Valence-State Atoms in Molecules. 6. Universal Ionic−Covalent Potential Energy Curves (original) (raw)
Related papers
Valence states and a universal potential energy curve for covalent and ionic bonds
Chemical Physics Letters, 1995
The relations between the diatomic spectroscopic constants and a novel parameter Dvs, the valence state dissociation energy, are investigated. An energy scaling by Dvs, instead of the spectroscopic D e, allows the formulation of a universal three-parameter valence state potential energy curve (VS-PEC). The second and higher derivatives at the equilibrium 2 distance, R e, are simple functions of a valence state parameter z = keRe/Dv s. The analysis of the VS-PECs of 25 single and multiple bonded molecules covering the whole range of polarity reveals a far greater similarity near the minimum than previously reported. In comparison with the Morse and Rydberg potentials, the accuracy of the calculated vibration-rotation coupling and anharmonicity constants is improved by an order of magnitude.
For 39 diatomic ionic and non-ionic molecules, the anomalous behaviour of the spectral parameters ae and wexe with respect to the bond type is reviewed. It is shown that on using a "universal" Sutherland parameter defined as A = ^kerc 2 /Dion, the anomalous behaviour disappears. Hard spectroscopic evidence is thus presented, for the first time to the author's knowledge, that just one bond type, in fact an ionic one, can account, in first approximation, for the spectral behaviour of both non-ionic and ionic bonds, H2 included.
The Journal of Physical Chemistry A, 1999
Achieving an unprecedented degree of universality, the valence state potential energy curve (VS-PEC) reproduces the inner branch of 50 "experimental" RKR-PECs to an accuracy of 1.14% average unsigned deviation. The scaled RKR curves of 50 molecules with calculated partial charges, 0 e δ < 0.9, coalesce into virtually a single curve in the Coulson-Fischer range, 0.5 R e e R e 1.5R e , when plotted against q VS) z 1/2 (R-R e)/R e , with z) R e ν e /2B e 2. The ground-state dissociation energy D e can be predicted from the equilibrium spectroscopic constants B e , ν e , R e and the calculated VS promotion energy.
Journal of Physical Chemistry A, 2004
The polarizable valence-state-atoms-in-molecules (pVSAM) model describes the electron-pair bond in A-B molecules by superposing core-polarized A + B -, A -B + , and A:B structures, whose weights are determined by electronegativity equalization. The polarizable valence state potential energy curve (pVS-PEC) is derived through the systematic improvement of the valence state potential energy curve (VS-PEC) von Szentpály, L. J. Phys. Chem. A 1999, 103, 9313] and is given as U(R) ) -[(K 1 /R) + (K 2 /R 4 ) + (K 3 /R 7 )] + (T/R) exp(-λR). The first bracketed term contains the Coulomb, charge-induced dipole, and induced dipole-induced dipole terms, derived from weighted ionic and covalent bond-charge contributions. The potential is tested on a broad variety of homonuclear diatoms and heteronuclear halides and hydrides (a total of 52 molecules). The accuracies of the dimensionless vibration-rotation coupling constant (F) and the anharmonicity constant (G) for the halides of the alkali and coinage metals are significantly better than those of the Morse, Rydberg, simple bond-charge, and Rittner potentials. Adding core polarization to the VS-PEC reduces the average unsigned errors in the spectroscopic constants of 47 diatomic molecules from 17.1% to 7.5% in F and 18.9% to 7.8% in G, whereas those of the Morse potential amount to 32.6% and 31.4%, respectively. hydrides, has been tentatively rationalized 4,5 by the increasing importance of core-polarization and core-valence intershell correlation, both of which may be accounted for by a corepolarization potential (CPP). 17,18 To prove the point, we extend the VS-PEC into a form that takes core polarization into consideration. The polarizable valence-state-atoms-in-molecules (pVSAM) model links the ionic and covalent descriptions of the bond, using the concept of configuration mixing among contributing ionic and covalent (bond-charge) structures within the framework of the valencestate-atoms-in-molecules (VSAM) model. By incorporating polarization terms, a greater degree of accuracy is expected, and the advantage of physical interpretation is maintained, illustrated, and exploited.
