Intramolecular Hydrogen Bond Energy Research Articles

Quantification of hydrogen bond energy based on equations using spectroscopic, structural, QTAIM-based, and NBO-based descriptors which calibrated by the molecular tailoring approach

ContextHydrogen bonds critically influence the structure and properties of both organic molecules and biomolecules, as well as supramolecular assemblies. For this reason, the development and elaboration of methods for quantitative assessment of hydrogen bond energy is an urgent challenge. In this study, using a large series of hydroxycarbonyl aliphatic compounds with the O‒H···O = C intramolecular hydrogen bond, a bank of hydrogen bond descriptors was created, including spectroscopic, structural, QTAIM-based, and NBO-based parameters. It was shown that the O‒H vibration frequency, OH chemical shift as the spectroscopic descriptors, the O···H hydrogen bond length, O···O distance, and O‒H covalent bond length as the structural descriptors, the electron density and its Laplacian, electron potential energy density in the hydrogen bond critical point, the electron density at the ring critical point as the QTAIM-based descriptors change in a correlated manner. The same correlation is found in change of the charge transfer energy through a hydrogen bond, the occupancy of the O‒H bond antibonding orbital, the Wiberg indices of the O···H hydrogen bond, and the O‒H covalent bond, as well as the polarization of the O‒H bond, which are the NBO-based descriptors. It was also recognized that the specified descriptors from the spectroscopic, structural, QTAIM-based, and NBO-based categories are functionally related to the values of intramolecular hydrogen bond energy, quantified via the molecular tailoring approach. This allowed one to obtain a system of equations for quantitative estimation of intramolecular hydrogen bond energy based on the spectroscopic, structural, QTAIM, and NBO descriptors, which makes such quantification more dependable and reliable.MethodsTo obtain the spectroscopic descriptors, the vibrational spectra and shielding constants were calculated using the GIAO method. Structural descriptors were obtained for the equilibrium geometry of molecules, calculated at the MP2(FC)/6–311 + + (2d,2p) level using the Gaussian 09 program. The QTAIM-based descriptors were calculated using the AIMAll program within the framework of the quantum theory “Atoms in Molecules.” The NBO-based descriptors were calculated using the NBO 3.1 program implemented into Gaussian 09. To quantify the energy of intramolecular hydrogen bonds, molecular fragmentation was used within the molecular tailoring approach.

Theoretical and experimental study of resonance-assisted hydrogen bond in o-hydroxyl diaryl schiff bases

In this paper, the intramolecular hydrogen bond (IMHB) energies (EHB) in fifty 2-OH-XArCH=NArY (2-OH-XBAY) compounds containing resonance-assisted hydrogen bond (RAHB) system and fourteen XArCH=NAr-2′-OH (XBA-2′-OH) compounds without RAHB system were estimated by molecular tailoring approach (MTA) and cis-trans method (CTM) at the theoretical level of DFT B3LYP/6–311++G(d, p). The above compounds were synthesized and their 1H NMR and 13C NMR spectra were determined, then the chemical shifts of H δH on OH and chemical shifts of C δC on CH=N were assigned. The relationship between the δH of the bridged hydrogen atom on OH, the δC of the carbon atom on the CH=N bond in the RAHB ring and the estimated EHB was investigated. The results show that: (1) There is a good linear relationship between the IMHB energy calculated by CTM and that calculated by MTA. (2) There is a good linear relationship between the calculated O-H bond length dO-H and the N···H distance dN···H. (3) For 2-OH-XBAY compounds with fixed X and changing Y, the chemical shift of the bridge hydrogen atom δH(OH) has a good linear relationship with the IMHB energy ECTM, but when both X and Y change, the linear relationship is not good. (4) There is no good linear relationship between the chemical shift of C on CH=N δC(CH=N) and the IMHB energy ECTM. (5) The IMHB energy ECTM of 2-OH-XBAY cannot be expressed by only employing δH(OH), and it is necessary to employ both δH(OH) and δC(CH=N) for expressing their ECTM. (6) As a comparison, there is no RAHB in XBA-2′-OH, and in such case, it is unsuitable to use δH(OH) to express the EHB.

