Hydrogen bonding in Schiff bases--NMR, structural and experimental charge density studies

A series of sixteen Schiff bases (derivatives of salicylaldehydes and aryl amines) was studied to reveal the influence of substituents and the length of the linker on the properties of the H-bonding formed. In theory, two groups of compounds, derivatives of 2-(2-hydroxybenzylidenoamine)phenol) and 2-hydroxy-N-(2-hydroxybenzylideno)benzylamine, can form different types of H-bonds using one or two hydroxyl groups present in the molecules. Two other groups of compounds, derivatives of 4-(2-hydroxybenzylidenoamine)phenol and N-(2-hydroxybenzyideno)benzylamine, can form only one type of H-bond. It was confirmed by (15)N and (13)C NMR experiments, that in all cases only traditional, H-bonded six-membered chelate rings were formed. The positions of the hydrogen atom in the rings depend on the substituent and phase. Generally, the OH H-bond form dominates in solution, with exception of the nitro derivatives, where the NH tautomer is present. In the solid state the tautomeric equilibrium is strongly shifted to the NH form. Only for the 5-Br derivative of one compound was the reverse relationship found. According to the results of experimental charge density investigations, two intramolecular H-bonds in the 5-methoxy derivative of 2-hydroxy-N-(2'-hydroxybenzylideno)benzylamine) differ significantly in terms of charge density properties. The intra- and intermolecular H-bonds formed by the deprotonated oxygen atom from 2-OH group are strong, with significant charge density concentration at the bond critical point and a straight, well-defined bond path, whereas the second intramolecular H-bond formed by the oxygen atom from the 2'-OH group is quite weak, with ca. five times smaller charge density concentration than in the previous case and a bent bond path. In terms of energy densities, the latter H-bond appears to be a non-bonding interaction, with total energy density being slightly positive. In terms of source contributions to the density at the H-bond critical point from the atoms involved, the intermolecular, linear H-bond is very strong and charge-assisted in the source function classification, the N(1)-H(1N)···O(1) H-bond is medium-strength, while the third H-bond is extremely weak.     

4,4'-, 5,5'-, and 6,6'-dimethyl-2,2'-bipyridyls: the structures, phase transitions, vibrations, and methyl group tunneling of their complexes with chloranilic acid

The crystal and molecular structures of 4,4(')- and 6,6(')-dimethyl-2,2(')-bipyridyl complexes with 2,5-dichloro-3,6-dihydroxy-p-benzoquinone (chloranilic acid, CLA) have been determined and compared with those of the complex with the 5,5(')-derivative, which is known to possess interesting antiferroelectric properties. In the crystalline state, all three compounds form hydrogen bonded chains with N(+)-H···O(-) and O-H···N bridges on both sides of the bipyridyl constituent. The comparison of three derivatives indicates that the N(+)-H···O(-) hydrogen bonds are shortest for the 5,5(')-dimethyl complex. The 4,4(')- and 6,6(')-derivatives do not show any ferroelectric feature. The 6,6(')-one is, however, characterized by a continuous phase transition, revealed in the differential scanning calorimetry, dilatometric, and dielectric characteristics. The tunneling splitting measured by neutron backscattering in the energy range ±30 μeV for the neat dimethyl bipyridyls and their complexes with CLA indicates that the different splittings are primarily due to the crystal packing effect and that charge transfer between interacting compounds plays only a minor role.     

