Luc G. Vanquickenborne

IDENTIFICATION OF THE LOWER TRANSITIONS IN THE SPECTRA OF URANYL COMPLEXES.

The typical low‐energy spectra of the uranyl complexes—both in solution and in the solid state—are interpreted by using group theory and qualitative molecular orbital theory. A comparative study of the effects of ligation on the spectrum shows that the observed facts can be rationalized by means of only one possible identification scheme. The band system situated at 20 000–22 000 cm−1 is due to a 1Σg+ → 1Φg transition, while the band system at 22 500–27 000 cm−1 is due to a 1Σg+ → 1Δg transition.

Influence of building block aromaticity in the determination of electronic properties of five-membered heterocyclic oligomers

This theoretical study investigates the influence of the building block aromaticity in the determination of electronic properties of five-membered heterocyclic oligomers. More specifically, we considered some fundamental energetic and electronic properties such as energy gaps, vertical ionization energies and static polarizability tensors of oligomers (up to octamers) built from five-membered heterocycles such as cyclopentadiene, pyrrole, furan, silole, (planar) phosphole and thiophene. Our computations are based on ab initio quantum mechanical methods including (time-dependent) density functional theory. We have chosen NICS as a quantitative criterion for measuring aromaticity and making a distinction between aromatic and non-aromatic building blocks.

The Coordination of CuII in Zeolites − Structure and Spectroscopic Properties

Zeolites loaded with transition metal ions are promising heterogeneous catalysts. Knowledge about the location and structure of the metal centers is of paramount importance for the understanding of the catalytic potential of these materials. In this work, the spectroscopic studies of the coordination of CuII in zeolite A, ZK4, X, Y and mordenite are reviewed. Experimentally, diffuse reflectance spectroscopy (DRS) and electron spin resonance (ESR) spectroscopy have been applied to study the coordination of CuII in zeolites. Ab initio calculations on model clusters, representing the CuII sites in zeolites, are used for the interpretation of the experimental data. The combination of experimental spectroscopic information with theoretical results leads to a new and profound insight into the CuII−zeolite interaction.

Can silacetylene be observed? A theoretical treatment of the tunneling effect

Abstract The potential energy surface for the 1,2-hydrogen shift converting silavetylene to silylidene, HSiCH→Si=CH 2 , has been calculated by means of ab initio molecular orbital methods up to the QCISD (T) level with 6-311++G(3df, 3pd) basis set. The geometries and energetics obtained were utilized to calculate the rate constants for the isomerization process including the tunneling correction over a wide range of temperatures. The tunneling probability has been treated using the WKB method with an analytical polynomial function of the energy potential. Our results suggest that the detection of silacetylene (HSiCH) might be possible with an instrument with nanosecond resolution. Detection of deuterium-substituted silacetylene could be easier. The half-life ( t 1 2 ) of HSiCH are about 10 −8 and 5 × 10 −6 s, respectively. The heats of formation at 0 K are predicted to be ΔH f 0 (H 2 C=Si)= 84.5±2 kcal/mol and ΔH f 0 (HSiCH) = 118.3±2 kcal/mol.

On the Coupling Scheme in Uranyl Complexes

Simple theoretical considerations of the ligand field type have been applied to the electronic structure of insular uranyl complexes. The study of the consequences of equatorial ligation is used as a means to test the applicability of different coupling schemes for the isolated uranyl ion. From the analysis of the spectra of a large number of different uranyl complexes of Dnh symmetry, it was previously shown how the effect of equatorial ligation consists in lifting certain degeneracies of the free uranyl electronic energy levels and introducing variations in the corresponding transition probabilities. In this note the equatorial field operator is successively applied to uranyl wavefunctions derived in a Russell‐Saunders approximation, in an ω ‐ω coupling approximation, and in an intermediate scheme. Only Russell‐Saunders coupling allows a satisfactory explanation of the observed splitting and intensity pattern.

Properties of phosphorus compounds by density functional theory: CH3P species as a test case

