Proton Ordering of Ice and Gas‐Hydrate Frameworks. Nanostructural Approach

This paper reviews the state of research into lowtemperature proton ordering of ice and the results of theoretical predictions of energetically preferred proton configurations of regular water systems based on the topological model of strong and weak Hbonds. All systems under study have noncrystalline proton ordering. It is concluded that in the context of proton ordering problems, ice is the object of nanostructural analysis.     

Transfer Reactions of Rydberg (Quasi-Rydberg Molecular) Electrons in Condensed Media: IV. Large Proton Displacements upon Phosphorescence Quenching via Highly Excited Triplet States T* of Substituted Aromatic Solute Molecules in Solutions at 77 K

Intramolecular phosphorescence quenching via states T* in aromatic solute molecules containing N–H (diphenylamine (DPA) or carbazole), O–H (naphthol), etc. bonds was observed in methylcyclohexane at 77 K. The quantum yield of quenching measured for DPA increases with increasing the energy of the T* state. As in the case of external electron acceptors, the quenching and photodissociation are associated with the capture of excited * electrons onto polarized bonds N–H⁺, O–H⁺and with the formation of triplet complexes (for example, Ph₂N···H*, where H* is the excited hydrogen atom). The complexes can be deactivated via configurations with large proton displacement distances (Ph₂N^–···H⁺).     

Vibrational spectra and structure of some oxalic acid/substituted pyridine (methyl-, amino-, and dihalogeno-) complexes: Hydrogen bond features

The Raman and infrared spectra of some polycrystalline substituted pyridine/oxalic acid complexes have been investigated and assignments in terms of group frequencies are given. Various hydrogen bonds (NH?O, OH?O, OH?N) are distinguished and crystal structures are proposed. For the stronger bases (methyl- or aminopyridines with pk_a ≈ 6) proton transfer occurs. The 1/1 complex contains infinite chains of hydrogen oxalate ions linked by strong OH?O hydrogen bonds with vOH between 2000 and 800 cm⁻¹. R_OH?O distances are 2.47–2.62 Å). The substituted pyridinium cations are linked to the chain backbone by medium NH?O hydrogen bonds with NH?O lengths of 2.71–2.81 Å. The 3,5-dichloropyridine forms a 2/1 adduct without proton transfer, in accordance with its pk_a (0.6), and strong OH?N hydrogen bonds occur (vOH about 2000 cm⁻¹ and ). Finally, the 2,6-dihalogenopyridine derivatives do not form complex with oxalic acid, presumably because of steric hindrance.     

On the Distribution of π Bonds in Cyclofusene

We present a type of coronafusene termed cyclofusene, in which each hexacycle shares exactly two nonadjacent edges with other hexacycles. Cyclofusene has exactly four configurations of bonds such that each bond belongs to the inner or outer boundary. In each of these configurations, the outer boundary has six more bonds than the inner boundary. The number of shared bonds in any mixed configuration is even. Let m be the number of shared bonds in a mixed configuration for a cyclofusene with exactly k linear chains. Then m k. Furthermore, there exists a mixed configuration with exactly k shared bonds.     

MNDO calculations of systems containing hydrogen bonds

A modified MNDO method, which can be used in the studies on structures with hydrogen bonds X-H-X, X, X = N, O, is described. Results for a wide range of molecular complexes are reported. Energies of hydrogen bonds are reproduced with useful accuracy. The modified MNDO seems to give more reliable values of hydrogen bond energies and barrier heights of proton transfers than 4-31G ab initio model.     

Quantum chemical study of the π-electronic states of the DNA base pairs

The results of calculations of the lowest -electronic states of the DNA base pairs with the SCF MO LCAO method both without taking into account configuration interaction and taking into account all the singly excited configurations are presented. The first excited singlet state and the first triplet state of both pairs in a one-configurational approximation are shown to be the states where the excitation is localized on one of the bases. The calculations in a multi-configurational interaction give qualitatively the same result. The possibility of the UV irradiation-induced mutations, the mechanism of which is caused by the proton tunneling on the hydrogen bonds in DNA, is discussed. The results of calculations are compared with the proton transfer constants in the base pairs.

