Raphael J. F. Berger

A Stable Derivative of the Global Minimum on the Si6H6 Potential Energy Surface

The higharomatic stabilization that is conferred by the cyclic delocal-ization of six p electrons distinguishes benzene from itsapproximately 200 theoretically known isomers and accountsfor the ubiquitous occurrence of the benzene motif in manyareas of chemistry. Despite impressive progress regarding thesynthesis of stable compounds with Si Si p bonds,

On the Molecular and Electronic Structures of AsP3 and P4

The molecular and electronic structures of AsP(3) and P(4) have been investigated. Gas-phase electron diffraction studies of AsP(3) have provided r(g) bond lengths of 2.3041(12) and 2.1949(28) A for the As-P interatomic distances and the P-P interatomic distances, respectively. The gas-phase electron diffraction structure of P(4) has been redetermined and provides an updated value of 2.1994(3) A for the P-P interatomic distances, reconciling conflicting literature values. Gas-phase photoelectron spectroscopy provides experimental values for the energies of ionizations from the valence molecular orbitals of AsP(3) and P(4) and shows that electronically AsP(3) and P(4) are quite similar. Solid-state (75)As and (31)P NMR spectroscopy demonstrate the plastic nature of AsP(3) and P(4) as solids, and an extreme upfield (75)As chemical shift has been confirmed for the As atom in AsP(3). Finally, quantum chemical gauge-including magnetically induced current calculations show that AsP(3) and P(4) can accurately be described as strongly aromatic. Together these data provide a cohesive description of the molecular and electronic properties of these two tetraatomic molecules.

Calculations of magnetically induced current densities: theory and applications

A review of computational studies of magnetically induced current density susceptibilities in molecules and their relation to experiments is presented. The history of the investigation of magnetically induced current densities and ring currents in molecules is briefly covered. The theoretical development of relativistic and nonrelativistic computational approaches for computing current densities in closed‐shell and open‐shell molecules is discussed and different state of the art methods to interpret calculated current densities are reviewed. Numerical integration approaches to assess global, semilocal, and local aromatic properties of multiring molecules are presented and demonstrated on free‐base trans‐porphyrin. We show that numerical integration of the current density combined with guiding visualization techniques of the current flow is a powerful tool for studies of the aromatic character of complicated molecular structures such as annelated aromatic and antiaromatic rings. Representative applications are reported illustrating the importance of careful current density studies for organic and inorganic chemistry. The applications include calculations of current densities and current strengths for aromatic, antiaromatic, and nonaromatic molecules of different kind. Current densities in spherical, cylindrical, tetrahedral, toroidal, and Möbius‐twisted molecules are discussed. The aromatic character, current pathways, and current strengths of porphyrins are briefly highlighted. Aromatic properties of inorganic molecules are assessed based on current density calculations. Current strengths as a noninvasive tool to determine strengths of hydrogen bonds are discussed. WIREs Comput Mol Sci 2016, 6:639–678. doi: 10.1002/wcms.1270

Silver and Gold Complexes with a New 1,10‐Phenanthroline Analogue N‐Heterocyclic Carbene: A Combined Structural, Theoretical, and Photophysical Study

Unusual coordination for gold: an imidazolium salt was synthesized and used as a precursor for an N-heterocyclic carbene, which can be considered as the carbene analogue of 1,10-phenanthroline. Like the diimine congener, this ligand gives luminescent metal complexes. Remarkably, the Au(III) complex features a gold atom in an unusual environment: it is surrounded by six donor atoms, two of which interact electrostatically with the Au atom.

N,N‐Dimethylaminopropylsilane: A Case Study on the Nature of Weak Intramolecular Si⋅⋅⋅N Interactions