Atom–atom partitioning of intramolecular and intermolecular Coulomb energy
The Journal of Chemical Physics, 2001
An atom-atom partitioning of the ͑super͒molecular Coulomb energy is proposed on the basis of the topological partitioning of the electron density. Atom-atom contributions to the molecular intraand intermolecular Coulomb energy are computed exactly, i.e., via a double integration over atomic basins, and by means of the spherical tensor multipole expansion, up to rank Lϭl A ϩl B ϩ1ϭ5. The convergence of the multipole expansion is able to reproduce the exact interaction energy with an accuracy of 0.1-2.3 kJ/mol at Lϭ5 for atom pairs, each atom belonging to a different molecule constituting a van der Waals complex, and for nonbonded atom-atom interactions in single molecules. The atom-atom contributions do not show a significant basis set dependence ͑3%͒ provided electron correlation and polarization basis functions are included. The proposed atomatom Coulomb interaction energy can be used both with post-Hartree-Fock wave functions and experimental charge densities in principle. The Coulomb interaction energy between two molecules in a van der Waals complex can be computed by summing the additive atom-atom contributions between the molecules. Our method is able to extract from the supermolecule wave function an estimate of the molecular interaction energy in a complex, without invoking the reference state of free noninteracting molecules. We provide computational details of this method and apply it to (C 2 H 2 ) 2 ; (HF) 2 ; (H 2 O) 2 ; butane; 1,3,5-hexatriene; acrolein and urocanic acid, thereby covering a cross section of hydrogen bonds, and covalent bonds with and without charge transfer.
The journal of physical chemistry. A, 2017
Bond dissociation energies and resonance energies for HnA-BHm molecules (A, B = H, C, N, O, F, Cl, Li, and Na) have been determined in order to re-evaluate the concept of electronegativity in the context of modern valence bond theory. Following Pauling's original scheme, and using the rigorous definition of the covalent-ionic resonance energy provided by the breathing orbital valence bond method, we have derived a charge-shift corrected electronegativity scale for H, C, N, O, F, Cl, Li, and Na. Atomic charge shift character is defined using a similar approach resulting in values of 0.42, 1.06, 1.43, 1.62, 1.64, 1.44, 0.46, and 0.34 for H, C, N, O, F, Cl, Li, and Na respectively. The charge-shift corrected electronegativity values presented herein follow the same general trends as Pauling's original values with the exception of Li having a smaller value than Na (1.57 and 1.91 for Li and Na respectively). The resonance energy is then broken down into components derived from th...
The physical origin of large covalent?ionic resonance energies in some two-electron bonds
Faraday Discussions, 2007
This study uses valence bond (VB) theory to analyze in detail the previously established finding that alongside the two classical bond families of covalent and ionic bonds, which describe the electron-pair bond, there exists a distinct class of charge-shift bonds (CS-bonds) in which the fluctuation of the electron pair density plays a dominant role. Such bonds are characterized by weak binding, or even a repulsive, covalent component, and by a large covalent-ionic resonance energy RE CS that is responsible for the major part, or even for the totality, of the bonding energy. In the present work, the nature of CS-bonding and its fundamental mechanisms are analyzed in detail by means of a VB study of some typical homonuclear bonds (H-H, H 3 C-CH 3 , H 2 N-NH 2 , HO-OH, F-F, and Cl-Cl), ranging from classical-covalent to fully charge-shift bonds. It is shown that CSbonding is characterized by a covalent dissociation curve with a shallow minimum situated at long interatomic distances, or even a fully repulsive covalent curve. As the atoms that are involved in the bond are taken from left to right or from bottom to top of the periodic table, the weakening effect of the adjacent bonds or lone pairs increases, while at the same time the reduced resonance integral, that couples the covalent and ionic forms, increases. As a consequence, the weakening of the covalent interaction is gradually compensated by a strengthening of CS-bonding. The large RE CS quantity of CS-bonds is shown to be an outcome of the mechanism necessary to establish equilibrium and optimum bonding during bond formation. It is shown that the shrinkage of the orbitals in the covalent structure lowers the potential energy, V, but excessively raises the kinetic energy, T, thereby tipping the virial ratio off-balance. Subsequent addition of the ionic structures lowers T while having a lesser effect on V, thus restoring the requisite virial ratio (T/ÀV = 1/2). Generalizing to typically classical covalent bonds, like H-H or C-C bonds, the mechanism by which the virial ratio is obeyed during bond formation is primarily orbital shrinkage, and therefore the charge-shift resonance energy has only a small corrective effect. On the other hand, for bonds bearing adjacent lone pairs and/or involving electronegative atoms, like F-F or Cl-Cl, the formation of
