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Racemic mixtures of twelve common a-amino acids and three chiral drugs were tested for the separation of their enantiomers by ion mobility spectrometry (IMS)-quadrupole mass spectrometry (MS). Separations were tested by introducing chiral selectors in the mobility spectrometer buffer gas. (R)-α-(trifluoromethyl) benzyl alcohol, (R)-tetrahydrofuran-2-carbonitrile, (L)-ethyl lactate, methyl (S)-2-chloropropionate, and the R and S enantiomers of 2-butanol and 1-phenyl ethanol were evaluated as chiral selectors. Experimental conditions were varied during the tests including buffer gas temperature, concentration, and type of chiral selectors, analyte concentration, electrospray voltage, electrospray (ESI) solvent pH, and buffer gas flow. The individual enantiomers yielded different drift times for periods of up to 8 hours in a few experiments; such drift times were sufficiently different (~ 0.3 ms) to partially resolve the enantiomers in racemic mixtures, but these mixtures always yielded a single mobility peak at the experimental conditions tested with a drift time similar to that of one of the enantiomers. Energy calculations of the chiral selector –ion interactions showed that these separations are unlikely using 2-butanol as chiral selector but they might be feasible depending on the nature of chiral selectors and the type of enantiomers.

In some dermal diseases with evident skin dehydration and desquamation, the natural ratio of CER[NP]:CER[AP] is altered in the extracellular matrix of the stratum corneum by increasing the concentration of CER[AP]. The extracellular matrix of the stratum corneum is composed of several stacked lipid bilayers. Molecular dynamics simulations were used to investigate the molecular nanostructure of CER[NP], CER[AP], cholesterol and lignoceric acid models of the extracellular matrix of the stratum corneum with a nativelike CER[NP]:CER[AP] 2:1 ratio and a CER[NP]:CER[AP] ratio of 1:2. Despite the very minor chemical difference between CER[NP] and CER[AP], which is only a single OH group, it was possible to observe differences between the structural influence of the two ceramides. In the models with 1:2 ratio, the higher CER[AP] content leads to a larger inclination of the acyl chains and a smaller overlap in the lamellar midplane, with a small increase of the repeat distance compared to the model with higher CER[NP] concentration. Because CER[AP] forms more H-bonds than CER[NP], the total number of hydrogen bonds in the headgroup region is larger in the models with higher CER[AP] concentration, reducing the mobility of the lipids towards the centre of the bilayer and resulting in less overlap and increased tilt angles.

The selective N-acylation of 1,2-amino alcohols has been proposed to occur through the proton shuttle mechanism. However, the O-acetylation of propranolol catalyzed by Candida antarctica lipase B is an exception. We investigated the relation between the chemoselectivity of this reaction and the acyl group length. For this purpose, we compared the acyl groups: ethanoyl, butanoyl, octanoyl, and hexadecanoyl. We studied the Michaelis complexes between serine-acylated Candida antarctica lipase B and propranolol, employing a computational approach that involved sampling Michaelis complex conformations through ensemble docking plus consensus scoring and molecular dynamics simulations. The conformations were then classified as near attack conformations for acylation of the amino or hydroxy group. The relative populations of these two classes of conformations were found to be consistent with the experimentally observed chemoselective O-acetylation. We predict that increasing the length of the hydrocarbon chain of the acyl group will cause O-acylation to be unfavorable with respect to N-acylation. The nucleophilic attack of propranolol to the acylated lipase was found to be more favorable through the classical mechanism when compared with the proton shuttle mechanism.

We employed a computational modeling approach to study the Michaelis complexes of (R)- and (S)-propranolol with serine-acylated Candida antarctica lipase B using four acyl groups: ethanoyl, butanoyl, octanoyl and hexadecanoyl. Our methodology involves sampling Michaelis complex conformations, first through ensemble docking using consensus scoring, and second by molecular dynamics simulations employing a quantum mechanics/molecular mechanics approach. The conformations are then categorized into two classes of near attack conformations, according to the distance of (a) the amino and (b) the hydroxy group of propranolol to the catalytic residues. The relative populations of these two classes of conformations was found to be consistent with the experimentally-observed exclusive chemoselectivity toward O-acylation with ethanoyl. Furthermore, we predict that increasing the length of the hydrocarbon chain of the acyl group will cause O-acylation to be unfavorable.

