Can multivalency be expressed kinetically? The answer is yes

Inspired by the concept of multivalency and in pursuit of ever more intricate artificial molecular machines, we investigated the strict self-assembly of a triply threaded two-component superbundle, starting from a tritopic receptor in which three benzo[24]crown-8 macrorings are fused onto a triphenylene core and a trifurcated trication wherein three bipyridinium units are linked 1,3,5 to a central benzenoid core. The result of the investigation was quite unexpected and surprising. It transpired that the rapid formation of a doubly threaded two-component complex was followed by an extremely slow conversion (a week at 253 K in CD3COCD3 to reach equilibrium) of this kinetically controlled product into a thermodynamically controlled one, namely a triply threaded two-component superbundle. This intriguing observation begs the question: are there instances in nature where multivalency is expressed as a kinetically controlled process, prior to an equilibrium state being reached, and if so, what are the biological implications, if any?     

To what question is the clamped-nuclei electronic potential the answer?

An examination is made of how the nuclear motion Hamiltonian arises from a consideration of solutions to the eigenvalue problem for the full Coulomb Hamiltonian and the role played by the usual clamped-nuclei electronic Hamiltonian in the construction of such solutions.     

When does a hydrogen bond become a van der Waals interaction? a topological answer

The hydrogen bond (H-bond) is among the most important noncovalent interaction (NCI) for bioorganic compounds. However, no “energy border” has yet been identified to distinguish it from van der Waals (vdW) interaction. Thus, classifying NCIs and interpreting their physical and chemical importance remain open to great subjectivity. In this work, the “energy border” between vdW and H-bonding interactions was identified using a dimer of water, as well as for a series of classical and nonclassical H-bonding systems. Through means of the quantum theory of atoms in molecules and in particular the source function, it was possible to clearly identify the transition from H-bonding to vdW bonding via analysis of the electronic structure. This “energy border” was identified both on elongating the interatomic interaction and by varying the contact angle. Hence, this study also redefines the “critic angle” previously proposed by Galvão et al. (J. Phys. Chem. A 2013, 117, 12668). Consequently, such “energy border” through an analysis of atomic basins volume variation was possible to identify the end of long-range interactions. © 2019 Wiley Periodicals, Inc.     

The 1,2,3-tridehydrobenzene triradical: 2B or not 2B? The answer is 2A!

The molecular and electronic structure of 1,2,3-tridehydrobenzene was investigated by a variety of computational methods. The two lowest electronic states of the triradical are the (2)B(2) and (2)A(1) doublet states characterized by different interactions of the unpaired electrons. Vertically, the two states are well separated in energy-by 4.9 and 1.4 eV, respectively. However, due to different bonding patterns, their equilibrium structures are very different and, adiabatically, the two states are nearly degenerate. The adiabatic energy gap between the (2)B(2) and (2)A(1) states is estimated to be 0.7-2.1 kcal/mol, in favor of the (2)A(1) state. Harmonic vibrational frequencies and anharmonic corrections were calculated for both states. Comparison with the three experimentally observed IR transitions supports the assignment of the (2)A(1) ground state for the triradical with a weakly bonding distance of 1.67-1.69 A between the meta radical centers.     

Chelatoaromaticity--existing: yes or no? An answer given by spatial magnetic properties (through space NMR shieldings--TSNMRS)

The spatial magnetic properties (through space NMR shieldings--TSNMRS) of metal complexes (with ligands such as acetylacetone, 3-hydroxy-pyran(4)one) and "metallobenzenes" have been calculated by the GIAO perturbation method and visualized as Iso-Chemical-Shielding Surfaces (ICSS) of various sizes and directions. The TSNMRS values, thus obtained, can be successfully employed to quantify and visualize partial aromaticity of the metallocyclic ring by comparison with the spatial magnetic properties of the corresponding non-complexed ligands in comparable structural and electronic situations, and benzene, respectively. Because anisotropy/ring current effects in (1)H NMR spectra proved to be the molecular response property of TSNMRS, the results obtained concerning partial "chelatoaromaticity" are experimentally ensured.     