Rapid degradation of chloroquine derivatives in a novel advanced reduction process: Role of intramolecular hydrogen bond in the formation of excited triplet state compound

The role of triplet excited state compounds (TESC) in contaminants degradation by advanced reduction processes (ARPs) is rarely reported. In this study, Salicylic acid derivatives (SAD, e.g. Salicylic Acid (SA) and Sulfosalicylic acid (SSA)) were applied in ultraviolet activated sulfite process (UV/Sulfite) to generate TESC and enhance the decomposition of chloroquine derivatives (CQD, antiviral drugs being used for treating COVID-19 infection, e.g. chloroquine phosphate (CP)). The results indicated that the pseudo 1st order rate constant of CP (kobs-CP) increased by 2.76 and 6.26 fold, which increased from 9.10 × 10–4 s−1 in UV/Sulfite process to 2.51 × 10–3 s−1, 5.72 × 10–3 s−1 in UV/Sulfite/SA and UV/Sulfite/SSA process respectively. Similar phenomena were observed in other CQDs. Transient absorption spectra demonstrated that both triplet excited state SAD (3SAD*) and hydrated electron (eaq–) were the reactive intermediates in UV/Sulfite/SAD process, and contribute to the abatement of CP. The enhancement of SAD on contaminants degradation in UV/Sulfite/SAD process could attribute to the strong acceleration of eaq– toward the formation of 3SAD* in its photosensitive process, where SAD with intramoleuclar hydrogen bond would not undergo a directly photoionization process after absorbing a photon but undergo intersystem crossing to transform into 3SAD* which was then ionized to eaq– by biphotonic process, and constitute a hydrated electrons mediated photosensitization cycle with the supplement of eaq– from UV/Sulfite process. The higher CP degradation rate in UV/Sulfite/SSA process than that in UV/Sulfite/SA process was ascribed to a higher second order rate constant of 3SSA* toward CP and a higher yield of 3SSA*, which resulted from its higher intramolecular hydrogen bond energy enhancing photosensitization and photoionization.

Effect of electrical stimulation on red meat Neu5Gc content reduction: a combined experimental and DFT study

The hazardous substance Neu5Gc (N-glycolylneuraminic acid), which is rich in red meat, is related to chronic inflammation but is hard to eliminate. Here, electrical stimulation, as a food-friendly nonthermal processing technology, was applied to red meat samples to reduce the Neu5Gc content. To explore the Neu5Gc structure changes during this process, electronic structure parameters were evaluated, and AIM (atom in molecules) theory and DFT (density function theory) calculations were further used. The results showed that the content of Nue5Gc in red meat can be reduced by (74.24 ± 0.69)% at 120V for 50s, with little impact on the meat texture and color. Theoretical calculations indicated that the Neu5Gc molecule becomes very unstable under electrical stimulation by increasing the OH bond length, reactive activity, strength of intermolecular dipole forces and total energy through reducing the values of bond dissociation energy and strength of intramolecular hydrogen bonds. Overall, this research provides an economical method to effectively control red meat safety.

Hydrogen bonding in the crystal of 1,1'-((1E,1'E)-(pyridine-3,4-diylbis(azanylylidene))bis(methanylylidene))-bis(naphthalen-2-ol) acetonitrile solvate: combined experimental and theoretical study

The title compound, 1,1'-((1E,1'E)-(pyridine-3,4-diylbis(azanylylidene))bis(methanylylidene))­bis(naphthalen-2-ol) (1), was synthesized and structurally characterized. The compound cocrystallized with one MeCN molecule. Interestingly, one of two salicylaldehyde Schiff base fragments exists in enol form, while the other one — in a ketone form. Moreover, cocrystallized acetonitrile molecule forms hydrogen bonding with three hydrogen atoms of the dye molecule. The nature and energies of intermolecular and intramolecular hydrogen bonds were studied theoretically using DFT calculations and topological analysis of the electron density distribution within the formalism of Bader's theory (QTAIM method).