CalB is a triacylglycerol hydrolase (E.C.3.1.1.3) used in the O-acylation of the beta-adrenergic blocking agent (R,S)-propranolol. The catalytic mechanism involves two steps: enzyme acylation and enzyme deacylation. The enantioselectivity of the O-acylation of (R,S)-propranolol originates from the second step, where the acyl-enzyme transfers the acyl group to the racemic substrate. This step occurs via an initial Michaelis complex (MCC) and a tetrahedral intermediate (TI-2). To gain more insight into the molecular basis of this reaction, we performed an exhaustive conformational sampling along the reaction coordinate of the enantioselective step of the reaction (MCC→TI-2→EPC) applying a QM/MM MD protocol (SCC-DFTB/CHARMM) in combination with umbrella sampling and the weighted histogram analysis method. To identify finite temperature effects we compare the PMF and the potential energy pathway. It is found that the effect of the finite temperature in this reaction is a destabilization of the tetrahedral intermediate and an increase of the barrier height of its formation. This increase is higher for the S-enantiomer.

The impact of altering the solvent pH value on the photodynamic action of thionine has been studied computationally by means of density functional theory and multi-reference interaction methods. To this end, we have investigated the electronic structure of the ground and excited states of diprotonated (TH₂²+) and neutral imine (T) forms of thionine (TH+). It is well known experimentally that the T₁ state of TH+ undergoes acid-base equilibrium reactions resulting in a pronounced pH effect for the efficiency of singlet oxygen (¹O₂) production. Our results show that the energy transfer reactions from the T₁ state of TH₂²+ and T to ³O₂ correspond to reversible equilibrium processes, whereas in TH+ this process is very exothermic in vacuum (-0.66 eV) and in aqueous solution (-0.49 eV). These facts explain the experimental observation of a much smaller efficiency of ¹O₂ production for TH₂²+ than for TH+. Moreover, we found that the pH value significantly affects the intersystem crossing (ISC) kinetics impacting the concentration of triplet state species available for energy transfer. In very acidic aqueous solution (pH 10⁷s-1). According to these results, maximum ISC efficiency is expected for intermediate acidic pH values (TH+, ∼10⁹ s-1).

Classical molecular dynamics (MD) simulations and combined quantum mechanics/molecular mechanics (QM/MM) calculations were used to investigate the origin of the enantioselectivity of the Candida antarctica lipase B (CalB) catalyzed O-acetylation of (R,S)-propranolol. The reaction is a two-step process. The initial step is the formation of a reactive acyl enzyme (AcCalB) via a tetrahedral intermediate (TI-1). The stereoselectivity originates from the second step, when AcCalB reacts with the racemic substrate via a second tetrahedral intermediate (TI-2). Reaction barriers for the conversion of (R)- and (S)-propranolol to O-acetylpropranolol were computed for several distinct conformations of TI-2. In QM/MM geometry optimizations and reaction path calculations the QM region was described by density functional theory (B3LYP/TZVP) and the MM region by the CHARMM force field. The QM/MM calculations show that the formation of TI-2 is the rate-determining step. The energy barrier for transformation of (R)-propran...

While the optical spectra of the acene series up to pentacene provide textbook examples for the annulation principle, the spectra of the larger members are much less understood. The present work provides an investigation of the optically allowed excited states of the acene series from pentacene to nonacene, the largest acene observed experimentally, using the density functional based multireference configuration method (DFT/MRCI). For this purpose, the ten lowest energy states of the B2u and B3u irreducible representations were computed. In agreement with previous computational investigations, the electronic wave functions of the acenes acquire significant multireference character with increasing acene size. The HOMO → LUMO excitation is the major contributor to the (1)La state (p band, B2u) also for the larger acenes. The oscillator strength decreases with increasing length. The (1)Lb state (α band, B3u), so far difficult to assign for the larger acenes due to overlap with photoprecursor bands, becomes almost insensitive to acene length. The (1)Bb state (β band, B3u) also moves only moderately to lower energy with increasing acene size. Excited states of B3u symmetry that formally result from double excitations involving HOMO, HOMO-1, LUMO, and LUMO+1 decrease in energy much faster with system size. One of them (D1) has very small oscillator strength but becomes almost isoenergetic with the (1)La state for nonacene. The other (D2) also has low oscillator strength as long as it is higher in energy than (1)Bb. Once it is lower in energy than the (1)Bb state, both states interact strongly resulting in two states with large oscillator strengths. The emergence of two strongly absorbing states is in agreement with experimental observations. The DFT/MRCI computations reproduce experimental excitation energies very well for pentacene and hexacene (within 0.1 eV). For the larger acenes deviations are larger (up to 0.2 eV), but qualitative agreement is observed.

