Valence (chemistry)

About: Valence (chemistry) is a research topic. Over the lifetime, 24937 publications have been published within this topic receiving 645252 citations. The topic is also known as: valency.

Ab initio theoretical predictions of C28, C28H4, C28F4, (Ti@C28)H4, and M@C28 (M=Mg, Al, Si, S, Ca, Sc, Ti, Ge, Zr, and Sn)

Abstract: Recent experiments have demonstrated that C28 is the smallest fullerene cage that successfully traps elements in its inside. In this work, we have studied the electronic structures, equilibrium geometries, and binding energies of the title molecules at the self‐consistent field (SCF) Hartree–Fock level of theory employing basis sets of double‐zeta quality. The empty C28 fullerene is found to have a 5A2 open‐shell ground state and behaves as a sort of hollow superatom with an effective valence of 4, both toward the outside and inside of the carbon cage. The theoretical evidence suggests that C28H4 and C28F4 should be stable molecules. The possibility of simultaneous bonding from the inside and outside of the C28 shell, as in (Ti@C28)H4, is also explored. Our calculations show that the binding energy of the M@C28 species is a good indicator of the success in experimentally trapping the metal atoms (M) inside the fullerene cage. Based on these results, we propose that elements with electronegativities smaller than 1.54 should form endohedral fullerenes larger than a minimum size which depends on the ionic radius of the trapped atom. This qualitative model, correctly reproduces the available experimental evidence on endohedral fullerenes.

Theory of Auger Neutralization of Ions at the Surface of a Diamond-Type Semiconductor

Abstract: The two-electron, Auger-type transitions which occur when an ion of sufficiently large ionization energy is neutralized at the atomically clean surface of a diamond-type semiconductor are discussed. Consideration of the basic elements of the problem leads to a computing program which enables one to calculate the total electron yield and kinetic energy distribution of ejected electrons in terms of a number of parameters. It is possible to account for the experimental results for singly-charged noble gas ions incident on the (111) faces of Si and Ge and the (100) face of Si. The fit of theory to experiment is unique in its principal features yielding numerical results concerning: (1) the state density function for the valence bands of Si and Ge, (2) the energy dependence of the matrix element as it is determined by symmetry of the valence band wave functions, (3) the effective ionization energy near the solid surface, (4) energy broadening, and (5) electron escape over the surface barrier. Over-all width of the valence band is found to be 14-16 ev for both Si and Ge. Width of the degenerate $p$ bands is 5.1 ev in Si, 4.3 ev in Ge. The matrix element for $p$-type valence electrons is 0.3 times that for $s$-type valence electrons. Effective ionization energy is 2.2 ev less than the free-space value for 10-ev ${\mathrm{He}}^{+}$ ions and decreases linearly with ion velocity. Energy broadening is small for 10-ev ions and increases approximately linearly with ion velocity. Probability of electron escape is several times that predicted for an isotropic distribution of excited electrons incident on a plane surface barrier. A general theory of Auger neutralization is given in which the conclusions of the fit to experiment are interpreted. Investigation of the matrix element as a Coulomb interaction integral involving wave functions whose general characteristics are known but which are not explicitly evaluated leads to an understanding of its principal dependences on energy and angle.

Divalent surface state on metallic samarium

Abstract: It is shown that the first atomic layer of trivalent metallic samarium has a large divalent component. The valence transition is attributed to a narrowing of the 5d band which populates the low-lying 4f/sup 6/ state.

Does covalency really increase across the 5f series? A comparison of molecular orbital, natural population, spin and electron density analyses of AnCp(3) (An = Th-Cm; Cp = eta(5)-C5H5)

Abstract: The title compounds are studied with scalar relativistic, gradient-corrected (PBE) and hybrid (PBE0) density functional theory. The metal–Cp centroid distances shorten from ThCp3 to NpCp3, but lengthen again from PuCp3 to CmCp3. Examination of the valence molecular orbital structures reveals that the highest-lying Cp π2,3-based orbitals transform as 1e + 2e + 1a1 + 1a2. Above these levels come the predominantly metal-based 5f orbitals, which stabilise across the actinide series such that in CmCp3 the 5f manifold is at more negative energy than the Cp π2,3-based levels. Mulliken population analysis shows metal d orbital participation in the e symmetry Cp π2,3-based orbitals. Metal 5f character is found in the 1a1 and 1a2 levels, and this contribution increases significantly from ThCp3 to AmCp3. This is in agreement with the metal spin densities, which are enhanced above their formal value in NpCp3, PuCp3 and especially AmCp3 with both PBE and PBE0. However, atoms-in-molecules analysis of the electron densities indicates that the An–Cp bonding is very ionic, increasingly so as the actinide becomes heavier. It is concluded that the large metal orbital contributions to the Cp π2,3-based levels, and enhanced metal spin densities toward the middle of the actinide series arise from a coincidental energy match of metal and ligand orbitals, and do not reflect genuinely increased covalency (in the sense of appreciable overlap between metal and ligand levels and a build up of electron density in the region between the actinide and carbon nuclei).