J. Appl. Cryst.の発刊に際して

The hydrogen bond is the most important of all directional intermolecular interactions. It is operative in determining molecular conformation, molecular aggregation, and the function of a vast number of chemical systems ranging from inorganic to biological. Research into hydrogen bonds experienced a stagnant period in the 1980s, but re-opened around 1990, and has been in rapid development since then. In terms of modern concepts, the hydrogen bond is understood as a very broad phenomenon, and it is accepted that there are open borders to other effects. There are dozens of different types of X-H.A hydrogen bonds that occur commonly in the condensed phases, and in addition there are innumerable less common ones. Dissociation energies span more than two orders of magnitude (about 0.2-40 kcal mol(-1)). Within this range, the nature of the interaction is not constant, but its electrostatic, covalent, and dispersion contributions vary in their relative weights. The hydrogen bond has broad transition regions that merge continuously with the covalent bond, the van der Waals interaction, the ionic interaction, and also the cation-pi interaction. All hydrogen bonds can be considered as incipient proton transfer reactions, and for strong hydrogen bonds, this reaction can be in a very advanced state. In this review, a coherent survey is given on all these matters.

The hydrogen bond in the solid state.

Developing highly efficient catalysts for the oxygen reduction reaction (ORR) is key to the fabrication of commercially viable fuel cell devices and metal–air batteries for future energy applications. Herein, we review the most recent advances in the development of Pt-based and Pt-free materials in the field of fuel cell ORR catalysis. This review covers catalyst material selection, design, synthesis, and characterization, as well as the theoretical understanding of the catalysis process and mechanisms. The integration of these catalysts into fuel cell operations and the resulting performance/durability are also discussed. Finally, we provide insights into the remaining challenges and directions for future perspectives and research.

Recent advancements in Pt and Pt-free catalysts for oxygen reduction reaction

The C-H‚‚‚O hydrogen bond is so well-established in structural chemistry that it seems difficult now to believe that when Sutor proposed the existence of this type of hydrogen bond in the early 1960s,1,2 her suggestion was drowned in scepticism, if not outright hostility.3 It was only two decades later, with Taylor and Kennard’s paper, that the subject was properly revived.4 Shortly thereafter, an Account appeared from this laboratory describing the role of the C-H‚‚‚O interaction in crystal engineering.5 Subsequently, one felt confident enough to term these erstwhile “interactions” hydrogen bonds, in a second Account.6 A recent invitation to contribute another Account and the many recent efforts in this direction by my students and postdoctorals have led to the present paper. It is clearly no longer necessary to justify the relevance of C-H‚‚‚O hydrogen bonds, so widely invoked are they in small-molecule and biological crystallography. The presence of O-atoms in a large majority of organic molecules means that this hydrogen bond is widespread, even if not identified in many cases. However, other questions concerning these weak hydrogen bonds could be posed: (1) What is their upper distance limit? (2) Are very short, bent bonds significant? (3) Why do C-H‚‚‚O bonds sometimes disturb the strong O-H‚‚‚O and N-H‚‚‚O network? Alternatively, why do hydrogen bond donors and acceptors not always pair in descending order of strength? (4) How important is cooperativity for weak hydrogen bonds? (5) Are C-H‚‚‚O hydrogen bonds responsible for crystal packing, or are they the forced consequences of packing? (6) Are weak hydrogen bonds robust enough for supramolecular synthesis and crystal engineering? (7) Does the C-H‚‚‚O hydrogen bond have any biological significance? These difficult questions cannot be answered fully. This Account attempts to address some of them, but better answers can only follow from further work.

The C-h···o hydrogen bond: structural implications and supramolecular design.

: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

Evidence for resonance-assisted hydrogen bonding from crystal-structure correlations on the enol form of the .beta.-diketone fragment

All cases of strong (2.50≤d(O--O)≤2.65 A) and very strong (d(O--O) <2.50 A) O-H---O hydrogen bonds whose geometries are known from accurate neutron or X-ray diffraction studies are reviewed and classified in chemical classes belonging to three fundamental types: (A) -O-H---O-, or negative charge assisted hydrogen bonding, (-)CAHB; (B)=O---H + --O=, or positive charge assisted hydrogen bonding, (+)CAHB; and (C) -O-H---O=, where the two oxygens are interconnected by a system of π-conjugated double bonds, or resonance-assisted hydrogen bonding, RAHB

Evidence for resonance-assisted hydrogen bonding. 4. Covalent nature of the strong homonuclear hydrogen bond. Study of the O-H--O system by crystal structure correlation methods

Evidence for resonance-assisted hydrogen bonding. 2. Intercorrelation between crystal structure and spectroscopic parameters in eight intramolecularly hydrogen bonded 1,3-diaryl-1,3-propanedione enols

β-Diketone enols are known to form intramolecular ···OC−CC−OH··· resonance-assisted hydrogen bonds (RAHBs) with O···O distances as short as 2.39−2.44 A. However, even the most accurate diffraction studies have not been able to assess with certainty whether these very strong hydrogen bonds (H-bonds) are to be described as proton-centered O···H···O bonds in a single-well (SW) potential or as the dynamic or static mixing of two O−H···O ⇌ O···H−O tautomers in a double-well (DW) one. This contribution reexamines the problem and shows that diffraction methods are fairly able to assess the SW or DW nature of the H-bond formed and, in the second case, its dynamic or static nature, provided a Bayesian approach is used which associates a number of experimental techniques (X-ray crystallography at variable temperature, difference Fourier maps, least-squares refinement of proton populations, Hirshfeld's rigid-bond test) with a reasonable prior, that is the full set of possible proton-transfer (PT) pathways for the O−...

Covalent versus Electrostatic Nature of the Strong Hydrogen Bond: Discrimination among Single, Double, and Asymmetric Single-Well Hydrogen Bonds by Variable-Temperature X-ray Crystallographic Methods in β-Diketone Enol RAHB Systems

The thermodynamic parameters (ΔG o , ΔH o , ΔS o ) of the drug-receptor binding equilibrium derived from equilibrium constant measurements at different temperatures and van't Hoff plots are reviewed. The analysis involves 186 independent experiments performed on 136 ligands binding to 10 biological receptors and, for comparison, to DNA and to two different enzymes. ΔH o and ΔS o values correlate according to the regression equation ΔH o (kcal mol -1 )=-9.5+27845 o (kcal K -1 mol -1 ) with a correlation coefficient of 0.981. The correlating equation is of the form ΔH o =βΔS o and is expected for a case of enthalpy-entropy compensation with a compensation temperature β=278 K

Valeria Ferretti

Papers

Evidence for resonance-assisted hydrogen bonding from crystal-structure correlations on the enol form of the .beta.-diketone fragment

Evidence for resonance-assisted hydrogen bonding. 4. Covalent nature of the strong homonuclear hydrogen bond. Study of the O-H--O system by crystal structure correlation methods

Evidence for resonance-assisted hydrogen bonding. 2. Intercorrelation between crystal structure and spectroscopic parameters in eight intramolecularly hydrogen bonded 1,3-diaryl-1,3-propanedione enols

Covalent versus Electrostatic Nature of the Strong Hydrogen Bond: Discrimination among Single, Double, and Asymmetric Single-Well Hydrogen Bonds by Variable-Temperature X-ray Crystallographic Methods in β-Diketone Enol RAHB Systems

Enthalpy-entropy compensation in drug-receptor binding