The effect of substituting the intra-cyclic sulphur of thionine by oxygen (oxonine) and selenium (selenine) on the intersystem crossing (ISC) efficiency has been studied using high level quantum mechanical methods. The ISC rate constants are considerably increased when going from O towards Se while the fluorescence rate constants remain unchanged. For the three dyes, all accessible ISC channels are driven by vibronic spin-orbit coupling (SOC) between ππ* states. The interplay between the ground and low-lying excited states has been investigated in order to determine the dominant relaxation pathways. In oxonine the relaxation to the ground state after photoexcitation in water proceeds essentially via fluorescence from the S1(πHπL*) bright state (kF = 2.10 × 10(8) s(-1)), in agreement with the high experimental fluorescence quantum yield. In aqueous solution of thionine, the ISC rate constant (kISC ∼ 1 × 10(9) s(-1)) is one order of magnitude higher than fluorescence (kF = 1.66 × 10(8) s(-1)) which is consistent with its high triplet quantum yield observed in water (ϕT = 0.53). Due to a stronger vibronic SOC in selenine, the ISC rate is very high (kISC ∼ 10(10) s(-1)) and much faster than fluorescence (kF = 1.59 × 10(8) s(-1)). This suggests selenine-based dyes as very efficient triplet photosensitizers.

We have carried out a computational study on the reactivity of catechol (1,2-dihydroxybenzene) towards superoxide radical anion (O2˙−) in water, N,N-dimethylformamide (DMF), pentyl ethanoate (PEA) and vacuum using density functional theory and the coupled cluster method. Five reaction mechanisms were studied: (i) sequential proton transfer followed by hydrogen atom transfer (PT-HT), (ii) sequential hydrogen atom transfer followed by proton transfer (HT-PT), (iii) single electron transfer (SET), (iv) radical adduct formation (RAF) and (v) concerted double proton-transfer electron-transfer (denoted as global reaction, GR). Our results show that catechol and superoxide do not react via a sequential reaction mechanism (initial PT, initial HAT or SET). Instead, the reaction proceeds via a concerted double proton-transfer electron-transfer mechanism yielding hydrogen peroxide and catechol radical anion. The protons are transferred asynchronously between the σ orbitals of the catechol oxygen atoms to superoxide, while the electron is transferred between oxygen π orbitals in the same direction. The calculated rate constants in aqueous media agree with the experimental values reported in the literature. This suggests that the mechanism proposed in this work is adequate to describe this reaction. In addition, our results show that the reaction exhibits a large tunneling effect.

Candida antarctica lipase B (CalB) displays moderate enantioselectivity when it catalyzes the acetylation of (R,S)-propranolol, favoring the faster transformation of the R-propranolol. With the aim to better understand the enantioselectivity of this reaction, we have performed a molecular dynamics (MD) study of the enzyme substrate complexes. Reactive enzyme substrate complexes were identified for both enantiomers of propranolol, which differ in their temporal stability and in their ability to reach the corresponding transition states (TS). Reactive complexes of R-propranolol present a better ability to be transformed by CalB than those of S-propranolol. This allows us to explain the enantioselectivity. Analysis of the enzyme–substrate interactions suggests that the CH-π interactions between the naphthyl rings of propranolol and the residues of the CalB binding pocket may play an important role in stabilizing the transition states involved in the transformation of the R-propranolol. The residues Ile189, Ala282 and Leu278 were identified as key residues for the enantioselectivity of CalB.

The chemo- and enantioselectivity of the Candida antarctica lipase B (CalB)-catalyzed acetylation reaction of (R,S)-propranolol using vinyl acetate as acyl donor and toluene as organic solvent was studied. Because of the poor solubility of propranolol in toluene small quantities of methanol were added as cosolvent. The effects of the propranolol/vinyl acetate ratio, the enzyme purification procedure and the methanol concentration on the reaction were investigated. The reactions occurring in the system were quantitatively investigated using 1H and 13C NMR spectroscopy. The major reactions were the hydrolysis and alcoholysis of vinyl acetate, as a consequence of the presence of residual water and methanol in the reaction medium. Furthermore, the NMR analysis confirmed that O-acetyl-propranolol was formed exclusively. The reaction was also found to be enantioselective favoring the faster transformation of the R-propranolol. In addition to the experiments, molecular modeling was used to study the formation of the reactive Michaelis complexes between propranolol and acetylated CalB, using a combined molecular docking and molecular dynamics (MD) procedure. Only for the O-acetylation we found binding modes of the substrate leading to formation of the product, which explains the experimentally observed chemoselectivity of CalB.