Interaction Types in C6H5(CH2)nOH-CO2 (n = 0-4) Determined by the Length of the Side Alkyl Chain.

C6H5(CH2)nOH-CO2 complexes have been investigated using rotational spectroscopy (n = 0-2) complemented by quantum chemical calculations (n = 0-4), which implies that the side alkyl chain length can determine the types of intermolecular interactions. Unlike the in-plane C···O tetrel bond in phenol-CO2, the π*CO2···πaromatic interaction has been shown to link CO2 to phenylmethanol and 2-phenylethanol, which is, to the best of our knowledge, the first time it has been demonstrated by rotational spectroscopy. Further elongations of the side alkyl chain gradually increase the energies of intramolecular hydrogen bonds in 3-phenylpropanol and 4-phenylbutanol so that CO2 cannot break it. CO2 will be pushed farther from the monomers and link with the -OH group through a dominating C···O tetrel bond. Our observations would allow, with the choice of the proper length of the side alkyl chain, new strategies for engineering C···πaromatic-centered noncovalent bonding schemes for the capture, utilization, and storage of CO2.

Estimation of resonance assisted hydrogen bond (RAHB) energies using properties of ring critical points in some dihydrogen-bonded complexes

Intramolecular hydrogen bond (IHB) interactions have investigated in aza-boron-hydrogen rings after formation of dihydrogen bond (DHB) interactions with some substituted aza-boron-hydrogen molecules. These molecules include electron-donating and electron-withdrawing groups. The IHB energies in the dihydrogen-bonded complexes have estimated using a method based on properties of ring critical points (RCPs). There is linear relationship between results of this method and molecular tailoring approach (MTA). Geometrical parameters, spectroscopic information, electronic properties based on atoms in molecules (AIM) method, and natural bond orbital (NBO) analysis have used to find effects of DHB on strength of the IHB interaction. Moreover, aromaticity of resonance-assisted hydrogen bond (RAHB) rings have evaluated using the para delocalization index (PDI), the aromatic fluctuation index (FLU), the average two center index (ATI), and the nucleus-independent chemical shift (NICS) to find relation between π-electron delocalization (π-ED) and properties of these rings. Results show that DHB leads to increase of strength of the IHB interaction. In addition, the PDI benchmark has the best correlation with the IHB descriptors. Additionally, localized molecular orbital energy decomposition analysis (LMO-EDA) reveals that the key components of interaction energies are exchange and electrostatic energy, respectively. Finally, new relations obtained for estimation of the IHB energies based on RCP characteristics and quantum mechanical descriptors.

Absorption performance and reaction mechanism study on a novel anhydrous phase change absorbent for CO2 capture

• A novel phase change absorbent was proposed for CO 2 capture with low energy consumption. • Ethanol promoted phase change behavior and increased CO 2 loading. • Reaction of CO 2 capture with [N 1111 ][Gly]/ethanol was a three-step reaction. • Intramolecular hydrogen bonds affected products’ solubility, causing phase change. Carbon capture and utilization technologies are urged more than ever due to the excessive emissions of CO 2 that could lead to serious environmental problems. Phase change absorbents are quite competitive in reducing the energy consumption of carbon capture. In this work, tetramethylammonium glycinate ([N 1111 ][Gly]), an efficient absorbent, was dissolved in ethanol and 1-propanol, respectively, to absorb CO 2 . After CO 2 absorption, the saturated solution separated into two phases. CO 2 was enriched in the lower phase, so that only the CO 2 -rich phase needed to be regenerated and the energy consumption of regeneration could be thus reduced. Under the optimal conditions, the CO 2 loading of fresh absorbents could reach 0.85 mol CO 2 /mol IL. On the basis of the 13 C NMR analysis and quantum chemical calculation, a three-step reaction mechanism of CO 2 capture with [N 1111 ][Gly]/ethanol was proposed. CO 2 reacted with [N 1111 ][Gly] to form carbamate, which was similar to the two-step Zwitterionic mechanism of MEA with CO 2 . Then the formed carbamate could react with ethanol to form ethyl carbonate and [N 1111 ][Gly], which resulted in higher CO 2 loading. Moreover, hydrogen bond characteristics were analysed to clarify the phase change mechanism. The number and bonding energy of intramolecular hydrogen bonds were increased after capturing CO 2 , leading to the formation of precipitation.

