Christoph Heinemann

The performance of density‐functional/Hartree–Fock hybrid methods: Cationic transition‐metal methyl complexes MCH+3 (M=Sc–Cu,La,Hf–Au)

Hybrid methods, including a mixture of Hartree–Fock exchange and density functional exchange‐correlation treatment have been applied to the cationic methyl complexes MCH+3 of the first and third‐row transition metals (M=Sc–Cu,La,Hf–Au). Bond dissociation energies and optimum geometries obtained with the ‘‘Becke‐Half‐and‐Half‐Lee–Yang–Parr’’ and ‘‘Becke‐3‐Lee–Yang–Parr’’ functionals and from calibration calculations employing quadratic configuration interaction with single and double excitations and with a perturbative estimate of triple excitations are reported. A comparison of the results for the 3d‐block species to earlier high‐level ab initio calculations and experimental data is carried out in order to assess the reliability of hybrid methods as a practical tool in organometallic chemistry. Furthermore, the bond dissociation energies of the cationic 5d‐block transition‐metal methyl complexes, many of which have not been investigated so far, are predicted.

Ethylenedione: An intrinsically short-lived molecule

Ethylenedione C2O2 is one of the elusive small molecules which have remained undetected even after numerous attempts with different experimental techniques, This is surprising, since theoretical studies predicted the triplet state of C2O2 to be stable towards spin-allowed dissociation and hence long-lived. Here we report a comprehensive study of charged and neutral ethylenedione by means of charge reversal and neutralization -reionization mass spectrometry. These experimental results, in conjunction with theoretical calculations, suggest that neutral ethylenedione is intrinsically short-lived rather than being elusive, Both the singlet and triplet states of C2O2 are predicted to dissociate rapidly into two ground-state CO molecules, and for the triplet species, this dissociation involves facile curve-crossing to the singlet surface within a few nanoseconds.

Pt+-mediated activation of methane: theory and experiment

Abstract A combined theoretical and experimental study on the Pt+-mediated activation of methane is presented. Dehydrogenation of CH4 by thermalized Pt+ cations (Pt+ + CH4 ← PtCH2+ + H2) proceeds along a doublet ground state potential energy surface and is found to be reversible under the conditions of Fourier transform ion-cyclotron resonance mass spectrometry. The recently reported oxidation of the cationic platinum carbene PtCH2+ by O2 produces electronically excited Pt+ cations, which are detected in the 4 F 9 2 state by means of charge-transfer bracketing experiments.

Hydrocarbon activation by “bare” uranium cations: formation of a cationic uranium-benzene complex from three ethylene units☆

The ability of the atomic uranium cation U+ to activate a variety of saturated and unsaturated hydrocarbon molecules is investigated. Both CH and CC bond activation processes proceeds at markedly higher kinetic efficiencies compared with the lower congener Nd+ from the lanthanide series. Formation of a cationic uranium-benzene complex occurs in three consecutive dehydrogenation reactions between U+ and ethylene. The mechanism of this metal-mediated cyclotrimerization is enlightened by kinetic measurements, collision-induced dissociation experiments and ion-molecule reactions of the intermediate species.

Relativistic effects in the cationic platinum carbene PtCH+2

Fully relativistic four‐component Dirac–Fock Coulomb calculations in conjunction with a second‐order perturbational estimate for the correlation energy have been performed in benchmark calculations on geometric and electronic structures as well as the binding energy of the cationic platinum carbene complex PtCH+2. The relativistic stabilization of this species amounts to as much as 50 kcal/mol and the combination of relativistic and correlation effects shorten the Pt–C bond length by nearly 1 bohr, changing the bond order from one to two. The relative importance of spin‐free and spin‐dependent relativistic effects on the geometry, the electronic structure, and the binding energy is evaluated by comparison to the Douglas–Kroll method. Relativistic effective core potentials are shown to describe the spin‐free effects reliably. The best theoretical estimate for the bond dissociation energy underestimates the experimental value by 13% due to truncation errors in the one‐ and n‐particle space treatments. The m...

An approximate method for treating spin-orbit effects in platinum

Abstract Spin-orbit coupling in platinum-containing species can be treated via a one-electron spin-orbit operator and a single scaling parameter Z eff (Pt) in conjunction with an effective core potential for the description of scalar relativistic effects. Our calibration calculations cover the five low-lying electronic states of platinum hydride PtH and the lowest fourteen levels in the atomic spectrum of the platinum atom Pt. Here, qualitative and semi-quantitative agreement between the presented semi-empirical approach and four-component Dirac-Fock calculations is found if Z eff (Pt) is chosen between 950 and 1200. Further applications concern the low-lying levels of the platinum cation Pt + , the theoretical determination of ground states for the diatomic oxides PtO and PtO + as well as spin-orbit effects in the cationic carbene complex PtCH 2 + .

