Charge-assisted intramolecular hydrogen bonds in disubstituted cyclohexane derivatives

In this paper, the N(+)-H···N, N(+)-H···O, and O-H···O(-) charge-assisted intramolecular hydrogen bonds (CAHBs) are investigated using different theoretical approaches. Monocharged cyclohexyldiamines (CHDA), aminocyclohexanols (ACHO), and cyclohexanediols (CHDO) are used as model compounds. Geometry optimizations at the MP2/aug-cc-pVDZ level are used to find the equilibrium structures for all possible H-bonded conformers. CAHBs are characterized geometrically and spectroscopically, and their energy is evaluated by means of homodesmic reactions. By comparison with the neutral forms, the presence of the charge is found to have a deep influence on the geometric and energetic H-bond parameters. In addition, these parameters are strongly dependent on the type of the groups involved as well as on their relative position in the cyclohexyl ring. For the systems under study, the H-bond energies vary from -23 to -113 kJ mol(-1), being classified from moderate to strong H-bonds. These H-bonds are also characterized by the application of the NBO and AIM theories. NBO analysis reveals that the energy corresponding to the charge transfer between the lone-pairs of the electron donor group and the antibonding orbitals of the acceptor group represents an important contribution in the H-bond stabilization. From the application of the AIM theory it is possible to see that these H-bonds possess some covalence which varies according to the type and relative position of the intervenient groups.     

The proton potential in short hydrogen bonds

The state of knowledge of the proton potential in systems for which double minimum and symmetrical, single minimum type curves have been envisaged is critically reviewed. In particular, the proposed model potentials for carboxylic acids, for KDP type ferroelectrics and for the shortest hydrogen bonds are confronted with experimental data, particularly from vibrational spectra. The early attempts at explaining the complex features in the AH stretching region by level splitting in one-dimensional double minimum potentials have not been successful. The interest in the profile of the potential surface is shifted from this aspect to its role in anharmonic interaction based general band-shaping mechanisms.     

Kinetics of hydrogen isotope exchange in molecules with strong intramolecular hydrogen bonds

Rate constants and activation energies of hydrogen exchange in solution between methanol and molecules with intramolecular H-bonds have been measured. It has been established that the rate-determining step is the dissociation of this bond.

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Effect of intramolecular hydrogen bonds on the kinetics of proton exchange of thiocarboxylic acids with phenols

The kinetics and mechanism of proton exchange between thiocarboxylic acids and o-substituted phenols with intramolecular hydrogen bonds (IMHB) were studied by dynamic PMR spectroscopy. Fast uncatalyzed exchange in cyclic binary complexes is observed for phenols with weak IMHB. In systems with strong IMHB fast exchange occurs only in the presence of an alkaline catalyst, the reaction of which with the phenol leads to cleavage of the IMHB. Concerted transfer of two protons and a cation is realized in the final step of the reaction. According to the proposed mechanism, the measured energy of activation of proton exchange is the sum of the energies of activation of the steps involving cleavage of the IMHB and strictly proton transfer.Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 24, No. 1, pp. 49–56, January–February, 1988.     

An NMR study of hydrogen exchange in p-hydroxybenzoic acid

The compound has been used over a temperature range in dimethylsulfoxide, with trichloroacetic acid and triethylamine as catalysts. The rate constant for uncatalyzed bimolecular exchange at 25 C is k_N=131 M^–1 sec^–1; the constant for acid-catalyzed exchange is k_A=0.9 · 10⁴ M^–1 sec^–1; and that for base-catalyzed exchange is k_B=0.5 · 10⁴ M^–1 sec^–1. The activation energy for uncatalyzed exchange is 5.75 kcal/mole. The exchange rates in dimethylformamide and acetone are higher, on account of differences in the hydrogen bond. The exchange rates of the isomers fall in the sequence ortho > para > meta, which is due to conjugation and intramolecular hydrogen bonding. The exchange mechanism is discussed. Simple relationships in dimensionless parameters are given for calculating the exchange rate from the shape of the NMR signal.     

Influence of intramolecular hydrogen bonds in the enzyme-catalyzed regioselective acylation of quinic and shikimic acid derivatives

Selective mono-functionalization of 3-epi, 4-epi-, and 5-epi quinic and shikimic acid derivatives has been accomplished by enzymatic acylation with Candida antarctica lipase A (CAL-A). We propose that the selectivity of this lipase is related to both the inherent receptor selectivity and the degree of intramolecular hydrogen bonding in the ligand. Conformational analysis of the hydroxyl protons has been carried out by ¹H NMR spectroscopy. We have shown that exchange of the hydroxyl protons by acid catalysis provides a useful method for the detection of intramolecular hydrogen bonds. The interpretation of exchange rates and coupling constants determines the direction of the H-bonds as conditioned by the relative acceptor and donor properties of the hydroxyl groups. The selectivity of the acylation agrees fully with the effectiveness of H-bonding networks in polyol compounds and with the higher reactivity of the equatorial hydroxyl groups.     