Effect of the H-bonding on aromaticity of purine tautomers

Four tautomers of purine (1-H, 3-H, 7-H, and 9-H) and their equilibrium H-bonded complexes with F(-) and HF for acidic and basic centers, respectively, were optimized by means of the B3LYP/6-311++G(d,p) level of theory. Purine tautomer stability increases in the following series: 1-H < 3-H < 7-H < 9-H, consistent with increasing aromaticity. Furthermore, the presence of a hydrogen bond with HF does not change this order. For neutral H-bonded complexes, the strongest and the weakest intermolecular interactions occur (-14.12 and -10.49 kcal/mol) for less stable purine tautomers when the proton acceptor is located in the five- and six-membered rings, respectively. For 9-H and 7-H tautomers the order is reversed. The H-bond energy for the imidazole complex with HF amounts to -14.03 kcal/mol; hence, in the latter case, the fusion of imidazole to pyrimidine decreases its basicity. The ionic H-bonds of N(-)···HF type are stronger by ~10 kcal/mol than the neutral N···HF intermolecular interactions. The hydrogen bond N(-)···HF energies in pyrrole and imidazole are -32.28 and -30.03 kcal/mol, respectively, and are substantially stronger than those observed in purine complexes. The aromaticity of each individual ring and of the whole molecule for all tautomers in ionic complexes is very similar to that observed for the anion of purine. This is not the case for neutral complexes and purine as a reference. The N···HF bonds perturb much more the π-electron structure of five-membered rings than that of the six-membered ones. The H-bonding complexes for 7-H and 9-H tautomers are characterized by higher aromaticity and a much lower range of HOMA variability.     

Electronic properties of hydrogen-bonded complexes of benzene(HCN)(1-4): comparison with benzene(H2O)(1-4)

The electronic properties, specifically, the dipole and quadrupole moments and the ionization energies of benzene (Bz) and hydrogen cyanide (HCN), and the respective binding energies, of complexes of Bz(HCN)(1-4), have been studied through MP2 and OVGF calculations. The results are compared with the properties of benzene-water complexes, Bz(H(2)O)(1-4), with the purpose of analyzing the electronic properties of microsolvated benzene, with respect to the strength of the CH/π and OH/π hydrogen-bond (H-bond) interactions. The linear HCN chains have the singular ability to interact with the aromatic ring, preserving the symmetry of the latter. A blue shift of the first vertical ionization energies (IEs) of benzene is observed for the linear Bz(HCN)(1-4) clusters, which increases with the length of the chain. NBO analysis indicates that the increase of the IE with the number of HCN molecules is related to a strengthening of the CH/π H-bond, driven by cooperative effects, increasing the acidity of the hydrogen cyanide H atom involved in the π H-bond. The longer HCN chains (n ≥ 3), however, can bend to form CH/N H-bonds with the Bz H atoms. These cyclic structures are found to be slightly more stable than their linear counterparts. For the nonlinear Bz(HCN)(3-4) and Bz(H(2)O)(2-4) complexes, an increase of the binding energy with the number of solvent molecules and a decrease of the IE of benzene, relative to the values for the Bz(HCN) and Bz(H(2)O) complexes, respectively, are observed. Although a strengthening of the CH/π and OH/π H-bonds, with increasing n, also takes place for the Bz(H(2)O)(2-4) and Bz(HCN)(3-4) nonlinear complexes, Bz proton donor, CH/O, and CH/N interactions are at the origin of this decrease. Thus CH/π and OH/π H-bonds lead to higher IEs of Bz, whereas the weaker CH/N and CH/O H-bond interactions have the opposite effect. The present results emphasize the importance of both aromatic XH/π (X = C, O) and CH/X (X = N, O) interactions for understanding the structure and electronic properties of Bz(HCN)(n) and Bz(H(2)O)(n) complexes.     

Computational studies of the structure and cation-anion interactions in 1-ethyl-3-methylimidazolium lactate ionic liquid

Quantum chemical calculations of the structures and cation-anion interaction of 1-ethyl-3-methylimidazolium lactate ([Emim][LAC]) ion pair at the B3LYP/6-31++G** theoretical level were performed. The relevant geometrical characteristics, energy properties, intermolecular H-bonds (H-bonds), and calculated IR vibrations with respect to isolated ions were systematically discussed. The natural bond orbital (NBO) and atoms in molecule (AIM) analyses were also employed to understand the nature of the interactions between cation and anion. The five most stable geometries were verified by analyzing the relative energies and interaction energies. It was found that the most of the C-H···O intermolecular H-bonds interactions in five stable conformers have some covalent character in nature. The elongation and red shift in IR spectrum of C-H bonds which involve in H-bonds is proved by electron transfers from the lone pairs of the carbonyl O atom of [LAC] to the C-H antibonding orbital of the [Emim]+. The interaction modes are more favorable when the carbonyl O atoms of [LAC] interact with the C2-H of the imidazolium ring and the C-H of the ethyl group through the formation of triple H-bonds.     

