The Ionic Character of Diatomic Molecules
TL;DR: In this paper, a relation between the electronegativity difference of two bonded atoms and the ionic character of the bond is obtained for singly bonded diatomic molecules in the gaseous state.
Abstract: A relation between the electronegativity difference of two bonded atoms and the ionic character of the bond is obtained for singly bonded diatomic molecules in the gaseous state. The relation is based primarily on the wide variety of nuclear quadrupole coupling constants recently measured by microwave techniques. However, dipole moments of diatomic molecules are also used and discussed. Both quadrupole coupling constants and dipole moments indicate strongly that bonds involving electronegativity differences greater than 2 are almost completely ionic. Certain effects of hybridization, overlap, and polarization on considerations of ionicity are discussed.
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TL;DR: In this article, an expression for the magnetic shielding tensor is obtained by the use of single-determinant Hartree-Fock molecular wave functions for nuclei of atoms in which the change in the second-order (paramagnetic) contribution is dominant.
Abstract: An expression for the magnetic shielding tensor is obtained by the use of single‐determinant Hartree‐Fock molecular wave functions. For nuclei of atoms in which the change in the second‐order (paramagnetic) contribution is dominant, LCAO theory is employed to express the shielding in terms of localized bond parameters (ionic character, hybridization, and double bonding) and to compare it with the related treatments of the quadrupole coupling constant. Application of the formulation to the multifluorobenzenes provides an explanation of the available experimental chemical shift data and permits the prediction of shift values for other compounds. Of particular interest is the demonstration that double bonding in the C–F bond plays an important role in the fluorobenzenes. Also, the presence of an ``ortho'' effect in the shift is isolated by a comparison of experimental and theoretical results and tentatively explained in terms of charge repulsions.
211 citations
TL;DR: In this article, a theoretical and experimental study was made of the shift in atomic core-electron binding energies caused by the chemical environment, and two models were presented to account for these "chemical shifts".
Abstract: A theoretical and experimental study was made of the shift in atomic core‐electron binding energies caused by the chemical environment. Two models are presented to account for these “chemical shifts.” The first uses an energy cycle to break the core‐electron binding energies into a free‐ion contribution and a classical Madelung energy contribution. The Madelung energy contributes a significant part of the binding‐energy shift. It can, in principle, be evaluated rigorously although there is some ambiguity as to a surface correction. The reference level for binding energies must also be considered in comparing theory with experiment (or in comparing experimental shifts with one another). Electronic relaxation could also introduce errors of ∼1 eV in shift measurements. The second, more approximate, model consists of a “charged‐shell” approximation for bonding electrons in atomic complexes. It gives semiquantitative estimates of shifts and demonstrates the relationship between bond polarity and core‐electron ...
187 citations
TL;DR: In this paper, B10, B11, and O17 NMR spectra were used to study B2O3 glass and the results were consistent with a structure composed of randomly oriented boroxol rings.
Abstract: B10, B11, and O17 NMR were used to study B2O3 glass. B10 NMR was employed to determine the most probable values and the Gaussian widths (σ) of the distributions of the quadrupole coupling constant (Qcc) and the asymmetry parameter (η). The resulting values are Q0cc=5.51 MHz, σQcc=0.21 MHz, η0=0.12, and ση=0.043. These values are consistent with results using B11 NMR. The O17 NMR spectra are obtained and analyzed using two sites: site 1 is characterized by Q0cc=4.69 MHz, σQcc=0.10 MHz, η0=0.58; site 2 is characterized by Q0cc=5.75 MHz, η0=0.4, and ση=0.2. Only small distributions are present for site 1 and for Qcc of site 2. A Townes–Daily calculation was employed, using the boroxol ring model of B2O3 glass, to determine the charge distributions and the B–O–B angle outside the boroxol rings. The results are consistent with a structure composed of randomly oriented boroxol rings.
182 citations
TL;DR: The analysis of electric field gradients (EFGs) using first principles theory along with model calculations has been studied in this article, where simple atomic orbital (AO) models for the EFG are developed in the spirit of the Townes-Dailey (TD) analysis and applied to various sets of spn hybrid orbitals and to atomic d orbital shells.
