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.

Crystal Engineering and Organometallic Architecture.

Hydrogen bonding is an important interaction playing a key role in chemical, physical, and biochemical processes. One can mention numerous examples such as the role of hydrogen bonding in enzymatic catalysis, arrangement of molecules in crystals, crystal engineering, proton transfer reactions, and also its important role in life processes. Hence, its nature is often the subject of investigations and polemics. One of the first definitions of hydrogen bonding was formulated by Pauling who stated that “under certain conditions an atom of hydrogen is attracted by rather strong forces to two atoms, instead of only one, so that it may be considered to be acting as a bond between them. This is called the hydrogen bond”. Pauling also pointed out that the hydrogen atom is situated only between the most electronegative atoms and it usually interacts much stronger with one of them. The latter interaction is a typical covalent bond (A-H). The interaction between hydrogen and another electronegative atom is much weaker and mostly electrostatic in nature; it is a nonbonding interaction (H 3 3 3B). This system is often designated as AH 3 3 3B where the B-center (acceptor of proton) should possess at least one lone electron pair; A-H is called the protondonating bond. Pauling stated that sometimes the H 3 3 3B interaction possesses characteristics of the covalent bond. The [FHF] ion is an example where the proton is inserted between two negative fluorine ions, accurately in the middle of the F 3 3 3 F distance. Hence, both H 3 3 3 F interactions are equivalent. This is in line with an early conclusion of Lewis that “an atom of hydrogen may at times be attached to two electron pairs of two different atoms” and with the statement of Latimer and Rodebush that “the hydrogen nucleus held by two octets constitutes a weak bond”. The latter statements correspond to recent studies on proton bound homodimers, that is, systems where the proton is inserted between two closed-shell moieties and where it often interacts equivalently with both of them. Chan and co-workers analyzed recently what factors determine whether the protonbound homodimer has a symmetric or an asymmetric hydrogen bond. In the other study, it is discussed what conditions should be fulfilled for the proton situated accurately in the midpoint of the donor-acceptor distance. The high level calculations up to CCSD(T)/6-311þþ(3df,3pd)//CCSD/6-311þþ(3df,3pd) were performed on the [FHF] ion and systems with O-H 3 3 3O or N-H 3 3 3N hydrogen bonds. The latter study is supported by the experimental X-ray and neutron diffraction data because there are numerous crystal structures with short O-H 3 3 3O hydrogen bonds and the proton situated in the central position or nearly so. Also recently, homogeneous and heterogeneous short and strong hydrogen bonds (SSHBs) as well as the proton bound homodimers were analyzed theoretically at MP2/aug-ccpVDZ þ diffuse(2s,2p) level. Among various topics, the matter was raised if hydrogen bonding is an electrostatic or covalent interaction. The following question arises: what does the covalency of hydrogen bonding mean? The decomposition of the interaction energy is useful to analyze hydrogen bonding and particularly to answer the latter question. One of the first decomposition schemes introduced is one of Morokuma and Kitaura, where the interaction energy is calculated within the Hartree-Fock one-electron approximation and it is decomposed into the following components: the exchange energy, EEX (arising from repulsive forces), and the other components, which might be a result of attractive forces: the polarization energy, EPL, the charge transfer energy, ECT, and the electrostatic energy, EES. If a method is applied where the correlation of electrons is taken into account, then the correlation energy may be included. One of the most important attractive components of the correlation energy is the dispersive energy. Different H-bonded systems were analyzed early by Umeyama and Morokuma who stated that: “The energy components are strongly distance dependent. At a relatively small separation, ES, CT, and PL can all be important attractive components, competing against a large EX repulsion. At larger distances for the same complex the short-range attractions CT and PL are usually unimportant and ES is the only important attraction.” One can see that the “covalency of interaction”may be connected with short H 3 3 3B distances where terms other than the electrostatic attractive one are important.

What is the covalency of hydrogen bonding

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

Stereochemistry of the R1(X:)C(sp2)-N(sp3)R2R3 fragment. Mapping of the cis-trans isomerization path by rotation around the carbon-nitrogen bond from crystallographic structural data.

Preparation et caracterisation spectrometrique des complexes du titre. Etude radiocristallographique du complexe de technetium(V)

Reactivity of 2-(2-hydroxyphenyl)benzothiazoline with the oxotetrahalometalate(V) complexes [MOX4]- (M = technetium, rhenium; X = Cl, Br). Synthesis and characterization of new oxo-M(V) complexes containing 2-(2-hydroxyphenyl)benzothiazole. Crystal structure of tetraphenylarsonium oxotrichloro[2-(2-hydroxyphenyl)benzothiazolato]technetate(V)

Fabrizio Bellucci

Papers

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

Stereochemistry of the R1(X:)C(sp2)-N(sp3)R2R3 fragment. Mapping of the cis-trans isomerization path by rotation around the carbon-nitrogen bond from crystallographic structural data.

Re-determination of the crystal structure of NaBr·Sucrose·2H2O