Effective Core Potential Basis Sets Research Articles

Density Functional Study of Tetra-Atomic Clusters and Complexes of the Group 16 Elements: Trends in Structure and Bonding

Se4 and Te4 clusters, the intermolecular weakly bound charge-transfer complexes (Se2)(O2) and (Te2)(O2), and the (X2)(Y2) and (XY)(YX) mixed compounds (X, Y = S, Se, Te) were studied using density functional theory with all-electron and effective core potential basis sets. All systems were analyzed in terms of interactions of the diatomic molecules. The energetically favorable “face to face” binding of the diatomic moieties leads to delocalization of the electron density in the high-lying occupied π*-antibonding orbitals of the tetra-atomic products. The cis structures are minima for all systems considered as a result of this delocalization. For the clusters and mixed compounds, the cis tetra-atomic structures formed by the diatomic molecules with equal or similar electronegativities correspond to global minima. The diatomic fragments in these compounds are nonparallel and have shorter bond lengths than their free diatomic values. For the clusters, the other low-lying isomers ordered according to their re...

Experimental Investigations and Ab Initio Studies of Tellurium(II) Dithiolates, Te(SR)(2).

The reaction between Te(O(i)Pr)(4) and HSR offers a new and effective route to tellurium dithiolates, Te(SR)(2). Te(S(i)Pr)(2) (1) and Te(S(t)Bu)(2) (2) are stable compounds whereas Te(SPh)(2) (3) slowly decomposes at room temperature to give Te and Ph(2)S(2). IR spectra of 1-3 and ab initio calculations (HF/3-21G(d) and MP2 with double-zeta polarization effective core potential basis set) show nu(as)(Te-S) and nu(s)(Te-S) to be around 340 and 380 cm(-)(1), respectively. UV spectra exhibit similar lambda(max) (346-348 nm) for all three compounds, with the greater extinction coefficient of 3 accounting for its different and more intense color. Analysis of the molecular orbitals of the model compound Te(SCH(3))(2) shows that the phototransition is likely to be of n(p)(Te)-sigma(Te-S) type, thus rationalizing the instability of 3 when irradiated. Single-crystal X-ray diffraction of 1-3 revealed the following basic structural parameters: 1 d(av)(Te-S) 239.4(1) and d(av)(S-C) 183.8(5) pm, angle(STeS) 99.61(4) and angle(av)(TeSC) 105.8(3) degrees, tau(CSTeS) 77.0(2) and 90.3(2) degrees; 2 d(Te-S) 239.1(1) and d(S-C) 186.4(2) pm, angle(STeS) 103.88(2) and angle(TeSC) 107.6(1) degrees, tau(CSTeS) 78.01(8) degrees; 3 d(Te-S) 240.6(2) and d(S-C) 177.4(7) pm, angle(STeS) 100.12(6) angle(TeSC) 103.2(2) degrees, tau(CSTeS) 69.0(3) and tau(CCSTe) 81.6(6) degrees. Geometries of model compounds Te(SH)(2) and Te(SCH(3))(2) optimized at the MP2 level exhibit d(Te-S), angle(STeS), and tau(XSTeS) (X = H, C) values similar to those of 1-3. Natural bond orbital analysis revealed n(p)(S(1))-sigma(Te-S(2)) hyperconjugation as the cause for the CSTeS torsion angles being close to 90 or -90 degrees. Thermochemical calculations on the HF and MP2 level proved Te(SH)(4) to be unstable with respect to Te(SH)(2) and HSSH, thus rationalizing the reduction of Te(IV) to Te(II) when Te(O(i)Pr)(4) or TeO(2) are reacted with thiols. NMR spectra reveal ligand exchange reactions between different tellurium(II) dithiolates and between Te(SR)(2) and HSR'. These types of reaction offer other routes to tellurium(II) dithiolates.

