Theoretical study of the intermolecular hydrogen bond interaction for furan-HCl and furan-CHCl_3 complexes

The importance of intermolecular interactions in biology and material science has prompted chemists to explore the nature of the variety of such interactions. The strongest of these interac-tions are the hydrogen bonds, which play an important role in determining the molecular confor-mation, crystal packing, and the structure of biological systems such as nucleic acids. Extensive experimental and theoretical efforts[1—5] have been devoted to the studies of this type of interac-tions, such as …     

Studies of hydrogen bonding—XXVII. NMR studies of hydrogen bonding and solvent effect

The OH proton chemical shifts are reported of dilution study of 2-methyl-6-t-butylphenol (2M6TBP) and 2,6-di-t-butylphenol (2.6DTBP), and of the 1:1 association between the phenols and hexamethylphosphoramide (HMPA) in C₆H₁₂, CCl₄, CH₃NO₂, CH₃CN and C₆H₆ at 25, 35 and 50°C. Also the association of the phenols with CH₃NO₂, CH₃CN and CCl₄ has been studied. The phenol OH chemical shift, δ_f, found by extrapolation to zero concentration, shows the highest upfield shift in benzene. Contrary, on adding HMPA, the complex OH chemical shift, δ_x, shows the lowest downfield shift in benzene.The K_ass, δ_x, ΔH and ΔS values have been evaluated by using the Hiquchi plot method. Linear relationships were obtained between Δδ_x and ΔH, and log K_ass for the system 2M6TBP/HMPA in the various solvents. Furthermore, the difference in H-bond energy of a given system in an inert solvent (C₆H₁₂) and in aprotic solvent (CH₃CN), corresponds to the enthalpy of association of the given donor with CH₃CN in C₆H₁₂.The K_ass value for the system 2M6TBP/HMPA is 10 times greater in C₆H₁₂ than in C₆H₆ and CCl₄, 100 times than in CH₃NO₂ and 300 times than in CH₃CN.     

Spectroscopic study of hydrogen bonding in the enol form of β-diketones—II. Symmetry of the hydrogen bond

The effects of proton potential on the i.r. and Raman spectra of β-diketones are discussed. The spectroscopic behaviour of the asymmetric hydrogen bond stretching mode and of the out-of-plane hydrogen bond bending modes indicates a relatively high barrier double minimum potential.From the observed O…O distances and these predicted from the spectroscopic results for intermolecular H-bonds, a bent hydrogen bond was concluded. The spectroscopic data show a great change in the internal modes of β-diketones upon deuteration. This was attributed to the lengthening of the O…O distance which is a known characteristic of a double minimum potential.     

Electronic structure of hydantoin studied by He I photoelectron spectroscopy and MNDO quantum—chemical calculations

The He I photoelectron spectra of hydantoin, 1-methyl-hydantoin, 3-methylhydantoin and 1,3-dimethylhydantoin are reported. Displacement of the bands upon the N-methylsubstitution indicates that HOMO and the third highest occupied MO are localized on the nitrogen atoms, the former on the amidic nitrogen and the latter on the imidic one. The second and the fourth highest occupied MO's are oxygen lone pairs. The MNDO method provides the same picture.     

Functionalized proline with double hydrogen bonding potential: highly enantioselective Michael addition of carbonyl compounds to β-nitrostyrenes in brine

Simple synthetic manipulation of S-proline allows access to prolinamides 5-7 as organocatalysts capable of double hydrogen bonding for enantioselective Michael addition reactions of carbonyl compounds to β-nitrostyrenes. It is shown that prolinamide catalyst 7 leads to addition products with a high diastereo- as well as enantioselectivity. The transition state structure involving the binding of electrophilic nitrostyrene via two H-bonds is believed to be further stabilized by π,π stacking interactions mediated by the tosyl ring.     