Finding the direct energy-structure correlations in intramolecular aromaticity assisted hydrogen bonding (AAHB)

A predictive model for intramolecular hydrogen bond energy (EHB) calculation of polyaromatic ortho-hydroxyaldehydes based on a set of small, functionalized hydrocarbons is developed. The complete data set of 18 compounds was used for this study. The model is based on one of four optional categories of molecular descriptors: geometric, spectroscopic, bond order and topological indices. The model of Wiberg bond indices (WBIs) as descriptors of the CC involved bond based on stepwise regression has acceptable prediction abilities for 14 structures of ortho-hydroxyformylobenzo[a]pyrene derivatives already at the semiempirical level. The presented correlation enables a significantly more rapid and quantitative description of the hydrogen bonding strength than the much more time-consuming MTA method. Thus, WBIs are shown to provide a reliable means for fast prescreening of the energy of chelate hydrogen bonds potentially for any polyaromatic derivatives.

Assessment of O–H⋯O and O–H⋯S intramolecular hydrogen bond energies in some substituted pyrimidines using quantum chemistry methods

Resonance-assisted hydrogen bond (RAHB) theory was studied in some substituted pyrimidines in which encompass O–H⋯Y unit (Y= O and S). Alteration of substituents (R 1, R2, R3 = H, C2H, C2F) on pyrimidine ring changes properties of electron charge density at this ring and influences indirectly on strength of intramolecular hydrogen bond (IHB) interactions in the mentioned structures. Then, IHB energies were estimated using cis-trans method (CTM), related rotamers method (RRM), Espinosa’ method (EM), and a viewpoint based on properties of electron charge densities at ring critical point (RCP) of RAHB ring. Moreover, the estimated IHB energies with these methods were compared with those obtained using modified Espinosa’ method (MEM), Iogansen’s relationship, and chemical shift-based method to find more consistent method with the proposed viewpoint based on RCP characteristics. The linear correlations between the all estimated IHB energies and some hydrogen bonding descriptors such as geometrical, spectroscopic, topological, and molecular orbital factors were examined. Results indicated that the IHB energies that obtained by way of MEM and Iogansen’s relationship have better correlations with hydrogen bonding descriptors. Also, there are good consistencies between results of these two methods with viewpoint based on properties of RCPs. Therefore, IHB energies can suitably estimate using properties of RCPs in heterocyclic molecular systems especially in cases that rotation around C–C/CC bonds makes additional interactions in isomers and influences on accuracy of calculated IHB energies using approaches such as CTM and RRM.

Intramolecular Hydrogen Bond Energy and Its Decomposition—O–H∙∙∙O Interactions

The method to calculate the energy of intramolecular hydrogen bond is proposed and tested for a sample of malonaldehyde and its fluorine derivatives; the corresponding calculations were performed at the ωB97XD/aug-cc-pVTZ level. This method based on relationships found for related intermolecular hydrogen bonds is compared with other approaches which may be applied to estimate the intramolecular hydrogen bond energy. Particularly, methods based on the comparison of the system that contains the intramolecular hydrogen bond compared with corresponding conformations where such interaction does not occur are discussed. The function-based energy decomposition analysis, FB-EDA, of the intramolecular hydrogen bonds is also proposed here.