We have examined the electronic and molecular structure of 3,7-diaminophenothiazin-5-ium dye (thionine) in the electronic ground state and in the lowest excited states. The electronic structure was calculated using a combination of density functional theory and multi-reference configuration interaction (DFT/MRCI). Equilibrium geometries were optimized employing (time-dependent) density functional theory (B3LYP functional) combined with the TZVP basis set. Solvent effects were estimated using the COSMO model and micro-hydration with up to five explicit water molecules. Our calculated electronic energies are in good agreement with experimental data. We find the lowest excited singlet and triplet states at the ground state geometry to be of π→π* (S(1), S(2), T(1), T(2)) and n→π* (S(3), T(3)) character. This order changes when the molecular structure in the electronically excited states is relaxed. Geometry relaxation has almost no effect on the energy of the S(1) and T(1) states (~0.02 eV). The relaxation effects on the energies of S(2) and T(2) are moderate (0.14-0.20 eV). The very small emission energy results in a very low fluorescence rate. While we were not able to locate the energetic minimum of the S(3) state, we found a non-planar minimum for the T(3) state with an energy which is very close to the energy of the S(1) minimum in the gas phase (0.04 eV above). When hydration effects are taken into account, the n→π* states S(3) and T(3) are strongly blueshifted (0.33 and 0.46 eV), while the π→π* states are only slightly affected (0.06 eV).

We report a combined quantum mechanical/molecular mechanical (QM/MM) study of the DsRed.M1 protein using as QM component the self-consistent charge density functional tight-binding (SCC-DFTB) method in molecular dynamics (MD) simulations and hybrid density functional theory (DFT, B3LYP functional) in QM/MM geometry optimizations. We consider different variants of the chromophore (including the cis- and trans-acylimine and peptide forms) as well as different protonation states of environmental residues. The QM/MM calculations provide insight into the role of nearby residues concerning their interactions with the chromophore and their influence on structural and spectroscopic properties. QM/MM optimizations yield a single conformer for the anionic acylimine chromophore, whereas there are distinct cis- and trans-conformers in the anionic peptide chromophore, the latter being more stable. The calculated vertical excitation energies (DFT/MRCI) for the anionic chromophores agree well with experiment. The published crystal structure of DsRed.M1 with an anionic acylimine chromophore indicates a quinoid structure, while the QM/MM calculations predict the phenolate form to be more stable.

Nuclear magnetic resonance (NMR) chemical shifts in solids may be calculated by ab initio methods approximating the solid state by molecular clusters. We employed this technique to obtain estimates of (15)N chemical shifts in NB(2)Si and NBSi(2) environments in the solid state. Such nitrogen environments are found in amorphous (Si/B/N-)ceramics which exhibit very interesting features such as high thermal and mechanical stability. We based our calculations on cutouts of hypothetical Si(3)B(3)N(7) crystals suggested by Kroll and Hoffmann [Silicon boron nitrides: hypothetical polymorphs of Si(3)B(3)N(7), Angew. Chem. Int. Ed. 37 (1998) 2527]. Taking the systematic errors of our calculations into account we expect the chemical shifts in NBSi(2) environments around -293+/-5ppm. Chemical shifts in NB(2)Si environments are expected at -272+/-6ppm. The range of the calculated chemical shifts in NBSi(2) environments coincides with experimental chemical shifts in molecular compounds. Experimental chemical shifts of NB(2)Si nitrogen in molecules appear at lower field than our calculated chemical shifts in the solid state.

A number of new diesterquats and anidoester quats were synthesized and tested for biodegradation and softening. Methyldiethanolamine, 3-methoxylpropylamine, diethanolaminopropylamine, aminoethylethanolamine, dimethylethanolamine, and diethanolamine were reacted, either “as is” or after alkoxylation, with tallow fatty acid and further converted into dialkyl quats. The diesterquat from diethanolisopropanolamine was by far the best of the ester quats synthesized in overall biodegradation, softening, and cost/ease of manufacture. It appears that two ester groups are needed for ready biodegradation as opposed to an amide-ester combination. Increasing unsaturation of the alkyl chains within a molecule increases its rate of biodegradation slightly.