Understanding hydrogen bonding of hydroxamic acids with some amino acid side chain model molecules

The hydrogen-bonding abilities of a few amino acid side chains have been studied through aggregation of methylamine, methanol, and acetic acid (as model molecules) with formo- and thioformo- hydroxamic acids using ab initio calculations. Forty six aggregates representing all possible H-bond interactions between these amino acid side chain groups and two most stable keto and enol tautomeric forms of both hydroxamic acids have been optimized. Although participation of conventional H-bond donors and acceptors leads to significant stabilization energies, yet C–H···O, C–H···N, S–H···O, and S–H···N etc. unconventional H-bonds also contribute to stabilize interactions in many aggregates. Strength of H-bonds of the molecules with formo- and thioformo- hydroxamic acid studied follows the order acetic acid > methylamine > methanol. A comparative study of atomic charges and orbital interactions employing NBO analysis has been carried out to explore the role of bond polarizations, charge transfer, and electron delocalizations as contributors to stabilization energy.     

Weak H-bonds. Comparisons of CH···O to NH···O in proteins and PH···N to direct P···N interactions

Whereas CH···O H-bonds are usually weaker than interpeptide NH···O H-bonds, this is not necessarily the case within proteins. The nominally weaker CH···O are surprisingly strong, comparable to, and in some cases stronger than, the NH···O H-bonds in the context of the forces that hold together the adjacent strands in protein β-sheets. The peptide NH is greatly weakened as proton donor in certain conformations of the protein backbone, particularly extended structures, and forms correspondingly weaker H-bonds. The PH group is a weak proton donor, but will form PH···N H-bonds. However, there is a stronger interaction in which P can engage, in which the P atom, not the H, directly approaches the N electron donor to establish a direct P···N interaction. This approach is stabilized by the same sort of electron transfer from the N lone pair to the P-H σ* antibond that characterizes the PH···N H-bond.     

Explorations of the nature of the coupling interactions between vitamin C and methylglyoxal: a DFT study

As the first step toward understanding the augment role of vitamin C (Vc) for the anticancer effect of methylglyoxal (MG), the nature of the coupling interactions between Vc and MG has been systematically investigated at the B3LYP/6-311++G** level of theory in combination with the atoms in molecules (AIM) theory, natural bond orbital (NBO) method, and energy decomposition analysis (EDA). The possible stable complexes have been located on their potential energy surface (PES). Most of them are characterized by one or two intermolecular H-bonds with the binding energies varying from −11.1 to −2.0 kcal/mol. AIM analyses suggest that all the intermolecular H-bonds have been predominated by the electrostatic interaction. A good linear correlation between the intermolecular H-bond distance and the electron density as well as its Laplacian at the bond critical point of the intermolecular H-bond has been observed. Depending on the selected coupling modes between Vc and MG, the origin of the blue-shifts of the stretching vibrational frequencies of different C–H bonds has been elucidated. Additionally, the inherent reason for the positive role of Vc in the anticancer process for MG has been verified through the investigation of the one-electron oxidation behaviors of the most stable complex.     