A Critical Overview of Current Theoretical Methods of Estimating the Energy of Intramolecular Interactions.

This article is probably the first such comprehensive review of theoretical methods for estimating the energy of intramolecular hydrogen bonds or other interactions that are frequently the subject of scientific research. Rather than on a plethora of numerical data, the main focus is on discussing the theoretical rationale of each method. Additionally, attention is paid to the fact that it is very often possible to use several variants of a particular method. Both of the methods themselves and their variants often give wide ranges of the obtained estimates. Attention is drawn to the fact that the applicability of a particular method may be significantly limited by various factors that disturb the reliability of the estimation, such as considerable structural changes or new important interactions in the reference system.

Fragmentation method reveals a wide spectrum of intramolecular hydrogen bond energies in antioxidant natural products

Very strong and weak IHBs in curcumin.

Stabilization of backbone-circularized protein is attained by synergistic gains in enthalpy of folded structure and entropy of unfolded structure.

Backbone circularization is an effective technique for protein stabilization. Here, we investigated the effect of a connector, an engineered segment that connects two protein termini, on the conformational stability of previously designed circularized variants of granulocyte colony-stimulating factor (G-CSF). Heat tolerance and chemical denaturation analyses revealed that aggregation resistance and thermodynamic stability of the circularized variants were superior to those of linear G-CSF. Crystal structure and molecular dynamics (MD) simulation of the most thermodynamically stable variant (C166) revealed a high number of intramolecular hydrogen bonds in both the connector region and Helix D adjacent to the connector region in the folded structure. MD simulations and theoretical calculations involving different force fields indicated a reduction in the main chain entropy of C166 in the unfolded state and increase in the intramolecular hydrogen bond energy of C166 in the folded structure. Although backbone circularization is usually considered to alter chain entropy of the unfolded state, the data indicated that it could also improve the conformational enthalpy of the folded state. Further structural examination of the connector region confirmed that protein design based on a statistical analysis of local structures is an effective approach for predicting an optimum connector length to improve the conformational stability of backbone-circularized proteins. Protein design using backbone circularization with an optimum connector length will be useful for the development of effective and safe protein therapeutics. DATABASE: Structural data are available in Protein Data Bank under the accession number 5ZO6.

On the performance of molecular tailoring approach for estimation of the intramolecular hydrogen bond energies of RAHB systems: a comparative study

In the current research, the performance of molecular tailoring approach (MTA) for estimation of the intramolecular hydrogen bond (IMHB) energies of the simple resonance-assisted hydrogen bond (RAHB) systems was theoretically investigated. First, a wide range of malonaldehyde derivatives (36 members) including the F, Cl, Br, CN, NO2, ethen (-CH=CH2), ethin (-C ≡ CH), CF3, OCH3, C2H5, CH3, and Ph substitutions at R1, R2, and R3 positions were considered. Then, all of these molecules at MP2/6-311++G(d, p) level of theory have been optimized and their MTA energies were calculated. Furthermore, various qualitative descriptors of IMHB such as structural, spectroscopic, topological, and molecular orbital parameters were considered, and all of correlations between these factors and MTA energies were explored. According to their regression coefficients (R2), the linear characteristic of correlations obeys the following order:$$ {H}_{\mathrm{H}\dots \mathrm{O}}>{d}_{\mathrm{O}-\mathrm{H}}>{E}_{\mathrm{CT}}>{V}_{\mathrm{H}\bullet \bullet \bullet \mathrm{O}}>{\rho}_{\mathrm{H}\bullet \bullet \bullet \mathrm{O}}>{\nu}_{\mathrm{O}-\mathrm{H}}>{d}_{\mathrm{H}\bullet \bullet \bullet \mathrm{O}}>{d}_{\mathrm{O}\bullet \bullet \bullet \mathrm{O}} $$$$ 0.952\kern1.5em 0.940\kern1.2em 0.936\kern0.75em 0.914\kern1.2em 0.901\kern1.2em 0.834\kern1em 0.789\kern1.3em 0.755 $$ These correlation coefficients have compared with the corresponding R2 values of other models such as RRM, RBM, GCM, IRM, and OCM, which leads to the following order of linearity:$$ \mathrm{MTA}\ge \mathrm{RRM}>\mathrm{RBM}>\mathrm{GCM}>\mathrm{IRM}>\mathrm{OCM}. $$ Finally, the significance of π-electron delocalization (π-ED) of RAHB rings is also evaluated by the geometrical factor of Gilli (λ) and the harmonic oscillator model of aromaticity (HOMA) that presents the excellent linear correlations with MTA energies, which may be implied on the validity of RAHB theory.

