David Danovich

Charge-shift bonding and its manifestations in chemistry

Electron-pair bonding is a central chemical paradigm. Here, we show that alongside the two classical covalent and ionic bond families, there exists a class of charge-shift (CS) bonds wherein the electron-pair fluctuation has the dominant role. Charge-shift bonding shows large covalent-ionic resonance interaction energy, and depleted charge densities, and features typical to repulsive interactions, albeit the bond itself may well be strong. This bonding type is rooted in a mechanism whereby the bond achieves equilibrium defined by the virial ratio. The CS bonding territory involves, for example, homopolar bonds of compact electronegative and/or lone-pair-rich elements, heteropolar bonds of these elements among themselves and with other atoms (for example, the metalloids, such as silicon and germanium), hypercoordinated molecules, and bonds whose covalent components are weakened by exchange-repulsion strain (as in [1.1.1]propellane). Here, we discuss experimental manifestations of CS bonding in chemistry, and outline new directions demonstrating the portability of the new concept.

Dihydrogen contacts in alkanes are subtle but not faint

Alkane molecules are held together in the crystal state by purportedly weak homonuclear R–H···H–R dihydrogen interactions. In an apparent contradiction, the high melting points and vaporization enthalpies of polyhedranes in condensed phases require quite strong intermolecular interactions. Two questions arise: ‘How strong can a weak C–H···H–C bond be?’ and ‘How do the size and topology of the carbon skeleton affect these bonding interactions?’ A systematic computational study of intermolecular interactions in dimers of n-alkanes and polyhedranes, such as tetrahedrane, cubane, octahedrane or dodecahedrane, showed that attractive C–H···H–C interactions are stronger than usually thought. We identified factors that account for the strength of these interactions, including the tertiary nature of the carbon atoms and their low pyramidality. An alkane with a bowl shape was designed in the search for stronger dihydrogen intermolecular bonding, and a dissociation energy as high as 12 kJ mol−1 is predicted by our calculations. Intermolecular non-polar H···H interactions between polyhedrane molecules may be as attractive as classical hydrogen bonds. A theoretical study identifies the chemical and structural factors that favour such attractive interactions.

On The Nature of the Halogen Bond

The wide-ranging applications of the halogen bond (X-bond), notably in self-assembling materials and medicinal chemistry, have placed this weak intermolecular interaction in a center of great deal of attention. There is a need to elucidate the physical nature of the halogen bond for better understanding of its similarity and differences vis-à-vis other weak intermolecular interactions, for example, hydrogen bond, as well as for developing improved force-fields to simulate nano- and biomaterials involving X-bonds. This understanding is the focus of the present study that combines the insights of a bottom-up approach based on ab initio valence bond (VB) theory and the block-localized wave function (BLW) theory that uses monomers to reconstruct the wave function of a complex. To this end and with an aim of unification, we studied the nature of X-bonds in 55 complexes using the combination of VB and BLW theories. Our conclusion is clear-cut; most of the X-bonds are held by charge transfer interactions (i.e., intermolecular hyperconjugation) as envisioned more than 60 years ago by Mulliken. This is consistent with the experimental and computational findings that X-bonds are more directional than H-bonds. Furthermore, the good linear correlation between charge transfer energies and total interaction energies partially accounts for the success of simple force fields in the simulation of large systems involving X-bonds.

Ionization potentials of porphyrins and phthalocyanines. A comparative benchmark study of fast improvements of Koopman's Theorem

The vertical ionization potentials (IPs) of a variety of free-base and zinc porphyrins and free-base and zinc phthalocyanine, including all those for which experimental ultraviolet photoelectron spectral (UPS) data are presently known, are computed using six semiempirical molecular orbital methods. Koopmans Theorem (KT), second order outer valence Greens function methods with a large number of active orbitals (OVGF), and explicit computation of the relative energies of neutral species and vertically ionized radical cations (ΔSCF IP) are used in combination with both PM3 and AM1 parameterizations, and the results are compared to experimental data. On average, both the OVGF and ΔSCF IP approximations reproduce the first vertical IPs, as determined by UPS, far more accurately than KT at minimal extra computational costs. Over the full set of available experimental data, the average error for the lowest IP with both OVGF and ΔSCF IP is only ca. 40% of that of KT (AM1 data, AM1 being generally more accurate than PM3). Inclusion of higher order terms in the OVGF treatment (third order truncation or full expression of the self-energy part) does not affect the computed IPs significantly, but inclusion of a large number of active orbitals in the OVGF technique is shown to be essential for this class of molecules. In agreement with the experimental data, zinc porphyrins and zinc phthalocyanines are computed to be better electron donors than their free-base analogues. Conformational differences of the peripheral substituents have no significant effects on the valence IPs.

