Stereoselectivity in nucleophilic addition to unsaturated ligands bound to molybdenum : Allylic alkylation of cyclohexanone

The stereochemistry and regiochemistry of nucleophilic addition to olefinic, allylic, or diene moieties can be controlled in reactions of molybdenum complexes. The synthesis of a wide range of -allylic cyclohexanones is feasible using (η⁵-cyclopentadienyl)Mo(CO)(NO)(allyl) cations. The stereoselective preparation of (RS,SR)-2-(1-methyl-2-butenyl)cyclohexanone from the reaction of 1-pyrrolidino-1-cyclohexene with [CpMo(CO)(NO)(η³-1,3-dimethylallyl)]BF₄ illustrates the methodology.     

New cycloaddition reactions of 1-phenyl-4-vinylpyrazole

1-Phenyl-4-vinylpyrazole reacts with methyl propiolate and N-phenylmaleimide giving via the Diels-Alder 1:1 adducts, products (4) and (8), and also the 1:2 adducts (5), (6) and (9) resulting from an “ene” reaction of the initially forced cycloadducts. The obtention of the adducts (5) and (6) in equimolecular amounts is a good example of the non-regioselective character of the “ene” reaction. The reaction with tetracyanoethylene takes place through the olefinic substituent giving the π² + π² adduct (10).     

The mechanism of pyrolysis of polycycloalkyl sulfonates : The homo-retroene reaction in the noradamantane, adamantane and diamantane series

The minor product, 4-protoadamantene (5), from the gas phase pyrolysis of 2-adamantyl mesylate (3) was shown by labelling experiments not to arise from the major product, 2,4-dehydroadamantane (4), but rather by a concerted process (homo-retroene reaction). In the formation of 4, 1,3-elimination is favored by at least a 92:8 margin over a competitive route involving -elimination to a carbene and then C-H insertion. Equatorial and axial 2-noradamantyl mesylates pyrolyze to give predominantly 4-brendene (11) and triaxane (12), respectively, as required by concerted 1,3-eliminations, and not by an ion pair mechanism. The cross-over product appears in each case to result from the epimerizarion of the starting material by a wall-catalyzed process. In agreement with the results of the noradamantyl substrates, pyrolysis of 3-diamantyl mesylate (16) gave results which suggest that product stability helps to determine the course of the reaction. The products with the least amount of strain were obtained, namely 3,5-dehydrodiamantane (17) and pentacyclo[8.3.1.0^2,8.0^4,13.0^7,12]tetradec-5-ene (19, protodiamantene). The ease of separation of the cyclopropanic and olefinic products makes these reactions synthetically useful.     

A water soluble nanostructured platinum (0) metal catalyst from platinum carbonyl molecular clusters: Synthesis, characterization and application in the selective hydrogenations of olefins, ketones and aldehydes

High nuclearity platinum carbonyl cluster anions (Chini's clusters) have been used as precursors to prepare a platinum nanocatalyst. The ionic polyelectrolyte poly(diallyldimethylammonium chloride) has been used as the support material for anchoring [Pt₃₀(CO)₆₀]²⁻ via ion-pairing and subsequent stabilization of the nanoparticles. The polymer-supported material has been studied by spectroscopy (NIR, ¹³C NMR, and IR) and TEM before and after its use as a water soluble hydrogenation catalyst. The nanocatalyst is found to be effective for the chemoselective hydrogenation of olefinic, aldehydic and ketonic double bonds. For most of the substrates isolation of the product and reuse of the catalyst are extremely easy due to the automatic phase separation of the products from the catalyst. The spectral features of the fresh catalyst show retention of the carbonyl ligands and molecular identity of the parent cluster, but after use the carbonyl ligands appear to be lost. TEM of the supported material before and after use as a catalyst shows the presence of platinum nanoparticles with majority (≥70%) of the particles in the range of 2–6 nm. Smaller particles are dominant in the used catalyst and this observation is rationalized on the basis of the known reactivity of Chini's clusters with dihydrogen.     