1H NMR studies of solvent and substituent effects on strong intramolecular hydrogen bonds

Chemical shifts of H-bonded protons in tetrabutylammonium hydrogen maleate and 14-substituted picolinic acid N-oxides have been measured in a number of dry solvents, of different activity, in order to distinguish between symmetrical single minimum and asymmetrical hydrogen bonds. In tetrabutylammonium hydrogen maleate the resonance was observed at 20.70 ppm and its was independent of the nature of the solvent used. The chemical shift value of picolinic acid N-oxide varies with the solvent. These observations suggest that the hydrogen bond is symmetrical in tetrabutylammonium hydrogen maleate but that it is asymmetrical in picolinic acid N-oxide. The chemical shifts of substituted picolinic acid N-oxides were correlated with σ_p, σ_m and Δ_pK_a. The substituent and solvent effects are compared and the position of the intramolecular H-bonded protons in picolinic acid N-oxides are estimated and discussed.     

Cooperativity in intramolecular bifurcated hydrogen bonds: an ab initio study

Molecular orbital and density functional theory calculations are performed on some di- and tetrasubstituted derivatives of anthraquinone, dihydrophenazine, and acridone to investigate cooperativity in a pair of bifurcated hydrogen bonds occurring in the same molecule. The various structures were selected as convenient model systems for three-center hydrogen bonding of both H...A...H and A...H...A types. In the former type, the C=O moieties in anthraquinone and acridone act as bifurcated hydrogen bond acceptors, and substituted OH groups act as hydrogen bond donors. In the latter type, the N-H moieties in dihydrophenazine, acridones act as bifurcated hydrogen bond donors, and the carbonyl oxygens of substituted CHO groups act as hydrogen bond acceptors. Different indicators of cooperativity reveal that two intramolecular bifurcated hydrogen bonds simultaneously present in the same molecule significantly reinforce each other.     

A study of the conformational states of cyclopeptide systems IV. NMR spectra of cyclohexapeptides constructed of alanine and glycine residues: Chemical shifts and intramolecular hydrogen bonds

Summary 1. The NMR spectra of cyclohexapeptides constructed of L(D)-alanine and glycine residues have been studied in (CD₃)₂SO, CF₃COOH, and H₂O solutions.2. In all the compounds studied, the amide bonds assume the trans configuration.3. In polar solvents, the cyclohexapeptides assume the "pleated-sheet" conformation characterized by two trans-annular hydrogen bonds of the 41 type.4. The dominating structure of the cyclohexapeptides is in equilibrium with two analogous structures accompanied by the migration of the system of H bonds with respect to the ring.For the preceding communication, see [1].M. M. Shemyakin Institute of the Chemistry of Natural Compounds of the Academy of Sciences of the USSR. Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 323–338, May–June, 1971.     

Cooperative interaction of hydrogen bonds in proton exchange between the bases of nucleic acids

The tautomeric equilibrium in the H-bonded pair guanine-cytosine was studied by the semiempirical MNDO/HB quantum-chemical method. It was shown that hydration of the bases leads to an appreciable reduction of the activation barrier in the synchronous exchange of protons between the bases. Desolvation of the active center of the polymerase reduces the probability of error in the synthesis of the complementary chain of the nucleic acid by 10⁶ times. Deprotonation of the phosphate groups of the nucleotides destabilizes the transition state in proton exchange. The genetic information is conserved as a result of the formation of nucleoprotein complexes with an ionic bond between the phosphate residue of the saccharophosphate skeleton of the nucleic acid and an amino (imino) group of the protein.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1828–1832, August, 1991.     

Possibility of intramolecular hydrogen bonds in 8-methoxypsoralene

Variants of the formation of weak intramolecular hydrogen bonds of C-H…O type in 8-methox-ypsoralene (8-MOP) were considered. Quantum-chemical calculations showed the possibility of an intramolecular hydrogen bond between the protons of the methoxy group and both (furan and pyrone) neighboring oxygen atoms of the psoralene system. The energy gain of this binding was detected by DFT, but not found by the Hartree-Fock method. The bond with pyrone oxygen is energetically more favorable, though the difference in energy between the two types of minima found on PES was small. This interaction had earlier been recorded for linear 8-substituted furocoumarins other than 8-MOP. The conclusion was drawn that the calculated energy barriers on the PES of methoxy group rotation were small enough (2.5 kcal/mol in the Hartree-Fock method, 1.1 kcal/mol in PBE, and 0.9 kcal/mol in B3LYP) to state that the methoxy group rotates freely, creating a steric hindrance for two close-lying oxygens of the psoralene structure, which are not involved in lone electron pair-π system interactions.     

Conformational analysis of intramolecular bonded amino alcohols : The conformational free energies of some intramolecular OH ⋯ N hydrogen bonds

Equilibrium positions between intramolecular OH ? N hydrogen bonded and free OH forms of some 3-piperidinols, decahydroisoquinolinols, a decahydroquinolinol, lupinine and N-methyl-3-piperidinemethanol have been determined from dilute solution IR spectral data at 33°. Conformational free energies of the H-bonds (ΔG°_OH?N, attractive) have been calculated. The results suggest a linear relationship between the apparent value of ΔG°_OH?N, as defined by the method of calculation, and the strength of the OH ? N bond expressed as Δν, within the limits of 0·5 ± 0·2kcal/mole per 100 cm^?1, from Δν 90 to 350 cm^?1. For cis-decahydroisoquinoline (N-Me or N-H) systems, a 0·4 kcal/mole difference has been calculated between the two possible ring-fused conformations, in favor of the so-called steroid form. For the corresponding cis-decahydroqumoline equilibrium, a 0·8 kcal/mole difference has been calculated, in favor of the nonsteroid form.