Hydrogen bonding interactions between <Emphasis Type="Italic">N</Emphasis>,<Emphasis Type="Italic">N</Emphasis>-dimethylformamide and cysteine: DFT studies of structures,properties, and topologies

The hydrogen bonding interactions between cysteine and N,N-dimethylformamide (DMF) were studied at the extended hybrid functional DFT-X3LYP/6-311++G(d,p) level regarding their geometries, energies, vibrational frequencies, and topological features of the electron density. The quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analyses were employed to elucidate the interaction characteristics in the complexes. The results show that two intermolecular hydrogen bonds (H-bonds) are formed in one complex except few complexes with one intermolecular H-bond. The H-bonds involving O atom of DMF as H-bond acceptor usually are red-shifting H-bonds, while the blue-shifting H-bond usually involve methyl of DMF or methenyl of cysteine moiety as H-bond donors. Both hydrogen bonding interaction and structural deformation play important roles in the relative stabilities of the complexes. Due to the π-bond cooperativity, the strongest H-bond is formed between hydroxyl of cysteine moiety and O atom of DMF, however, the serious deformation counteract the hydrogen bonding interaction to a great extent. The complex involves a stronger hydrogen bonding interaction as well as the smaller deformation is the most stable one. The electron density (ρ_b) as well as its Laplacian (∇²ρ_b) at the H-bond critical point predicted by QTAIM is strongly correlated with the H-bond structural parameter (δR _H···Y) and the second-perturbation energies E(2) in the NBO scheme.     

Hydrogen bonding of formo- and thioformohydroxamic acid with methanethiol and methaneselenol as amino acid side chain groups

Hydrogen (H-) bonding ability of most stable keto and enol tautomers of formo- and thioformohydroxamic acids has been investigated by optimizing their 1:1 aggregates with MeSH and MeSeH as model molecules for sulfur and selenium containing amino acid side chains. Although enol is the most stable conformer of thioformohydroxamic acid, yet the most stable aggregate in both hydroxamic acids (HAs) being formed with keto conformer suggests that H-bonding can influence specific conformational dominance. In the aggregates, HAs preferably act as H-bond donor and S/Se of MeSH and MeSeH act as H-bond acceptor. The S···H and Se···H H-bonds although disfavored by electrostatics yet are favored by significant charge transfer. H-bonding preference and strength of interaction of HAs with MeSH and MeSeH is remarkably similar but markedly different from MeOH. AIM and NBO analysis have been employed to understand the role of electron delocalization, bond polarizations, charge transfer etc. as contributors to stabilization energy.     

Theoretical studies on the coupling interactions in H2SO4···HOO˙···(H2O)n (n = 0-2) clusters: toward understanding the role of water molecules in the uptake of HOO˙ radical by sulfuric acid aerosols

A detailed knowledge of coupling interactions among sulfuric acid (H(2)SO(4)), the hydroperoxyl radical (HOO˙), and water molecules (H(2)O) is crucial for the better understanding of the uptake of HOO˙ radicals by sulfuric acid aerosols at different atmospheric humidities. In the present study, the equilibrium structures, binding energies, equilibrium distributions, and the nature of the coupling interactions in H(2)SO(4)···HOO˙···(H(2)O)(n) (n = 0-2) clusters have been systematically investigated at the B3LYP/6-311++G(3df,3pd) level of theory in combination with the atoms in molecules (AIM) theory, natural bond orbital (NBO) method, energy decomposition analyses, and ab initio molecular dynamics. Two binary, five ternary, and twelve tetramer clusters possessing multiple intermolecular H-bonds have been located on their potential energy surfaces. Two different modes for water molecules have been observed to influence the coupling interactions between H(2)SO(4) and HOO˙ through the formations of intermolecular H-bonds with or without breaking the original intermolecular H-bonds in the binary H(2)SO(4)···HOO˙ cluster. It was found that the introduction of one or two water molecules can efficiently enhance the interactions between H(2)SO(4) and HOO˙, implying the positive role of water molecules in the uptake of the HOO˙ radical by sulfuric acid aerosols. Additionally, the coupling interaction modes of the most stable clusters under study have been verified by the ab initio molecular dynamics.     