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Zusammenfassung Die Ergebnisse von Elektronen-Rechnungen für DNA-Basenpaare (ohne CI für den Grundzustand) werden mitgeteilt. Der erste Singulett- und Triplettzustand sind jeweils Zustände mit lokalisierter Anregung. Auch die Wechselwirkung mit mehrfach angeregten Konfigurationen ändert daran nichts. Ferner wird die Möglichkeit von UV-induzierten Mutationen diskutiert und die Resultate der Rechnungen mit Protonentransfer-Konstanten der Basenpaare verglichen.

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Résumé Calculs des plus bas états électroniques des paires de base du DNA avec la méthode SCF MO LCAO sans et avec interaction de configurations avec tous les monoexcités. On montre que le premier état excité singulet et le premier état excité triplet des deux paires dans l'approximation à une configuration sont les états où l'excitation est localisée sur l'une des deux bases. Dans le cas multiconfigurationnel on obtient qualitativement le même résultat. Discussion de la possibilité de mutations induites par irradiation UV, provoquées par transfert tunnel du proton dans les liaisons hydrogène. Les résultats du calcul sont comparés avec les constantes de transfert du proton dans les paires de bases.

     

Hydrogen bonding: A molecular orbital treatment by the EHT and the CNDO/2 methods of methanol and of formic acid

By both the EHT and the CNDO/2 calculations, the linear dimer of methanol is found to be more stable than the cyclic dimer. The hydrogen bonds in the trimer are stronger than those in linear dimers. The proton potential function, charge densities, and overlap populations in the linear dimer of methanol have been obtained. The CNDO/2 calculations show that the cis-form of formic acid is more stable than the trans-form, in agreement with experimental data. The cyclic dimer of formic acid is more stable than the open dimer. The -form of formic acid trimer is more stable than the -form. The proton potential function and the charge densities in the cyclic dimer of formic acid have been obtained. The CNDO/2 method gives more realistic proton potential functions for the dimers of methanol and formic acid. The O ... O stretching force constant in the dimers of methanol and formic acid have been estimated to be 0.13 × 10⁵ dynes/cm and 0.27 × 10⁵ dynes/cm, respectively, in agreement with experimental data.     

Complexation of monosubstituted benzenes and methanes by cyclotetrachromotropylene in aqueous solution: Substituent effects on π-π and Ch-π interactions

The stability constantsK of 11 complexes formed in aqueous solution between several monosubstituted benzenes (C₆H₅X) and methanes (CH₃X) as guests and cyclotetrachromotropylene as host were determined by proton NMR spectroscopy. Variations ofK with the substituent X are attributed to the electronic effect of X and the presence of C–H or aromatic bonds, if any, interacting with the host bonds.     

Dynamic Properties of Local Structures in Aqueous Model Systems

A method for describing structural and dynamic inhomogeneity in aqueous model systems is proposed. The method is based on the construction of ranked distributions of measured quantities (lifetime of hydrogen bonds and local configurations of water molecules). Asymptotic estimates were obtained for the lifetimes of hydrogen bonds and local configurations of hydrogenbonded water molecules selected by a dynamic criterion for hydrogen bonds.     

Carboxamides of Dihydropyridin-2(1 H)-ones

3-Carboxamides and 3-carboxanilides of 6-alkyl and 6-aryldihydropyridin-2(1H)-ones have been prepared via different reaction pathways. All synthesized amides show hydrogen bonds in their NMR spectra. The 4-hydroxy compounds were obtained as a mixture of tautomers. Their configurations were elucidated by NMR experiments.     