A study of the possible intersystem crossing (ISC) mechanisms (S→T) in thionine (3,7-diamino-phenothiazin-5-ium), which is conducive to the efficient population of the triplet manifold, is presented. The radiationless deactivation channels {S(1),S(2)(π → π*) → T(1),T(2)(π → π*)} have been examined. Since the direct ISC does not explain the high triplet quantum yield in this system, attention has been centered on the vibronic spin-orbit coupling between the low-lying singlet and triplet (π → π*) states of interest. An efficient population transfer from the S(1)(π(H) → π(L)*) state to the T(2)(π(H-1) → π(L)*) state via this channel is confirmed. The calculated ISC rate constant for this channel is k(ISC) ≈ 3.35 × 10(8) s(-1), which can compete with the radiative depopulation of the S(1)(π(H) → π(L)*) state via fluorescence (k(F) ≈ 1.66 × 10(8) s(-1)) in a vacuum. The S(1)(π(H) → π(L)*) → T(1)(π(H) → π(L)*) and {S(2)(π(H-1) → π(L)*) → T(1),T(2)(π → π*)} ISC channels have been estimated to be less efficient (k(ISC) ≈ 10(5)-10(6) s(-1)). Based on the computed ISC rate constants and excited-state solvent shifts, it is suggested that the efficient triplet quantum yield of thionine in water is primarily due to the S(1)(π(H) → π(L)*) → T(2)(π(H-1) → π(L)*) channel with a computed rate constant of the order of 10(8)-10(9) s(-1) which is in accord with the experimental finding (k(ISC) = 2.8 × 10(9) s(-1)).

The far-red fluorescent protein HcRed was investigated using molecular dynamics (MD) and combined quantum mechanics/molecular mechanics (QM/MM) calculations. Three models of HcRed (anionic chromophore) were considered, differing in the protonation states of nearby Glu residues (A: Glu214 and Glu146 both protonated; B: Glu214 protonated and Glu146 deprotonated; C: Glu214 and Glu146 both deprotonated). SCC-DFTB/MM MD simulations of model B yield good agreement with the available crystallographic data at ambient pH. Bond lengths in the QM region are well reproduced, with a root mean square (rms) deviation between experimental and average MD data of 0.079 A; the chromophore is almost co-planar, which is consistent with experimental observation; and the five hydrogen bonds involving the chromophore are conserved. QM/MM geometry optimizations were performed on representative snapshot structures from the MD simulations for each model. They confirm the structural features observed in the MD simulations. According to the DFT(B3LYP)/MM results, the cis-conformation of the chromophore is more stable than the trans-form by 9.1-12.9 kcal mol(-1) in model B, and by 12.4-19.9 kcal mol(-1) in model C, consistent with the experimental preference for the cis-isomer. However, in model A when both Glu214 and Glu146 are protonated, the stability is inverted with the trans-form being favored. The different protonation states of the titratable active-site residues Glu214 and Glu146 thus critically influence the manner in which the relative stability and degree of planarity of the cis- and trans-conformers vary with pH. Coupled with the known correlation of chromophore conformation with fluorescence efficiency, this work provides a detailed structural basis for the observed phenomenon that red fluorescent proteins such as HcRed, mKate and Rtms5 show bright fluorescence at high pH.

Quantum refinement is an improvement upon the molecular mechanics (MM)-based crystallographic refinement. In the latter, X-ray data are supplemented with additional chemical information through MM force fields, whereas quantum refinement describes crucial regions of interest in the macromolecule by quantum mechanics (QM) instead of MM. In this paper, we report the implementation of quantum refinement in the ChemShell QM/MM framework and its application in an investigation of the chromophore structure of the red fluorescent protein DsRed.M1. Both mechanical and electrostatic QM/MM embedding schemes are implemented and tested. In the quantum refinement of DsRed.M1, the anionic red acylimine chromophore adopts a nearly orthogonal arrangement (rather than a cis or trans form), and the bond lengths in the acylimine moiety are more consistent with a phenolate (rather than a quinoid) structure. These findings are in contrast to the structure deduced from a standard crystallographic refinement (PDB: 2VAD), but in agreement with our earlier results from a purely theoretical QM/MM study. On the other hand, the quantum refinement of the anionic acylimine form of DsRed.M1 yields a hydrogen bonding network around the chromophore, especially with regard to the arrangement of the water molecules and the Glu148 residue, that is closer to the 2VAD structure than to the previously optimized QM/MM structure. In our earlier study the initial classical molecular dynamics (MD) simulations during QM/MM setup apparently exaggerated the mobility of the water molecules around the chromophore. On the basis of the present results, it seems likely that the Glu148 residue is protonated in the DsRed.M1 protein. The calculation of electronic excitation energies allows for further assessment of the proposed structures, especially in the chromophore region. Using a combination of density functional theory and multireference configuration interaction (DFT/MRCI), we find excellent agreement between experiment and theory only for the structures obtained from quantum refinement and from QM/MM optimization, but not for the 2VAD structure. The present case study on DsRed.M1 thus demonstrates the merits of combining reliable theoretical and experimental information in the determination of protein structures.

