Variation of radiative lifetimes of NH2(Ã2A1) with rotational levels in the (0, 8, 0) and (0, 9, 0) vibration bands

Radiative lifetimes from the first electronically excited state of the amidogen free radical, NH(2)(A?(2)A(1)), are reported for rotational states in selected vibrational levels ν(2)' using laser-induced fluorescence. Thermal collision of argon, Ar(?)((3)P(0), (3)P(2)) metastable atoms in a microwave discharge-flow system with ammonia (NH(3)) molecules produced ground state NH(2)(X?(2)B(1)). The radiative lifetimes for the deactivation of NH(2)(A?(2)A(1)) were determined by measuring the decay profiles of NH(2)(A?(2)A(1)?→?X?(2)B(1)). In addition to the Fermi resonances with the ground state that lengthen the radiative lifetimes, a systematic increase in the radiative lifetimes with rotational quantum number was observed. Furthermore, the average radiative lifetimes of the (0, 9, 0) Γ, τ(1) = 18.65 ± 0.47 μs and (0, 8, 0) Φ, τ(2) = 23.72 ± 0.65 μs levels were much longer than those of the (0, 9, 0) Σ, τ(3) = 10.62 ± 0.47 μs, and (0, 8, 0) Π, τ(4) = 13.55 ± 0.55 μs states suggesting increased mixing of the first electronic excited and the ground states.     

Theoretical analysis of initial adsorption of high-κ metal oxides on In(x)Ga(1-x)As(0 0 1)-(4×2) surfaces

Ordered, low coverage to monolayer, high-κ oxide adsorption on group III rich InAs(0 0 1)-(4×2) and In(0.53)Ga(0.47)As(0 0 1)-(4×2) was modeled via density functional theory (DFT). Initial adsorption of HfO(2) and ZrO(2) was found to remove dangling bonds on the clean surface. At full monolayer coverage, the oxide-semiconductor bonds restore the substrate surface atoms to a more bulklike bonding structure via covalent bonding, with the potential for an unpinned interface. DFT models of ordered HfO(2)/In(0.53)Ga(0.47)As(0 0 1)-(4×2) show it fully unpins the Fermi level.     

Model studies of the chemisorption of hydrogen and oxygen on the Au (1 0 0) surface

 The convergence of chemisorption energy for hydrogen and oxygen on gold clusters is studied. Two theoretical approaches have been employed; wavefunction methods at the self-consistent-field second–order M?ller–Plesset level and density functional theory and the two methods are compared. Relativistic effective core potentials exploited in the former approach were developed in this work. Received: 25 October 1999 / Accepted: 21 February 2001 / Published online: 11 October 2001     

X-ray structure of (N-heterocyclic)(η4-diene)-dicarbonyliron(0) compounds

(N-Heterocyclic)(η⁴-diene)dicarbonyliron(0) compounds, where the N-heterocycles are quinoline and pyrazine and the diene is the diethyl ester of the hexa-2,4-dien-1,6-dioic acid (diethyl muconate) were studied by X-ray diffraction. The measurements demonstrated that the N-heterocycle occupies different positions of a tetragonal pyramid in these compounds. Quinoline occupies a basal position and pyrazine an apical position. This is discussed in terms of the π-bonding ability of the ligands.     

Synthesis and selective silylation of ω-hydroxy functionalized (η-alkenyl)carbene complexes of chromium(0) and tungsten(0) Part 11. The chemistry of metallacyclic alkenylcarbene complexes

Tungsten(0) carbene complexes of the type (OC)₅WC(NMeCH₂CHCHCH₂OH)R 2 (R=Me: 2a; R=Ph: 2b) were generated by aminolysis of (OC)₅WC(OMe)R with cis-NHMeCH₂CHCHCH₂OH. Like their Cr-congeners 1, complexes 2 exist at room temperature as mixtures of Z- and E-isomers with regard to the C-N bond. The metallacyclic complexes (OC)₄WC(η²-NMeCH₂CHCHCH₂OH)R (4) were obtained in good yields upon photo-decarbonylation of 2. Deprotonation/silylation of the complexes (OC)₄MC(η²-NMeCH₂CHCHCH₂OH)Me (M=Cr: 3a; M=W: 4a) with one equivalent of ⁿBuLi/Me₃SiCl gave (OC)₄MC(η²-NMeCH₂CHCHCH₂OSiMe₃)CH₃ (M=Cr: 5; M=W: 6), whereas with two equivalents of ⁿBuLi/Me₃SiCl complexes (OC)₄MC(η²-NMeCH₂CHCHCH₂OSiMe₃)CH₂SiMe₃ (M=Cr: 7; M=W: 8) were formed. Hydrolysis of the latter yielded selectively (OC)₄MC(η²-NMeCH₂CHCHCH₂OH)CH₂SiMe₃ (M=Cr: 9; M=W: 10). The complexes 1-10 were analyzed in solution by one- and two-dimensional NMR spectroscopy (¹H, ¹³C, ²⁹Si, ¹H/¹H COSY, ¹H/¹H NOESY, ¹³C/¹H HETCOR).     

