The hydroperoxy radical as a hydrogen bond acceptor. HOO-HCl complexes--Ab initio study

Protonacceptor properties of the HOO radical were investigated previously by means of ab initio as well as topological Atoms in Molecules (AIM) and Electron Localization Function (ELF) methods. It was pointed out that in the radical there are three nonequivalent positions most susceptible to protonation, and on this basis three structures of possible hydrogen bonded complexes were proposed. Results reported in the present article concern all possible 1:1 complexes formed by HCl and HOO molecules, and fully confirm suppositions given on the basis of the above-mentioned investigations. There are three various complexes referring to the local minima, and the transition structure predicted by topological methods has been found as well. The cyclic structure appeared to be the most stable one, which confirms conclusions given in the experimental article. Apart from structure optimization, harmonic as well as anharmonic spectra of the complexes have been simulated. Anharmonicity of H-Cl stretching vibration was of special interest, as the frequency of this vibration characterizes the Cl-H...O hydrogen bond in these complexes. To obtain values of these frequencies the one-dimensional Hamiltonian has been diagonalized numerically. The potential for this Hamiltonian has been taken from a set of single-point scanning of the part of the Potential Energy Surface (PES) connected with this vibration. The potential calculated on the MP2 level leads to the result close to the experimental value, whereas the B3LYP method is inappropriate for the purpose of PES investigation of these complexes.     

Highly activated Michael acceptor by an intramolecular hydrogen bond as a fluorescence turn-on probe for cyanide

An activated Michael acceptor type of probe by an intramolecular hydrogen bond has shown a selective fluorescence turn-on response to cyanide through a conjugated addition of the nucleophilic anion to the enone probe with a 1300-fold increase in its fluorescence intensity.     

The hydrogen bond donor and acceptor ability of thioformic acid

The molecular orbital (MP2) and density functional theory calculations (B3LYP) using 6-311++G(2d,2p) basis set have been carried out on adducts of the cis and trans conformers of two tautomeric forms of thioformic acid (TFA) with water. Thirteen adducts of TFA with water have been optimized. Similar calculations have been carried out on adducts of formic acid with water and seven energy minima have been obtained for them. Our findings indicate that the specific interactions with water play an important role in the conformational stability as thiol form of TFA is the most stable form in gas phase, while it is the thione form which is the most stable in 1:1 adduct. The H-bond acceptor ability of S and O at the thiocarbonyl and carbonyl positions in TFA and FA, respectively, has been compared and observed to be only slightly lower in the former. However, the H-bond donor ability of S–H has been observed to be nearly half to that of O–H. The contributors to stabilization energies of adducts are explored by analyzing geometrical variations, atomic charges, and electron delocalizations.     

Basicity of uranyl oxo ligands upon coordination of alkoxides

Uranium(VI) alkoxide complexes are prepared via metathesis reactions of [UO2Cl2(THF)2]2 with potassium alkoxides in nonaqueous media. The dark red compound U[OCH2C(CH3)3]6, 1, results from redistributive exchange of oxo and neopentoxide ligands between more than one uranium species. Single-crystal X-ray diffraction analysis of 1 reveals a monomer in which the uranium is coordinated in a pseudooctahedral fashion by six neopentoxide ligands. Imposition of steric congestion at the metal center prevents oxo-alkoxide ligand exchange in the reactions using more sterically demanding alkoxides. Simple metathesis between uranyl chloride and alkoxide ligands occurs in the synthesis of golden yellow-orange UO2(OCHPh2)2(THF)2, 2, and yellow UO2[OCH(tBu)Ph]2(THF)2, 3. Single-crystal X-ray diffraction analysis of 2 reveals a monomer in which the uranium is coordinated in a pseudooctahedral fashion by two apical oxo ligands, two diphenylmethoxide ligands occupying trans positions, and two tetrahydrofuran ligands. Coordination of diisopropylmethoxide allows for synthesis of a more complex binary alkoxide system. Single-crystal X-ray diffraction analysis of watermelon red [UO2(OCH(iPr)2)2]4, 4, reveals a tetramer in which each uranium is coordinated in a pseudooctahedral fashion by two apical oxo ligands, one terminal alkoxide, two bridging alkoxide ligands, and one bridging oxo ligand from a neighboring uranyl group. These compounds are characterized by elemental analysis, 1H NMR, infrared spectroscopy, and, for 1, 2, and 4, single-crystal X-ray diffraction analysis. Luminescence spectroscopy is employed to evaluate the extent of aggregation of compounds 2-4 in various solvents. Vibrational spectroscopic measurements of 2-4 imply that, in contrast to the case of uranyl complexes prepared in aqueous environments, coordination of relatively strongly donating alkoxide ligands allows for enhancement of electron density on the uranyl groups such that the uranyl U=O bonds are weakened. Crystal data are as follows. 1: monoclinic space group C2/m, a = 10.6192(8) A, b = 18.36(1) A, c = 10.6151(8) A, beta = 109.637(1) degrees, V = 1949.1(3) A3, Z = 2, dcalc = 1.297 g cm-3. Refinement of 2065 reflections gave R1 = 0.045. 2: monoclinic space group P2(1)/c, a = 6.1796(4) A, b = 15.669(1) A, c = 16.169(1) A, beta = 95.380(1) degrees, V = 1558.7(2) A3, Z = 2, dcalc = 1.664 g cm-3. Refinement of 3048 reflections gave R1 = 0.036. 4: tetragonal space group I4, a = 17.8570(6) A, b = 17.8570(6) A, c = 11.4489(6) A, V = 3650.7(3) A3, Z = 2, dcalc = 1.821 g cm-3. Refinement of 1981 reflections gave R1 = 0.020.     

