The TCNE-benzene complex: A CNDO approach

CNDO/2 calculations on the TCNE-benzene complex are reported. A stable complex is found which exhibits a relatively large stabilization energy (0.2 a.u.) at a short interplanar separation (1.75 Å); the binding apparently arises solely through charge transfer. Mulliken population analyses were performed by reinterpreting the CNDO orbitals as Löwdin orbitals. Sample calculations on small organic molecules and first row diatomics indicate the procedure to be satisfactory. It is shown that generally only overlap populations that are summed over the orbitals of the atoms in question reflect the symmetry of the molecule.

---

Zusammenfassung Die Ergebnisse von CNDO/2-Rechnungen an Tetracyanaoäthylen-Benzol-Komplexen werden mitgeteilt. Es wird ein stabiler Komplex gefunden, der eine relativ große Stabilisierungsenergie (0,2 A.E.) bei geringem Abstand (1,75 Å) der Molekülebenen besitzt; die Bindung entsteht anscheinend nur durch Ladungsübertragung. Eine Populationsanalyse nach Mulliken wurde mit Hilfe der Interpretation der CNDO-Orbitale als Löwdin-Orbitale durchgeführt. Berechnungen an Beispielen wie kleinen organischen Molekülen und zweiatomigen Molekülen aus Elementen der ersten Reihe zeigen, daß die Methode befriedigende Ergebnisse liefert. Es wird gezeigt, daß im allgemeinen nur die Überlappungs-Populationen, die über die Orbitale der betrachteten Atome summiert werden, die Symmetrie des Moleküls widerspiegeln.

---

Résumé Calculs CNDO/2 sur le complexe TCNE-benzène. Un complexe stable apparaît pour une séparation interplan courte (1,75 Å) avec une énergie de stabilisation relativement forte (0,2 u.a); la liaison provient apparemment du seul transfert de charge. Une analyse de population de Mulliken a été effectuée en réinterprétant les orbitales CNDO comme orbitales de Löwdin. Des calculs échantillonés sur de petites molécules organiques et des molécules diatomiques de la première ligne montrent que le procédé s'avère satisfaisant. On montre qu'en général, seules les populations de recouvrement sommées sur les orbitales des atomes en question reflètent la symétrie de la molécule.

---

---

Supported in part by NASA University Sustaining Grant NGR 34-001-005 and National Science Foundation Grant GP-8298.     

Conformations of methylcyclooctane. A combined iterative force field—CNDO approach

A combined iterative force field—CNDO molecular orbital approach to conformations of methylcyclooctane is described. This hybrid method involves a full relaxation force-field calculation of conformer structures, followed by a single CNDO calculation on each structure.     

Automated site-directed drug design: An assessment of the transferability of atomic residual charges (CNDO) for molecular fragments

Summary In this paper a database of atomic residual charges has been constructed for all the molecular fragments defined previously in a combinatorial search of the Cambridge Structural Database. The charges generated for the atoms in each fragment are compared with charges calculated for whole molecules containing those fragments. The fragment atomic charges lie within 1 S.D. of the mean for 68%, and within 2 S.D. for 91%, of the atoms whose charges were computed for whole molecules. The actual charges on any atom are strongly influenced by the adjacent connected atoms. There is a large spread of atomic residual charge within the fragments database.     

The use of the CNDO method in spectroscopy

The Pariser approximation for the two center Coulomb repulsion integrals Γ _rshas been replaced by the Nishimoto-Mataga approximation in the original CNDO/S method. This modification has significantly improved the calculated position of the benzene ¹ B _1u ←¹ A _1g(¹ L _a ) electronic transition in benzenoid compounds. The calculation of transition moments of n — π ^* transitions is also considered. These moments vanish formally in any theory employing the ZDO approximation since integrals of the form 〈2s¦er2p〉 vanish even when the 2s and 2p atomic orbitals are on the same center. In this work the ZDO approximation is abandoned in the evaluation of the electronic transition moment resulting in calculated intensities for n — π ^*, ¹W←¹A, transitions which are in good agreement with experiment.     

Calculation of polarizabilities by the CNDO method

The polarizabilities of a number of small and medium size molecules are calculated using CNDO/2 wave functions. The first-order and the second-order perturbation equations are solved by variational procedures. The results are encouraging for the component along the internuclear axes and in the molecular plane. The effect of deorthogonalization of the CNDO/2 orbitals is discussed.     

Semi-classical approach to intensity spectrum of atomic hydrogen

The intensity of the dipole spectrum of atomic hydrogen is re-examined with the aid of classical, semi-classical and quantum-mechanical approximations. Quantum-mechanical data can be approached semi-classically on condition the differences between the discrete time periods of hydrogen levels are applied in the calculations.     

An attempt to approach the solutions of the N-electron CNDO hamiltonian for conformational studies of conjugated systems

While dealing with the same N-electron hamiltonian, the SCF approximation and the third order PCILO results completely disagree about the conformation of conjugated systems. An attempt is made, using the CIPSI algorithm, to approach the exact solution of the N-electron CNDO hamiltonian for the butadiene molecule starting from either a priori localized, SCF localized or SCF delocalized MO's. The partial CI's performed from SCF delocalized MO's give arbitrary results when increasing the number of doubly excited determinants and Pancir?'s recent results is fortuitous; on the contrary the results obtained from a priori and SCF localized MO's have better convergence and consistence; from our best calculations the final solution seems to be a rather flat potential curve, the stable minimum being the trans planar minimum, with a second gauche minimum for θ = 120-150° (ΔH ≈ 1 kcal/mole), separated by a weak barrier (ΔH ≈ 2 kcal/mole). The third order PCILO solution is in much better agreement with this estimate of the exact solution than the SCF one.     