A comparison of different density functional theory (DFT) and molecular orbital (MO) methods for calculating molecular and energetic properties of low‐coordinated phosphorus compounds is reported. While DFT methods include both Becke–Lee–Yang–Parr (BLYP and B3LYP) nonlocal functionals, MO methods involve second‐order perturbation theory (MP2), quadratic configuration interaction [QCISD(T)], and coupled‐cluster theory [CCSD(T)], in conjunction with the 6‐31G(d,p), 6‐311++G(3df,2p), and 6‐311++G(3df,3pd) basis sets. Properties examined include geometrical parameters of the different CH3P equilibrium structures (phosphaethene, phosphinocarbene, methylphosphinidene, and a phosphacarbyne) and relevant transition structures for isomerisations and rearrangements in both the lowest‐lying singlet and triplet states, vibrational wave numbers, relative energies, barrier heights, and singlet–triplet energy gaps. In addition, the heat of formation, ionization energy, and proton affinity of phosphaethene are also evaluated. Overall, the B3LYP method, when employed with a large basis set, yields energetic results comparable to the CCSD(T) results. Nevertheless, both DFT methods fail to predict the behavior of the addition/elimination reactions of the hydrogen atom in the triplet state.A comparison of different density functional theory (DFT) and molecular orbital (MO) methods for calculating molecular and energetic properties of low‐coordinated phosphorus compounds is reported. While DFT methods include both Becke–Lee–Yang–Parr (BLYP and B3LYP) nonlocal functionals, MO methods involve second‐order perturbation theory (MP2), quadratic configuration interaction [QCISD(T)], and coupled‐cluster theory [CCSD(T)], in conjunction with the 6‐31G(d,p), 6‐311++G(3df,2p), and 6‐311++G(3df,3pd) basis sets. Properties examined include geometrical parameters of the different CH3P equilibrium structures (phosphaethene, phosphinocarbene, methylphosphinidene, and a phosphacarbyne) and relevant transition structures for isomerisations and rearrangements in both the lowest‐lying singlet and triplet states, vibrational wave numbers, relative energies, barrier heights, and singlet–triplet energy gaps. In addition, the heat of formation, ionization energy, and proton affinity of phosphaethene are also evalu...

How reliable are ab initio calculations?: The structure and conformation of chlorocarbonyl isocyanate (CIC(O)NCO) revisited

Abstract The theoretical analysis of the conformations ofchlorocarbonyl isocyanate was reinvestigated. Our results show that ab initio molecular orbital (MO) calculations are reliable in determining the cis-trans energy difference CL C(O)NCO (the trans conformer being the more stable), in agreement with experiment. We thus answer positively the recently posed question, “How reliable are ab initio calculations?” (H.-G. Mack, H. Oberhammer and C.O. Della Vedova, J. Mol. Struct., (Theochem), 200 (1989) 277). The failure of the calculations by Mack et al. to reproduce the cis-trans separation gap might be due to the geometrical parameters employed in their work. (In the present work, we employed the MP2/6-31G(d,p) optimized geometries.) We extended the scope of the problem by carrying out similar calculations for HC(O)NCO and FC(O)NCO. The calculated geometries were improved by using a correction procedure in order to predict the corresponding rg structures. In the two latter species, the cis conformers are predicted to have slightly higher stabilities than the trans conformers.

Phosphinidene Transition Metal Complexes: A Combined Ab Initio MO-DFT Study of Cr(CO) 5 -PR

Ab initio MO and DFT calculations have been performed on phosphinidene complexes of the type Cr(CO)5–PR, with R = H, CH3, SiH3, NH2, PH2, OH, and SH. The formation of the Cr–P bond essentially arises from a ligand-to-metal charge transfer. While a significant π backdonation is also observed for the Cr(CO)5–PH, Cr(CO)5–PCH3, and Cr(CO)5–PSiH3 complexes, this is less the case for Cr(CO)5–POH, Cr(CO)5–PSH, and Cr(CO)5–PNH2, and the backbonding almost disappears for Cr(CO)5–PPH2. In both the lowest lying singlet and triplet states, all complexes exhibit a staggered conformation. CASSCF/CASPT2 calculations performed with the ANO basis sets indicate a closed-shell singlet ground state along the whole series. The binding energy between Cr(CO)5 and PR ranges from 216 kJ/mol for Cr(CO)5–PNH2 to 127 kJ/mol for Cr(CO)5–PSiH3 (B3LYP values). In general, the B3LYP-DFT scheme yields reasonable qualitative and quantitative results when compared with CASPT2(12/12).

EXCHANGE INTERACTIONS IN TI2CL93- : A CRITICAL ANALYSIS

The electronic nature of the exchange interactions in Ti23− is investigated from a theoretical point of view, using two different methods: (i) a parametric model Hamiltonian acting in the space of the (t2g)1−(t2g)1 pair states of the ground configuration; (ii) detailed ab initio calculations of the CASSCF and CASPT2 type. Comparison of the results allows the hopping integrals between the trigonal components of the t2g shells, as well as in intercentre Coulomb integrals, to be determined. Both ‘through-bond’ and ‘through-space’ exchange interactions appear to be present. The conclusions are compared with existing exchange models and with experimental results.

Structure-property relationships in phosphole oligomers: a theoretical insight

Abstract The influence of cyclic π-conjugation and interannular rotation on the vertical ionization energies and energy gaps of phosphole oligomers has theoretically been investigated which provided some useful structure–property relationships. While vertical ionization energies were obtained from negative HOMO-energies using the Hartree–Fock method, energy gaps were calculated using a DFT/hybrid method (B3LYP). Combination of geometric (JULG and d Cα ) and magnetic (NICS) criteria gives a qualitative indication about the competition between the cyclic- and the carbon backbone π-conjugation. Results of this preliminary theoretical study suggest that phosphole oligomers have interesting tunable electronic properties.