Trimethylglycine complexes with carboxylic acids and HF: solvation by a polar aprotic solvent

A series of strong H-bonded complexes of trimethylglycine, also known as betaine, with acetic, chloroacetic, dichloroacetic, trifluoroacetic and hydrofluoric acids as well as the homo-conjugated cation of betaine with trifluoroacetate as the counteranion were investigated by low-temperature (120-160 K) liquid-state NMR spectroscopy using CDF(3)/CDF(2)Cl mixture as the solvent. The temperature dependencies of (1)H NMR chemical shifts are analyzed in terms of the solvent-solute interactions. The experimental data are explained assuming the combined action of two main effects. Firstly, the solvent ordering around the negatively charged OHX region of the complex (X = O, F) at low temperatures, which leads to a contraction and symmetrisation of the H-bond; this effect dominates for the homo-conjugated cation of betaine. Secondly, at low temperatures structures with a larger dipole moment are preferentially stabilized, an effect which dominates for the neutral betaine-acid complexes. The way this second contribution affects the H-bond geometry seems to depend on the proton position. For the Be(+)COO(-)···HOOCCH(3) complex (Be = (CH(3))(3)NCH(2)-) the proton displaces towards the hydrogen bond center (H-bond symmetrisation, O···O contraction). In contrast, for the Be(+)COOH···(-)OOCCF(3) complex the proton shifts further away from the center, closer to the betaine moiety (H-bond asymmetrisation, O···O elongation). Hydrogen bond geometries and their changes upon lowering the temperature were estimated using previously published H-bond correlations.     

Preferential formation of the different hydrogen bonds and their effects in tetrahydrofuran and tetrahydropyran microhydrated complexes

The role of cycloether-water (c-w) and water-water (w-w) hydrogen bonds (H-bonds) on the stability of the tetrahydrofuran THF/(H(2)O)(n) and the tetrahydropyran THP/(H(2)O)(n) complexes with n = 1-4 was investigated herein using the density functional and ab initio methods and the atoms in molecules theory. Geometry optimizations for these complexes were carried out with various possible initial guess structures. It was revealed that the major contributions of the mono and dihydrated complexes came from c-w H-bonds. A competition between c-w and w-w H-bonds contribution was observed for trihydrated complexes. For most of tetrahydrated complexes, the inter-water H-bonds provided the greatest contribution, whereas the c-w contributions were small but not negligible. It was confirmed that to produce a hydrophobic hydration of cycloethers, the C-H···O(w) H-bond should be associated with a network of H-bonds that connects both portions of the solute, through the formation of a bifunctional H-bond. A linear correlation is obtained for the sum of electron density at the bond critical points (ρ(b)) with the interaction energy (ΔE) and with the solute-solvent interaction energy (ΔE(s-w)) of the microhydrated complexes. In addition, a new way to estimate the energetic contribution as well as the preferential formation of the different H-bonds based completely on ρ(b) was found. Even more, it allows to differentiate the contribution from c-w interactions in both hydrophilic and hydrophobic contributions, it is therefore a useful tool for studying the hydration of large biomolecules. The analysis of the modifications in the atomic and group properties brought about by successive addition of H(2)O molecules allowed to pinpoint the atoms or molecular groups that undergo the greatest changes in electron population and energetic stabilization. It was identified that the remarkable stabilization of the water oxygen atoms is crucial for the stabilization of the complexes.     

Intramolecular hydrogen bond energy and cooperative interactions in α-, β-, and γ-cyclodextrin conformers

Accurate estimation of individual intramolecular hydrogen bond (H-bond) energies is an intricate task for multiply H-bonded systems. In such cases, the hydrogen bond strengths could be highly influenced by the cooperative interactions, for example, those between hydroxyl groups in sugars. In this work, we use the recently proposed molecular tailoring approach-based quantification (Deshmukh, Gadre, and Bartolotti, J Phys Chem A 2006, 110, 12519) to the extended systems of cyclodextrins (CDs). Further, the structure and stability of different conformers of α-, β-, and γ-CDs are explained based on the energetics and cooperative contribution to the strength of these H-bonds. The estimated O-H···O H-bond energies in the various CD conformers are found to vary widely from 1.1 to 8.3 kcal mol(-1). The calculated energy contributions to cooperativity toward the H-bond strengths fall in the range of 0.25-2.75 kcal mol(-1).     