CNDO calculations of diamagnetic shielding in molecules

An approximate procedure for the calculation of diamagnetic shielding in molecules is presented. The method proposed is based on the ‘complete neglect of differential overlap’ (CNDO) molecular wave functions and is formulated according to the zero differential overlap (ZDO) approximation. The results obtained with several CNDO-type wave functions for diatomic and polyatomic molecules are in very good agreement with non-empirical SCF calculations. The ¹⁴N diamagnetic shielding constants in several molecules were computed and some approximations usually adopted in the interpretation of ¹⁴N chemical shifts are critically discussed. It was shown that in some cases the observed ¹⁴N chemical shifts cannot be interpreted solely in terms of the paramagnetic contribution to the shielding constant.     

A new approach to the problem of atomization in electrothermal atomic absorption spectrometry

We established that the partial pressure of oxygen in the graphite furnace is several orders of magnitude higher than is explained by the thermodynamic equilibrium of the O₂ + 2C = 2CO reaction. Taking this into account has led us to some new conclusions for thermal destruction, atomization and dissociation of the oxygen-containing compounds. It explains why some elements are reduced to the metal on graphite, while other elements are vaporized as the oxide. For the elements vaporized as the oxide, it is shown that there is good agreement between the calculated thermal dissociation temperature of the metal oxide and the observed appearance temperature. For these metals, the atomization of the oxides proceeds by their thermal dissociation without direct participation of carbon in the reduction process. The presence of oxygen in the purge gas accounts for anomalous curvature in, for example, the Sn calibration curve, large variations in sensitivity for some elements (Ga, Ge, Sb, Se, Sn) comparing gas-stop and full-flow modes of atomization and the enhancement of sensitivity for some elements in oxygen-activated graphite furnaces.     

Alternative implementation of the unitary group approach to the atomic shell theory

Subduction coefficients adapted to the group chain, which appeared in Racah's treatment of fⁿ configurations, are defined and calculated in the unitary group approach. The coefficients are then utilized to construct successively adapted term functions and evaluate other interesting coefficients. In addition the simplified expressions for the Coulomb and spin-orbit operators are obtained in terms of generators.     

CNDO/2 calculations of the interactions of formamide and imidazole: Another failure of CNDO/2

The interaction energy curve of the formamide-imidazole system resulting from the CNDO/2 method exhibits a superfluous minimum at a much smaller separation than the sum of the van der Waals radii of the closest atoms, and a low maximum at this latter distance. The relationship of the interaction energies using CNDO/2 and those using the atomic charges on the two molecules is investigated for this system.     

An efficient approach to calculation of zero-flux atomic surfaces and generation of atomic integration data

A new robust method for variational determination of atomic zero-flux surfaces is presented. The zero-flux surface sheets are expressed in terms of variational trial functions in prolate spheroidal coordinates. The trial functions are optimized with a Newton procedure to satisfy the zero-flux condition on a grid. The data required for radial integrations are generated by an adaptive quadrature procedure that employs model electron densities and utilizes an original third-order algorithm for linear search. Results of test calculations involving variational determination of atomic surfaces are presented for a representative set of 20 molecules. The new approach is both less time consuming and substantially more accurate than the previously published algorithms. © 1995 John Wiley & Sons, Inc.     

CNDO study of the properties of the OH group

Using the CNDO method, a number of hydroxy-containing systems were calculated. The results are compared with IR spectroscopic data on the frequencies and acidity of surface OH groups and their dependence on the electronegativity of the atom to which the OH group is bonded.

---

. - OH- , .

     

CNDO Studies on the Intermolecular Coulomb Interaction in TCNQ-Stack

The coulomb interaction among TCNQ- molecules in different stack structures was calculated by quantum chemistry CNDO method. The results of the calculation would be used to explain the change rule of the electricity property of the TCNQ complex very well.     

Improved local density functional approach for atomic systems

Through a new local density approximation to the kinetic energy density functional introduced by us recently, a simple Thomas–Fermi-like scheme for the direct calculation of electron density in atoms is proposed. The calculated density is nonsingular at the nucleus and the energy values are in very good agreement with the corresponding Hartree–Fock results for atoms. © 1994 John Wiley & Sons, Inc.     

CNDO calculations of N-methyl substituted acrylamides

CNDO calculation is made for N-methyl acrylamide (both incis andtrans configurations) and N,N-dimethyl acrylamide. The charges, bond orders and dipole moments are discussed and compared with those of acrylamide. Thetrans form of N-methyl acrylamide is found to be more stable thancis isomer by 4.5 kcal/mole.     

A few level approach for the electronic partition function of atomic systems

A simplified model to calculate partition functions and thermodynamic properties of atomic species is presented. This model consists in grouping the atomic states in few virtual levels. Their statistical weights and energies are calculated summing or mediating over the states belonging to each group. The partition function is calculated considering the virtual levels to follow the Boltzmann distribution. A theoretical foundation of the model has been described for a general case and verified for hydrogen, oxygen and nitrogen atoms. Two- and three-level models are adequate to keep the error within a few percent. A second order correction term can also be added to further reduce the error without changing the advantages of the model.     

CNDO bonding parameters in transition metal atoms

The method of calculating CNDO bonding parameters developed recently is extended to transition metal atoms. It is shown that one of the approximations introduced earlier can also be deduced by a more complete treatment of the imbalance problem in CNDO-MO theory. The conventionally calibrated bonding parameters indirectly incorporate important contributions from two-particle interactions. The parameters developed are used to compute the coefficients of metal-to-ligand transfer of spin in many hexafluro metallate ions of transition metals. The results are compared with those obtained by conventional CNDO-MO calculation. Comparison of the computed bonding parameters with other available values is also made.