An ab initio study of the structures and enthalpies of the hydrogen-bonded complexes of the acids H2O,H2S,HCN, and HCl with the anions OH−, SH−, CN−, and Cl−

Ab initio calculations at second-order Møller-Plesset perturbation theory with the 6-31 + G(d,p) basis set have been performed to determine the equilibrium structures and energies of a series of negative-ion hydrogen-bonded complexes with H₂O, H₂S, HCN, and HCl as proton donors and OH^–, SH^–, CN^–, and Cl^– as proton acceptors. The computed stabilization enthalpies of these complexes are in agreement to within the experimental error of 1 kcal mol^–1 with the gas-phase hydrogen bond enthalpies, except for HOHOH^–, in which case the difference is 1.8 kcal mol^–1. The structures of these complexes exhibit linear hydrogen bonds and directed lone pairs of electrons except for complexes with H₂O as the proton donor, in which cases the hydrogen bonds deviate slightly from linearity. All of the complexes have equilibrium structures in which the hydrogen-bonded proton is nonsymmetrically bound, although the symmetric structures of HOHOH^– and ClHCl^– are only slightly less bound than the equilibrium structures. MP2/6-31 + G(d,p) hydrogen bond energies calculated at optimized MP2/B-31 + G(d,p) and at optimized HF/6-31G(d) geometries are similar. Using HF/6-31G(d) frequencies to evaluate zero-point and thermal vibrational energies does not introduce significant error into the computed hydrogen bond enthalpies of these complexes provided that the hydrogen-bonded proton is definitely nonsymmetrically bound at both Hartree-Fock and MP2.     

The role of intra- and intermolecular hydrogen bonds in the formation of β-cyclodextrin head-to-head and head-to-tail dimers. The results of ab initio and semiempirical quantum-chemical calculations

The conformation of a free -cyclodextrin molecule optimized by the MNDO/PM3 quantum-chemical calculations has C₇ symmetry. The right orientation of the interglucose hydrogen bonds in -cyclodextrin, in which the 2-OH groups act as the proton donors and the O atoms of the nearby 3"-OH groups function as the proton acceptors, is advantageous for thermodynamic reasons. The ring of seven H bonds thus formed stabilizes the symmetrical form of -cyclodextrin. The -cyclodextrin head-to-head dimer has D ₇ symmetry and consists of molecules whose 2-OH groups partcipate as proton donors in the formation of fourteen complementary intermolecular hydrogen bonds. The energy of H bonds in the -cyclodextrin monomer and dimer was estimated to be 1.0--1.4 kcal mol^–1. Of the two possible -cyclodextrin dimers, the head-to-tail dimer is more thermodynamically stable. The thermodynamic preference of the right orientation of the inter-glucose H bonds in -cyclodextrin was confirmed by the MP2/6-31G(d,p)//6-31G(d,p) ab initio calculations for maltose (-glucodioside). The maltose molecule with inter-glucose H bonds of the type 2-OHO(3")-H is more stable than the structure with the H-(2)OH-O(3") orientation of H bonds with a difference of 2.7 kcal mol^–1. According to the MNDO/PM3 method, the maltose structure with the right H bond orientation is more stable by 3.1 kcal mol^–1.     

Carboxamides of Dihydropyridin-2(1<Emphasis Type="Italic">H</Emphasis>)-ones

Summary. 3-Carboxamides and 3-carboxanilides of 6-alkyl and 6-aryldihydropyridin-2(1H)-ones have been prepared via different reaction pathways. All synthesized amides show hydrogen bonds in their NMR spectra. The 4-hydroxy compounds were obtained as a mixture of tautomers. Their configurations were elucidated by NMR experiments.Received December 16, 2002; accepted December 20, 2002 Published online June 2, 2003     

Effect of intermolecular O-H ⋯ O hydrogen bonding on the molecular structure of phenol: An ab initio molecular orbital study

The effect of intermolecular O-H O hydrogen bonding on the molecular structure of phenol has been studied by SCF ab initio MO calculations at the HF/6-31G * level. The systems investigated are eight phenol-water complexes and the dimer and trimer of phenol. Optimized geometries show that hydrogen bond formation causes a consistent pattern of changes in the structure of the molecule. When phenol acts as a proton donor, the expected increase ofr (O-H) is accompanied by a slight decrease ofr(C-O) and of the internal ring angles at theipso andpara positions, and by an increase ofr(C _ipso © _ortho ). These changes suggest that the relative contribution of polar canonical forms to the electronic structure of the molecule increases upon hydrogen bond formation, since this enhances the strength of the interaction. The opposite changes occur when phenol acts as a proton acceptor, except forr(O-H), which is the same as in the free molecule. If phenol acts as a proton donorand as a proton acceptor, the two hydrogen bonds become stronger due to a synergic effect. In this case, however, the structural deformation of the molecule is less pronounced than in the previous cases, due to the opposite effect of the two hydrogen bonds. The available experimental evidence on gas-crystal structural differences for phenol is critically reviewed, also in the light of the present results on gas-phase complexes.     