Electronic spectra of guanine in the gas phase and in water were studied by quantum mechanical/molecular mechanical (QM/MM) methods. Geometries for the excited-state calculations were extracted from ground-state molecular dynamics (MD) simulations using the self-consistent-charge density functional tight binding (SCC-DFTB) method for the QM region and the TIP3P force field for the water environment. Theoretical absorption spectra were generated from excitation energies and oscillator strengths calculated for 50 to 500 MD snapshots of guanine in the gas phase (QM) and in solution (QM/MM). The excited-state calculations used time-dependent density functional theory (TDDFT) and the DFT-based multireference configuration interaction (DFT/MRCI) method of Grimme and Waletzke, in combination with two basis sets. Our investigation covered keto-N7H and keto-N9H guanine, with particular focus on solvent effects in the low-energy spectrum of the keto-N9H tautomer. When compared with the vertical excitation energies of gas-phase guanine at the optimized DFT (B3LYP/TZVP) geometry, the maxima in the computed solution spectra are shifted by several tenths of an eV. Three effects contribute: the use of SCC-DFTB-based rather than B3LYP-based geometries in the MD snapshots (red shift of ca. 0.1 eV), explicit inclusion of nuclear motion through the MD snapshots (red shift of ca. 0.1 eV), and intrinsic solvent effects (differences in the absorption maxima in the computed gas-phase and solution spectra, typically ca. 0.1-0.3 eV). A detailed analysis of the results indicates that the intrinsic solvent effects arise both from solvent-induced structural changes and from electrostatic solute-solvent interactions, the latter being dominant.

We have examined the electronic and molecular structure of 1H-phenalen-1-one (phenalenone) in the electronic ground state and in the lowest excited states, as well as intersystem crossing. The electronic structure was calculated using a combination of density functional theory and multi-reference configuration interaction. Intersystem crossing rates were determined using Fermi's golden rule and taking direct and vibronic spin-orbit coupling into account. The required spin-orbit matrix elements were obtained applying a non-empirical spin-orbit mean-field approximation. Our calculated electronic energies are in good agreement with experimental data. We find the lowest excited singlet states to be of the npi* (S1) and pipi* (S2) type. Energetically accessible from S1 are two triplet states of the pipi* (T1) and npi* (T2) type, the latter being nearly degenerate to S1. This ordering of states is retained when the molecular structure in the electronically excited states is relaxed. We expect very efficient intersystem crossing between S1 and T1. Our calculated intersystem crossing rate is approximately 2 x 10(10) s(-1), which is in excellent agreement with the experimental value of 3.45 x 10(10) s(-1). Our estimated phosphorescence and fluorescence rates are many orders of magnitude smaller. Our results are in agreement with the experimentally observed behavior of phenalenone, including the high efficiency of 1O2 production.

Structural, magnetic and transport properties of undoped and Mn-doped quasi one-dimensional ZnO nano-wires formed as dendrite crystals of different width have been studied. All Mn-doped nanowires (Mn content 16% and 27 %) exhibit a ferromagnetic behavior, which is significantly temperature dependent. A small magnetic signal detected up to the room temperature is supposed to be due to small amounts of magnetic impurities. Analysis of the temperature dependencies of magnetization shows that the observed magnetic properties can be caused by the host ferromagnetic matrix with Curie temperature TC  40 K.

Ammonolysis of the monomeric, base-stabilized trisaminoalane Me 3 N-Al[N(H)Dipp)] 3 (Dipp=2,6- i Pr 2 –C 6 H 3 ) yielded Al–N oligomers, which were characterized in detail by solid state NMR spectroscopy ( 1 H, 13 C, 15 N, 27 Al) and TGA/DTA. Pyrolysis of as-prepared oligomers at different temperatures in an argon steam yielded carbon-containing black solids, whereas pyrolysis under a steady flow of NH 3 produced pure aluminum nitride (AlN). The role of the pyrolysis temperature and the influence of NH 3 on the formation of crystalline materials were investigated. As-prepared AlN was characterized by solid state NMR spectroscopy ( 15 N, 27 Al), X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). Theoretical calculations were performed in order to identify potential reaction intermediates.

