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.

Valence bond studies of the dependence upon substituents of c13-h and si29-h coupling

Abstract: An interpretation is presented for the additivity of substituent effects on the C13–H coupling constant, which has been observed previously in the high‐resolution NMR spectra of substituted methanes Each atom or group X is assigned a characteristic ``affinity'' for s character in the carbon hybrid orbital of the C—X bond Distribution of s character among the carbon orbitals in accord with the relative s affinities of the four substituents leads to the observed additivity relation provided that the total s character is conserved The valence‐bond approach used with this model gives a linear relation between the s character of the carbon hybrid orbital involved in the C–H bond (αH2) and the observed C13–H coupling constant (JCH=500 αH2) Also, it allows the determination of the s character of the other carbon orbitals The dependence of the s character of the C—X bond on the electronegativity of X is discussed in terms of electron spin and charge correlation It is noted that the hybridization changes sho

Structural diversity and electronic properties of Cu2SnX3 (X = S, Se): A first-principles investigation

Abstract: The ternary semiconductors Cu(2)SnX(3) (X = S, Se) are found frequently as secondary phases in synthesized Cu(2)ZnSnS(4) and Cu(2)ZnSnSe(4) samples, but previous reports on their crystal structures and electronic band gaps are conflicting. Here we report their structural and electronic properties as calculated using a first-principles approach. We find that (i) the diverse range of crystal structures such as the monoclinic, cubic, and tetragonal phases can all be derived from the zinc-blende structure with tetrahedral coordination. (ii) The energy stability of different structures is determined primarily by the local cation coordination around anions, which can be explained by a generalized valence octet rule. Structures with only Cu(3)Sn and Cu(2)Sn(2) clusters around the anions have low and nearly degenerate energies, which makes Cu and Sn partially disordered in the cation sublattice. (iii) The direct band gaps of the low-energy compounds Cu(2)SnS(3) and Cu(2)SnSe(3) should be in the range of 0.8-0.9 and 0.4 eV, respectively, and are weakly dependent on the long-range structural order. A direct analogy is drawn with the ordered vacancy compounds found in the Cu(In,Ga)Se(2) solar-cell absorbers.

Theoretical Study of the Electronic Spectra of Cyclopentadiene, Pyrrole, and Furan

Abstract: The electronic spectra of the title molecules have been studied using a newly proposed quantum chemical approach for ab initio calculations of dynamic electron correlation effects in molecular systems: multiconfigurational second-order perturbation theory (CASPT2). For cyclopentadiene and furan, the calculations comprise three valence excited singlet states and, in addition, the la2 - 3s, 3p, and 3d Rydberg states, thus providing a full assignment of the spectra in the energy range below 8.0 eV. For pyrrole, the 2bl - 3s. 3p, and 3d components of the Rydberg series have been added. The four lowest triplet states have also been studied in all three molecules. The computed excitation energies deviate from experiment by less than 0.17 eV in all cases where an assignment is possible. It is shown that the two main features in the spectra are caused by the valence excited states 1Bz (5.27, 5.92,6.04 eV) and 'A: (7.89, 7.46, 7.74 eV), where the calculated energies for the two states in the three molecules are given in parentheses. In addition, the 'A; state has been determined to appear near 6 eV in all three molecules. These results differ drastically from earlier theoretical predictions but are in agreement with experimental data. A number of new assignments of the Rydberg states are suggested.

ESCA studies of molecular core and valence levels in the gas phase

Abstract: The study of molecules in the gas phase by ESCA offers several advantages. Particularly, the opportunity to make direct comparisons between theoretical and experimental results is of importance. From such comparisons a more complete understanding of several fundamental phenomena in ESCA has been gained. The present paper reviews some recent work in Uppsala along this line. The effect of electron reorganization on core binding energies in atoms and molecules is analyzed. Chemical shifts in ESCA and n.m.r. are discussed. The existence of a general relation involving these chemical shifts and nuclear spin-rotation constants is pointed out. A model for analyzing ESCA intensities of the valence electrons in molecules is presented. It is shown that the analysis may be useful in identifying the molecular orbitals.For most applications of ESCA it is highly desirable to improve the resolution and the intensity. An instrument, particularly designed to achieve this for gases, is discussed. It can also handle solid samples. It makes use of a high-emission, fine-focus electron gun, a rotating anode and a double focusing quartz crystal. The first electron spectrum recorded with this new instrument is presented.

Macroscopic Polarization Change via Electron Transfer in a Valence Tautomeric Cobalt Complex

Abstract: Polarization change induced by directional electron transfer attracts considerable attention owing to its fast switching rate and potential light control. Here, we investigate electronic pyroelectricity in the crystal of a mononuclear complex, [Co(phendiox)(rac-cth)](ClO4)·0.5EtOH (1·0.5EtOH, H2phendiox = 9, 10-dihydroxyphenanthrene, rac-cth = racemic 5, 5, 7, 12, 12, 14-hexamethyl-1, 4, 8, 11-tetraazacyclotetradecane), which undergoes a two-step valence tautomerism (VT). Correspondingly, pyroelectric current exhibits double peaks in the same temperature domain with the polarization change consistent with the change in dipole moments during the VT process. Time-resolved Infrared (IR) spectroscopy shows that the photo-induced metastable state can be generated within 150 ps at 190 K. Such state can be trapped for tens of minutes at 7 K, showing that photo-induced polarization change can be realized in this system. These results directly demonstrate that a change in the molecular dipole moments induced by intramolecular electron transfer can introduce a macroscopic polarization change in VT compounds. Polarization change from directional electron transfer attracts considerable attention owing to its fast switching rate and potential light control. Here, the authors provide a proof-of-concept of electronic pyroelectricity induced by intramolecular electron transfer in the single crystal of a valence tautomeric compound.