Lg Vanquickenborne

A mass spectrometric and ab initio molecular orbital characterization of thionitrosyl hydride (H-N=S)

A combination of tandem mass spectrometry experiments (collisional activation and neutralization-reionization mass spectrometry) and ab initio molecular orbital calculations was used to provide evidence for the stability of thionitrosyl hydride, H-N=S, and to characterize it. Experimentally, HNS has been generated using three different precursors. The (HNS) potential energy surface has been explored in the lowest-lying singlet, triplet and doublet ionized states using (PU)MP4/6-311++G(2df,2pd) energies based on (U)MP2/6-31G(d,p) geometries. Predictions for several spectroscopic properties of NHS have been made on the basis of MO calculations and empirical corrections. These include geometry, rotational constants, vibrational wavenumbers, dipole moment, 14N nuclear quadrupole coupling constant, triplet-singlet energy gap, electron and proton affinities, ionization and π* ← n transition energies.

A THEORETICAL STUDY OF THE REACTION OF SIH2 WITH C2H2 AND C2D2

Abstract Potential energy surfaces of the reaction of SiH 2 and C 2 H 2 (and C 2 D 2 ) have been calculated by means of ab initio molecular orbital theory at the QCISD/6-311G++(2df, 2p)//MP2/6-31G(d, p) level with corrections for the triple excitations to the QCISD energies. The barrier heights for the two reaction channels of the adduct, thus calculated, were further utilized for the dynamical calculation of the rate constants in the framework of quantum statistical Rice-Ramsperger-Kassel theory. Contributions of the rate constants of the various pathways to the total rate constant ( K T ) for the disappearance of the reactants are critically examined and compared with experiment. The pressure dependence of K T (C 2 H 2 ) is primarily due to the formation of silirene. K T (C 2 D 2 ) is consistently higher than K T (C 2 H 2 ). The standard heat of formation of silirene is predicted to be 72.1 ± 3 kcal/mol. Rearrangement of silirene to vinylsilylene requires an activation energy smaller than that to silylacetylene.

The effect of the crystal environment on the metal–ligand interaction and the ligand field spectrum of CrF3−6

The octahedral CrF3−6 cluster is studied in two different crystal environments. The available experimental data reveal that the cluster with the largest Cr–F bond distance is at the same time characterized by the largest value of the spectrochemical strength. This paradoxical fact has been shown to be due to the effect of the two different Madelung potentials. In K2NaCrF6, the central Cr ion is situated at a maximum of the Madelung potential; in CrF3, it is situated at a minimum. For the different crystal environments it has also been shown that the increase in covalency, that goes with the introduction of correlation, parallels the covalency that is present already at the Hartree–Fock level.

Ring–chain rearrangements of phosphirane

Ab initio molecular orbital calculations have been used to explore the C2H5P potential-energy surface. Geometries were optimized at the MP2/6-31G(d,p) level while relative energies were estimated using QCISD(T)/6-311 G(d,p) calculations and corrected for zero-point energies. Among C2H5P isomers, phosphirane 1, vinylphosphine, 2, 1-phosphapropene, 3, and 2-phosphapropene, 9, are low-energy isomers and have similar energy content. 1 lies only 5 kJ mol–1 above the most stable isomer, 9. This is in line with the experimentally observed equilibrium between substituted phosphiranes and vinylphosphines. The 1–2 rearrangement is a single-step process with an energy barrier of 235 kJ mol–1, which is not inconsistent with experimental thermal reactions at 500 °C. The 1–3 interconversion seems possible via ethylphosphinidene, 5, but 5 is not an equilibrium structure. The 2–3 isomerization is possible via either an antarafacial or a suprafacial 1,3-hydrogen shift with a barrier height of ca. 263 kJ mol–1. This represents the first example of an accessible suprafacial 1,3-hydrogen shift. The 1,3-hydrogen shift in 2-phosphapropene remains an antarafacial mode with an energy barrier of 288 kJ mol–1(in comparison with 345 kJ mol–1 in propene). Phosphinoethylidene, 4, is a high-energy isomer lying 201 kJ mol–1 above 1 and separated from 2 and 3 by moderate energy barriers (61 and 58 kJ mol–1). 4 is not involved in interconversions of 1, 2 and 3. While 1, 2-H2 loss from 3 giving CH3—CP or CH2CPH is unlikely, cycloreversion of 1 giving an alkene plus a phosphinidene is a realistic thermal process and thereby a possible mechanism for the formation of CH3—CP upon thermolysis of vinylphosphirane at 700 °C. Conversion of phosphirane to 9 is possible by two distinct two-step pathways: the first involves (methylphosphino) methylene, 8, as an intermediate whereas the second pathway involves bis(methylene)phosphorane, 10. The latter is favoured over the former. Overall, these ring–chain isomerizations of phosphirane constitute a novel type of reaction of phosphorus compounds which do not exist in either carbon or nitrogen analogues.