We present an enhanced reverse Monte Carlo approach that includes fitting to NMR data in the form of chemical shifts in addition to the usually used scattering data. Furthermore, the internal energy is accounted for in the cost function to prevent unphysical structures. This approach was applied to generate structural models of amorphous Si3B3N7, which is a prototype of a new class of high performance ceramics exhibiting interesting features like high thermal and mechanical stability. We fitted our structural models in direct space to radial distribution functions from x-ray, neutron and electron scattering experiments, and to the 15N NMR spectrum of the ceramic. This spectrum could not be interpreted before since it exhibits a broad structureless signal which is a superposition of peaks related to different chemical environments NBxSi3−x (x = 0–3) whose chemical shifts were only partly known experimentally. Therefore we based the calculation of NMR data in the reverse Monte Carlo optimizations on previous theoretical work that was done in our group. All generated models reproduce the experimental radial distribution functions very well. This good agreement does not deteriorate when the NMR data are taken into account. Fitting the models to 15N NMR chemical shifts in addition to scattering data results in structural changes that not only improve the agreement with the experimental magic-angle spinning (MAS) NMR spectrum but yield also significantly better second-nearest neighbour coordination statistics.

The equilibrium geometries and first bond dissociation energies of the homoleptic complexes M(EMe)4 and M(CO)4 with M = Ni, Pd, Pt and E = B, Al, Ga, In, Tl have been calculated at the gradient corrected DFT level using the BP86 functionals. The electronic structure of the metal-ligand bonds has been examined with the topologial analysis of the electron density distribution. The nature of the bonding is revealed by partitioning the metal-ligand interaction energies into contributions by electrostatic attraction, covalent bonding and Pauli repulsion. The calculated data show that the M-CO and M-EMe bonding is very similar. However, the M-EMe bonds of the lighter elements E are much stronger than the M-CO bonds. The bond energies of the latter are as low or even lower than the M-TlMe bonds. The main reason why Pd(CO)4 and Pt(CO)4 are unstable at room temperature in a condensed phase can be traced back to the already rather weak bond energy of the Ni-CO bond. The Pd-L bond energies of the complexes with L = CO and L = EMe are always 10 — 20 kcal/mol lower than the Ni-L bond energies. The calculated bond energy of Ni(CO)4 is only Do = 27 kcal/mol. Thus, the bond energy of Pd(CO)4 is only Do = 12 kcal/mol. The first bond dissociation energy of Pt(CO)4 is low because the relaxation energy of the Pt(CO)3 fragment is rather high. The low bond energies of the M-CO bonds are mainly caused by the relatively weak electrostatic attraction and by the comparatively large Pauli repulsion. The σ and π contributions to the covalent M-CO interactions have about the same strength. The π bonding in the M-EMe bonds is less than in the M-CO bonds but it remains an important part of the bond energy. The trends of the electrostatic and covalent contributions to the bond energies and the σ and π bonding in the metal-ligand bonds are discussed. Theoretische Studien anorganischer Verbindungen. 17 Warum sind die homoleptischen Diyl-Komplexe M(InR)4 mit M = Pt und Ni stabile Verbindungen obwohl nur Ni(CO)4 nicht aber Pt(CO)4 isoliert werden kann? Eine theoretische Untersuchung von M(EMe)4 und M(CO)4(M = Ni, Pd, Pt; E 5 B, Al, Ga, In, Tl) Die Geometrien und erste Dissoziationsenergien der homoleptischen Komplexe M(EMe)4 und M(CO)4 mit M = Ni, Pd, Pt und E = B, Al, Ga, In, Tl wurden mit gradientenkorrigierten Dichtefunktionalen unter Verwendung von BP86 berechnet. Die elektronische Struktur der Metall-Ligand-Bindungen ist mit der topologischen Analyse der Elektronendichteverteilung untersucht worden. Die Natur der chemischen Bindung wurde durch eine Energieanalyse aufgeklart, mit deren Hilfe die Beitrage der elektrostatischen Anziehung, der kovalenten Bindung und der Pauli-Abstosung zu den Metall-Ligand-Wechselwirkungen ermittelt wurden. Die Ergebnisse zeigen, dass die M-CO- and M-EMe-Bindungen sehr ahnlich sind. Die M-EMe-Bindungen der leichteren Elemente E sind aber wesentlich starker als die M-CO-Bindungen. Die Metall-Carbonyl-Bindungen haben eine ahnlich geringe Bindungsenergie wie die M-TlR-Bindungen. Die Erklarung, warum Pd(CO)4 und Pt(CO)4 in kondensierter Phase bei Raumtemperatur instabil sind, findet ihren Ursprung in der bereits geringen Bindungsenergie des Ni(CO)4. Die Pd-L-Bindungsenergien der Komplexe mit L = CO und L = EMe sind immer 10 — 20 kcal/mol niedriger als die Bindungsenergien der Ni-L-Bindungen. Da die berechnete Bindungsenergie des Ni(CO)4 lediglich Do = 27 kcal/mol betragt, sinkt die Bindungsenergie des Pd(CO)4 auf nur noch Do = 12 kcal/mol. Die erste Bindungsdissoziationsenergie des Pt(CO)4 ist niedrig, weil die Relaxationsenergie des Pt(CO)3-Fragments recht hoch ist. Die relativ niedrigen Wechselwirkungsenergien der M-CO-Bindungen konnen mit der im Vergleich zur M-ER-Bindung schwachen elektrostatischen Anziehung und der starken Pauli-Abstosung erklart werden. Die kovalente M-CO-Bindung hat etwa gleichstarke σ- und π-Beitrage. Die σ-Beitrage in den M-EMe-Bindungen sind schwacher als in den M-CO-Bindungen aber sie bleiben wichtig fur die Bindungsenergie. Die Trends der elektrostatischen und der kovalenten Beitrage zu den Bindungsenergien und die σ- und π-Anteile der Metall-Ligand-Bindungen werden diskutiert.