Ba2In2O4(OH)2: Proton sites, disorder and vibrational properties

The structure of Ba₂In₂O₄(OH)₂ is analysed by the explicit full optimization of a large number of possible proton arrangements using periodic density functional theory. It is shown that the experimental assignments in which protons appear to be located at high symmetry positions with unphysical bond lengths do not correspond to minima on the potential energy hypersurface. The apparent sites are averages of a number of possible proton locations involving a set of possible local structural environments in which the internuclear separations are more realistic. Such problems with structural refinements are common where profile refinement programs place the atoms at the average position due to dynamic and/or static disorder. Thus while the calculations support a previous neutron diffraction analysis of the structure in that the average structure contains two different proton sites, they also reveal substantial information about the local environments of the protons. In all optimizations, the protons moved from the average positions suggested in the neutron diffraction study with calculated O–H and OHO distances consistent with those observed in other oxides. The energies of different proton distributions vary significantly so the protons are not randomly distributed. We also present an analysis of the vibrational properties of the O–H bonds. Since the strength of the hydrogen bonds is closely related to the local structural environments of the protons, a range of vibrational frequencies is obtained providing a prediction of the vibrational spectra. In O–HO linkages, O–H stretching modes soften with increasing HO hydrogen bond strength, while the in-plane and out-of-plane bending or libration modes stiffen. Together, our results show how modern theoretical methods can provide a clearer understanding of the structure and dynamics of a complex inorganic material.     

Microscopic Models for Proton Transfer in Water and Strongly Hydrogen-Bonded Complexes with a Single-Well Proton Potential

A new mechanism and formalism for proton transfer in donor-acceptor complexes with long hydrogen bonds introduced recently [1], is applied to a proton transfer in liquid water. Structural diffusion of hydroxonium ions is regarded as totally adiabatic process, with synchronous hindered translation of two closest water molecules to and from the reaction complex as crucial steps. The water molecules induce a gated shift of the proton from the donor to the acceptor in the double-well potential with simultaneous breaking/formation of hydrogen bonds between these molecules and the proton donor and acceptor. The short-range and long-range proton transfer as structural diffusion of Zundel complexes is also considered. The theoretical formalism is illustrated with the use of Morse, exponential, and harmonic molecular potentials. This approach is extended to proton transfer in strongly hydrogen-bonded donor-acceptor complexes. In contrast to the above model [1], the short hydrogen bond between the donor and acceptor moieties, however, completely erodes the barrier along the proton transfer mode. This introduces some physical pattern differences from proton transfer reactions in truly double-well potentials with a finite proton transfer barrier at the transition configuration with respect to the environmental nuclear coordinates. The differences apply particularly to the origin of the kinetic isotope effect. We discuss explicitly details of the excess proton conductivity in aqueous solution, but the concepts and formalism apply broadly to acid-base reactions, proton conduction channels, and other strongly hydrogen-bonded O- and N-proton donor-acceptor systems.     

Supramolecular amide and thioamide synthons in hydrogen bonding patterns of N-aryl-furamides and N-aryl-thiofuramides

The supramolecular synthon of amide group in the primary and secondary amides is well recognized to be infinite chains of the C(4) type formed by the intermolecular hydrogen bond of the type N–HO=C. On the other hand, there is a lack of structural data for the thioamides. Three compounds belonging to the class of N-aryl-fura-mides (N-(4-bromophenyl)-5-bromo-2-furancarboxamide, N-(4-chlorophenyl)-5-bromo-2-furancarboxamide) and to the class of N-aryl-thiofuramide (N-(4-methoxyphenyl)-2-furanthiocarboxamide) are prepared and characterized by the NMR spectroscopy in solution; molecular and crystal structures in the solid state have been determined by X-ray single crystal diffractometry and the structures in the gas phase by DFT and AM1 calculations. The investigation is carried out in order to establish supramolecular amide and thioamide synthons of hydrogen bonding patterns in these crystal structures. The geometry of the N–HO=C and the N–HS=C type of hydrogen bonds are compared due to the possibility of the N–H amide group to form intramolecular hydrogen bond with the furan oxygen atom, thus, commonly, leading to the three-center hydrogen bond pattern. The competition between the S=C proton acceptor of thioamides and the other proton acceptors (such as methoxy group) for the amide N–H proton donor group has been investigated. In that context, the above-mentioned compounds are correlated with the others of this class, structurally determined, so far.     

