Estimation of intramolecular hydrogen bond energy via molecular tailoring approach

A novel method, based on the molecular tailoring approach for estimating intramolecular hydrogen bond energies, is proposed. Here, as a case study, the O-H...O bond energy is directly estimated by addition/subtraction of the single point individual fragment energies. This method is tested on polyhydroxy molecules at MP2 and B3LYP levels of theory. It is seen to be able to distinguish between weak ( approximately 1 kcal mol(-1)) and moderately strong ( approximately 5 kcal mol(-1)) hydrogen bonds in polyhydroxy molecules.     

Intramolecular interactions and intramolecular hydrogen bonding in conformers of gaseous glycine

Ab initio calculations at the MP2/6‐311++G** level of theory led recently to the identification of 13 stable conformers of gaseous glycine with relative energies within 11 kcal/mol. The stability of every structure depends on subtle intramolecular effects arising from conformational changes. These intramolecular interactions are examined with the tools provided by the Atoms In Molecules (AIM) theory, which allows obtaining a wealth of quantum mechanics information from the molecular electron density ρ( r ). The analysis of the topological features of ρ( r ) on one side and the atomic properties integrated in the basins defined by the gradient vector field of the density on the other side makes possible to explore the different intramolecular effects in every conformer. The existence of intramolecular hydrogen bonds on some conformers is demonstrated, while the presence of other stabilizing interactions arising from favorable conformations is shown to explain the stability of other structures in the potential energy surface of glycine. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 702–716, 2001     

Intramolecular hydrogen bonding in calixarenes

The construction of a molecular cavity for recognition and catalysis requires either covalent synthesis or intermolecular self-assembly of complementary units. Intramolecular hydrogen bonding is another tool to control the cavity-forming process. When properly positioned within the same molecular structure, hydrogen bonding sites are responsible for the formation, preorganization, and binding ability of the host. The most typical examples from the supramolecular chemistry of calixarenes, the key cavity-containing building blocks, and derived from them receptor molecules are discussed.     

Acetylene aggregates via cluster-building algorithm and molecular tailoring approach

Acetylene clusters are prototypical of simple non-aromatic systems bonded through C–H…π interactions. The present work explores structures and properties of acetylene clusters (C₂H₂) _n , n = 8 and 10, employing cluster-building algorithm and molecular tailoring approach (MTA). The former uses electrostatics guidelines for building (C₂H₂)₈ and (C₂H₂)₁₀ structures. These clusters are treated at MP2 level of theory with correlation-consistent basis sets using MTA. The Hessian matrix and vibrational spectra for the best five structures of (C₂H₂)₈ and (C₂H₂)₁₀ are computed employing MTA. Actual calculations on these clusters using conventional methods employing large basis sets are prohibitively difficult to perform. All the frequencies for these structures extracted using MTA-based Hessian matrix are found to be real, confirming their local minimum nature. This study points to the possibility of using the present approach for exploring structures, energetics and vibrational spectra of even larger clusters at higher levels of theory.     

Intramolecular hydrogen bonding in some acyclic alcohols

The extent of intramolecular hydrogen bonding, occurring in dilute CCl₄ solutions, of members of the series (n = 2–5) HO.(CH₂)_n.OH and MeO.(CH₂)_n.OH has been determined. The results permit the interpretation of the patterns of hydrogen bonding in the three monomethyl ethers of butane-1,2,4-triol and in 1,4-dimethoxybutan-2-ol. In these compounds, where hydrogen bonding allows the formation of rings of different sizes the sequence of preference 5> 6> 7 was observed.     

Intramolecular hydrogen bond energy in polyhydroxy systems: a critical comparison of molecular tailoring and isodesmic approaches

The intramolecular hydrogen bond (H-bond) energies in several polyhydroxy systems are estimated using an isodesmic/homodesmic reaction approach as well as a molecular tailoring approach (MTA) [Deshmukh, M. M.; Gadre, S. R.; Bartolotti, L. J. J. Phys. Chem. A 2006, 110, 12519]. It is shown that the isodesmic/homodesmic reaction approach as advocated in the literature does not give true H-bond energy but includes the effect of strain energy due to the formation of a ring structure. Such a ring strain is duly accounted for in the MTA method. The isodesmic H-bond energies are found to be smaller than their MTA energy counterparts typically by the strain energy. The MTA is applied to decitol, a system with more than five different H-bonds for which an application of an appropriate isodesmic reaction is extremely difficult. It has been shown that the MTA method is able to predict not only the H-bond energies but also the trends in conformational energies for three different conformers of decitol studied in the present work.     

Intramolecular hydrogen bonding and anion binding of N-benzamido-N'-benzoylthioureas

N-(p-Dimethylamino)benzoyl-N'-phenylthiourea as an N-acylthiourea is known to be unable to bind anions due to a strong intramolecular hydrogen bond (IHB). We show here that by inserting an amido group in the N'-phenyl side the newly designed N-benzamido-N'-benzoylthioureas, despite this IHB too, bind strongly to anions with binding constants on the order of 10(6)-10(7) mol(-1) L. Results suggest that potential anion receptors or organocatalysts could be developed on the basis of this framework with a wide structural diversity.     

