Energy-adjustedab initio pseudopotentials for the second and third row transition elements

Summary Nonrelativistic and quasirelativisticab initio pseudopotentials substituting the M^(Z–28)+-core orbitals of the second row transition elements and the M^(Z–60)+-core orbitals of the third row transition elements, respectively, and optimized (8s7p6d)/[6s5p3d]-GTO valence basis sets for use in molecular calculations have been generated. Additionally, corresponding spin-orbit operators have also been derived. Atomic excitation and ionization energies from numerical HF as well as from SCF pseudopotential calculations using the derived basis sets differ in most cases by less than 0.1 eV from corresponding numerical all-electron results. Spin-orbit splittings for lowlying states are in reasonable agreement with corresponding all-electron Dirac-Fock (DF) results.     

Complexes of 2,5-dihydroxy-1,4-benzoquinone and chloranilic acid with second and third row transition elements

The synthesis of new 2,5-dihydroxy-1,4-benzoquinone (H2DHBQ) and chloranilic acid (H2CA) complexes cis-[Mo2O5L2]2– (L = DHBQ, CA), [W2O5(DHBQ)2]2–, [WO2(CA)2]2–, trans-[UO2(DHBQ)]·H2O, and trans-[UO2(CA)2]2– is described. Raman, i.r., 1H and 13C n.m.r spectra of the ligands and their complexes are reported and their structures discussed.     

Nonempirical electron-correlation calculations on ALik and ALi k+1 + clusters formed with elements from the second and third periods

Institute for New Chemical Problems, USSR Academy of Sciences. Translated from Zhurnal Strukturnoi Khimii, Vol. 29, no. 3, pp. 12–21, May–June, 1988.     

One-center expansion for pseudopotential matrix elements

Semilocal pseudopotential operators can be expressed as a linear combination of nonlocal (projection) operators. Pseudopotential operator integrals over a molecular basis set are therefore reduced to linear combinations of overlap integrals products. Molecular calculations indicate that sufficient precision can be achieved with a limited number of nonlocal operators. Analytic derivatives of pseudopotential integrals are easily deduced and implemented in a standard quantum chemistry program.     

Applications of the INDO method to some radicals containing second row elements

The INDO molecular orbital method has been extended to allow for the inclusion of second row atoms. Equilibrium geometries have been calculated for several radicals containing Si and Cl and their hyperfine coupling constants compared with results from ESR experiments. The calculations give chlorine hyperfine splittings in organic radicals in good agreement with recently reported experimental values.

---

Zusammenfassung Die INDO Molekularorbital-Methode wurde so erweitert, daß sie auch auf Atome der 2. Periode anwendbar ist. Es wurden die Gleichgewichtsgeometrien für einige Si und Cl enthaltende Radikale berechnet. Die ebenfalls errechneten Hyperfeinkopplungs-Konstanten werden mit Resultaten aus ESR Experimenten verglichen. Für organische Radikale erhält man Chlor-Hyperfein-Aufspaltungen in guter Übereinstimmung mit neueren experimentellen Werten.

     

pi bonding in second and third row molecules: testing the strength of Linus's blanket

The flexibility of valence bond (VB) theory provides a new method of calculating pi-bond energies in the double-bonded species H(m)A=BH(n), where A, B = C, N, O, Si, P, S. This new method circumvents the problems usually associated with obtaining pi-bond strengths by targeting only the pi bond, while all other factors remain constant. In this manner, a clean separation between sigma- and pi effects can be achieved which highlights some expected trends in bond strength upon moving from left to right and up and down the Periodic Table. Intra-row pi bonds conform to the classic statement by Pauling [L. Pauling, The Natiure of the Chemical Bond, Cornell University Press, Ithaca, 1960, 3rd edition] regarding the relationship of heteronuclear bond strengths to their homonuclear constituents whereas inter-row pi bonds do not. This variance with Pauling's statement is shown to be due to the constraining effect of the underlying sigma bonds which prevents optimal p(pi)-p(pi) overlap. While Pauling's statement was based on the assumption that the resonance energy (RE) would be large for heteronuclear and small for homonuclear bonds, we have found large REs for all bonds studied herein; this leads to the conclusion that REs are dependent not only on the electronegativity difference but also the electronegativity sum of the constituent atoms. This situation where the bond is neither covalent nor ionic but originates in the covalent-ionic mixing has been termed charge shift (CS) bonding [S. Shaik, P. Maitre, G. Sini, P. C. Hiberty, J. Am. Chem. Soc. 1992, 114, 7861]. We have shown that CS bonding extends beyond single sigma bonds in first row molecules, thus supporting the idea that CS-bonding is a ubiquitous bonding form.     

