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.

Higher Order Speciation Effects on Plutonium L3 X-ray Absorption Near Edge Spectra

Abstract: Pu L3 X-ray near edge absorption spectra for Pu(0−VII) are reported for more than 60 chalcogenides, chlorides, hydrates, hydroxides, nitrates, carbonates, oxy-hydroxides, and other compounds both as solids and in solution, and substituted in zirconolite, perovskite, and borosilicate glass. This large database extends the known correlations between the energy and shape of these spectra from the usual association of the XANES with valence and site symmetry to higher order chemical effects. Because of the large number of compounds of these different types, a number of novel and unexpected behaviors are observed, such as effects resulting from the medium and disorder that can be as large as those from valence.

Valence bond concepts applied to the molecular mechanics description of molecular shapes. 3. applications to transition metal alkyls and hydrides

Abstract: Recently we reported a qualitative, valence bond derived model for describing the shapes of transition metal complexes, with a focus on metal hydrides and alkyls. This model, based on the concepts of hybridization and resonance, rationalizes the unusual and varied shapes of hydride and alkyl complexes with transition metals. This paper demonstrates the quantitative incorporation of these valence bond concepts into molecular mechanics algorithms. The resulting force field method (HV-VB) accurately describes the structures of alkyls and hydride complexes of the transition metals. For a wide variety of crystallographically characterized molecules, the HV-VB computations faithfully reproduce the observed structures.

High-level electron correlation calculations on formamide and the resonance model

Abstract: The question of planarity and the validity of the amide resonance model have been investigated in formamide on the basis of high-level quantum chemical calculations. Complete geometry optimizations were performed for the equilibrium structure and for the 90°-rotated transition state at the MBPT(2), MBPT(4), CCSD, and CCSD(T) electron correlation levels, with basis sets up to cc-PVTZ. While electron correlation tends to give nonplanar equilibrium, the final result at the CCSD(T)/PVTZ level is an exactly planar structure, as proven by the absence of imaginary vibrational frequencies. The crucial parameter in the geometry, the C-N bond length is calculated at 1.354 A. For the barrier to internal rotation around the C-N bond our best estimate, including the zero-point-energy correction, is 15.2 ( 0.5 kcal/mol. To check predictions of the resonance model, we have analyzed geometric changes, charge shifts from Mulliken population analysis, and the nature of relevant valence orbitals and also calculated NMR chemical shieldings as a function of internal rotation. In contrast to recent suggestions by Wiberg et al .( J .Am. Chem. Soc. 1987, 109, 5935; 1992, 114, 831; Science 1991, 252, 1266) that ﷿-resonance would not play a significant role in explaining the rotational barrier in formamide, we have found no compelling evidence to doubt the validity of the amide resonance model.