Theoretical studies on the formation of mercury complexes in solution and the dissolution and reactions of cinnabar

Expanding upon our previous studies of the properties of Au complexes, we present calculations for several Hg (super 2+) species in aqueous solution and for molecular models for cinnabar. Hydration effects are treated with a combination of supermolecule calculations containing several explicit water molecules and polarizable continuum calculations. We focus upon the following problems: (1) calculation of the stabilities of HgL 2 , L = F (super -) , Cl (super -) , OH (super -) , SH (super -) , and CN (super -) and HgCl n (super 2-) n n = 1-4; (2) development of a molecular model for cinnabar of the form Hg 3 S 2 (SH) 2 ; and (3) dissolution or adsorption reactions using this cinnabar model. The absolute and relative formation enthalpies of the HgL 2 species can be satisfactorily reproduced at the Hartree-Fock plus Moller-Plesset second order correlation correction level using relativistic effective core potential basis sets if the hydration of neutral HgL 2 is explicitly taken into account. Evaluating the energetics for the series of complexes HgCl n (super 2-) n is more difficult, because great accuracy is needed in the large hydration energies and some of the species are highly nonspherical. The Hg 3 S 2 (SH) 2 species shows an equilibrium structure very much like that in cinnabar. The relative energetics for dissolution of cinnabar by H 2 O, H 2 S, SH (super -) , and SH (super -) +elemental S are correctly reproduced using this model molecule. Calculations on Hg 3 S 2 ClI provide a model for understanding the adsorption of I (super -) ions on cinnabar surfaces in the presence of Cl (super -) .

Quantum chemical study of low pressure gas phase cationic polymerization reactions in the Ti + and isobutylene system

Ab initio quantum chemical calculation of the low-pressure Ti +/isobutylene system in the initiation reaction of gas-phase cationic polymerization has been carried out by using the Hartree–Fock method with Hay–Wadt's effective core potential (ECP) and Huzinaga's MIDI-4p basis set, second-order Møller–Plesset (MP2) perturbation, single and double excitation configuration interactions (SDCI), B3LYP density-functional method, and coupled cluster [CCSD(T)] method. For the dπ-complex TiC 4H 8 + ( 1) and first product TiC 4H 6 + ( 2), the quartet state is more stable than the doublet. The reaction enthalpy from reactant to first product is 23.7 kcal/mol at the CCSD(T)/HUZSP*//B3LYP/HUZSP* level. We found a new allyl-complex TiHC 4H 7 + ( 3) as an intermediate of the initiation reaction and two transition states, and a remarkably interesting mechanism of hydrogen elimination that is completely consistent with the experimental result by Daly and El-Shall [J. Phys. Chem. 98 (1994) 696].

Ab Initio Molecular Orbital Study of XO2+(X = F, Cl, Br, I) Systems

The depletion of stratospheric ozone has resulted in an increasing interest in the study of the possible reaction mechanisms responsible for its depletion. The structures and relative stabilities of the cationic forms of the halogen dioxides have been studied by means of ab initio molecular orbital calculations. For fluorine- and chlorine-containing compounds the geometries and the harmonic vibrational frequencies of all possible isomers were calculated at the QCISD/6-311+G(2d) level of theory. For bromine- and iodine-containing compounds the effective core-potential basis sets of Hay and Wadt, modified to include a set of diffuse functions and two sets of polarization functions, were employed. For all systems the final energies were obtained at the QCISD(T)/6-311+G(3df) level of theory. In addition, multiconfiguration-based methods have also been used. The relative stabilities of structures XOO{sup +} and OXO{sup +} are greatly reduced relative to those observed for the corresponding neutral species. In fact, for Cl and I derivatives, the lowest energy isomer corresponds to the symmetric OXO{sup +} open-chain species. The corresponding cyclic structures arise as local minima on the respective potential energy surfaces, but they lie much higher in energy than the OXO{sup +} open-chain form or the XOO{sup +} isomer. There are significant differencesmore » in bonding between XOO{sup +} and OXO{sup +}, the X-O interaction in OXO{sup +} being more covalent than in XOO{sup +}. There are also trends along the series that reflect the pronounced disparity between the electron affinity of F{sup +} and those of the heavier atoms of the group. FOO{sup +} species can be viewed as F({sup 2}P)-O{sub 2}{sup +} complexes, whereas XOO{sup +}(X = Br, I) species can be regarded as X{sup +}({sup 3}P)-O{sub 2} complexes. The OXO{sup +} open-chain species have an electron charge distribution similar to that of the ozone molecule, reflecting the same number of valence electrons in each case.« less

Intermolecular Self-Interactions of the Titanium Tetrahalides TiX4 (X = F, Cl, Br)