Gaussian basis sets for the fifth row elements,Mo-Cd,and the sixth row elements W-RN

Gaussian basis sets, consisting of 17 s-type, 12 p-type, and 8 d-type functions, for the fifth row elements, Mo? Cd, and 19 s-type, 14 p-type (16 p-type), 10 d-type and 5 f-type functions for the sixth row elements, W? Rn, are presented. The basis sets are of double zeta quality, and are optimized to .002 a.u. in the energy. The energies are compared with D.Z. STO basis sets.     

An evaluation of the performance of diffuse function-augmented basis sets for second row elements,Na-Cl

The energetic effects of the addition of diffuse functions to molecules with second-row elements are much less dramatic than those for their first-row counterparts. Although diffuse functions on second-row elements have little effect on the geometries and vibrational frequencies of neutral molecules, significant changes are found for anions. While the largest basis set, 6-31 + G*, generally performs best, the results at 3-21 + G* are comparable, and this basis can be recommended for practical applications.     

NMR studies of some second and third row transition metal complexes of monothio-β-diketones

A ¹³C and ¹⁹F NMR study of twenty-four ruthenium, rhodium, palladium and platinum complexes containing a difluoromethyl or a trifluoromethyl substitutent(R′) on the monothio-β-diketone, RCSCH₂COR′, is reported. The R-substituents are 2′-thienyl, 2′-naphthyl, phenyl, p-fluorophenyl or p-methylphenyl. The ¹³C NMR data show the chemical shift of the diketonate ring carbons to be geometry dependent. Similarly, the ¹⁹F NMR spectra show chemical shift data which are also metal dependent. The thiocarbonyl and methine carbon's shieldings are also dependent on the nature of the R-group. The rhodium and platinum complexes show carbon-metal and carbon-fluorine spin coupling. The paramagnetic ruthenium(III) complexes give ¹⁹F NMR spectral resonances which are broad and shifted upfield from the corresponding diamagnetic rhodium, palladium and platinum complexes. ¹³C and ¹⁹F NMR data supports a facial octahedral geometry for the rhodium(III) complexes.     

Numerical instabilities in the computation of pseudopotential matrix elements

Steep high angular momentum Gaussian basis functions in the vicinity of a nucleus whose inner electrons are replaced by an effective core potential may lead to numerical instabilities when calculating matrix elements of the core potential. Numerical roundoff errors may be amplified to an extent that spoils any result obtained in such a calculation. Effective core potential matrix elements for a model problem are computed with high numerical accuracy using the standard algorithm used in quantum chemical codes and compared to results of the MOLPRO program. Thus, it is demonstrated how the relative and absolute errors depend an basis function angular momenta, basis function exponents and the distance between the off-center basis function and the center carrying the effective core potential. Then, the problem is analyzed and closed expressions are derived for the expected numerical error in the limit of large basis function exponents. It is briefly discussed how other algorithms would behave in the critical case, and they are found to have problems as well. The numerical stability could be increased a little bit if the type 1 matrix elements were computed without making use of a partial wave expansion.     

Gaussian basis sets for the first and second row atoms

Gaussian basis sets consisting for first row atoms of 7 s-type and 3 p-type and for second row atoms of 10 s-type and 6 p-type functions with optimized exponents are reported. These basis sets consists of at least two functions per atomic orbital.

---

Zusammenfassung Es werden für die Atome der ersten und zweiten Reihe Basissätze aus Gaußfunktionen mitgeteilt, die aus 7 Funktionen vom s-Typ und 3 Funktionen vom p-Typ für die Elemente der ersten Reihe und 10 Funktionen vom s-Typ und 6 Funktionen vom p-Typ für die Elemente der zweiten Reihe mit optimierten Exponenten bestehen. Diese Basissätze bestehen aus wenigstens zwei Funktionen pro Atomorbital.

---

Résumé Une base de 7 gaussiens du type s et 3 du type p est presenté pour les éléments du premier rang et de 10 gaussiens du type s et 6 du type p pour des éléments du deuxième rang; les exposants sont optimisés. Les bases consistent au moins en deux fonctions par orbital atomique.