Theoretical and experimental studies of radiative lifetimes of excited electronic states in CO

The electronic dipole transition moment functions of the A ²Π-X ²Σ⁺, B ²Σ⁺-X ²Σ⁺ and B ²Σ⁺-A ²Π transitions and the dipole moment function of the X ²Σ⁺ state of CO⁺ have been calculated using large contracted CI wavefunctions. The computed transition moment functions together with experimental potential energy curves were used to obtain radiative lifetimes of the excited electronic states B ²Σ⁺ and A ²Π. Radiative lifetimes of vibrational levels of the X ²Σ⁺ state were derived from the calculated dipole moment function. The high-frequency deflection technique was used to obtain radiative lifetimes of the ν′ = 0, 1,2 and 3 vibrational levels of the B ²Σ⁺ state and also radiative lifetimes of individual rotational levels of ν′ =0. The calculated radiative lifetimes are shorter than the measured ones by about 10%. The experimental ν′ dependence is reproduced by theoretical calculation. The calculated radiative lifetimes for the A ²Π state are in excellent agreement with lifetimes measured with an ion trap technique.     

Ab initio study of the electronic spectrum of dichlorocarbene CCl2

The equilibrium geometries, excitation energies, force constants and vibrational frequencies for seven low-lying electronic states X ¹A₁, ¹B₁, ³B₁, ¹A₂, ³A₂, ¹B₂ and ³B₂ of dichlorocarbene CCl₂ have been calculated at the MRSDCI level with a double-zeta plus polarization basis set. Our calculated equilibrium geometry for the X ¹A₁ state, excitation energy for X ¹A₁ → ¹B₁ and vibrational frequencies for the X ¹A₁ and ¹B₁ states are in good agreement with experimental data. The electronic transition dipole moments, oscillator strengths for the ¹B₁ → X ¹A₁ and ¹B₂ → X ¹A₁ transitions, radiative lifetimes for the ¹B₁ and ¹B₁ states are calculated using MRSDCI wavefunctions, predicting results in reasonable agreement with experiment.     

MRSDCI STUDIES OF LOW-LYING ELECTRONIC STATES OF THE N_2F~+ MOLECULE

Ab initio electronic structure calculations are reported for low-lying electronic states,X ~1Σ~+andA ~1Π of the N_2F~+ molecule.Geometric parameters for the ground state X ~1Σ~+ are predicted by means of mul-tireference single and double excitation configuration interaction(MRSDCI)calculations with a double zeta pluspolarization(DZ+P)basis set.Vertical excitation energy for these two electronic states is determined usingMRSDCI/DZ+P calculations at the ground state equilibrium geometry.The oscillator strength for the X~1Σ+→A ~1Π transition and the radiative lifetime for the A~1Π state are calculated based on the MRSDCI wavefunc-tions.     

New phosphabenzenes by [4 + 2] cycloaddition of stannoles to 1-phospha-1-alkynes — determination of signs of coupling constants [J(P, C), J(P, H), J(P, Si), J(Sn, P)]

Stannoles bearing dialkylboryl groups in 3-position react with 1-phospha-1-alkynes P≡C---^tBu (1) and P≡C---CH₂^tBu (2) by [4 + 2] cycloaddition and elimination of stannylene to give phosphabenzenes in high yield. The stannylenes oligomerise to give [R¹₂Sn]_n with n ≥ 7 (R --- Me, Et, -(CH₂)₅₋ or, in the case of R¹ = ^tBu, react with the stannole itself. All phosphabenzenes are characterized by their consistent sets of NMR data. The absolute signs of the coupling constants ⁿJ(³¹P, ¹H), ⁿJ(³¹P, ¹³C), ²J(³¹P, ²⁹Si) and ²J(¹¹⁹Sn, ³¹P) were determined by appropriate ID and 2D NMR experiments.     

Matrixreaktion von GaF mit o-atomen: ir-spektrum von OGaF

It is possible to synthesize OGaF in an argon matrix by co-condensation of GaF with O atoms. The stretching vibrations of this compound are found at 943 and 690 cm⁻¹ (¹⁶O⁶⁹GaF). An exact calculation of the force field is possible with the aid of the observed ¹⁶O/¹⁸O and ⁶⁹Ga/⁷¹Ga shifts. The following force constants are found: f(GaO) = 6.32; f(GaF) = 4.69; f(GaO/GaF) = −0.1 [10²Nm⁻¹]     

Automatic scanning of chromatograms by multi-scaling with a gamma-ray spectrometer

The location of gamma-emitting isotopes on Chromatographic thin layers is achieved, together with qualitative and quantitative analysis, by using only a gamma-ray spectrometer. This technique is applied to the determination of the Chromatographic behaviour of nanogram amounts of ⁵¹Cr, ⁵⁴Mn, ⁵⁹Fe, ⁶⁰Co, ⁶⁵Zn and ¹⁰⁹Cd thiocyanate complexes on silica gel. Separations are made of Co, Zn and Cd from Cr, Mn and Fe.     