A Response to the Critical Comments on “One Molecule, Two Atoms, Three Views, Four Bonds?”

Sason : Frenking and Hermann have criticized our work in Nature Chemistry and the trialogue in Angewandte Chemie on quadruple bonding in C2, [1, 2] and raised some useful points, which I thought I may be able to answer with some hard facts. In the trialogue, I argued that one can put an experimental value on the bond energy of the 4th bond in C2, using the difference between the successive bond dissociation energies (BDEs) of the two C H bonds of acetylene. Using the experimental and theoretical data, Henry and I showed that BDE(CH-1) = 133.5 kcalmol 1 and BDE(CH-2) = 116.7 kcalmol , which leads to BDE(4th) = 16.8 kcalmol 1 as the experimental bond energy of the 4th bond. Gernot argues reasonably, that this bond energy determination is based on the assumption that the other bonds in C2 are not affected, and since the C C distance changes from 1.208 in HCCC to 1.243 in C2, Gernot feels that the assumption was wrong, and the BDE difference, BDE(CH-1) BDE(CH-2) = 16.8 kcal mol , reflects in fact the relaxation of the C2 molecule rather than the strength of the 4th bond. To test this, David has now calculated the energy change of C2 from 1.210 to 1.245 , using CCSD(T)/aug-cc-pV5Z and CCSD(T)/CBS (CBS—complete basis set limit). The values are 1.116 kcal mol 1 and 1.115 kcal mol . Valence bond (VB) calculations led him to a consistent value of 1.22 kcalmol . These values show that Gernot is mistaken in his concerns, and that the experimental BDE(4th) value is 15.7–15.8 kcal mol , in perfect agreement with the original VB calculations. Gernot later argues that the difference between the two BDE(CH) values reflects the fact that, as C2 is formed upon second C H bond dissociation, the reference state of the carbons atoms changes from S to the P ground state, and this is reflected in a BDE lowering. As we show in the following two paragraphs, the electronic configuration of the carbon atoms is 78–81% 2s2p. The state is virtually S, and there is no basis for assuming it is P. Thus, showing that the CC bondlength change has negligible effect and refuting the argument about the reference-state change, the only factor that is responsible for the lowering of the second C H bond dissociation is the 4th bond in C2. Gernot criticizes our estimate of the intrinsic bond energy, Din (the intrinsic bond energy, going from the bottom of the energy well to the fragments in their reference states as in the molecule) of C2 versus HCCH. [1] He argues that, while the reference state for the HC fragments is undoubtedly Sg , the reference state for C in C2 is the P ground state of the carbon atom (with a 2s2p population), while we use in Ref. [1] the S state (with a 2s2p population). Gernot bases his argument on the fact that in both molecules the potential energy curves for dissociation correlate to his favored fragment states; Sg +

Does solvation cause symmetry breaking in the I3− ion in aqueous solution?

We seek to answer the question posed in the title by simulation of the tri-iodide ion in water, modeling the intermolecular interactions by classical potentials. The decrease in solvation free energy as a function of the dipole moment of the ion is calculated using an extended dynamics simulation method. This decrease is approximately quadratic in the ion dipole. Symmetry breaking occurs if this decrease is greater than the energy required to polarize the ion. We use ab initio calculations on an isolated ion to find the electronic and vibrational contributions to the polarizability, from which the polarization energy can be calculated. The solvated ion is found to be more stable when displaced along the asymmetric stretching coordinate, due to contributions of this deformation to the molecular dipole. As a test of the model’s reliability, it is used to derive solvation force autocorrelation functions from which time scales for vibrational energy and phase relaxation are estimated. The results are demonstr...

