Estimates of the energy of intramolecular hydrogen bonds

A method for the estimation of the energy of intramolecular hydrogen bonds in conjugated systems existing in a variety of conformations is presented. The method is applied to determine the intramolecular hydrogen bond energy in 3-aminopropenal and 3-aminopropenthial. According to the proposed estimation scheme, the intramolecular H-bond energies are found to be of the order of 5-7 kcal/mol. These results are compared with those obtained by using other estimation schemes as well as with the recent results by other authors. Also, the H-bond energies in dimers and trimers of the two molecules are calculated and compared with the corresponding data for internally hydrogen-bonded monomers. This comparison shows that the bond equalization effect is primarily due to proton donor-proton acceptor proximity. In comparison with intermolecular hydrogen bonds, the rigidity of the chelate skeleton enhances this proximity effect. The same effect can be seen in systems with intermolecular hydrogen bonds, although its magnitude is diminished because of the absence of additional forces which pull the proton donor and proton acceptor groups toward each other. No specific resonance-assisted origin of the intramolecular hydrogen bond energy seems to be needed to elucidate the energetics of these bonds.     

A scheme estimating the energy of intramolecular hydrogen bonds in diols

The relative energies of conformers of 1,2-ethanediol, 1,3-propanediol, and 1,4-butanediol are split into a sum of five different terms including the intramolecular OH?O interaction. This scheme allows to estimate the energy of the O-H?O intramolecular hydrogen bond of the tGG′g and gGG′g conformers of 1,3-propanediol, the g′GG′Gt and g′GG′Gg conformers of 1,4-butanediol, and the energy of the non-bonded O-H?O interaction in the g′Gt, g′Gg and g′Gg′ conformers of 1,2-ethanediol. This scheme provides pure hydrogen bond energies without assuming the geometry and/or electronic features to be constant between the conformation having a IHB and a reference conformation. The fitted energies show a perfect linear correlation with the corresponding r(H?O)⁻¹ values. QTAIM atomic electron population and energies of the donor hydrogen calculated along the H-O-C-C internal rotation are found to be linearly correlated. These linear correlations display small changes at the BCP formation in 1,3-propanediol.     

The nature and energy characteristics of intramolecular hydrogen bonds in crystals

The review concerns the results of systematic X-ray diffraction studies of the electron density distribution in the crystals of compounds with strong intramolecular hydrogen bonds N-H...O, O-H...O, O-H...N, and N-H...S. The advantages of the topological analysis of the electron density distribution function in the analysis of the nature and estimation of the H-bond energies directly from experimental data are discussed. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 1–14, January, 2006.     

Bridging QTAIM with vibrational spectroscopy: the energy of intramolecular hydrogen bonds in DNA-related biomolecules

Physical properties of over 8000 intramolecular hydrogen bonds (iHBs), including 2901 ones of the types OH···O, OH···N, NH···O and OH···C, in 4244 conformers of the DNA-related molecules (four canonical 2'-deoxyribonucleotides, 1,2-dideoxyribose-5-phosphate, and 2-deoxy-D-ribose in its furanose, pyranose and linear forms) have been investigated using quantum theory of atoms in molecules (QTAIM) and vibrational analysis. It has been found that for all iHBs with positive red-shift of the proton donating group stretching frequency the shift value correlates with ρ(cp)-the electron charge density at the (3,-1)-type bond critical point. Combining QTAIM and spectroscopic data new relationships for estimation of OH···O, OH···N, NH···O and OH···C iHB enthalpy of formation (kcal mol(-1)) with RMS error below 0.8 kcal mol(-1) have been established: E(OH···O) = -3.09 + 239·ρ(cp), E(OH···N) = 1.72 + 142·ρ(cp), E(NH···O) = -2.03 + 225·ρ(cp), E(OH···C) = -0.29 + 288·ρ(cp), where ρ(cp) is in e a(0)(-3) (a(0)- the Bohr radius). It has been shown that XHY iHBs with red-shift values over 40 cm(-1) are characterized by the following minimal values of the XHY angle, ρ(cp) and nubla(2)ρ(cp): 112°, 0.005 e a(0)(-3) and 0.016 e a(0)(-5), respectively. New relationships have been used to reveal the strongest iHBs in canonical 2'-deoxy- and ribonucleosides and the O(5')H···N(3) H-bond in ribonucleoside guanosine was found to have the maximum energy (8.1 kcal mol(-1)).     

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.     

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.     

Acidities in cyclohexanediols enhanced by intramolecular hydrogen bonds

Equilibrium gas-phase acidities of the six isomeric cyclohexanediols were measured in a Fourier transform ion cyclotron resonance mass spectrometer. Although all six cyclohexanediols have the same functional groups and similar structures, the acidities vary over 11 kcal/mol. This large difference is due mostly to the balance between hydrogen bonding and geometric strain. To understand the origins of the acidity differences in more detail, the conformations and energetics of the cyclohexanediols were studied using density functional theory, which gave good agreement with the experimental acidities. Finally, methanol-methoxide and methanol-methanol interactions were used as a model for the hydrogen bonding.     