Effect of hydrogen bonding and hydrophobic interaction on the formation of supramolecular hydrogels formed by L-phenylalanine derivative hydrogelator

A new hydrogelator,pyridinium bromide salt of N-6-bromohexanoyl-L-phenylamino octadecane,was synthesized.Supramo- lecular hydrogels can be formed through the self-assembly of this hydrogelator in water.In this work,D_2O was used instead of H_2O as solvent for FT-IR measurement due to the fact that it is impossible to obtain useful FT-IR information on the hydrogen bonding in water.The investigation of FT-IR and steady-state fluorescence indicated that the driving forces for the self-assembly were mainly hydrogen bonding and hydrophobic interaction.Based on the data of XRD and molecular modeling,the possible mechanism of the formation of hydrogelator aggregates was proposed.     

Effect of intermolecular O-H ⋯ O hydrogen bonding on the molecular structure of phenol: An ab initio molecular orbital study

The effect of intermolecular O-H O hydrogen bonding on the molecular structure of phenol has been studied by SCF ab initio MO calculations at the HF/6-31G * level. The systems investigated are eight phenol-water complexes and the dimer and trimer of phenol. Optimized geometries show that hydrogen bond formation causes a consistent pattern of changes in the structure of the molecule. When phenol acts as a proton donor, the expected increase ofr (O-H) is accompanied by a slight decrease ofr(C-O) and of the internal ring angles at theipso andpara positions, and by an increase ofr(C _ipso © _ortho ). These changes suggest that the relative contribution of polar canonical forms to the electronic structure of the molecule increases upon hydrogen bond formation, since this enhances the strength of the interaction. The opposite changes occur when phenol acts as a proton acceptor, except forr(O-H), which is the same as in the free molecule. If phenol acts as a proton donorand as a proton acceptor, the two hydrogen bonds become stronger due to a synergic effect. In this case, however, the structural deformation of the molecule is less pronounced than in the previous cases, due to the opposite effect of the two hydrogen bonds. The available experimental evidence on gas-crystal structural differences for phenol is critically reviewed, also in the light of the present results on gas-phase complexes.     

π-Complexation and C—H hydrogen bonding in the formation of colored cocrystals

The present study evaluates the potential combination of charge-transfer electron-donor–acceptor π–π complexation and C—H hydrogen bonding to form colored cocrystals. The crystal structures of the red 1:1 cocrystals formed from the isomeric pyridines 4- and 3-{2-[4-(dimethylamino)phenyl]ethynyl}pyridine with 1-[2-(3,5-dinitrophenyl)ethynyl]-2,3,5,6-tetrafluorobenzene, both C₁₄H₄F₄N₂O₄·C₁₅H₁₄N₂, are reported. Intermolecular interaction energy calculations confirm that π-stacking interactions dominate the intermolecular interactions within each crystal structure. The close contacts revealed by Hirshfeld surface calculations are predominantly C—H interactions with N, O, and F atoms.     

A DFT study on the effect of hydrogen bonding on the reaction of a μ-benzoquinone diruthenium complex with acetylene

A DFT study was carried out to investigate the reaction mechanisms of a model μ-benzoquinone diruthenium complex {CpRu(μ-H)}₂(μ-η²:η²-C₆H₄O₂), derived from the experimental compound {Cp*Ru(μ-H)}₂(μ-η²:η²-C₆H₃RO₂) (R = H or R = Me, Cp* = η⁵-C₅Me₅), with acetylene both in aprotic and protic solvents. Results of calculations show that the influence of the solvent methanol on the reaction is mainly on the step of acetylene coordination. Enhanced hydrogen bonding is the reason for acceleration of the reaction in protic solvent, which is supported by NBO charge analysis.     