C and H nuclear magnetic shielding and spin–spin coupling constants of C-enriched bromomethane in the gas phase

¹³C and ¹H NMR spectral parameters are investigated for ¹³CH₃Br in gaseous matrices. It is found that both the ¹³C and ¹H chemical shifts of ¹³CH₃Br are linearly dependent on solvent density. Similar dependence is also detected for one-bond spin–spin coupling, ¹J(CH). For the first time the ¹³C and ¹H magnetic shielding constants and ¹J(CH) spin–spin coupling are obtained for an isolated ¹³CH₃Br molecule together with the coefficients responsible for solute–solvent molecular interactions in gaseous matrices. The present experimental results confirm the accuracy of some recent ab initio calculations of nuclear magnetic shielding performed for bromomethane.     

Tunnel effects on inter- and intramolecular hydrogen transfer reactions of transient dihydro- and hexahydrocarbazoles

We examine inter- and intramolecular hydrogen-transfer processes in two related metastable dihydrocarbazoles in nonpolar solvents of different viscosity and compare them with similar transfer processes in transient hexahydrocarbazoles. N-ethyldiphenylamine (A′) and N-ethyl-2,6-dimethyldiphenylamine (A) photocyclize in their triplet states, yielding the triplet states of the zwitterionic dihydrocarbazoles ³Z′* and ³Z*, respectively, which subsequently relax to their metastable singlet ground states ¹Z′ and ¹Z′. In spite of their similarity, the two transients ¹Z′ and ¹Z stabilize by completely different pathways: the unsubstituted transient ¹Z′ is converted into N-ethylcarbazole (C) and an N-ethyltetrahydrocarbazole (THC) by a bimolecular disproportionation reaction. The methylsubstituted intermediate ¹Z is converted into a stable dihydrocarbazole (D) by a sigmatropic, intramolecular [1,8]-H-shift and by an intermolecular, mutual hydrogen-exchange reaction within the encounter complex ¹(ZZ) which yields two molecules of D. The rates of the intra- and of the intermolecular transfer reaction of ¹Z are governed by tunnel effects. The rate of the intramolecular tunnel process does not depend on solvent friction and becomes temperature independent at low temperatures. The rate of the intermolecular, reaction-controlled exchange reaction ¹(ZZ) → 2 ¹D becomes also temperature independent if the solvent is fluid enough. In more viscous solvents the reaction becomes diffusion controlled and, therefore, strongly temperature dependent. The intermolecular disproportionation reaction 2 ¹Z′ → C + THC is also reaction controlled but no tunnel effects are observed.     

Direkte und indirekte metallierung von endo-dicylopentadien. Sn- und C-NMR studie stannylierter folgeprodukte

endo-Dicyclopentadiene (1) can be metalated by use of simple procedures with good overall yields. The attack occurs at the various vinyl, rather than at the allyl, positions of 1 as was confirmed by trapping the carbanions with Me₃SnCl. When t-BuLi/TMEDA are used, the 8- and 9-stannyl derivatives (3 and 4) are formed, whereas an excess of n-BuLi/t-BuOK leads to doubly stannylated derivatives with Me₃Sn groups in position 4/8 (6), 4/9 (7), and 3/9 (8) in addition to 3 and 4. Furthermore the latter reaction yields 5,5-bis(trimethylstannyl)cyclopentadiene (5). With stoichiometric amounts of n-BuLi/t-BuOK the formation of 3 and 4 predominates over that of 5–8. 5 is obtained from 1 after deprotonation at the allyl position, followed by an extremely fast retro-Diels-Alder reaction and then by further deprotonation. This follows from two experiments: (1) exo- and endo-5-trimethylstannyl-endo-dicyclopentadiene (11 and 12) which are synthesized from 1 in three steps give cyclopentadienyllithium and 1 when treated with methyllithium at −78°C; (2) cyclopentadiene reacts with an excess of n-BuLi/t-BuOK and Me₃SnCl to give 5. When 12 is heated syn-10-trimethylstannyl-endo-dicyclopentadiene (13) is obtained. The eight stannyl derivatives of 1 are identified mainly from the following NMR parameters: δ(¹¹⁹Sn), δ(¹³C), δ(¹H), ⁿJ(^119/117Sn---¹³C), and ⁶J(¹¹⁹Sn---^119/117Sn). The ¹³C NMR satellite spectrum of 1 yields the isotope shifts ¹Δ¹³(i(¹³C(j)) and ¹J(¹³C---¹³C). The latter lead to the revision of earlier signal assignments.     