Journal of Physical Chemistry A, 2002
A semiempirical approach for constructing a universal ionic-covalent (UIC) potential energy curve is presented, and two related UIC functions are discussed. In the vicinity of the equilibrium bond length, the attraction between the atoms in the molecule (AIM) is modeled as purely Coulombic, -C/R, as implied by the asymptotic reference to the promoted valence-state energy of partially charged atoms Szentpály, L. v. J. Phys. Chem. A 1999, 103, 9313]. The partial charge is calculated by electronegativity equalization. Along the dissociation coordinate R, we model the decreasing contribution of "ionic structures" as a "soft" Coulson-Fischer transition: the composite UIC function is generated by continuously reducing the weight of the valencestate potential energy function by the admixture of a modified Morse function. Average unsigned errors of 1.42% and 1.16% of D e are obtained by comparing our five-parameter UIC and UIC R curves with the full Rydberg-Klein-Rees, or ab initio, curves of 42 covalent or polar diatomic molecules (from H 2 to NaCl). The evaluation of the rotation-vibration coupling constant, R e , requires only three parameters and yields an average unsigned error of 6.37% for 50 molecules.
New empirical potential energy function for diatomic molecules
Journal of Molecular Structure: THEOCHEM, 2004
A new empirical potential energy function for diatomic molecules, which has the simple form of VðrÞ ¼ ðar b þ mÞ=ð1 2 e nr Þ is introduced, where a; b; m and n are variational parameters of the function. To obtain the parameters of this function, it was fitted to the points, which were obtained previously from the RKR calculations for a wide range of diatomic molecules in both their ground and excited states. The reliability of the proposed function was checked by calculation of some spectroscopic constants such as bond length ðR e Þ; dissociation energy ðD e Þ; force constant ðK e Þ; rotational constant ðB e Þ; vibrational frequency ðv e Þ; anharmonicity constant ðv e x e Þ and vibration -rotation coupling constant ða e Þ for these molecules. For v e x e and a e the results were compared with those of the Morse, Rosen -Morse, Rydberg, Pöschl -Teller, Linnett, Frost -Muslin, Varshni, Lippincott and Rafi functions. Our results are consistent with or have less error than those of the above-mentioned functions. q
Physical Review A, 1992
The Kohn-Sham energy with exact exchange [using the exact Hartree-Fock (HF) exchange but an approximate correlation-energy functional] may be computed very accurately by adding the correlation obtained from the HF density to the total HF energy. Three density functionals are used: local spin density (LSD), LSD with self-interaction correction, and LSD with generalized gradient correction. This scheme has been extended (Lie-Clementi, Colle-Salvetti, and Moscardo-San-Fabian) to be used with general-valence-bond (GVB) energies and wave functions, so that the extra correlation included in the GVB energy is not counted again. The effect of all these approximate correlations on HF or GVB spectroscopic constants (R"co"and D,) is studied. Approximate relations showing how correlation affects them are derived, and may be summarized as follows: (1) the effect on R, and co, depends only on the correlation derivative at R"and (2) the effect on D, depends mainly on the correlation difference between quasidissociated and equilibrium geometries. A consequence is that all correlation corrections tested here give larger co, and D, and shorter R, than the uncorrected HF or GVB values. This trend is correct for D, for both HF and GVB. For R, and co" it is correct in most cases for GVB, but it often fails for the HF cases. A comparison is made with Kohn-Sham calculations with both exchange and correlation approximated. As a final conclusion, it is found that, within the present scheme, a qualitatively correct HF or GVB potential-energy curve, together with a correlation-energy approximation with correct dissociation behavior, is crucial for obtaining good estimates of spectroscopic constants.