Complexes of TETROL with selected heterocyclics: unconventional host–guest hydrogen bonding and the correlation with host selectivity

Here we investigate and compare the more salient characteristics of host–guest complexes of (+)-(2R,3R)-1,1-4,4-tetraphenylbutane-1,2,3,4-tetraol (TETROL) with four heterocyclic guests, morpholine, piperidine, pyridine and dioxane. These guests each formed inclusion compounds with TETROL, and host:guest ratios were either 1:2 or 1:1. Single crystal diffraction experiments revealed unprecedented host behaviour in the presence of both piperidine and dioxane with respect to the mode of host–guest hydrogen bonding employed. Furthermore, by utilizing ¹H-NMR spectroscopy or gas chromatography (as applicable) as methods for analysing complexes obtained from competition experiments, we were able to identify the host selectivity order, and were gratified to discover that this order correlated precisely with host–guest hydrogen bond distance.     

π-Hydrogen bonding wins over conventional hydrogen bonding interaction: a jet-cooled study of indole···furan heterodimer

In this study, we have explored the conformational landscape of the indole···furan dimer in a supersonic jet by using resonant two-photon ionization (R2PI) and IR-UV double-resonance spectroscopic techniques combined with dispersion-corrected density functional theory (DFT) calculations. Only one conformer of the dimer has been observed in the experiment. DFT/B97-D level calculation shows that N-H···π hydrogen-bonded conformer (T') is energetically more stable than the N-H···O hydrogen-bonded conformer (HB). Natural bond orbital (NBO) calculation also shows that the hydrogen-bonding interaction in the HB conformer is very weak. Finally, the structure of the observed dimer has been determined to be tilted T-shaped N-H···π hydrogen-bonded (T') from very excellent agreement between experimental and theoretical N-H stretch frequency. The most significant finding of this study is the first-time observation of a N-H···π bound conformer of a dimer, which wins over a conventional hydrogen-bonded conformer of the dimer.     

Aromaticity and π-bond covalency: prominent intermolecular covalent bonding interaction of a Kekulé hydrocarbon with very significant singlet biradical character

An anthracene-linked bisphenalenyl Kekulé molecule with very significant singlet biradical character has shown a prominent covalent bonding interaction between molecules in a molecular aggregate. High aromatic stabilization energy in the anthracene linker is responsible for the significant singlet biradical character.     

Relativistic effect of iodine in <Superscript>13</Superscript>C NMR chemical shifts of iodomethanes from quantum chemical calculations within the framework of the full four-component relativistic Dirac—Coulomb scheme

¹³C NMR shielding constants (chemical shifts) of iodomethanes were calculated within the framework of the full four-component relativistic Dirac—Coulomb scheme. As the number of iodine atoms in the molecule increases, the relativistic counterpart of the ¹³C NMR chemical shift increases from a few tens to several hundreds of ppm. Calculations of ¹³C NMR chemical shifts of organoiodine compounds should be performed at the relativistic level using relativistic Dyall’s basis sets dyall.vXz and dyall.xvXz (x = a, c, ac, ae; X = 2, 3, 4) of at least triple-zeta quality or at the correlated non-relativistic level taking into account relativistic corrections. Solvent effects are not of prime importance; however, taking into account the solvent corrections causes the mean absolute error of determination of the ¹³C NMR chemical shifts to decrease by 1—2 ppm.     

Mechanism of a chemical classic: quantum chemical investigation of the autocatalyzed reaction of the serendipitous wöhler synthesis of urea

The detailed reaction pathways for the ammonium cyanate transformation into urea (W?hler's reaction) in the gas phase, in solution, and in the solid state have exhaustively been explored by means of first-principles quantum chemical calculations at the B3LYP level of theory using the 6-31G(d,p) basis set. This serendipitous synthesis of urea is predicted to proceed in two steps; the first step involves the decomposition of the ammonium cyanate to ammonia and isocyanic or cyanic acid, and the second one, which is the main reaction step (and probably the rate-determining step), involves the interaction of NH(3) with either isocyanic or cyanic acid. Several alternative pathways were envisaged for the main reaction step of W?hler's reaction in a vacuum involving the formation of "four-center" transition states. Modeling W?hler's reaction in aqueous solution and in the solid state, it was found that the addition of NH(3) to both acids is assisted (autocatalyzed) by the active participation of extra H(2)O and/or NH(3) molecules, through a preassociative, cooperative, and hydrogen-transfer relay mechanism involving the formation of "six-center" or even "eight-center" transition states. The most energetically economic path of the rate-determining step of W?hler's reaction is that of the addition of NH(3) to the C=N double bond of isocyanic acid, directly affording urea. An alternative pathway corresponding to the anti-addition of ammonia to the Ctbd1;N triple bond of cyanic acid, yielding urea's tautomer HN=C(OH)NH(2), seems to be another possibility. In the last case, urea is formed through a prototropic tautomerization of its enolic form. The energies of the reactants, products, and all intermediates along with the barrier heights for each reaction path have been calculated at the B3LYP/6-31G(d,p) level of theory. The geometry optimization and characterization of all of the stationary points found on the potential energy hypersurfaces was performed at the same level of theory.     

