Monika Srebro

Does a Molecule-Specific Density Functional Give an Accurate Electron Density? The Challenging Case of the CuCl Electric Field Gradient.

In the framework of determining system-specific long-range corrected density functionals, the question is addressed whether such functionals, tuned to satisfy the condition -ε(HOMO) = IP or other energetic criteria, provide accurate electron densities. A nonempirical physically motivated two-dimensional tuning of range-separated hybrid functionals is proposed and applied to the particularly challenging case of a molecular property that depends directly on the ground-state density: the copper electric field gradient (EFG) in CuCl. From a continuous range of functional parametrizations that closely satisfy -ε(HOMO) = IP and the correct asymptotic behavior of the potential, the one that best fulfills the straight-line behavior of E(N), the energy as a function of a fractional electron number N, was found to provide the most accurate electron density as evidenced by calculated EFGs. The functional also performs well for related Cu systems.

Optical Rotation Calculated with Time-Dependent Density Functional Theory: The OR45 Benchmark

Time-dependent density functional theory (TDDFT) computations are performed for 42 organic molecules and three transition metal complexes, with experimental molar optical rotations ranging from 2 to 2 × 10(4) deg cm(2) dmol(-1). The performances of the global hybrid functionals B3LYP, PBE0, and BHLYP, and of the range-separated functionals CAM-B3LYP and LC-PBE0 (the latter being fully long-range corrected), are investigated. The performance of different basis sets is studied. When compared to liquid-phase experimental data, the range-separated functionals do, on average, not perform better than B3LYP and PBE0. Median relative deviations between calculations and experiment range from 25 to 29%. A basis set recently proposed for optical rotation calculations (LPol-ds) on average does not give improved results compared to aug-cc-pVDZ in TDDFT calculations with B3LYP. Individual cases are discussed in some detail, among them norbornenone for which the LC-PBE0 functional produced an optical rotation that is close to available data from coupled-cluster calculations, but significantly smaller in magnitude than the liquid-phase experimental value. Range-separated functionals and BHLYP perform well for helicenes and helicene derivatives. Metal complexes pose a challenge to first-principles calculations of optical rotation.

Theoretical Analysis of Bonding in N-Heterocyclic Carbene−Rhodium Complexes

The natural orbitals for chemical valence and the Ziegler-Rauk bond energy decomposition analysis were used to describe the donor/acceptor character of the N-heterocyclic carbenes (NHC)-metal bond in two groups of square-planar rhodium(I) complexes: (NHC)RhCl(cod) (1-X; cod = 1,5-cyclooctadiene) and (NHC)RhCl(CO)(2) (2-X), with a group X = H, Cl, NO(2), or CN located on the NHC ligand. The results show that the NHC-metal bond consists of the components originating from donation (sigma symmetry) and back-donation (two contributions of the pi symmetry, out-of-plane and in-plane, accompanied by one sigma-back-bonding component). The charge-flow measures from NOCV indicate that the total back-bonding contribution is of comparable importance to donation. The out-of-plane pi component contributes to ca. 50% of the total back-bonding charge-flow. The energy measures from the Ziegler-Rauk analysis show that the total back-bonding energy corresponds to ca. 40% of the orbital interaction energy. The ligand trans to NHC (CO or cod) strongly affects the back-bonding component; for the complexes 1-X, the back-donation is substantially enhanced compared to 2-X. The back-bonding component increases with an increase in the pi-withdrawing ability of X for both, 1-X and 2-X. However, this effect is relatively small. Back-bonding components of the two bonds involving the metal are strongly coupled; an increase in NHC-Rh leads to a decrease in Rh-olefin/CO(trans). The changes in the back-bonding are too small to be followed by the trends in bond energies, which are finally determined by the electrostatic and Pauli repulsion energy.