N,N-Dimethylaminopropylsilane H(3)Si(CH(2))(3)NMe(2) was synthesised by the reaction of (MeO)(3)Si(CH(2))(3)NMe(2) with lithium aluminium hydride. Its solid-state structure was determined by X-ray diffraction, which revealed a five-membered ring with an SiN distance of 2.712(2) A. Investigation of the structure by gas-phase electron diffraction (GED), ab initio and density functional calculations and IR spectroscopy revealed that the situation in the gas phase is more complicated, with at least four conformers present in appreciable quantities. Infrared spectra indicated a possible SiN interaction in the Si-H stretching region (2000-2200 cm(-1)), as the approach of the nitrogen atom in the five-membered ring weakens the bond to the hydrogen atom in the trans position. Simulated gas-phase IR spectra generated from ab initio calculations (MP2/TZVPP) exhibited good agreement with the experimental spectrum. A method is proposed by which the fraction of the conformer with a five-membered ring can be determined by a least-squares fit of the calculated to experimental absorption intensities. The abundance of this conformer was determined as 23.7(6) %, in good agreement with the GED value of 24(6) %. The equilibrium SiN distance predicted by theory for the gas-phase structure was highly variable, ranging from 2.73 (MP2) to 3.15 A (HF). The value obtained by GED is 2.91(4) A, which could be confirmed by a scan of the potential-energy surface at the DF-LCCSD[T] level of theory. The nature of the weak dative bond in H(3)Si(CH(2))(3)NMe(2) can be described in terms of attractive inter-electronic correlation forces (dispersion) and is also interpreted in terms of the topology of the electron density.

The Pentamethylcyclopentadienylsilicon(II) Cation as a Catalyst for the Specific Degradation of Oligo(ethyleneglycol) Diethers

Covalent silicon(IV) compounds of the type R3SiX (R= organic substituent, X= electronegative group) have long been used as catalysts for several organic reactions, mainly in carbon–carbon bond-forming processes (such as Diels–Alder reactions, aldol condensations). Some ionic silicon(IV) compounds containing R3Si + cations have also been applied as catalysts in C C bond formation reactions and in some other transformations. The most important examples are collected in Scheme 1.

Dismutational and global-minimum isomers of heavier 1,4-dimetallatetrasilabenzenes of Group 14.

Aromatic species with heavier Group 14 elements show remarkable differences in terms of stability, structure, and reactivity. Herein we report our experimental and theoretical investigations regarding isomers of germanium- and tin-containing benzene analogues E2Si4R6 (E=Ge, Sn). The germanium-substituted dismutational isomer with a tricyclic six-membered scaffold is isolable, but unlike the homonuclear Si6 analogue slowly rearranges even at room temperature to give the propellane-type global minimum isomer. In case of E=Sn the dismutational isomer may be an intermediate on the pathway to the propellane-type species obtained, but cannot be detected even at low temperature. Unprecedentedly large chemical shift anisotropies in the (29)Si NMR spectra that increase from the Si6 species through Ge2Si4 to Sn2Si4 are rationalized by progressively larger paramagnetic-term contributions to the chemical shift tensor as a result of diminishing HOMO-LUMO gaps, which are also reflected in the absorption spectra, as well as by appearance and symmetry of these frontier orbitals.

Strong Intramolecular Si−N Interactions in the Chlorosilanes Cl3−nHnSiOCH2CH2NMe2 (n = 1−3)

The compounds Cl 3SiOCH 2CH 2NMe 2 ( 1) and Cl 2HSiOCH 2CH 2NMe 2 ( 2) were prepared by reactions of lithium 2-(dimethylamino)ethanolate with SiCl 4 and HSiCl 3. The analogous reaction with H 2SiCl 2 gave ClH 2SiOCH 2CH 2NMe 2 ( 3), but only in a mixture with Cl 2HSiOCH 2CH 2NMe 2 ( 2), from which it could not be separated. All compounds were characterized by IR and NMR ( (1)H, (13)C, (29)Si) spectroscopy, 1 and 2 by elemental analyses and by determination of their crystal structures. Cl 3SiOCH 2CH 2NMe 2 ( 1) and Cl 2HSiOCH 2CH 2NMe 2 ( 2) crystallize as monomeric ring compounds with pentacoordinate silicon atoms participating in intramolecular Si-N bonds [2.060(2) A ( 1), 2.037(2) A ( 2)]. The dative bonds in 1 and 2 between the silicon and nitrogen atoms could also be proven to exist at low temperatures in solution in (1)H, (29)Si-HMBC-NMR experiments by detection of the scalar coupling between the (29)Si and the protons of the NCH 2 and NCH 3 groups. A function describing the chemical shift delta exp (29)Si dependent on the chemical shifts of the individual equilibrium components, the temperature, and the free enthalpy of reaction was worked out and fitted to the experimental VT-NMR data of 1 and 2. This provided values of the free reaction enthalpies of Delta G = -28.8 +/- 3.9 kJ x mol (-1) for 1 and Delta G = -22.3 +/- 0.4 kJ x mol (-1) for 2 and estimates for the chemical shifts of open-chain (index o) and ring conformers (index r) for 1 of delta r = -94 +/- 2 ppm and delta o = -36 +/- 5 ppm and for 2 of delta r = -82 +/- 1 ppm and delta o = -33 +/- 4 ppm. The value of delta r for 1 is very close to that obtained from a solid-state (29)Si MAS NMR spectrum. Quantumchemical calculations (up to MP2/TZVPP) gave largely differing geometries for 1 (with a Si...N distance of 3.072 A), but well reproduced the geometry of 2. These differences are due to Cl...H and Cl...C repulsions and solid state effects, which can be modeled by conductor-like screening model calculations and also rationalized in terms of the topology of the electron density, which was analyzed in terms of the quantum theory of atoms in molecules.