A quantum chemical study on the mechanism of the consecutive addition of HCN to propionitrile

MINDO/3 MO method has been used to study the mechanism of the consecutive addition of HCN to propionitrile. The results obtained for the first five steps show that the reaction is exothermic, and step 1 is the rate determining step.     

Reaction dynamics and statistical theory for the growth of hydrogen bonding clusters

The similarities between the formation of hydrogen bonds and polycondensation reactions are stated from the statistical viewpoint, and then taking the hydrogen bonding system of AaDd type as an example, the growing process of hydrogen bonding clusters is investigated in terms of the theory of reaction dynamics and statistical theory for polymeric reactions. The two methods lead to the same conclusions, stating that the statistical theory for polymerization is applicable to the hydrogen bonding systems. Based on this consideration, the explicit relationship between the conversions of proton-donors and proton-acceptors and the Gibbs free energy of the system under study is given. Furthermore, the sol-gel phase transition is predicted to take place in some hydrogen bonding systems, and the corresponding generalized scaling laws describing this kind of phase transition are obtained.     

Host–guest complexes of mixed glycol-phenanthroline cryptands: prediction of ion selectivity by quantum chemical calculations IV

Abstract Structures and complex-formation energies, calculated with DFT (B3LYP/LANL2DZp) for the cryptands [2.2.phen] and [2.phen.phen] with endohedrally complexed alkali and alkaline earth metal ions, were utilized to predict their ion selectivity. Both cryptands [2.2.phen] and [2.phen.phen] have a cavity size smaller than [2.2.2], [phen.phen.phen] and [bpy.bpy.bpy], and prefer to bind K⁺ and Sr²⁺, whereas [2.2.phen] that is larger than [2.phen.phen], has a preference for Ba²⁺, and [2.phen.phen] favours Na⁺ and Ca²⁺. The cryptand flexibility is mainly attributed to the presence of CH₂–N_SP3···N_SP3–CH₂ groups. Graphical abstract Host–Guest Complexes of mixed Glycol-Phenanthroline Cryptands—Prediction of Ion Selectivity by Quantum Chemical Calculations III Ralph Puchta* and Rudi van Eldik Keywords Cation selectivity Host–guest DFT DFT-studies allow a sensitive analysis of selectivity and cage size. Calculations predict a favourable binding of K⁺, Sr²⁺ and Ba²⁺ by [2.2.phen], and binding of K⁺, Na⁺, Ca²⁺ and Sr²⁺ by [2.phen.phen]. The cryptands fold around the ions by twisting their torsion angles in order to reach the best coordination mode for each cation. For “Prediction of ion selectivity by quantum chemical calculations III” see, R. Puchta, R. van Eldik. Aust. J. Chem. 60, 889–897 (2007).     

Structure and property of the hydrogen bonding complex between triazines and water

The study of the intermolecular interactions that drive the solvation of six-membered nitrogenated aromatic rings is of particular importance since they are known to constitute key building blocks of pro- teins and nucleotides[1―5]. The investigation of the 1:1 adduct of these molecules with water will be the first step in the understanding of such interactions. These molecules possess two different proton-acceptor sites: the ring π cloud and the lone pairs of electrons on the nitrogen atoms…