonstrates that C-H...O hydrogen bonds can be categorized as typical hydrogen bonds from the viewpoint of GIE, irrespective of the hybridizing state of carbon and inter- or intramolecular hydrogen bond.     

Aromatic C-H...O interactions in a series of bindone analogues. NMR and quantum mechanical study

The aromatic C-H...O hydrogen bonding within the series of the structurally relative indenone derivatives has been studied. The presence of the hydrogen bonds is corroborated by the large low-field chemical shifts of the protons involved in the hydrogen bond observed experimentally and reproduced by quantum mechanical calculations. Further confirmation is provided by analysis of the orbital overlap coefficients, (13)C NMR chemical shifts, and one-bond spin-spin coupling constants J((13)C-(1)H). The relationship between molecular geometry and (1)H NMR chemical shifts of involved protons has a complex nature, but the C-H...O distance is the principal factor.     

Anion complexation via C-H...X interactions using a palladacyclic receptor

A novel organometallic receptor binds anions in solution and in the solid state, with complexes stabilised through a series of C-HX interactions, as evidenced by 1H NMR spectroscopy, X-ray crystallography and computational models.     

Theoretical studies for Lewis acid-base interactions and C-H...O weak hydrogen bonding in various CO2 complexes

Comprehension of the basic concepts for the design of CO2-philic molecules is important due to the possibility for "green" chemistry in supercritical CO2 of substitute solvent systems. Lewis acid-base interactions and C-H...O weak hydrogen bonding were suggested as two key factors in the solubility of CO2-philic molecules. To isolate the stabilization energy of weak hydrogen bonding from the overall binding energy, high-level quantum mechanical calculations were performed for the van der Waals complexes of CO2 with methane, methylacetate, dimethylether, acetaldehyde, and 1,2-dimethoxyethane. Structures and energies were calculated at the MP2 level of theory using the 6-31+G(d) and aug-cc-pVDZ basis sets with basis set superposition error corrections. In addition, the single-point energies were calculated using recently developed multilevel methods. This study shows that the Lewis acid-base interaction has a significant impact on the complex stability compared to the C-H...O weak hydrogen bond. The additional stabilization energy of the cooperative weak hydrogen bond with alpha-proton of the carbonyl group was negligible on the enhancement of supercritical CO2 solubility. However, the stabilization energy was larger for the ether group, such that it may have an important role in increasing the supercritical CO2 solubility. Additional formation of cooperative weak hydrogen bonds may not further increase the solubility due to the stability reduction by steric hindrance.     

Macrocyclization and molecular interlocking via Mitsunobu alkylation: highlighting the role of C-H...O interactions in templating

[reaction: see text] A series of diimide-based macrocycles have been prepared using Mitsunobu-mediated alkylation as the macrocyclization step. These macrocycles could not be incorporated into [2]catenanes using previously established building blocks and coupling methodology. However, when one of the macrocycle syntheses was conducted in the presence of a dinaphtho crown ether, catenane formation was achieved. This result is discussed in terms of the ability of the components to establish intermolecular C-H...O hydrogen-bonding contacts.