Nucleophilic degradation of fenitrothion insecticide and performance of nucleophiles: a computational study

Ab initio and density functional theory (DFT) calculations have been performed to understand the destruction chemistry of an important organophosphorus insecticide O,O-dimethyl O-(3-methyl-4-nitrophenyl) phosphorothioate, fenitrothion (FN), toward nucleophilic attack. Breaking of the P-OAr linkages through nucleophilic attack is considered to be the major degradation pathway for FN. One simple nucleophile, hydroxide (OH(-)), and two different α-nucleophiles, hydroperoxide (OOH(-)) and hydroxylamine anion (NH(2)O(-)), have been considered for this study. Nucleophilic attack at the two different centers, S(N)2@P and S(N)2@C, has been monitored, and the computed reaction energetics confirms that the S(N)2@P reactions are favorable over the S(N)2@C reactions for all the nucleophiles. All electronic structure calculations for the reaction are performed at DFT-B3LYP/6-31+G(d) level of theory followed by a refinement of energy at ab initio MP2/6-311++G(2d,2p) level. The effect of aqueous polarization on both the S(N)2 reactions is taken into account employing the conductor-like screening model (COSMO) as well as polarization continuum model (PCM) at B3LYP/6-31+G(d) level of theory. Relative performance of the two α-nucleophiles, OOH(-) and NH(2)O(-), at the P center has further been clarified using natural bond orbital (NBO), conceptual DFT, and atoms in molecules (AIM) approaches. The strength of the intermolecular hydrogen bonding in the transition states and topological properties of the electron density distribution for -X-H···S (X = O, N) intermolecular hydrogen bonds are the subject of NBO and AIM analysis, respectively. Our calculated reaction energetics and electronic properties suggest that the relative order of nucleophilicity for the nucleophiles is OOH(-) > NH(2)O(-) > OH(-) for the S(N)2@P, whereas for the S(N)2@C the order, which gets little altered, is NH(2)O(-) > OOH(-) > OH(-).     

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)).     

Electron density characteristics in bond critical point (QTAIM) versus interaction energy components (SAPT): the case of charge-assisted hydrogen bonding

Charge-assisted hydrogen bonds (CAHBs) of N-H···Cl, N-H···Br, and P-H···Cl type were investigated using advanced computational approach (MP2/aug-cc-pVTZ level of theory). The properties of electron density function defined in the framework of Quantum Theory of Atoms in Molecules (QTAIM) were estimated as a function of distance in H-bridges. Additionally, the interaction energy decomposition was performed for H-bonded complexes with different H-bond lengths using the Symmetry-Adapted Perturbation Theory (SAPT). In this way both QTAIM parameters and SAPT energy components could be expressed as a function of the same variable, that is, the distance in H-bridge. A detailed analysis of the changes in QTAIM and SAPT parameters due to the changes in H···A distance revealed that, over some ranges of H···A distances, electrostatic, inductive and dispersive components of the SAPT interaction energy show a linear correlation with the value of the electron density at H-BCP ρ(BCP). The linear relation between the induction component, E(ind), and ρ(BCP) confirms numerically the intuitive expectation that the ρ(BCP) reflects directly the effects connected with the sharing of electron density between interacting centers. These conclusions are important in view of charge density studies performed for crystals in which the distance between atoms results not only from effects connected with the interaction between atomic centers directly involved in bonding, but also from packing effects which may strongly influence the length of the H-bond.     