MRD CI study on the low-lying states of AlP

Large-scale MRD CI calculations assign to AlP the ground state X ³Σ⁻ (9σ²3π²) and a close-lying state 1 ³Π (9σ3π³) (T_e = 0.08 eV). Up to transition energies of 2.0 eV, other states are described by the configurations 9σ3π³ (1¹Π), 8σ²3π⁴ (1 ¹Σ⁺), 9σ²3π² (1 ¹Δ and 2 ¹Σ⁺) and 9σ3π²4π (1 ⁵Π). The 2 ³Π state, located at ≈ 2.30 eV, shows a shallow double minimum. Numerous perturbations are expected to induce predissociation upon 2 ³Π. Multiplets arising from the occupation 8σ²3π³4π are clustered in the 3.25–3.50 eV region. Quintet states with the configuration 8σ9σ3π³4π are bound, with T_e values (in eV) of 3.80 (1 ⁵Σ⁺), 4.44 (1 ⁵Δ) and 4.88 (3 ⁵Σ⁻), respectively. The 9σ → 4s Rydberg members ⁵Σ⁻ and ³Σ⁻ lie in the 4.58–4.72 eV energy region. The first ionization potential (ionization to X⁴Σ⁻ of AlP⁺, 9σ → ∞) is estimated to be 7.65 eV. Ionization to the 1 ²Σ⁻ and 1 ²Π states of AlP⁺ is suggested to occur between 8.0 and 8.8 eV. The dipole moments of X ³Σ⁻, 1 ¹Δ and 2 ¹Σ⁺ are close to 1.0 D, whereas the 1 ¹Σ⁺ state has μ = 3.49 D; 1 ³Π and 1 ¹Π have dipole moments from 2.45 to 2.91 D. All low-lying states show a polarity Al⁺P⁻. Finally, the electronic structure and transition energies of AlP are compared with those of the isoelectronic species BN, AIN, and SiP⁺.     

Effect of Zn2+ Ions on Upconversion Emission of Er3+ in Zn/Er:LiNbO3 Crystal

The effect of Zn2+ ions codoped on the upconversion emission of Er3+ ions in Er:LiNbO3 crystal under different excitation wavelength was reported.The upconversion emission spectra of Zn/Er:LiNbO3 follo...     

Determination of picogram amounts of technetium in environmental samples by neutron activation analysis

A procedure for the determination of trace amounts of ⁹⁹Tc in filter paper and vegetation samples by neutron activation analysis has been developed. The procedure consists of the following major steps: (a) pre-irradiation separation of technetium from the sample and purification of the technetium fraction; (b) thermal neutron irradiation of the ⁹⁹Tc fraction to produce ¹⁰⁰Tc ; (c) post-irradiation separation and purification of ¹⁰⁰Tc from other activated nuclides: (d) counting of the 16-sec ¹⁰⁰Tc in a low-background β-counter.

The estimated detection limits for ⁹⁹Tc by this procedure with irradiations at a thermal neutron flux of about 5.10¹³ n cm^-2 sec^-1 are: 5.lO^-12g ⁹⁹Tc in filter paper samples, and 9. 10^-12 g ⁹⁹Tc in vegetation samples.     

Electronic structure of the radicals NF and NCl. III. The radiative lifetimes of the bΣ and aΔ states