     

Nonempirical calculations of NQR spectrum parameters of azetidine

The components of the¹⁴N electric field gradient (EFG) tensor, the corresponding nuclear quadrupole coupling constant (NQCC) , and the asymmetry parameter of azetidine were calculated using the restricted Hartree-Foek-Roothaan method, The geometry of azetidine was optimized with the 4–31G basis set, and the values of the ring puckering angle () and the angle between the N-H bond and the CNC plane () were refined with the 6–31G^* basis set. The effect of choice of geometry on calculated NQR parameters was studied. To clarify the origin of EFG at the nitrogen atom nucleus, the contributions from individual bond orbitals and lone electron pairs to the EFG tensor componentseq _ii were calculated in the framework of the LMO approach. It was demonstrated that the 4-31G + 6–31G^*//6–31G^* level calculations give NQCC and values of azetidine that are in good agreement with the results of MW spectroscopy.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No, 12, pp. 2886–2889, December, 1996.     

Ab-initio and pseudo-potential calculations on some first,second and third row molecules. A comparative study

Calculations using a pseudo-potential approach on test systems containing first, second and third row elements have been performed. Orbital energies and equilibrium geometries are compared with results obtained from full SCF calculations.     

Energy-adjustedab initio pseudopotentials for the second and third row transition elements: Molecular test for M2 (M=Ag,Au) and MH (M=Ru,Os)

Summary Recently published nonrelativistic and quasirelativistic energy-adjustedab initio pseudopotentials representing the M^(Z–28)+ cores of the second row transition metal atoms and the M^(Z–60)+ cores of the third row transition metal atoms have been tested in SCF, CI(SD) and CEPA1 calculations of the spectroscopic constants (R _e ,D _e , and _e ) of the ground states of the neutral and singly charged silver and gold dimers, and in state averaged CASSCF and multi-reference CI(SD) calculations of the spectroscopic constants (R _e ,D _e , _e , _e , /R). Comparison is made with experimental and reliable theoretical data where available; in the case of the hydrides, additional calculations with pseudopotentials published by other groups have been made for comparison.     

Influence of the heteroatom on the structure,bonding and ring strain of a series of three-membered rings containing a second,third, fourth and fifth row elements: a theoretical investigation

Three-membered heterorings have received a great interest for the design of organic reactions and new active therapeutic agents. However, there is little information available in the literature about their structural properties, in particular for those containing third, fourth, and fifth row elements. With this in mind, structure, bonding, ring strain, and Mulliken charge distribution of a series of 22 saturated three-membered rings containing a second, third, fourth, and fifth row element were theoretically investigated. Calculations were carried out within the MP2, PBE1PBE, and CCSD approximations using Pople’s and correlation consistent basis sets. In general, structural predictions obtained by MP2 and coupled cluster are comparable with each other for the studied heterocycles, and their predictions are in good agreement with the little experimental data available. The structural parameters, ring strain, and Mulliken charges are strongly affected by the nature of heteroatom contained into ring skeleton, finding a consistent periodic relationship according to the row-group or row-period plot. The ring geometry was highly symmetric in most of the studied cases (C_2h), except for the rings containing V-group elements (C_s) whose molecular symmetry is distorted by the disposition out of molecular plane of H-heteroelement bond. Finally, the increase of heteroatomic radius increases significantly the molecular strain of these three-membered heterocycles, being especially notable in the four and fifth row element rings. Curiously, the rings containing tellurium, iodium, bromo, chloro, and sulfur presented a ring strain comparable to those common heterocycles containing second row element.     

Development of new pseudopotential methods: improved model core potentials for the first-row transition metals

We have recently developed new nonrelativistic and scalar-relativistic pseudopotentials for the first-row transition metal and several main-group elements. These improved Model Core Potentials were tested on a variety of transition metal complexes to determine their accuracy in reproducing electronic structures, bond lengths, and harmonic vibrational frequencies with respect to both all-electron reference data as well as experimental data. The new potentials are also compared with the previous model core potentials available for the first-row transition metals. The new potentials do a superior job at reproducing atomic data, reproduce molecular data as well as the previous version, and in conjunction with new main-group pseudopotentials that have L-shell structure of the valence basis set, they are slightly faster.