Straightforward access to mono- and bis-cycloplatinated helicenes displaying circularly polarized phosphorescence by using crystallization resolution methods

Enantiopure mono-cycloplatinated-[8]helicene and bis-cycloplatinated-[6]helicene derivatives were prepared through column chromatography combined with crystallization of diastereomeric complexes using a chiral ancillary sulfoxide ligand. The UV-visible spectra, circular dichroism, molar rotations, and (circularly polarized) luminescence activity of these new helical complexes have been examined in detail and analysed with the help of first-principles quantum-chemical calculations.

Applications of the ETS-NOCV method in descriptions of chemical reactions

AbstractThe present study characterizes changes in the electronic structure of reactants during chemical reactions based on the combined charge and energy decomposition scheme, ETS-NOCV (extended transition state–natural orbitals for chemical valence). Decomposition of the activation barrier, ΔE#, into stabilizing (orbital interaction, ΔEorb, and electrostatic, ΔEelstat) and destabilizing (Pauli repulsion, ΔEPauli, and geometry distortion energy, ΔEdist) factors is discussed in detail for the following reactions: (I) hydrogen cyanide to hydrogen isocyanide, HCN → CNH isomerization; (II) Diels-Alder cycloaddition of ethene to 1,3-butadiene; and two catalytic processes, i.e., (III) insertion of ethylene into the metal-alkyl bond using half-titanocene with phenyl-phenoxy ligand catalyst; and (IV) B–H bond activation catalyzed by an Ir-containing catalyst. Various reference states for fragments were applied in ETS-NOCV analysis. We found that NOCV-based deformation densities (Δρi) and the corresponding energies ΔEorb(i) obtained from the ETS-NOCV scheme provide a very useful picture, both qualitatively and quantitatively, of electronic density reorganization along the considered reaction pathways. Decomposition of the barrier ΔE# into stabilizing and destabilizing contributions allowed us to conclude that the main factor responsible for the existence of positive values of ΔE# for all processes (I, II, III and IV) is Pauli interaction, which is the origin of steric repulsion. In addition, in the case of reactions II, III and IV, a significant degree of structural deformation of the reactants, as measured by the geometry distortion energy, plays an important role. Depending on the reaction type, stabilization of the transition state (relatively to the reactants) originating either from the orbital interaction term or from electrostatic attraction can be of vital importance. Finally, use of the ETS-NOCV method to describe catalytic reactions allows extraction of information on the role of catalysts in determination of ΔE#. FigureContours of dominant deformation density contributions, Δρ1, together with the corresponding energies ΔEorb(1) obtained from the ETS-NOCV method for the transition state (TS) and the product (cyclohexene) of Diels-Alder cycloaddition of ethene to 1,3-butadiene. Black and white colors used in the representation of Δρ1 indicate carbon and hydrogen atoms, respectively. The blue/red colors in the contours corresponds to accumulation/depletion of electron density upon bond formation

QTAIM and ETS-NOCV analyses of intramolecular CH···HC interactions in metal complexes.

The topological analysis, based on the quantum theory of atoms in molecules (QTAIM) of Bader and the ETS-NOCV charge and energy decomposition method have been used to characterize coordination bonds, chelating rings, and additional intramolecular interactions in the ZnNTA and ZnNTPA complexes in solvent. The QTAIM and ETS-NOCV studies have conclusively demonstrated that the H-clashes (they are observed only in the ZnNTPA complex and classically are interpreted as steric hindrance destabilizing a complex) are characterized by (i) the electron flow channel between the H-atoms involved, as discovered by the ETS-NOCV analysis (on average, ΔE(orb) = -1.35 kcal mol(-1)) and (ii) QTAIM-defined a bond path that indicates the presence of a preferred quantum-mechanical exchange channel, hence, they should be seen as H-H intramolecular bonding interactions. The main reason for the formation of a weaker ZnNTPA complex was attributed to the strain energy (from both QTAIM and ETS-NOCV techniques) and the larger Pauli repulsion contribution found from the ETS-NOCV analysis. An excellent agreement between physical properties controlling the stability of the two complexes was found from the two techniques, QTAIM and ETS-NOCV.

Analysis of Optical Activity in Terms of Bonds and Lone-Pairs: The Exceptionally Large Optical Rotation of Norbornenone.