Ab initio calculations have been performed on the closed-shell molecules TiX4 and Ti2X8 (X = F, Cl, Br) in order to determine the magnitude and the nature of the intermolecular self-interactions of the titanium tetrahalides. Geometry optimizations have been carried out using an effective core potential basis set with polarization, including the effects of dynamic electron correlation through second-order perturbation theory (MP2). The importance of higher order correlation effects is examined through coupled cluster single-point energy calculations. Basis set effects are investigated using MP2 single-point energy calculations with large all-electron basis sets. Ti2F8 is predicted to be a bound C2h dimer with bridging bonds, lower in energy than the separated monomers by 10.5 kcal/mol. Ti2Cl8 and Ti2Br8 are predicted to be weakly bound dimers whose structures are that of associated monomers with overall D3d point group symmetry. Ti2Cl8 is lower in energy than separated monomers by 4.9 kcal/mol. Transition ...

Conformational analysis of the isomers of lewisite

Chlorovinyldichloroarsine, also known as lewisite, is a powerful vesicant that has been used in the past as a chemical weapon. Its extreme toxicity makes obtaining experimental data to characterize this notorious chemical system very challenging. In this work, ab initio calculations were carried out on the geminal, cis and trans isomers of lewisite at a variety of levels of theory employing both all-electron and effective-core potential basis sets. The aims were to ascertain the relative stability of these three isomeric forms of lewisite and to characterize their structures, dipole moments and conformational preferences. The trans isomer of lewisite was found to be the most stable and the geminal isomer the least stable. This is consistent with the experimental data available on the compositions of lewisite mixtures. Copyright  1999 John Wiley & Sons, Ltd.

Comprehensiveab initio quantum mechanical and molecular orbital (MO) analysis of cisplatin: Structure, bonding, charge density, and vibrational frequencies

We carried out an extensive series of ab initio quantum mechanical (QM) calculations using pure effective core potential (ECP) and hybrid Hartree–Fock (HF)/ECP basis sets, including various electron correlation treatments up to the MP4 level on cis-diamminedichloroplatinum(II), cisplatin, an important anticancer drug. The optimized geometric parameters and vibrational frequencies of cisplatin were compared with the experimental values, and the effects of varying basis sets and correlation (MP level) treatments on the geometric parameters and vibrational frequencies were analyzed. We also present a detailed description of the bonding in cisplatin using qualitative MO analysis to characterize the key intramolecular interactions in cisplatin. The calculated molecular electrostatic potential (MEP), and the electrostatic potential (ESP) charges for cisplatin were used to identify regions of electrophilic and nucleophilic character. The charge density and the Laplacian of charge density of cisplatin were calculated to determine its bonding relationships. Using basis set performance, we identified two hybrid basis sets (HF/6-311G* and MP2/6-311G*) as basis sets of choice for studying cisplatin's molecular properties. Furthermore, we recommend a hybrid ECP/HF approach with electron correlation for the most accurate physicochemical and electronic description of cisplatin and related compounds. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 365–382, 1999

Computational approaches to the study of some lanthanide (III)-polyazamacrocyclic chelates for magnetic resonance imaging

A set of parameters consistent with the CHARMM force field has been determined for molecular dynamics simulations of several DOTA– and DOTP–Ln(III) chelates. Bonding and van der Waals parameters were derived from the available experimental data and analogy to similar ones in the existing force field. Net atomic charges were derived from ab initio calculations at the Hartree–Fock level to reproduce molecular electrostatic potentials (ESPs), with an effective core potential (ECP) basis set for the metal ion and the 6-31G* basis set for the ligand atoms. The charges are consistent with the TIP3P water model. Preliminary molecular dynamics simulations of the lanthanide chelates in aqueous solution were performed using the Nose–Hoover thermostat at 300 K. The new parameters correctly predicted the molecular structures and stability of the chelates major and minor isomers. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 73: 237–248, 1999

Effective Nuclear Charges for the First- through Third-Row Transition Metal Elements in Spin−Orbit Calculations

The effective nuclear charges (Zeff), which are empirical parameters in an approximate spin−orbit Hamiltonian, are determined for the first- through third-row transition metal elements by using experimental results for the fine structure splittings in atomic terms. All calculations use multiconfiguration self-consistent-field (MCSCF) wave functions, whose active space includes nd and (n + 1)sp orbitals (n is the principal quantum number), with the effective core potential (ECP) basis sets proposed by Stevens et al., augmented by one set of polarization f functions. First-order or second-order configuration interaction (FOCI or SOCI) calculations were also performed in order to understand disagreements between the MCSCF results and the experimental ones.