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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Structural and energetic consequences of the formation of intramolecular hydrogen bonds

Formation of intramolecular hydrogen bonds leads to structural modifications in the whole molecule, which are discussed on the basis of B3LYP/6-31G(d,p) calculations. The energy and the structure of various hydrogen-bonded and open conformers are considered for two groups of ortho-substituted phenols–N-dimethylaminomethylphenols (Mannich bases) and N-methylbenzylideneamines (Schiff bases). The energy of intramolecular hydrogen bond formation in Mannich bases was corrected for non-bonded interactions within the molecules, based on a thermodynamic cycle. Structural data were used to estimate the fraction of the ortho-quinoid (keto) form in particular tautomers. It is shown that proton transfer in Schiff bases leads to an increase of this fraction to about 40%, while opening of the hydrogen bond in the proton transferred form increases the keto fraction to 70%.     

A Schiff-base porphyrin complex with double intramolecular hydrogen bonds

We have designed a porphyrin with a Schiff-base substituent as a model to study intramolecular hydrogen-bonding. The corresponding complex [Zn(SATPP)(CH₃OH)] has been synthesized and characterized by X-ray crystallography, ¹H NMR, and UV-Vis spectroscopy. The structure shows that there are three phenyl groups and one salicylideneaminophenyl group at the meso positions of the porphyrin, and the phenol oxygen is involved in double hydrogen bonds, one within the salicylideneaminophenyl and the other between coordinated methanol and phenol oxygen. ¹H NMR spectra suggest that the binding of methanol to zinc is an equilibrium process in solution and the equilibrium constant has been determined by UV-Vis measurements. The intramolecular hydrogen bond stabilizes the structure, and the binding affinity increases 10 times over the corresponding TPP (TPP, dianion of meso-5,10,15,20-tetraphenylporphyrin).     

Influence of intramolecular hydrogen bonds on the tautomeric equilibrium of 1,3-diketones

The paper deals with a quantitative characterization of the influence of multiple intramolecular hydrogen bonds on the tautomeric equilibrium of 1,3-diketones by means of NMR-spectroscopy. The contents of the keto- and two enol forms of 1-(o-hydroxyphenyl) -1,3-butandione and 1-(o-methoxyphenyl)-1,3-butandione in tetrachloromethane, deuterochloroforme, acetone-d₆ and acetonitrile-d₃ are compared.The intramolecular hydrogen bond between the phenolic hydroxyl group and the aromatic carbonyl group in 1-(o-hydroxyphenyl)-1,3-butandione shifts are keto-enol equilibrium toward the keto-tautomer and enol-enol equilibrium toward the tautomer with an enolized aliphatic carbonyl group.     

A theoretical analysis of the conformation of Ac-L-Ala-L-Pro-L-Ala-NHMe with intramolecular hydrogen bonds

Summary 1. The optimum conformations of Ac-L-Ala-L-Pro-L-Ala-NHMe with eight possible systems of intramolecular hydrogen bonds have been calculated by the method of theoretical conformational analysis.2. The structures found serve as canonical forms for the study of the conformational states of sections of peptide-protein systems with proline residues.3. It has been shown that the interaction of adjacent residues in the structure of the compound mentioned play the dominating role.M. M. Shemyakin Institute of Bioorganic Chemistry, Moscow. Translated from Khimiya Prirodynykh Soedinenii, No. 2, pp. 233–238, March–April, 1976.     

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.     

Perturbation of the intramolecular hydrogen bonds of glucose by Cu+ association

A density functional theory study of glucose and glucose–Cu⁺ complexes has been performed to investigate the changes undergone by the set of intramolecular hydrogen bonds of the neutral system upon Cu⁺ association. The geometries of the different species investigated were optimized at the B3LYP/6‐31G(d,p) level. The same level of theory was used to obtain the harmonic vibrational frequencies and to analyze the electron charge density by means of the atoms in molecules theory. We have shown that the interaction with Cu⁺ strongly perturbs the set of intramolecular hydrogen bonds of the neutral. Some of these changes are a direct consequence of the conformational changes induced by the metal, which result in the breaking of some of the existing bonds or in the formation of new ones. The most important point, however, is that the intramolecular hydrogen bonds that remain are perturbed to a different extent. In general, all hydrogen bonds in which the OH donor interacts directly with the metal cation are significantly stabilized while the remaining ones become weaker. These changes influence the relative stability of the complexes as well as its capacity to interact with other systems. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001