Intramolecular hydrogen bonding and cooperative interactions in carbohydrates via the molecular tailoring approach

In spite of many theoretical and experimental attempts for understanding intramolecular hydrogen bonding (H-bonding) in carbohydrates, a direct quantification of individual intramolecular H-bond energies and the cooperativity among the H-bonded networks has not been reported in the literature. The present work attempts, for the first time, a direct estimation of individual intramolecular O-H...O interaction energies in sugar molecules using the recently developed molecular tailoring approach (MTA). The estimated H-bond energies are in the range of 1.2-4.1 kcal mol(-1). It is seen that the OH...O equatorial-equatorial interaction energies lie between 1.8 and 2.5 kcal mol(-1), with axial-equatorial ones being stronger (2.0-3.5 kcal mol(-1)). The strongest bonds are nonvicinal axial-axial H-bonds (3.0-4.1 kcal mol(-1)). This trend in H-bond energies is in agreement with the earlier reports based on the water-water H-bond angle, solvent-accessible surface area (SASA), and (1)H NMR analysis. The contribution to the H-bond energy from the cooperativity is also estimated using MTA. This contribution is seen to be typically between 0.1 and 0.6 kcal mol(-1) when H-bonds are a part of a relatively weak equatorial-equatorial H-bond network and is much higher (0.5-1.1 kcal mol(-1)) when H-bonds participate in an axial-axial H-bond network.     

Two-way effects between hydrogen bond and intramolecular resonance effect: an ab initio study on complexes of formamide and its derivatives with water

Ab initio calculations up to MP2/aug-cc-pVTZ//MP2/cc-pVTZ level, including natural charge population and natural resonance theory analyses, have been carried out to study the two-way effects between hydrogen bond (H-bond) and the intramolecular resonance effect by using the H-bonded complexes of formamide ( FAO) and its derivatives ( FAXs, X represents the heavy atoms in the substituent groups, CH 2, NH, SiH 2, PH, and S) with water as models. Unlike NH 3 and NH 2CH 3 which prefer being H-bond acceptors ( HA) to form H-bond with water, the amino groups in the six monomers, because of the resonance effect, prefer being H-bond donors ( HD) rather HA. Six monomers can all form HD complexes with water, and only two ( FAC and FASi) with the weakest resonance effect are able to form HA complexes with water. The HD H-bond and resonance effect enhance each other (positive two-way effects) whereas the HA H-bond and resonance effect weaken each other (negative two-way effects). The H-bond energies in the six HD complexes are nearly linearly correlated with the weights of the dipolar resonance in Pauling's model and the N-C bond lengths; the correlation coefficients are 0.91 and 0.93, respectively. The positive two-way effects also happens in FAO-water complex, in which the FAO CO group serves as HA ( HA co ). Interestingly, when the HD and HA co H-bonds are present in FAO H-bond complex simultaneously, the enhancements are much more significant, and the energies of the two types of H-bonds are much larger than those when only one type of H-bond is present, reflecting the cooperative effects. By using the knowledge to the two-way effects, we computationally designed a molecule ( FAO- BH 3 ) to increase H-bond energy. Because of the oxygen lone pair donation to the empty pi orbital of BH 3, FAO- BH 3 has a much stronger resonance effect than FAO. As a result, the H-bond energy (-5.55 kcal/mol) in HD H 2O ... FAO- BH 3 complex is much greater than the -3.30 kcal/mol in the HD H 2O...FAO complex. The two-way effects can be rationalized as follows: the resonance effect leads to intramolecular charge shifts in the monomers which facilitate or prevent the charge donation or acceptation of their H-bond partners. Therefore, the H-bonds are strengthened or weakened. In reverse, the charge donations or acceptations of their H-bond partners facilitate or prevent the intramolecular charge shifts in the monomer moieties, which enhance or weaken the resonance effect. The understanding to the two-way effects may be helpful in drug design and refinement by modulating the H-bond strength and in building empirical H-bond models to study large biological molecules. The study supports Pauling's resonance model.