The First Carbonyl Bond Dissociation Energies of M(CO)5and M(CO)4(C2H2) (M = Fe, Ru, and Os): The Role of the Acetylene Ligand from a Density Functional Perspective

Recent kinetics experiments have shown that the rate of carbonyl substitution in complexes of the type M(CO)4(C2R2), where M = Fe, Ru, and Os, is accelerated by factors of 102−1013 over their respective pentacarbonyl complexes. These substitution reactions have been shown to be dissociative in nature and show a marked metal dependence of the rate. The origin of the increased reactivity of these alkyne complexes was studied with nonlocal density functional theory (BLYP functional), using both effective core potential (ECP) and all-electron basis sets, in conjunction with Frenking's charge decomposition analysis (CDA) scheme and Bader's atoms in molecules (AIM) analysis. We found that the BLYP/ECP method predicted geometries very close to experiment for both the parent carbonyl and alkyne complexes (in this study C2H2 was used). The calculated CO bond dissociation energies (BDEs) were also found to agree well with experiment and mirrored the metal dependence trends observed experimentally for both M(CO)5 an...

Theoretical Study of the Photodecomposition of Methyl Hg Complexes

Methyl mercury, an important pollutant, decomposes photolytically when exposed to sunlight. Methyl mercury chloride has been shown to yield CH3 and HgCl radicals upon photodecomposition under UV irradiation. We have calculated spectral transition energies for a number of methylmercury species using quantum mechanical methods, specifically the Hartree−Fock method, the Moller−Plessett second order perturbation theory method (MP2), and the configuration interaction singles method using polarized double-ζ relativistic effective core potential basis sets. We find that singlet to triplet absorptions occur at lower energy than that of singlet to singlet absorptions, by about 2−3 eV. The calculated singlet to triplet (S−T) energy is much lower for 1-coordinate CH3Hg+ than for the 2-coordinate species CH3HgL, where L=CH3-, OH2, OH-, Cl-, or SH-. Of the 2-coordinate species studied, CH3HgOH2+ and CH3HgSH show the lowest energy S−T transitions, with calculated maxima just below 5.0 eV (at the MP2 level). They should...

A Study of the X̃2A2 State of KO2 Using Ab Initio and Density Functional Theory: The Equilibrium Geometry and Vibrational Frequencies

Geometry optimization and harmonic vibrational frequency calculations at the CASSCF, CISD, UMP2, QCISD, CCSD(T), and B3LYP levels of theory were performed on the ground X̃2A2 state of KO2. Various augmented effective core potential and all-electron basis sets were employed, with the largest being the LANL2DZ effective core potential augmented by [6s6p3d1f], the all-electron basis set, [11s10p5d3f] for potassium, and aug-cc-pVTZ for oxygen. These calculations lead to the conclusion that the C2v X̃2A2 state of KO2 has an equilibrium bond angle of 32.3 ± 0.5°, a K−O bond length of 2.410 ± 0.005 Å, and an O−O bond length of 1.341 ± 0.001 Å. The B3LYP density functional method is also employed for NaO2 and LiO2, and it appears that this method does not suffer from symmetry breaking; a similar conclusion is reached for the QCISD method. Isotopic shifts for all three molecules are also reported and compared to available experimental values.

Ab initio calculations of the structures and energies of Ge(CH3)2 from tetramethylgermane in CVD

Ab initio calculations, at several levels of theory including density functional calculations, have been performed with a high level basis set for the two lowest energy states of Ge(CH3)2. The ground-state singlet configuration, at the B3PW91/“6-311G*” level of theory, is one in which the methyl hydrogen atoms are eclipsed. Similar calculations at the same level of theory, but using an effective core potential (ECP), do not predict the same geometry, indicating that the use of ECP basis sets should be avoided in organogermanium molecules. This is the first reported calculation of the configurations and energies of Ge(CH3)2 and is expected to be useful in the spectroscopic monitoring of CVD processes involving tetramethylgermane, a common precursor. Comparison is made to calculations for GeH2 performed at the same levels of theory. © 1998 John Wiley & Sons, Inc. Heteroatom Chem 9:195–200, 1998