Equilibrium studies of weakly bound Fe(L)+ complexes with L = Xe, CO2, N2 and CH4

Abstract The binding energies of Xe, N 2 , CO 2 and CH 4 to the Fe + cation have been studied by equilibrium measurements using Fourier-transform mass spectrometry. Reacting mass-selected Fe(CO 2 ) + with the mixtures N 2 /CH 4 , CO 4 , CO 2 /CH 4 , CO 2 /Xe CO 2 /N 2 and Xe/N 2 provides an internally consistent set of equilibrium constants and free energies ΔΔG (298 K). Consideration of B3LYP calculated thermal corrections suggests Fe + −Xe⩽Fe + −CO 2 ⩽Fe + − N 2 ⩽Fe + −CH 4 for the bond dissociation energies. Using the literature value for D 0 (Fe +  CH 4 ) as an achor point, D 0 (Fe + Xe) = 11.7 ± 1.5, D 0 (Fe +  CO 2 ) = 13.0 ± 1.3 and D 0 (Fe +  N 2 ) = 13.4 ± 1.1 kcal/mol are obtained.

Quantum chemical study on the equilibrium geometries of S3 and S−3, The electron affinity of S3 and the low lying electronic states of S−3

The geometries and relative stabilities of the open, C2v symmetric and closed, D3h symmetric forms of thiozone and its anion, the adiabatic electron affinity of S3 and the energies of the three low‐lying excited electronic states of the thiozone anion (A 2B2,B 2A1,C 2A2) at the optimized geometry of the X 2B1 ground state are computed employing coupled‐cluster [CCSD(T)], second‐order multireference perturbation theory (CASPT2), and multireference CI (MRCI and IC‐MRCI) methods using large atomic natural orbital basis sets. In addition, the saddle point for the open→closed isomerization on the neutral S3 potential energy surface is being studied. Surprisingly, the calculations do not show the expected underestimation of the experimentally determined electron affinity, in sharp contrast to test calculations on the sulfur atom, the disulfur molecule, and earlier results for ozone. Apart from this, thiozone and its anion behave in many respects qualitatively similar as ozone and O−3, while quantitatively v...

An asymmetric induction principle and biomimetics with photons via electron transfer

Isoprenoid polyalkene radicals, formed upon anti-Markovnikov addition of a nucleophile to their parent radical cations, which are readily accessible viu photo- induced electron transfer, undergo cascade cyclizations. The regioselectivity is efficiently controlled by the substitution pattern, i. e. the generally observed 6-endo-trig mode is replaced by 5-exo-trig, if electron-deficient double bonds are involved. Mechanistic studies revealed that these synthetically useful transformations, initiated by polyalkene radical cations being trapped by water, are propagated in a plain radical fashion and terminated upon either protonation of carbanions or hydrogen transfer. Cyclization products were used for a natural product total synthesis, i.e of stypoldione. Moreover, high asymmetric inductions in such transformations have been achieved by the use of chiral spirocyclic dioxinones, derived from the auxiliary (-)-menthone, remotely located from the initiation site of the cyclizations. These asymmetric photoinduced cyclizations are further examples of a more general enantiodivergent induction principle giving access to enantiomerically pure polycyclic terpenoids of complementary chiralities by means of the single chiral auxiliary (-)- menthone. Finally, the efficient application of solar radiation for photochemical purpose is demonstrated by the use of flat collectors which in contrast to concentrating technologies not only employ direct, but also diffuse radiation (the latter amounts to about 40% of the global radiation at central European latitude).

Interaction of the Fe+ cation with heavy noble gas atoms

The diatomic iron–noble gas complexes FeAr+, FeKr+, FeXe+, and FeRn+ have been theoretically investigated by means of quantum‐chemical calculations including an extensive treatment of electron correlation. Potential energy curves and spectroscopic constants for the lowest 4Δ and 6Δ states are derived from an open‐shell coupled‐cluster approach and the relative energies of all seven low‐lying electronic states are evaluated by the multireference configuration interaction method. While in FeAr+ the lowest quartet and sextet states are found to be energetically almost degenerate, the heavier Fe+–noble gas molecules are predicted to exhibit 4Φ ground states. From a qualitative point of view bonding in these species is shown to be electrostatic in origin with intrinsically higher interaction energies for the quartet as compared to the sextet states. For calibration purposes, also an accurate calculation of the 4F(4s03d7)–6D(4s13d6) energy difference in the atomic Fe+ cation is provided.