The direct (254 nm) and acetone-sensitized (300 nm) photolyses of 2,4,4-trideuteriobicyclo(3.2.1)octa-2,6-diene (D-1) gave exclusively 3,5,5-trideuteriotricyclo(4.2.0.0)oct-3-ene (D-7) and 2,6,6-trideuteriotricyclo(3.2.1.0)oct-3-ene (D-8) but no rearranged diene, namely, 2,8,8-trideuteriobicyclo(3.2.1)octa-2,6-diene (D-1{prime}). Pyrolysis (400{degree}C) and direct (334 nm) and benzophenone-sensitized (364 nm) photolyses of the azoalkane 3,5,5-trideuterio-9,10-diazatricyclo(4.4.0.0)deca-3,9-diene (D-9) afforded a mixture of tricyclooctenes D-7 (major product) and D-8 and bicyclooctadiene D-1, but no rearranged diene D-1{prime}. These three modes of denitrogenation of the azoalkane 2,2,7-trideuterio-4,5-diazatricyclo(4.3.1.0)deca-4,8-diene (D-10) led only to the tricyclooctene D-8 (major product) and equal amounts of the bicyclooctadienes D-1 and D-1{prime}. Distinct temperature profiles were observed in the product fingerprints of the direct photolysis (334 nm) of the azoalkanes 9 and 10. For both azoalkanes 9 and 10 the quantum yields of denitrogenation increased with rising temperature. In none of these transformations could bicyclo(3.3.0)octa-2,7-diene (2) be detected. These results imply that the di-{pi}-methane rearrangement of the bicyclooctadienes 1 (D-1) and the denitrogenations of the azoalkanes 9 (D-9) and 10 (D-10) are disjoint chemical events. The intervention of diazenyl diradicals (one-bond cleavage of the azoalkanes) is postulated to be responsible for this disparity. The intervention of cyclopropyldicarbinyl diradical 3 as bona fide reaction intermediate in the di-{pi}-methane rearrangement of bicyclooctadiene 1 is questioned.

In contrast to the direct or acetone-sensitized irradiation of the azoalkane (I), which leads exclusively to the product of cyclization (III), a rearranged product (IV) is formed in the presence of the electron acceptor triphenylpyrylium tetrafluoroborate (TPT) via the radical cation (II) and the TPT radical.

In contrast to the direct or acetone-sensitized irradiation of the azoalkane (I), which leads exclusively to the product of cyclization (III), a rearranged product (IV) is formed in the presence of the electron acceptor triphenylpyrylium tetrafluoroborate (TPT) via the radical cation (II) and the TPT radical.