Green's function methods for calculating ionization potentials, electron affinities, and excitation energies

Greens function (GF; electron propagator) methods represent a very useful set of tools for direct calculation of electron detachment (ionization potentials), electron attachment (electron affinities), excitation energies, electron transition probabilities, and other properties. The main idea of GF methods is that for description of various properties of a many‐body system, one does not need to describe all the particles of the system but rather needs information about one or two particles belonging to the system. The corresponding required quantities are the one‐ and two‐particle GFs. Within one‐ or two‐particle GF methods, the energy difference between an initial state and a state with one additional or one less electron is calculated directly, thus eliminating errors due to inconsistent treatment of the initial and final states.

An Excursion from Normal to Inverted C-C Bonds Shows a Clear Demarcation between Covalent and Charge-Shift C-C Bonds

What is the nature of the C-C bond? Valence bond and electron density computations of 16 C-C bonds show two families of bonds that flesh out as a phase diagram. One family, involving ethane, cyclopropane and so forth, is typified by covalent C-C bonding wherein covalent spin-pairing accounts for most of the bond energy. The second family includes the inverted bridgehead bonds of small propellanes, where the bond is neither covalent nor ionic, but owes its existence to the resonance stabilization between the respective structures; hence a charge-shift (CS) bond. The dual family also emerges from calculated and experimental electron density properties. Covalent C-C bonds are characterized by negative Laplacians of the density, whereas CS-bonds display small or positive Laplacians. The positive Laplacian defines a region suffering from neighbouring repulsive interactions, which is precisely the case in the inverted bonding region. Such regions are rich in kinetic energy, and indeed the energy-density analysis reveals that CS-bonds are richer in kinetic energy than the covalent C-C bonds. The large covalent-ionic resonance energy is precisely the mechanism that lowers the kinetic energy in the bonding region and restores equilibrium bonding. Thus, different degrees of repulsive strain create two bonding families of the same chemical bond made from a single atomic constituent. It is further shown that the idea of repulsive strain is portable and can predict the properties of propellanes of various sizes and different wing substituents. Experimentally (M. Messerschmidt, S. Scheins, L. Bruberth, M. Patzel, G. Szeimies, C. Paulman, P. Luger, Angew. Chem. 2005, 117, 3993-3997; Angew. Chem. Int. Ed. 2005, 44, 3925-3928), the C-C bond families are beautifully represented in [1.1.1]propellane, where the inverted C-C is a CS-bond, while the wings are made from covalent C-C bonds. What other manifestations can we expect from CS-bonds? Answers from experiment have the potential of recharting the mental map of chemical bonding.

A different story of benzene

Abstract The paper summarizes a collaborative research of the authors over more than a decade, as described by the speaker (S. Shaik) at the recent WATOC conference in Jerusalem during July 8–12, 1996. This is a story of the attempts of the authors to drive home the idea that the π-electronic component of benzene and other conjugated systems has a dual character. The π-component is, on the one hand, distortive along a localizing mode, and on the other hand, it is stabilized by resonance energy relative to a localized reference. The text of the paper is a modified transcript of the talk and as such, the paper conserves the active voice of the speaker S. Shaik and the personalized style of his talk.

The origins of the directionality of noncovalent intermolecular interactions

The recent σ‐hole concept emphasizes the contribution of electrostatic attraction to noncovalent bonds, and implies that the electrostatic force has an angular dependency. Here a set of clusters, which includes hydrogen bonding, halogen bonding, chalcogen bonding, and pnicogen bonding systems, is investigated to probe the magnitude of covalency and its contribution to the directionality in noncovalent bonding. The study is based on the block‐localized wavefunction (BLW) method that decomposes the binding energy into the steric and the charge transfer (CT) (hyperconjugation) contributions. One unique feature of the BLW method is its capability to derive optimal geometries with only steric effect taken into account, while excluding the CT interaction. The results reveal that the overall steric energy exhibits angular dependency notably in halogen bonding, chalcogen bonding, and pnicogen bonding systems. Turning on the CT interactions further shortens the intermolecular distances. This bond shortening enhances the Pauli repulsion, which in turn offsets the electrostatic attraction, such that in the final sum, the contribution of the steric effect to bonding is diminished, leaving the CT to dominate the binding energy. In several other systems particularly hydrogen bonding systems, the steric effect nevertheless still plays the major role whereas the CT interaction is minor. However, in all cases, the CT exhibits strong directionality, suggesting that the linearity or near linearity of noncovalent bonds is largely governed by the charge‐transfer interaction whose magnitude determines the covalency in noncovalent bonds.