<Emphasis Type="Italic">O</Emphasis>-vinylacetoxime as a hydrogen bond acceptor

According to quantum chemical calculations (B3LYP/6-311G*) and IR spectroscopy the basicity of oxygen atom of O-vinylacetoxime is substantially lower than that of O-ethylacetoxime and is comparable to the basicity of phenyl vinyl and diphenyl ethers. In CCl₄ solution, O-vinylacetoxime gives H-complexes wit methanol by formation of N···HO bonds. With phenol and trifluoroacetic acid under these conditions it enters in the reaction of electrophilic addition. O-Ethylacetoxime in inert media forms with methanol and phenol two types of H-complexes with the N···HO or O···HO bonds.     

Theoretical study of the Si–H group as potential hydrogen bond donor

The ability of the Si–H group as hydrogen bond (HB) donor has been studied theoretically. Most of the selected molecules include the Si–H group in a polar environment that could produce an electron deficiency on the hydrogen atom. In addition, analogous derivatives where the silicon atom has been replaced by a carbon atom have been considered. In all cases, ammonia has been used as HB acceptor. The calculations have been carried out at the MP2/6‐311++G** computational level. The electron density of the complexes has been characterized within the atoms in molecules (AIM) framework. A search in the Cambridge Structural Database (CSD) has been carried out to verify the existence of this kind of interactions in solid phase. The results of the theoretical study on these HB complexes between ammonia and the silicon derivatives provides long HB distances (2.4 to 3.2 Å) and small interaction energies (?2.4 to ?0.2 kcal/mol). In all cases, the HBs of the corresponding carbon analogs show shorter interaction distances corresponding to stronger complexes. The CSD search provides a small number of short interactions between Si and other heavy atoms in agreement with the small stabilizing energy of the Si–H?N HB and the lack of SiH bond in polar environment within the database. © 2002 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Isocyanides as hydrogen bond acceptors

A theoretical study of the capability of hydrogen isocyanide (HNC) as a hydrogen bond acceptor has been carried out. The geometry, interaction energy and electronic properties of the corresponding complexes with HF, HCl, HBr, H₂O, NH₃, HCCH, HCN and HNC itself indicate that it is able to form strong hydrogen bonds. A search in the Cambridge Structural Database has shown the presence of isocyanides involved in hydrogen bonds in solid phase. Finally, the comparison of the properties of HNC with its isomer, hydrogen cyanide, shows strong similarities between both compounds. Received: 13 October 1997 / Accepted: 11 December 1997     

Communication: Frequency shifts of an intramolecular hydrogen bond as a measure of intermolecular hydrogen bond strengths

Infrared-ultraviolet double resonance spectroscopy has been applied to study the infrared spectra of the supersonically cooled gas phase complexes of formic acid, acetic acid, propionic acid, formamide, and water with 9-hydroxy-9-fluorenecarboxylic acid (9HFCA), an analog of glycolic acid. In these complexes each binding partner to 9HFCA can function as both proton donor and acceptor. Relative to its frequency in free 9HFCA, the 9-hydroxy (9OH) stretch is blue shifted in complexes with formic, acetic, and propionic acids, but is red shifted in the complexes with formamide and water. Density functional calculations on complexes of 9HFCA to a variety of H bonding partners with differing proton donor and acceptor abilities reveal that the quantitative frequency shift of the 9OH can be attributed to the balance struck between two competing intermolecular H bonds. More extensive calculations on complexes of glycolic acid show excellent consistency with the experimental frequency shifts.     

Perturbation calculations on the hydrogen bond in the water dimer for different proton acceptor orientations

Using perturbation theory of intermolecular forces (including exchange effects) hydrogen-bond energies are calculated for a simple model representing the water dimer. Several orientations of the proton acceptor with respect to the O...O axis are considered and it is found that the optimum orientation depends sensitively on the oxygen lone-pair hybridization assumed. The main orientation-dependent energy term is found to be the classical electrostatic energy between the unperturbed molecules. The relation of the present results to the structures of the ices I and Ic, and to the results of recent SCF studies are discussed briefly.

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Zusammenfassung Die Energien von Wasserstoffbindungen werden für ein einfaches Modell des dimeren Wassers mit Hilfe der Störungstheorie für intermolekulare Kräfte (mit Austauscheffekten) berechnet. Verschiedene Orientierungen des Protonen-Akzeptors bezüglich der O ... O-Achse werden untersucht; man findet, daß die optimale Orientierung empfindlich von der angenommen Hybridisierung des einsamen Elektronenpaars des Sauerstoffs abhängt. Der hauptsächliche, von der Orientierung abhängige Energieterm ist die klassische elektrostatische Energie zwischen den beiden ungestörten Molekülen. Die Beziehung der vorliegenden Ergebnisse zu den Strukturen von Eis I und Ic sowie zu den Ergebnissen kürzlicher SCF-Rechnungen wird kurz diskutiert.

