A novel definition for the hydrogen bond is recommended here. It takes into account the theoretical and experimental knowledge acquired over the past century. This definition insists on some evidence. Six criteria are listed that could be used as evidence for the presence of a hydrogen bond.

Ab initio calculations are used to analyze the CH‚‚‚O interaction between F n H 3-n CH as proton donor and H 2 O, CH 3 OH, and H 2 CO as acceptor. The interaction is quite weak with CH 4 as donor but is enhanced by 1 kcal/mol with each F added to the donor. The CH‚‚‚O interaction behaves very much like a conventional OH‚‚‚O H-bond in most respects, including shifts in electron density that accompany the formation of the bond and the magnitudes of the various components of the interaction energy. The two sorts of H-bonds also gravitate toward a similar equilibrium geometry and are comparably sensitive to deformations from that structure. In a quantitative sense, while both CH‚‚‚O and OH‚‚‚O prefer a linear configuration, the former is somewhat more easily bent and is less sensitive to stretches from its equilibrium H-bond length. Whereas the OH bond has been shown to stretch and undergo a red shift in its vibrational frequency upon formation of a H-bond, the CH bond of the molecules studied here follows the opposite trend, a contraction and a blue shift. Analysis demonstrates that this opposite pattern is not due to any fundamental distinction between the two interactions, since the same sets of forces are acting on both. It is concluded that the CH‚‚‚O interaction can, indeed, be categorized as a true H-bond.

Abstract:The term "hydrogen bond" has been used in the literature for nearly a century now. While its importance has been realized by physicists, chemists, biologists, and material scientists, there has been a continual debate about what this term means. This debate has intensified following some important experimental results, especially in the last decade, which questioned the basis of the traditional view on hydrogen bonding. Most important among them are the direct experimental evidence for a partial covalent nature and the observation of a blue-shift in stretching frequency following X-HؒؒؒY hydrogen bond formation (XH being the hydrogen bond donor and Y being the hydrogen bond acceptor). Considering the recent experimental and theoretical advances, we have proposed a new definition of the hydrogen bond, which emphasizes the need for evidence. A list of criteria has been provided, and these can be used as evidence for the hydrogen bond formation. This list is followed by some characteristics that are observed in typical hydrogen-bonding environments.

Electronic structure and bonding in metal phthalocyanines (MetalϭFe, Co, Ni, Cu, Zn, Mg) is investigated in detail using a density functional method. The metal atoms are strongly bound to the phthalocyanine ring in each case, by as much as 10 eV. The calculated orbital energy levels and relative total energies of these D 4h structures indicate that Fe and Co phthalocyanines have 3 A 2g and 2 E g ground states, respectively, but that these states are changed upon interaction with strong-field axial ligands. The valence electronic structures of Fe and Co phthalocyanines differ significantly from those of the others. The HOMOs in Fe, Co, and Cu phthalocyanine are metal 3d-like, whereas in Ni and Zn phthalocyanines, the HOMO is localized on the phthalocyanine ring. The first ionization removes an electron from the phthalocyanine a 1u orbital in all cases, with very little sensitivity of the ionization energy to the identity of the metal. Whereas the first reduction in Fe and Co phthalocyanine occurs at the metal, it is the phthalocyanine that is reduced upon addition of an electron to the other systems. Fe, Ni, and Cu phthalocyanines have smaller HOMO-LUMO separations than do Zn and Co phthalocyanine. There is very little variation in atomic charges within the phthalocyanine from one metal to the next.

Among a wide range of noncovalent interactions, hydrogen (H) bonds are well known for their specific roles in various chemical and biological phenomena. When describing conventional hydrogen bonding, researchers use the notation AH···D (where A refers to the electron acceptor and D to the donor). However, the AH molecule engaged in a AH···D H-bond can also be pivoted around by roughly 180°, resulting in a HA···D arrangement. Even without the H atom in a bridging position, this arrangement can be attractive, as explained in this Account. The electron density donated by D transfers into a AH σ* antibonding orbital in either case: the lobe of the σ* orbital near the H atom in the H-bonding AH···D geometry, or the lobe proximate to the A atom in the HA···D case. A favorable electrostatic interaction energy between the two molecules supplements this charge transfer. When A belongs to the pnictide family of elements, which include phosphorus, arsenic, antimony, and bismuth, this type of interaction is called a pnicogen bond. This bonding interaction is somewhat analogous to the chalcogen and halogen bonds that arise when A is an element in group 16 or 17, respectively, of the periodic table. Electronegative substitutions, such as a F for a H atom opposite the electron donor atom, strengthen the pnicogen bond. For example, the binding energy in FH(2)P···NH(3) greatly exceeds that of the paradigmatic H-bonding water dimer. Surprisingly, di- or tri-halogenation does not produce any additional stabilization, in marked contrast to H-bonds. Chalcogen and halogen bonds show similar strength to the pnicogen bond for a given electron-withdrawing substituent. This insensitivity to the electron-acceptor atom distinguishes these interactions from H-bonds, in which energy depends strongly upon the identity of the proton-donor atom. As with H-bonds, pnicogen bonds can extract electron density from the lone pairs of atoms on the partner molecule, such as N, O, and S. The π systems of carbon chains can donate electron density in pnicogen bonds. Indeed, the strength of A···π pnicogen bonds exceeds that of H-bonds even when using strong proton donors such as water with the same π system. H-bonds typically have a high propensity for a linear AH···D arrangement, but pnicogen bonds show an even greater degree of anisotropy. Distortions of pnicogen bonds away from their preferred geometry cause a more rapid loss of stability than in H-bonds. Although often observed in dimers in the gas phase, pnicogen bonds also serve as the glue in larger aggregates, and researchers have found them in a number of diffraction studies of crystals.