Extension of the PS-GVB electronic structure code to transition metal complexes

We have developed a parameterization enabling ab initio electronic structure calculation via the PS-GVB program on transition-metal-containing systems using two standard effective core potential basis sets. Results are compared with Gaussian-92 for a wide range of complexes, and superior performance is demonstrated with regard to computational efficiency for single-point energies and geometry optimization. Additionally, the initial guess strategy in PS-GVB is shown to provide considerably more reliable convergence to the ground state. © 1997 John Wiley & Sons, Inc. J Comput Chem18: 1863–1874, 1997

A theoretical study of the photoinduced desorption of I — from a CF3I dimer

Ab initio SCF-MO calculations using effective core-potential basis sets are employed to evaluate ionization potentials and electron affinities for CF3I and the geometries and energies of the singlet and triplet states of the CF3I dimer. The calculated geometry of the single state of the dimer is in qualitative agreement with the experimental geometry for condensed phase CF3I. The calculated energy for vertical excitation from the singlet to the triplet state is 4.1 eV at the Hartree-Fock level and 4.5 eV after incorporation of correlation at the Moller-Plesset 2nd-order level, consistent with excitation by 193 nm (6.4 eV) light. The equilibrium geometry of the triplet consists essentially of a CF3I+, CF3I− ion pair, in which the CI bond distance in the anionic component has increased to 5.5Å, compared with 2.1Åin the neutral molecule. The calculated binding energy of the triplet ion pair is about 4 eV.

Regioselective reactions of isothiocyanates with the titanocene vinylidene fragment [Ti(CCH2)(η-C5Me 5)2]. Crystal and molecular structure of [Ti{SC(NC6H11)C CH2}(η-C5Me5 )2

R. Beckhaus, J. Sang, T. Wagner and U. Böhme, J. Chem. Soc., Dalton Trans., 1997, 2249 DOI: 10.1039/A701499F

Ab initio study of the adducts of carbon monoxide with alkaline cations

The interaction between CO (either via the C or the O end) and the alkaline cations (Li+, Na+, K+, Rb+, and Cs+) has been studied by means of six ab initio methods, featuring the classical Hartree–Fock, the second order Mo/ller–Plesset treatment of electron correlation, one local density functional and two gradient-corrected methods as well as a quadratic configuration interaction inclusive of single and double substitutions with a noniterative triples contribution to the energy. Basis sets adopted for CO, Li+, Na+, and K+ and the corresponding adducts are of triple-ζ valence quality augmented with a double set of polarization functions (d on C and O; p on the cations). For Rb+ and Cs+, Hay–Wadt effective core potential basis sets have been adopted. Calculated features are the binding energy, the frequency and intensity of the CO stretch, the bending mode, the cation-carbon (or oxygen) stretch, and the equilibrium geometry. Gradient-corrected density functional methods yield results nearly as good as the most expensive correlated method based on configurations interaction. A number of correlations are established among the observables. The role of electrostatics in the interaction is analyzed both by studying the molecular electrostatic potential of CO and by replacing the cation with a proton in the same position. Binding through the C end is invariably preferred, though, with increasing size of the cation, binding through the O end become progressively less unfavored. Experimental data concerning alkaline-cation substituted zeolites are compared with computational results, and an overall agreement is observed.

Ab initio calculations of titanium NMR chemical shifts including the use of effective core potential basis sets

Ab initio calculations of titanium NMR chemical shieldings are reported for titanium tetrahalides using a range of all-electron basis sets and effective core potential basis sets. Absolute shieldings are shown to be very dependent on basis set but chemical shifts are well reproduced even for small and effective core potential basis sets. In particular the inverse halogen dependence is reproduced. Second-order calculations on titanium tetrafluoride give much lower shieldings that Hartree–Fock calculations in contrast to second-order calculations for main group elements.

Ab initio calculations for some oxo-anions of chlorine, bromine and iodine

For the XO −, XO − 2 XO − 3, and XO − 4 ions of chlorine, bromine and iodine, ab initio calculations were performed at the SCF level, using an effective core potential basis set. The properties calculated include dipole moment, polarizability, force-constant matrix, dipole moment and polarizability derivatives with respect to vibrational symmetry coordinates.