The radiative lifetimes of the b¹Σ⁺ and a¹Δ states have been evaluated by perturbation expansions including X³Σ⁻, a¹Δ, b¹Σ⁺, 1^3,1Π, 2^3,1Π, 2³Σ⁻ and 2¹Σ⁺ states. All wavefunctions result from large MRD CI calculations. The b—X transition is dominated by the parallel transition moment; it is found to be much stronger than the a—X transition. The calculated radiative lifetimes of τ(¹Σ⁺)=18 ms, τ(¹Δ)=2.2 s for NF and τ(¹Σ⁺)=2.5–3.5 ms for NCl are in good accord with corresponding experimentally deduced values. The lifetime for the a¹Δ state in NCl is found to be τ(¹Δ)=1.1 s, ie. much longer than derived from a recent experiment. Its magnitude is consistent with the τ(b¹Σ⁺)/τ(a¹Δ) ratio of similar systems and with the decrease in lifetime from NF to NCl and is thus believed to be quite reliable. A detailed analysis of all contributions of the perturber states to the transition mechanism is made and comparison with the related data in SO, O₂ and S₂ is undertaken. The b-a transition probability dominated by the quadrupole transition is fairly constant in all the systems in the order of A = 0.013 (NF) - 0.0013 (S₂) s⁻¹.     

Fast, low-level detection of strontium-90 and strontium-89 in environmental samples by collinear resonance ionization spectroscopy

Environmental assessment in the wake of a nuclear accident requires the rapid determination of the radiotoxic isotopes ⁸⁹Sr and ⁹⁰Sr. Useful measurements must be able to detect 10⁸ atoms in the presence of about 10¹⁸ atoms of the stable, naturally occurring isotopes. This paper describes a new approach to this problem using resonance ionization spectroscopy in collinear geometry, combined with classical mass separation. After collection and chemical separation, the strontium from a sample is surface-ionized and the ions are accelerated to an energy of about 30 keV. Initially, a magnetic mass separator provides an isotopic selectivity of about 10⁶. The ions are then neutralized by charge exchange and the resulting fast strontium atoms are selectively excited into high-lying atomic Rydberg states by narrow-band cw laser light in collinear geometry. The Rydberg atoms are then field-ionized and detected. Thus far, a total isotopic selectivity of S > 10¹⁰ and an overall efficiency of ξ = 5 × 10⁻⁶ have been achieved. The desired detection limit of 10⁸ atoms ⁹⁰Sr has been demonstrated with synthetic samples.     

1,3,2-Diazastanna-[3]ferrocenophanes bearing alkyn-1-yl groups at tin and their 1,1-organoboration with triethylborane—molecular structure of a novel spirotin compound

2,2-Dichloro-1,3-bis(trimethylsilyl)-1,3,2-diazastanna-[3]ferrocenophane (3) reacts with lithium alkynides LiCCR¹ to give the corresponding di(alkyn-1-yl)tin derivatives 4a (R¹=^tBu) and 4b (R¹=SiMe₃). 1,1-Organoboration of 4 with triethylborane affords the spirotin compounds 5 which contain both a ferrocenophane and a stannacyclopentadiene ring. The crystal structure of 5b was determined by X-ray analysis. The compounds 4 and 5 were characterised in solution by multinuclear magnetic resonance (¹H-, ¹¹B-, ¹³C-, ¹⁵N-, ²⁹Si-, ¹¹⁹Sn-NMR), using pulsed field gradients in HMBC experiments for the ¹H detected ¹⁵N- and ¹¹⁹Sn-NMR signals. The compound 5b was also studied by solid-state ¹³C, ²⁹Si and ¹¹⁹Sn MAS NMR in order to correlate liquid and solid-state NMR data with the structural evidence.     

Ab initio study of the two competitive channels HO2 + H → H2 + O2 and HO2 + H → H2O + O

Saddle point geometries and barrier heights have been calculated for the H abstraction reaction HO₂(²A″)+H(²S) → H₂(¹Σ⁺_g)+O₂(³Σ⁻_g) and the concerted H approach-O removing reaction HO₂ (²A″)+H(²S) → H₂O(¹A₁)+O(³P) by using SDCI wavefunctions with a valence double-zeta plus polarization basis set. The saddle points are found to be of C_s symmetry and the barrier heights are respectively 5.3 and 19.8 kcal by including size consistent correction. Moreoever kinetic parameters have been evaluated within the framework of the TST theory. So activation energies and the rate constants are estimated to be respectively 2.3 kcal and 0.4×10⁹ ℓ mol⁻¹ s⁻¹ for the first reaction, 20.0 kcal and 5.4.10⁻⁵ ℓ mol⁻¹ s⁻¹ for the second. Comparison of these results with experimental determinations shows that hydrogen abstraction on HO₂ is an efficient mechanism for the formation of H₂ + O₂, while the concerted mechanism envisaged for the formation of H₂O + O is highly unlikely.