Nature of the broadening,the form of the spectral bands,and thev AH − RA...B correlation in liquids with hydrogen bonds

Conclusion The experimentally established correlation for crystals between the frequency of the OH (or OD) stretching vibrations and the interatomic separation R_O·O can also be used for each hydrogen bond in the liquid phase taken in isolation, if the equilibrium length R_e is used for R_O·O. The empirical correlation between the low-frequency shift of the band and its broadening results from the exponentialv _OH(R_e) relationship, while the distribution function of the frequencies in the vibrational spectrum P() corresponds to the distribution of the energies of the hydrogen bonds P(E). When the deflection of the equilibrium configurations of the H bonds can be neglected, P() is expressed unambiguously through the distribution of the lengths of the hydrogen bonds P(R_e) and makes it possible to determine their variance. Otherwise (the continuous network of strongly deflected H bonds in liquid water) the complex form of the spectral band and its temperature dependence can be described quantitatively by a simple equation of the Boltzmann type, in which the exponential part is the energy of the hydrogen bond (the depth of the potential well) making a contribution to the spectrum at the investigated frequency. The agreement between calculation and experiment reveals an important fact, i.e., the equality of the energies for the various configurations of the hydrogen bond producing one and the same frequencyv _OH.Institute of Chemical Kinetics and Combustion, Siberian Branch, Russian Academy of Sciences. Translated from Zhurnal Strukturnoi Khimii, Vol. 32, No. 6, pp. 72–80, November–December, 1991.     

Reactions of proton transfer and exchange with the participation of OH,SH, and CH acids and amines

A comparison of proton exchange reactions between OH, SH, and CH acids and the NH groups of trialkylammonium ions showed that regardless of the nature of the acid XH, the mechanism of exchange includes transfer of a proton in the ion pair N-H⁺ ... X^– as the slow step. At the fast steps of proton exchange XH- N⁺H, i.e., molecular exchange with breaking of a hydrogen bond X-H ... N and transfer of a proton along these bonds, differences appear in the properties of XH acids. In the sequence from OH to SH and CH acids, the hydrogen bonds X-H ... N are weakened. As a result of this, in the same sequence the kinetic acidity (k₂) decreases but the rate of molecular exchange (k_H) increases. The ratio between the values of k₂ and k_H is inverted when the strong bonds O-H ... N (k₂/k_H 1) are replaced by weak bonds C-H ... N (k₂/k_H 1). It was also established that the kinetic stability of the anions increases as the oxygen atoms are replaced by sulfur in the series RCOO^– < RCOS^– < R₂PSS^– as a result of the more effective delocalization of the negative charge on the diffuse orbitals of sulfur.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 23, No. 4, pp. 471–475, July–August 1987.     

Complexing ability of uranyl(VI) with some aroylhydrazone derivatives of 7-carboxaldehyde-8-hydroxyquinoline

Summary New dioxouranium(VI) complexes with the dibasic tridentate 7-carboxaldehyde-8-hydroxyquinoline-aroylhydrazone derivatives (1), have synthesized and is bonded to the axial oxygen of the uranyl moiety and is easily substituted by dimethylsulphoxide. The ligands contain intramolecular hydrogen bonds, one of which is from the enolic form of the hydrazone residue. The¹H n.m.r. spectra show that the position of the hydrogenbonded proton of the hydrazone moiety is substituent dependent. The F(U–O) (mydn/) and the bond length r(U–O) () of the UO bond are calculated from the i.r. data and related to the electronic properties of the substituents.