A comparative study of intramolecular hydrogen bond on N-formylformamide derivatives in ground and first singlet excited state: a DFT and TD-DFT study

Intramolecular hydrogen bond (IHB) of N-formylformamide derivatives in the both ground (GS) and first singlet excited state (S1) have been investigated by DFT and TD-DFT methods with the standard basis set 6-311++G (d,p), respectively. Also the potential energy density method was used to calculate of the IHB energies in the both GS and S1. Our calculations show that IHB energies in the GS are much higher than the S1 one. In order to explain this energy difference, molecular orbital analysis (MOS) has been employed. According to the MOS, it was found that electron density on antibonding orbital (δ O–H * ) increases after excitation, which can be a reason for the weakened IHB in the S1. In this work, we evaluated and compared the substitution effects on the IHB strength in the both GS and S1 by some of the IHB descriptors such as geometrical, topological and orbital charge transfer parameters. The results proved that the substitution effects on the both GS and S1 are same. As with replacing of the halogen atoms at R2 position the IHB energies increased while for the R2 and R1,2 decreased. Moreover, for the first time, various geometry—based resonance indices such as Gilli (Q and λ) parameters, original harmonic oscillator model of aromaticity, harmonic oscillator model of electron delocalization and harmonic oscillator model for heterocyclic with π-electrons delocalization were calculated and compared in the both GS and S1. Interestingly, it is found from the resonance indices that electron delocalization in the S1 increased that this result is against the RAHB theory in the studied molecules. Finally, the linear equations of the IHB energies versus the IHB descriptors parameters and resonance indices in the both GS and S1 indicate that the potential energy density is an inappropriate method for calculation of the IHB energies of the heterocyclic RAHB systems in the S1.

Benchmark calculations of intramolecular hydrogen bond energy based on molecular tailoring and function‐based approaches: Developing hybrid approach

AbstractThe energy of the intramolecular hydrogen bond has been recognized and may be estimated by the molecular fragmentation method and the method based on the functional dependence on the hydrogen bond descriptor. Both basic methods for estimating the energy of intramolecular hydrogen bonds are compared using a series of the hydroxycarbonyl aliphatic compounds as the benchmark. The functional dependencies are established that provide the best fit with the molecular fragmentation method. As both methods for estimating the energy of an intramolecular hydrogen bond have disadvantages, a new hybrid approach is proposed. In this approach, the average value of the intramolecular hydrogen bond energy calculated by the molecular fragmentation method and functional dependence method is used as the estimated parameter. The hybrid approach provides a compensation of the error in the evaluation of the interaction strength compared to the molecular fragmentation method or the functional dependencies method. Hence, it allows one to more reliably rank the strength of the intramolecular hydrogen bond in homogeneous series of molecules. Based on this approach, the strength of the О−Н···О=C intramolecular hydrogen bond in the studied compounds are classified in the energy scale in terms of weak, moderate, and strong interactions.