Abstract: This article is concerned with the analysis of electric field gradients (EFGs) using first–principles theory along with model calculations. Simple atomic orbital (AO )models for the EFG are developed in the spirit of the Townes–Dailey (TD) analysis and applied to various sets of spn hybrid orbitals and to atomic d orbital shells. These AO models are then combined with modern analysis methods rooted in first principles theory which provide accurate localized molecular orbital contributions to the EFG. It is shown by density functional computations how such analyses of the EFG for a variety of typical structural motifs can provide an intuitive way of understanding the chemical origin of the magnitude and the sign of EFG tensors at atomic nuclei, as well as of their orientation with respect to the molecular coordinate frame. The utility of graphical visualizations of EFG tensors is also emphasized. The systems that are investigated span the range from very small molecules (carbon and sulfur EFGs in CO, CS, OCS) to small- and medium-sized molecules (nitrogen and aluminum EFGs in ammonia, methyl-cyanide and -isocyanide, aluminum AlX3 model systems and various alumino-organic systems), to the metal atom field gradient in transition metal complexes with Ru and Nb and a variety of ligands. © 2010 Wiley Periodicals, Inc. Concepts Magn Reson Part A 36A: 84–126, 2010.
132 citations
TL;DR: The microwave spectra of C13H3CH2Cl35, CH3C13H2Cl37 and CH3CD2Cl36 have been examined and rotational constants assigned as discussed by the authors.
Abstract: The microwave spectra of C13H3CH2Cl35, CH3C13H2Cl35, CH3CD2Cl35, and CH2DCH2Cl35 have been examined and rotational constants assigned. Both a‐type and b‐type spectra have been observed. From these spectra and those previously reported for CH3CH2Cl35, CH3CH2Cl37, and CH2DCH2Cl35 a complete structure has been obtained by means of the substitution method. The bond distances and bond angles obtained are as follows: CC = 1.520 A, CCl = 1.788 A, CH (methyl) = 1.091 A, CH (methylene) = 1.089 A, CCCl = 111°2′, HCH (methyl) = 108°30′, HCH (methylene) = 109°12′, CCH (methylene) = 111°36′. The quadrupole coupling parameters and barrier to internal rotation have been re‐evaluated in terms of the above structure. The quadrupole coupling parameters are χbond = —68.80 Mc and ηbond = 0.035, if the angle between the a axis and the CCl bond is determined from the structure; or χbond = —71.24 Mc, if a cylindrical charge distribution is assumed near the chlorine nucleus. The barrier to internal rotation is 3685 cal/mole.
127 citations
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TL;DR: In this article, it was shown that this variation of electric field is usually simply related to the molecular electronic structure, being primarily dependent on the way in which valence electrons fill the lowest energy p-type orbits.
Abstract: Nuclear quadrupole coupling constants in molecules depend on the nuclear quadrupole moments and the variation in electrostatic field at the nucleus. It is shown that this variation of electric field is usually simply related to the molecular electronic structure, being primarily dependent on the way in which valence electrons fill the lowest‐energy p‐type orbits. Structural information which can consequently be obtained from known quadrupole coupling constants is discussed. Hybridization of the normal covalent bonds of N, Cl, and As with at least 15 percent s character is clearly shown. The alkali halides appear to be almost purely ionic; the quadrupole coupling data allow no more than 3 percent covalent character. In addition to molecular structure, some nuclear quadrupole moments are approximately evaluated by use of the theory developed here.
886 citations
TL;DR: In this article, a simple theory of alkali halide gas molecules in the spirit of Born-Mayer lattice theory is presented, where each molecule is constituted of ions, each of which is polarized by the electrostatic field of the other.
Abstract: A simple theory of alkali halide gas molecules in the spirit of Born‐Mayer lattice theory is presented. The molecule is considered to be constituted of ions, each of which is polarized by the electrostatic field of the other. This deformation polarization has two important consequences: (1) the net dipole moment of the molecule becomes appreciably lower than that given by the product of the electric charge e, and the internuclear separation a; (2) the repulsion constants (determined from empirical data) become different from the corresponding constants for the crystal. The theory yields satisfactory results for the binding energy, vibration frequency, and dipole moment, μ, in all instances where necessary data are available. In cases where the internuclear distances are not known experimentally, they are calculated from the theory using experimental binding energies. It is possible to assign to the ions Goldschmidt‐like radii, the sums of which reproduce reasonably well both the calculated and the observe...
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TL;DR: In this paper, the microwave spectra of all alkali halides excepting LiF, NaF, KF, RbF, and LiCl were analyzed in terms of molecular and nuclear constants.
Abstract: Data from the microwave spectra of all alkali halides excepting LiF, NaF, KF, RbF, and LiCl are given and analyzed in terms of molecular and nuclear constants. These yield internuclear distances and ionic radii for the gaseous alkali halides, molecular dipole moments, potential constants including four constants for some molecules, amount of covalent character from quadrupole coupling constants and from rotational effects, and mass ratios for isotopes of Li, K, Rb, Cl, and Br.
158 citations