Sila-Substitution of Alkyl Nitrates: Synthesis, Structural Characterization, and Sensitivity Studies of Highly Explosive (Nitratomethyl)-, Bis(nitratomethyl)-, and Tris(nitratomethyl)silanes and Their Corresponding Carbon Analogues

A series of analogous nitratomethyl compounds of carbon and silicon of the formula types Me(3)ElCH(2)ONO(2) (1a/1b), Me(2)El(CH(2)ONO(2))(2) (2a/2b), MeEl(CH(2)ONO(2))(3) (3a/3b), (CH(2))(4)El(CH(2)ONO(2))(2) (4a/4b), and (CH(2))(5)El(CH(2)ONO(2))(2) (5a/5b) were synthesized [El = C (a), Si (b); (CH(2))(4)El = (sila)cyclopentane-1,1-diyl; (CH(2))(5)El = (sila)cyclohexane-1,1-diyl]. All compounds were characterized by using NMR, IR, and Raman spectroscopy and mass spectrometry. In addition, the crystal structures of Me(2)C(CH(2)ONO(2))(2) (2a), (CH(2))(4)C(CH(2)ONO(2))(2) (4a), Me(2)Si(CH(2)ONO(2))(2) (2b), and (CH(2))(5)Si(CH(2)ONO(2))(2) (5b) were determined by single-crystal X-ray diffraction. The gas-phase structures of the C/Si analogues 1a and 1b were determined by electron diffraction and compared with the results of quantum chemical calculations at different levels of theory. The thermal stabilities of the C/Si pairs 1a/1b-5a/5b were investigated by using DSC. In addition, their friction and impact sensitivities were measured with standard BAM methods. The extreme sensitivities of the silicon compounds 1b-5b compared to those of the corresponding carbon analogues 1a-5a were discussed in terms of the structures of the C/Si analogues and possible geminal Si...O interactions.

Potassium hydroxylamine complexes.

Potassium complexes of N,N-dialkylhydroxylamines [KONR2, R=Me (1a), iPr (2a), CH2C6H5] were synthesized by the deprotonation of the corresponding N, N-dialkylhydroxylamines with KH. 1a and 1b [(KONMe2)(HONMe2)] dissolve in THF under the addition of an additional equiv of the parent hydroxylamine to give 1b and [(KONiPr2)(HONiPr2)(THF)] 2b. 1b, 2b and [(KONBn2)6(THF)4] (3) were characterized by NMR and IR spectroscopy, by elemental analyses, and by X-ray diffraction of single crystals. 1b and 2b crystallize as polymers, whereby compound 1b with smaller groups leads to higher coordination numbers at the potassium atoms (CN=7) and double-stranded more complex ladder-type aggregates, whereas 2b with the larger iPr groups contains potassium atoms with a coordination number of 5 and is a single-stranded polymer. The compound {[KON(CH2C6H5)2]6(THF)4} (3) exists in a hexameric bis-cubane-based form in the solid state. Quantum chemical calculations were undertaken to examine the nature of the hydrogen bonding in the (R2NO...H...ONR2) units of 1b and 2b, which is asymmetric in the first and symmetric in the second case.