Exploring Effective Factors on Energy Data of Some Benzofuran Derivatives

Benzofuran derivatives have many useful applications. Computational quantum chemistry method was used to study the relationship between energy data and molecular properties of the 5,6-dihydroxy-2-methyl-1-benzofuran-3-carboxylate derivatives (molecules 3a–3f). Results indicate that there is a good relationship between intramolecular hydrogen bond lengths and energy data of these molecules. Also, X-ray of molecule 3e was used to compare experimental and computational geometrical parameters. Chemical hardness, chemical potential and electronegativity values were calculated to recognize relation between energy data and reactivity of these molecules. Atomic net charges and molecular electrostatic potential values were employed for better insight regarding energy data of the molecules. Electronic charge densities were calculated using atoms in molecules method. The correlation coefficients between experimental and computational 13 C NMR chemical shifts were examined. Total spin– spin coupling constant and its components were evaluated to understand the contribution of these properties in energy data of the molecules. The relation between energy data of the molecules and aromaticity of the rings was also determined.

DFT and TD-DFT study of the enol and thiol tautomers of 3-thioxopropanal in the ground and first singlet excited states

In the first part of this paper, a comprehensive theoretical study of molecular structure, stability, intramolecular hydrogen bond (IMHB) and [Formula: see text]-electron delocalization ([Formula: see text]-ED) of the enol and thiol tautomers of 3-thioxopropanal (TPA) in the ground state is performed. In this regard, all of the plausible conformations of TPA at M06-2X/6-311[Formula: see text]G(d,p) are optimized and a variety of theoretical levels are employed to identify the global minimum. Our calculations show that E1 is the most stable form that is in contrast to the results of Gonzalez et al. [J Phys Chem 101: 9710, 1997]. In order to elucidate this duality, the IMHB and [Formula: see text]-ED of chelated forms (E1 and T1) have been extensively investigated. So, it is found that both of the IMHB analysis and [Formula: see text]-ED concepts emphasize on the E1, as the global minimum. In the second part of this study, a set of simple electron-withdrawing and electron-donating substituents such as CN, F, Cl, CH3 and NH2 have been considered to evaluate their effects on the IMHB of the first singlet excited state of E1 and T1 at TD-DFT/6–311[Formula: see text]G(d,p) level of theory. According to our analysis, it was found that the IMHB strength of the excited states are much weaker than the ground states. Surprisingly, the IMHB of thiol derivatives is stronger than the enol ones in contrast to the ground state. Furthermore, the substitution effects in the ground and excited states are significantly different. Finally, various linear correlations between the IMHB energies with geometrical, topological and molecular orbital parameters are obtained.

Energy of Intramolecular Hydrogen Bonding in ortho-Hydroxybenzaldehydes, Phenones and Quinones. Transfer of Aromaticity from ipso-Benzene Ring to the Enol System(s).

Intramolecular hydrogen bonding (HB) is one of the most studied noncovalent interactions of molecules. Many physical, spectral, and topological properties of compounds are under the influence of HB, and there are many parameters used to notice and to describe these changes. Hitherto, no general method of measurement of the energy of intramolecular hydrogen bond (EHB) has been put into effect. We propose the molecular tailoring approach (MTA) for EHB calculation, modified to apply it to Ar-O-H∙∙∙O=C systems. The method, based on quantum calculations, was checked earlier for hydroxycarbonyl-saturated compounds, and for structures with resonance-assisted hydrogen bonding (RAHB). For phenolic compounds, the accuracy, repeatability, and applicability of the method is now confirmed for nearly 140 structures. For each structure its aromaticity HOMA indices were calculated for the central (ipso) ring and for the quasiaromatic rings given by intramolecular HB. The comparison of calculated HB energies and values of estimated aromaticity indices allowed us to observe, in some substituted phenols and quinones, the phenomenon of transfer of aromaticity from the ipso-ring to the H-bonded ring via the effect of electron delocalization.