Halogen bonding refers to the non-covalent interactions of halogen atoms X in some molecules, RX, with negative sites on others. It can be explained by the presence of a region of positive electrostatic potential, the sigma-hole, on the outermost portion of the halogen's surface, centered on the R-X axis. We have carried out a natural bond order B3LYP analysis of the molecules CF(3)X, with X = F, Cl, Br and I. It shows that the Cl, Br and I atoms in these molecules closely approximate the [Formula: see text] configuration, where the z-axis is along the R-X bond. The three unshared pairs of electrons produce a belt of negative electrostatic potential around the central part of X, leaving the outermost region positive, the sigma-hole. This is not found in the case of fluorine, for which the combination of its high electronegativity plus significant sp-hybridization causes an influx of electronic charge that neutralizes the sigma-hole. These factors become progressively less important in proceeding to Cl, Br and I, and their effects are also counteracted by the presence of electron-withdrawing substituents in the remainder of the molecule. Thus a sigma-hole is observed for the Cl in CF(3)Cl, but not in CH(3)Cl.

A σ-hole bond is a noncovalent interaction between a covalently-bonded atom of Groups IV-VII and a negative site, e.g. a lone pair of a Lewis base or an anion. It involves a region of positive electrostatic potential, labeled a σ-hole, on the extension of one of the covalent bonds to the atom. The σ-hole is due to the anisotropy of the atom's charge distribution. Halogen bonding is a subset of σ-hole interactions. Their features and properties can be fully explained in terms of electrostatics and polarization plus dispersion. The strengths of the interactions generally correlate well with the magnitudes of the positive and negative electrostatic potentials of the σ-hole and the negative site. In certain instances, however, polarizabilities must be taken into account explicitly, as the polarization of the negative site reaches a level that can be viewed as a degree of dative sharing (coordinate covalence). In the gas phase, σ-hole interactions with neutral bases are often thermodynamically unfavorable due to the relatively large entropy loss upon complex formation.

A halogen bond is a highly directional, electrostatically-driven noncovalent interaction between a region of positive electrostatic potential on the outer side of the halogen X in a molecule R-X and a negative site B, such as a lone pair of a Lewis base or the pi-electrons of an unsaturated system. The positive region on X corresponds to the electronically-depleted outer lobe of the half-filled p-type orbital of X that is involved in forming the covalent bond to R. This depletion is labeled a sigma-hole. The resulting positive electrostatic potential is along the extension of the R-X bond, which accounts for the directionality of halogen bonding. Positive sigma-holes can also be found on covalently-bonded Group IV-VI atoms, which can similarly interact electrostatically with negative sites. Since positive sigma-holes often exist in conjunction with negative potentials on other portions of the atom's surface, such atoms can interact electrostatically with both nucleophiles and electrophiles, as has been observed in surveys of crystallographic structures. Experimental as well as computational studies indicate that halogen and other sigma-hole interactions can be competitive with hydrogen bonding, which itself can be viewed as a subset of sigma-hole bonding.

A positive π-hole is a region of positive electrostatic potential that is perpendicular to a portion of a molecular framework. It is the counterpart of a σ-hole, which is along the extension of a covalent bond to an atom. Both σ-holes and π-holes become more positive (a) in going from the lighter to the heavier atoms in a given Group of the periodic table, and (b) as the remainder of the molecule is more electron-withdrawing. Positive σ- and π-holes can interact in a highly directional manner with negative sites, e.g., the lone pairs of Lewis bases. In this work, the complexes of 13 π-hole-containing molecules with the nitrogen lone pairs of HCN and NH(3) have been characterized computationally using the MP2, M06-2X and B3PW91 procedures. While the electrostatic interaction is a major driving force in π-hole bonding, a gradation is found from weakly noncovalent to considerably stronger with possible indications of some degree of coordinate covalency.

We report the characterization of carbon nanodots (CNDs) synthesized under mild and controlled conditions, that is, in a microwave reactor. The CNDs thus synthesized exhibit homogeneous and narrowly dispersed optical properties. They are thus well suited as a testbed for studies of the photophysics of carbon-based nanoscopic emitters. In addition to steady-state investigations, time-correlated single-photon counting, fluorescence up-conversion, and transient pump probe absorption spectroscopy were used to elucidate the excited-state dynamics. Moreover, quenching the CND-based emission with electron donors or acceptors helped shed light on the nature of individual states. Density functional theory and semiempirical configuration-interaction calculations on model systems helped understand the fundamental structure-property relationships for this novel type of material.

It has been observed both experimentally and computationally that some divalently-bonded Group VI atoms interact in a noncovalent but highly directional manner with nucleophiles. We show that this can readily be explained in terms of regions of positive electrostatic potential on the outer surfaces of such atoms, these regions being located along the extensions of their existing covalent bonds. These positive regions can interact attractively with the lone pairs of nucleophiles. The existence of such a positive region is attributed to the presence of a "sigma-hole." This term designates the electron-deficient outer lobe of a half-filled p bonding orbital on the Group VI atom. The positive regions become stronger as the electronegativity of the atom decreases and its polarizability increases, and as the groups to which it is covalently bonded become more electron-withdrawing. We demonstrate computationally that the sigma-hole concept and the outer regions of positive electrostatic potential account for the existence, directionalities and strengths of the observed noncovalent interactions.

A covalently-bonded atom typically has a region of lower electronic density, a "σ-hole," on the side of the atom opposite to the bond, along its extension. There is frequently a positive electrostatic potential associated with this region, through which the atom can interact attractively but noncovalently with negative sites. This positive potential reflects not only the lower electronic density of the σ-hole but also contributions from other portions of the molecule. These can significantly influence both the value and also the angular position of the positive potential, causing it to deviate from the extension of the covalent bond. We have surveyed these effects, and their consequences for the directionalities of subsequent noncovalent intermolecular interactions, for atoms of Groups IV-VII. The overall trends are that larger deviations of the positive potential result in less linear intermolecular interactions, while smaller deviations lead to more linear interactions. We find that the deviations of the positive potentials and the nonlinearities of the noncovalent interactions tend to be greatest for atoms of Groups V and VI. We also present arguments supporting the use of the 0.001 a.u. contour of the electronic density as the molecular surface on which to compute the electrostatic potential.

The direct metalation of tetraphenylporphyrin with bare metal atoms (Co and Zn) was studied with X-ray photoelectron spectroscopy, scanning tunneling microscopy, and temperature-programmed reaction measurements on ordered monolayer films of the molecules adsorbed on a Ag(111) surface. The mechanism of this novel type of surface reaction was investigated using density functional theory (DFT) calculations for the related gas-phase reactions of the unsubstituted porphyrin with the metals Fe, Co, Ni, Cu, and Zn. The reaction starts with the formation of an initial complex, in which the metal atom is coordinated by the intact unreduced porphyrin. This complex resembles the sitting-atop complex proposed for porphyrin metalation with metal ions in solution. In two subsequent steps, the pyrrolic hydrogen atoms are transferred to the metal atom, forming H2, which is eventually released. The activation barriers of the H-transfer steps vary for the different metal atoms. DFT calculations suggest that metalations with Fe, Co, and Ni show two-state reactivity, while those with Cu and Zn proceed on a single potential energy surface. For metalation with Zn, we calculated a barrier of the first hydrogen transfer step of 32.6 kcal mol(-1), in good agreement with the overall experimental activation energy of 31 kcal mol(-1).