Intramolecular hydrogen bonding and conformation of 1,3-propanediol

Conformational differences in molecules of 1,3-propanediol with variations of medium, temperature, and phase composition appear in their IR and Raman spectra. In the gas phase and in dilute solutions of 1,3-propanediol in CCl₄ forms with intramolecular hydrogen bonding (IHB) and without it exist in comparable amounts. The difference in the weighted mean energies of the corresponding groups of conformers was found from the temperature dependence of the intensities of the bands for free _OH and bounded _OH. By calculating the conformational term of E^* and by using the energy values of the 25 conformers of the diol molecules, calculated by molecular mechanics methods, the IHB energy was determined for two of them.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2277–2284, October, 1991.     

Intramolecular hydrogen bonding in the o-(aminomethyl) phenols

Conclusions The strength of intramolecular hydrogen bonding in the o-(aminomethyl)phenols varies by some 5 kcal/mole, depending on the substituent at the nitrogen atom and in the benzene ring. Steric effects from the substituent determine the bonding strength.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 331–334, February, 1977.     

Intramolecular reorientations and the effects of thermal history and hydrogen bonding in four closely related organic molecular solids

We have measured proton spin—lattice relaxation rates at 30 MHz from 77 to 330 K in para-di-t-butylbenzene, 2,5-di-t-butylphenol, 2,6-di-t-butylphenol, 3,5-di-t-butylphenol and modifications of the latter two in which the hydroxyl proton has been replaced with a deuteron.The observed maxima in the relaxation rates are interpreted in terms of the reorientations of the t-butyl groups and their constituent methyl groups. The observed relaxation rate depends, to some degree, on the thermal history of the samples and in the phenols this is attributed, in part, to the formation of dimeric and polymeric forms via intermolecular hydrogen bonding forming OH... O bridges. Infrared spectra are run with the three phenols in both the dilute liquid and solid state and the results interpreted in terms of hydrogen bonding. Nuclear spin relaxation due to the flip-flop of the hydroxyl proton is observed in 2,6-di-t-butylphenol but not in 3,5-di-t-butylphenol as expected on the basis of the IR spectra.     

Intramolecular hydrogen bonding in 1-nitro-5,10-dihydrophenazine derivatives

The formation of free radicals only for derivatives with a nitro group in the 1 position was observed in the oxidation of a number of 5-substituted 5,10-dihydrophenazines with lead dioxide. A long-wave absorption band was observed in the electronic spectra of the derivatives with a nitro group in the 1 position. The assumption of the formation of an intramolecular hydrogen bond in dihydrophenazines with a nitro group in the 1 position was confirmed by quantum-chemical modeling of the three structures of the 1,3-dinitro-5-phenyl-5,10-dihydrophenazine molecule. By comparison of the integral intensities of the bands of the stretching vibrations of the N-H bond in 5-substituted dihydrophenazines it was concluded that this bond is depolarized in derivatives with an intramolecular hydrogen bond.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No, 2, pp, 263–265, February, 1978.     

Dipolar interactions and hydrogen bonding in supramolecular aggregates: understanding cooperative phenomena for 1st hyperpolarizability

Weak intermolecular forces like dipolar interactions and hydrogen-bonding lead to a variety of different packing arrangements of molecules in crystals and self-assemblies. Such differences in the arrangements change the extent of excitonic splitting and excitation spectra in the multichromophore aggregates. In this tutorial review, the role of such interactions in fine tuning the linear and 1st non-linear optical (NLO) responses in molecular aggregates are discussed. The non-additivity of these optical properties arise specifically due to such cooperative interactions. Calculations performed on dimers, trimers and higher aggregates for model systems provide insights into the interaction mechanisms and strategies to enhance the 1st hyperpolarizabilities of pi-conjugated molecular assemblies. Flexible dipole orientations in the alkane bridged chromophores show odd-even variations in their second-harmonic responses that are explained through their dipolar interactions in different conformations. Parameters for the optical applications of molecules arranged in constrained geometry, like in Calix[n]arene, have been elucidated. We also highlight the recent developments in this field of research together with their future prospects.     

Intramolecular hydrogen bonding and hydrogen atom abstraction in gas-phase aliphatic amine radical cations

The intramolecular hydrogen atom abstraction by the nitrogen atom in isolated aliphatic amine radical cations is examined experimentally and with composite high-level ab initio methods of the G3 family. The magnitude of the enthalpy barriers toward H-atom transfer varies with the shape and size of the cyclic transition state and with the degree of substitution at the nitrogen and carbon atoms involved. The lower barriers are found for 1,5- and 1,6-abstraction, for chairlike transition states, for abstraction reactions in ionized primary amines, and for abstraction of H from tertiary carbon atoms. In most cases, the internal energy required for 1,4-, 1,5-, and 1,6-hydrogen atom abstraction to occur is less than that required for gas-phase fragmentation by simple cleavage of C-C bonds, which explains why H-atom transfer can be reversible and result in extensive H/D exchange prior to the fragmentation of many low-energy deuterium labeled ionized amines. The H-atom transfer to nitrogen is exothermic for primary amine radical cations and endothermic for tertiary amines. It gives rise to a variety of distonic radical cations, and these may undergo further isomerization. The heat of formation of the gauche conformers of the gamma-, delta-, and epsilon-distonic isomers is up to 25 kJ mol(-1) lower than that of the corresponding trans forms, which is taken to reflect C-H-N hydrogen bonding between the protonated amino group and the alkyl radical site.