No-Pair Bonding in Coinage Metal Dimers

High-level ab initio calculations at the coupled cluster with single and double substitutions and perturbative treatment of triple substitutions, CCSD(T), level of theory have been carried out for the dimers of coinage metal atoms Cu, Ag, and Au in the ground 1Σg+ state and in the excited 3Σu+ state. All of the calculations have been carried out with the inclusion of scalar-relativistic effects via the normalized elimination of the small component (NESC) method. For the dimers in the triplet state, nonzero bond dissociation energies are obtained which vary from 1.3 kcal/mol for 3Cu2 to 4.6 kcal/mol for 3Au2. Taking into account that, in bulky high-spin copper clusters, the bond dissociation energy per atom increases steeply to the value of ca. 19 kcal/mol, the results obtained in the present paper suggest that the bond dissociation energy per atom in high-spin gold clusters may reach extremely high values exceeding 20 kcal/mol thus becoming comparable to the usual bonding due to the spin-pairing mechanism.

Towards the definition of the maximum allowable tightness of an electron transfer transition state in the reactions of radical anions and alkyl halides

A mechanistic continuum with a sharp changeover zone from electron transfer (ET) to substitution (SUB) reactions has been found in the analysis of the reactions of formyl radical anions with methyl halides (see below). The minimum C – C distance at which the transition state for ET collapses to that of SUB could be defined

Valence Bond Theory—Its Birth, Struggles with Molecular Orbital Theory, Its Present State and Future Prospects

This essay describes the successive births of valence bond (VB) theory during 1916–1931. The alternative molecular orbital (MO) theory was born in the late 1920s. The presence of two seemingly different descriptions of molecules by the two theories led to struggles between the main proponents, Linus Pauling and Robert Mulliken, and their supporters. Until the 1950s, VB theory was dominant, and then it was eclipsed by MO theory. The struggles will be discussed, as well as the new dawn of VB theory, and its future

Bonding with Parallel Spins: High-Spin Clusters of Monovalent Metal Atoms

Bonding is a glue of chemical matter and is also a useful concept for designing new molecules. Despite the fact that electron pairing remains the bonding mechanism in the great majority of molecules, in the past few decades scientists have had a growing interest in discovering novel bonding motifs. As this Account shows, monovalent metallic atoms having exclusively parallel spins, such as ¹¹Li₁₀, ¹¹Au₁₀, and ¹¹Cu₁₀, can nevertheless form strongly bound clusters, without having even one traditional bond due to electron pairing. These clusters, which also can be made chiral, have high magnetic moments. We refer to this type as no-pair ferromagnetic (NPFM) bonding, which characterizes the ^<i>n</i>+1M_<i>n</i> clusters, which were all predicted by theoretical computations. The small NPFM alkali clusters that have been “synthesized” to date, using cold-atom techniques, support the computational predictions.In this Account, we describe the origins of NPFM bonding using a valence bond (VB) analysis, which shows that this bonding motif arises from bound triplet electron pairs that spread over all the close neighbors of a given atom in the cluster. The bound triplet pair owes its stabilization to the resonance energy provided by the mixing of the local ionic configurations, [³M(↑↑)⁻]M⁺ and M⁺[³M(↑↑)⁻], and the various excited covalent configurations (involving p_<i>z</i> and d_<i>z</i>² atomic orbitals) into the repulsive covalent structure ³(M↑↑M) with the s¹s¹ electronic configuration. The NPFM bond of the bound triplet is described by a resonating wave function with “in–out” and “out–in” pointing hybrids. The VB model accounts for the tendency of NPFM clusters to assume polyhedral shapes with rather high symmetry. In addition, this model explains the very steep rise of the bonding energy per atom (<i>D</i>_e/<i>n</i>), which starts out small in the ³M₂ dimer (<1 kcal/mol) and reaches 12–19 kcal/mol for clusters with 10 atoms. The model further predicts that usage of heteroatomic clusters should increase the bonding energy of an NPFM cluster.These NPFM clusters are excited state species. We suggest here stabilizing these states and making them accessible, for example, by using magnetic fields, or a combination of magnetic and electric fields. The advent of NPFM clusters offers new horizons in chemistry and enriches the scope of chemical bonding. These prospects form a strong incentive to investigate the origins of the bound triplet pairs and further chart the territory of NPFM clusters, for example, in clusters of Be, Mg, or Zn, possibly in clusters of their monosubstituted species, and the group III metalloids, such as B, Al, as well as in transition metals such as Sc