Effect of fluorine and chlorine atoms on the stability of phosphino-substituted nitrenes and phosphinidenes

Abstract Various equilibrium structures for the XYPN and XYP2 isomeric systems (X, Y=H, F and C1) have been determined by means of ab initio molecular orbital calculations. Geometries were optimized at the MP2/6-31G** level while relative energies were estimated at the CISDQ/6-311G** + ZPE level. In general, monohalogenenation destabilizes phosphinonitrenes, XYP-N, or phosphinophosphinidenes, XYP-P, relative to their corresponding XP=NY or XP=PY isomers, whereas perhalogenation stabilizes the former appreciably. The perfluoro effect in F2PN is quite strong, making it the lowest energy form among F2PN isomers. The effect of the chlorine atom is less pronounced. F2PN is best regarded as a phosphonitrile, F2P≡N, characterized by a hypervalent phosphorus atom. Its peculiar stability is also attributable to its pronounced ionic character and to the intrinsic strength of the P-F bond. Perfluorophosphinophosphinidene (F2P-P) is also stabilized bat remains less stable than FP=PF; the P-P bond in the singlet F2P-P has a character between double and triple. Monohalogenophosphinophosphinidenes, HFP-P and HC1P-P, probably have singlet ground electronic states. The cis-effect favouring a Z isomer is operative in XP=NX but not in XP=PX. The vibrational spectrum of F2P=N has also been predicted.

1,3-HYDROGEN SHIFT IN PHOSPHAPROPENES - SUPRAFACIAL SIGMATROPIC REARRANGEMENTS

Ab initio molecular orbital calculations at the QCISD(T) / 6-311 + + G(d,P) / / MP2 / 6-31G(d,p) + ZPE level show that the suprafacial 1,3-hydrogen shifts in the 1-phosphapropene-vinylphosphine (H3Cue5f8CHue5fbPH⇌H2Cue5fbCHue5f8PH2) and degenerate 1,3-diphosphapropene (H2Pue5f8CHue5fbPH⇌HPue5fbCHue5f8PH2) rearrangements do not only exist but they also compete favourably with the corresponding antarafacial pathways.

The electronic structure of Cr2+,4+ in fluoride host materials

The electronic absorption spectra of Cr4+ in Rb2CrF6 and Cr2+ in KCrF3 have been studied using ab initio molecular orbital methods. Near‐degeneracy effects within the dn (n=2,4) manifold are treated using the complete active space (CAS) self‐consistent field method, while dynamical correlation is dealt with using both the averaged coupled‐pair functional (ACPF) method or second‐order perturbation theory (CASPT2). The ground state electronic structure of the CrFn−6 (n=2,3,4) clusters in the ionic crystals Rb2CrF6, K2NaCrF6, KCrF3 was analyzed first using Bader’s theory of atoms in molecules. The topological analysis of the charge density is consistent with an ionic picture of the chemical bond in all three compounds, although the contribution of covalent effects clearly increases in the series CrF4−6<CrF3−6<CrF2−6. The ligand field strength exerted by the first coordination sphere of fluorine ligands increases within the same series. The present results indicate that the ligand field strength in the d2 sys...

Theoretical evidence of a singlet α-oxocarbene intermediate in the retro-Wolff rearrangement of azafulvenone

Ab initio MO calculations at the CISDQ/6-31G** level suggest that the transformation of azafulvenone yielding isocyanovinyl ketene involves a singlet cyclic α-oxocarbene intermediate.

Charge transfer spectra of octahedral transition metal complexes

Abstract The charge transfer (CTLM) spectra of octahedral transition metal compounds can be analyzed by using a simple model involving crystal field theory. It Is shown that the metal and the ligand environment may be considered as two essentially separate, but weakly interacting entities. The charge transfer spectra of a dn system can be treated by using crystal field theory of the corresponding dn+1 systems. The method described here was used in the analysis of tetrahedral transition metal complexes in a previous paper. It seems therefore to be of rather wide applicability in the field of transition metal chemistry.

Theoretical analysis of the ligand field spectrum of K3CoF6

Abstract We present a detailed ab initio study of the ligand field spectrum of the CoF 3− 6 cluster in the K 3 CoF 6 crystal. The crystal environment around the octahedral CoF 3− 6 cluster is accounted for by a Madelung potential. The ground and ligand field excited states of K 3 CoF 6 have been studied with the inclusion of correlation effects. Each state was first optimized separately in a complete active space self-consistent field (CASSCF) calculation, including ten active electrons occupying the bonding e g and anti-bonding e * g and t * 2g shells. Further correlation was incorporated by using the averaged coupled pair functional approach (ACPF) with the CASSCF configuration space as reference space and where the maximum number of correlated electrons was up to twenty. The one quintet-quintet transition is already described well at the CASSF level while the spin-forbidden quintet-triplet transitions require the inclusion of dynamical correlation.