Norbornenone, which has both a C═O and a C═C chromophore in a rigid bicyclic hydrocarbon framework, exhibits optical rotation (OR) an order of magnitude larger than that of similar molecules with only one of these chromophores (e.g., α-pinene). Its OR is also very sensitive to approximations in electronic structure calculations. The present study demonstrates a novel approach to interpret optical rotation using familiar concepts of chemical bonding, aided by first-principles calculations. A theoretical procedure is developed for analyzing the OR tensor components of a molecule in terms of individual bonds and lone pairs. The link between the chemists bond and quantum mechanics is provided by localized molecular orbitals obtained from density functional theory (DFT) calculations. Delocalization of π orbitals is shown to play a crucial role in the large OR of norbornenone, as hinted by the DFT delocalization error inherent in many standard functionals and confirmed by detailed analysis. The OR contributions generated by the double bond in α-pinene are even stronger than that of norbornenone. The isotropic average, observed in solution or in gas phase, is small as a result of cancellation of tensor components with opposite signs. The electronic coupling and delocalization of the C═C π bond and the C═O oxygen π lone pair in norbornenone selectively enhance one of the OR tensor components, resulting in the exceptionally large negative isotropic OR.

Ruthenium‐Grafted Vinylhelicenes: Chiroptical Properties and Redox Switching

The properties of mono- and bis-Ru-vinyl[6]helicene complexes (2 a and 2 b, respectively), recently synthesized by using molecular engineering of helicenes based on the grafting of lateral organometallic substituents on the π-helical backbone through a vinyl bridge, are presented. These helicene derivatives are thoroughly characterized, with special attention given to their chiroptical properties and redox switching activity. The UV/Vis and electronic circular dichroism (ECD) spectra of P and M enantiopure species, both in the neutral and oxidized states ([2 a](·+), [2 b](·+), and [2 b](2+)), are analyzed with the aid of quantum-chemical calculations. The extended π-conjugation facilitated by the vinyl moiety, clearly visible in the electronic structures of 2 a,b, introduces new active bands in the ECD spectra that consequently lead to a significant increase in optical rotation of Ru-vinylhelicenes compared with the organic precursors. The vibrational circular dichroism (VCD) spectra were measured and calculated for both the organic and organometallic species and constitute the first examples of VCD for metal-based helicene derivatives. Finally, the redox-triggered chiroptical switching activity of 2 a,b is examined in detail by using ECD spectroscopy. The modifications of the ECD spectra in the UV/Vis and NIR region are well reproduced and rationalized by calculations.

Conformational Stability and Spin States of Cobalt(II) Acetylacetonate: CASPT2 and DFT Study.

Electronic structure and conformation of bis(acetylacetonate) cobalt(II), Co(acac)2, a prototypical mediator in controlled radical polymerization of olefins, is reinvestigated. The ab initio multiconfigurational CASSCF/CASPT2 method is used to resolve the doubts stemming from density functional theory results. We assign the quartet ground state for a single molecule and point at tetrahedral conformation as the preferred one. Several density functionals are tested against the ab initio calculations, and their performance is assessed. The strength of intermolecular interactions in the crystal structure composed of square-planar Co(acac)2 molecules ( Burgess , J. ; et al. Acta Crystallogr. 2000 , C56 , 649 - 650 ) is estimated to be sufficient for their planarization (suggested by Matyjaszewski , K. ; et al. Chem.-Eur. J. 2007 , 13 , 2480 - 2492 ).

Chiroptical Properties of Carbo[6]Helicene Derivatives Bearing Extended π-Conjugated Cyano Substituents

New carbo[6]helicene derivatives grafted with π-conjugated cyano-phenyl arms were synthesized in enantiopure forms and their π-conjugation examined by UV-vis spectroscopy. The influence of the π-conjugation on the circular dichroism spectra and molar rotations is discussed based on comparing experimental data with results from quantum-chemical calculations. The results highlight the fact that increasing the spatial extension of the π-system in a helicene molecule is an efficient way of increasing its molar rotation.