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For some recent CNDO/2 calculations on (H₂O)₂ which concentrate mainly on rotations of the donor see Ref. [5].     

Intramolecular hydrogen bond and hyperfine splitting at the hydroxyl group

The ESR spectra of 1, 2-, 1, 4-, 1, 5-, and 1, 8-dihydroxyanthrasemiquinone were measured. A hyperfine splitting at the protons of the hydroxy groups participating in the formation of stable intramolecular hydrogen bonds with the carbonyl oxygen atoms is observed for all radicals. The spin density was calculated by the MO LCAO method in the Hückel approximation, taking into consideration the configurational interaction according to McLachlan. Two models were used: the Pullman two-center models, one with and one without consideration of hydrogen bonds. The existence of the splittings indicates the participation of H-bonds in the transfer of the conjugation. A transfer of the spin density to the proton of the H-bond takes place principally from a proton-acceptor atom. It has been shown that the formation of intramolecular hydrogen bonds has a significant effect on the distribution of the spin density in the radical. Reasons are given for the advantage of the qualitative usefulness of the Pullman model in the calculation of H-bonds.     

Defining the bond energy of a strong hydrogen bond

Based on a recent definition of hydrogen-bond energy the hydrogen bond in [HCOO…H…F]^? is weaker than that in [F…H…F]^?, although the former still ranks as a very strong hydrogen bond.     

Nitrobenzene as hydrogen acceptor in Pd/C-catalyzed hydrogen transfer reaction

Nitrobenzene was found to work as an efficient hydrogen acceptor in the oxidation of allylic alcohols to give the corresponding enones in high yields.     

Tuning the hydrogen bond donor/acceptor isomerism in jet-cooled mixed dimers of aliphatic alcohols

Hydrogen bonded complexes between two different aliphatic alcohols exhibit donor/acceptor isomerism. In a supersonic jet expansion, the less stable isomer can isomerize to the more stable isomer if the energy difference is sufficiently large and the barrier sufficiently low. We show by FTIR jet spectroscopy that this is progressively the case for methanol/methanol-d1, methanol/ethanol, and methanol/tert-butyl alcohol, until no metastable donor/acceptor isomer persists in the expansion. Collisional relaxation experiments, 18O labeling and quantum chemical calculations are used to assign the spectra. Differences between energetical and spectroscopic acceptor strengths are discussed.     

Selective oxo functionalization of the uranyl ion with 3d metal cations

The linear uranyl dication [UO2]2+ can be bound in one of two coordination sites in the ditopic Pacman-shaped pyrrolic macrocyle H4L. Incorporation of Mn2+, Fe2+, or Co2+ cations in the second donor compartment affords the first uranyl complexes with a transition-metal-functionalized oxo group.     

Ability of As=S group to form the hydrogen bond

Conclusions The As=S group is capable of forming a hydrogen bond with phenol by functioning as the proton acceptor, in which connection the proton-acceptor capacity depends on the environment of the arsenic atom. The stability of the formed hydrogen bonds is somewhat higher than for the P=S group, but it is considerably lower than for the As=O group.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2778–2779, December, 1973.     

The effect of hydrogen bond acceptor properties of ionic liquids on their cellulose solubility

It is nowadays well-known that ionic liquids can dissolve cellulose. However, little systematic data has been published that shed light onto the influence of the ionic liquid structure on the dissolution of cellulose. We have conducted 1H NMR spectroscopy of ethanol in a large number of ionic liquids, and found an excellent correlation of the data obtained with the hydrogen acceptor properties (β-values). With this tool in hand, it is possible to distinguish between cellulose-dissolving and non-dissolving ionic liquids. A modulating effect of both, the anion of the non-dissolving ionic liquid and its cation was found in solubility studies with binary ionic liquid mixtures. The study was extended to other non-dissolving liquids, namely water and dimethylsulfoxide, and the effect of the cation was also investigated.     

Low-barrier hydrogen bond between phosphate and the amide group in phosphopeptide

As the conversion between the monoionic (1) and diionic (2) form of the phosphate occurs, the phosphorylated peptides or proteins can not only cause the formation of a hydrogen bond between the phosphate group and the amide group but also change the strength of the hydrogen bond to form low-barrier hydrogen bonds (LBHBs). This reversible protonation of the phosphate group, which changes both the electrostatic properties of the phosphate group and the strength of the hydrogen bond, provides a possible mechanism in regulating protein function.     

Transfer of electron density as a result of hydrogen bond formation

It has been found that the amount of charge transfer between donor and acceptor molecules in four sets of hydrogen‐bonded complexes may be adequately described as an exponential function of the equilibrium distance between the hydrogen atom and the nearest atom of the acceptor molecule. The exponential factors of the transfer are of the same order but somewhat larger than the factors found otherwise in the investigations of dynamic electron transfer. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009