Epoxidation of acyclic chiral allylic alcohols with peroxy acids: spiro or planar butterfly transition structures? A computational DFT answer

The mechanism of the epoxidation of two chiral allylic alcohols, i.e., 3-methyl-3-buten-2-ol and (Z)-3-penten-2-ol, with peroxyformic acid has been investigated by locating 20 transition structures with the B3LYP/6-31G* method and by evaluating their electronic energy also at the B3LYP/6-311+G**@B3LYP/6-31G* theory level. Relative stability of TSs, as far as electronic energy is concerned, is basis set dependent; moreover, it also depends on entropy and solvent effects. Free enthalpies, calculated by using electronic energy at the higher theory level and with inclusion of solvent effects, indicates that syn, exo TSs, where the olefinic OH group hydrogen bonds the peroxy oxygens of the peroxy acid, outweigh syn, endo TSs, where the peroxy acid carbonyl oxygen is involved in hydrogen bonding. In the former TSs the peroxy acid moiety maintains its planar geometry while in the latter ones a strong out-of-plane distortion of peroxy acid is observed. This distortion makes it viable an unprecedented 1,2-H shift, as a possible alternative to the 1,4-H shift, for the peroxy acid hydrogen. In fact, for one syn, endo TS IRC analysis demonstrated that the 1,2-H shift mechanism is actually operative. The geometry of all TSs substantially conforms to a spiro (i.e., with the peroxy acid plane almost perpendicular to the C=C bond axis) butterfly orientation of the reactants while no TS resembles, even loosely, the planar butterfly structure. Theoretical threo/erythro epoxide ratios are in fair accord with experimental data. Calculations indicate that threo epoxides derive mostly from TSs in which the olefinic OH assumes an outside conformation while erythro epoxides originate from TSs with the OH group in an inside position. Computational findings do not support the qualitative TS models recently proposed for these reactions.     

Can undoped calcium tetraborides exist? An answer from the comparison of its density functional theory electronic structure with that of rare-earth metal tetraboride

Periodic density functional theory calculations are used to discuss the existence of metal tetraborides MB4 with divalent metals. Tetraborides which contain metal atoms inserted in a three-dimensional boron network made of B6 octahedra and B2 dumbbells exhibit a pseudo energy gap for a count of 60 valence electrons per M4(B6)2(B2)2 formula unit. Such a count satisfies the stability electron requirement for B6(2-) (20 electrons) octahedra and B2(2-) (8 electrons) units and allows the filling of two supplementary low-lying bands deriving from the valence metallic d atomic orbitals. This favored electron count is not reached for CaB4 which is then formally deficient by one electron per metal atom. This indicates that CaB4 is unlikely to exist without n-doping.     

Are the carbon monoxide complexes of Cp(2)M (M = Ca,Eu, or Yb) carbon or oxygen bonded? An answer from DFT calculations

DFT calculations have been performed on the CO adducts of the bivalent lanthanides, Cp(2)M(CO)(x), where M is Eu or Yb and x is 1 or 2, the alkaline earth metallocene Cp(2)Ca(CO), and the methylisocyanide adducts of Yb. The calculated nu(CO) values are in agreement with experiment for Cp(2)M(CO) when M is Ca or Eu, but in striking disagreement when the CO is bound to the metal by way of the carbon atom in CO in the case of Yb. The calculated nu(CO) values for M = Yb are brought into agreement with experiment when the CO is allowed to bond to Cp(2)Yb by way of the oxygen atom.     

Why are [P(C6H5)4](+)N3- and [As(C6H5)4](+)N3- ionic salts and Sb(C6H5)4N3 and Bi(C6H5)4N3 covalent solids? A theoretical study provides an unexpected answer

A recent crystallographic study has shown that, in the solid state, P(C(6)H(5))(4)N(3) and As(C(6)H(5))(4)N(3) have ionic [M(C(6)H(5))(4)](+)N(3)(-)-type structures, whereas Sb(C(6)H(5))(4)N(3) exists as a pentacoordinated covalent solid. Using the results from density functional theory, lattice energy (VBT) calculations, sublimation energy estimates, and Born-Fajans-Haber cycles, it is shown that the maximum coordination numbers of the central atom M, the lattice energies of the ionic solids, and the sublimation energies of the covalent solids have no or little influence on the nature of the solids. Unexpectedly, the main factor determining whether the covalent or ionic structures are energetically favored is the first ionization potential of [M(C(6)H(5))(4)]. The calculations show that at ambient temperature the ionic structure is favored for P(C(6)H(5))(4)N(3) and the covalent structures are favored for Sb(C(6)H(5))(4)N(3) and Bi(C(6)H(5))(4)N(3), while As(C(6)H(5))(4)N(3) presents a borderline case.