(η6-Anisole) (η2-benzene) dicarbonylchromium(0): an intermediate in the photochemical carbonyl substitution of (η6-anisole)tricarbonylchromium(0) in benzene

The photochemical carbonyl substitution of (η⁶-anisole)Cr(CO)₃ has been investigated by laser flash photolysis. Both transient spectra and second-order rate constants for the reactions of transients with nucleophiles are found to be extremely variable depending upon solvents used. The coordination of benzene to the transient in cyclohexane forms the transient in benzene, indicating two discrete chemical species: (η⁶-anisole)Cr(CO)₂ and (η⁶-anisole)Cr(CO)₂(η²-benzene). The latter type of transient was observed also for fluorobenzene and mesitylene, leading to the assignment of a weak band in the visible region as η²-arene → Cr charge transfer. The existence of (η⁶-arene)Cr(CO)₂(η²-arene′) may throw light on what have been described as solvent effects in organometallic reactions.     

Insertionen in Wolfram(0)Kohlenstoff- und Wolfram(0)Wasserstoff-Bindungen

Trans-dicarbonylmethylnitrosyl-bis(triisopropylphosphite)tungsten (1) reacts with CO or CO₂ to give (CO)₂(ON)[P(O-i-Pr)₃]₂WCOCH₃ (2) or (OC)(ON)[P(O-i-Pr)₃]₂W-η²-OOCCH₃ (3), respectively.The hydride complex (OC)₂(ON)[P(O-i-Pr)₃]₂WH (4a) undergoes insertion by CO₂ or by activated acetylenes such as propiolic aldehyde or methyl propiolate to give (OC)₂(ON)[P(O-i-Pr)₃]₂OC(O)H (5) or (OC)₂(ON)]P(O-i-Pr)₃]₂CHCHR (R  CHO, 6; R  COOCH₃, 7). 4a adds BH₃ to yield a (OC)₂(ON)[P(O-i-Pr)₃]₂WHBH₃ system 8. This in turn can react with CO to give 4a and a (OC)₃(ON)P(O-i-Pr)₃WH complex (4b).     

Novel Synthesis of Haloaromatic Tertiary Alcohols Using (η6-Fluoroarene)tricarbonylchromium(0) Complexes

Regioselective synthesis of haloaromatic tertiary alcohols can be achieved by ortho deprotonation/electrophilic reactions of (η⁶- fluoroarene)Cr(CO)₃ complexes with halobenzophenones and metal removal. The coexistence of different halogens in the aromatic rings of these molecules make them unique within the group of tertiary alcohols.     

Electrodeposition of nano-sized nuclei of magnetic Co–Ni alloys onto n-Si (1 0 0)

Nanometer-sized nuclei of Co–Ni alloys were electrodeposited onto n-Si (1 0 0) electrodes from buffered solutions of the metallic ions. The energy levels of the interface Si/(Co(II),Ni(II))_aq were determined from Mott–Schottky plots. Electrodeposition of the alloys occurs by an anomalous mechanism and preferential deposition of Co took place at all the Co(II)/Ni(II) ratios investigated. The morphology and magnetic properties of the nuclei were studied by AFM and MFM, respectively. It is showed that nuclei of height <30 nm and low aspect ratio present a single-domain magnetic behaviour. Larger nuclei depict complex magnetic characteristics, with a progressive built-up of multi-domains.     

Study on the inclusion compound of β-cyclodextrin with (η5-cyclopentadienyl)tricarbonylmanganese(0)

The supramolecular inclusion compound of β-cyclodextrin (β-CD, host) with (η5-cyclopentadienyl)tricarbonylmanganese [MnCp(CO)3, guest] was obtained in a crystalline state. The host-guest compound is thermally stable and do not liberate the guest on heating at 100$ in vacuum. It was characteried by elemental analysis,1H NMR, differential scanning thermal (DSC) analysis and TLC. Continueous variation plot by NMR method shows that β-CD formed 1:1 inclusion compound with MnCp(CO)3. On the basis of 1H NMR spectra and the model building with Corey Pauling Koltum (CPK) models, the most probable inclusion mode is proposed.     

Studies on Flavonoids (XII) --Synthesis of 7-0-(β-D-Mannopyranosyl) -Flavone and Determination of Its Configuration

Introduction Flavonoids usually exist in the form of glycoside in nature. Different kinds of sugar and aglycone moieties will lead to different physiological activities.The natural flavonoid Dglycosides are usually β-anomers.     

Structures of three molybdenum(0)–tetracarbonyl–Schiff base compounds

The structures of three molybdenum(0)–tetracarbonyl–Schiff base compounds, determined by X-ray diffraction techniques, are reported. The Schiff bases are those derived from 2-acetylpyridine and 4-methoxyaniline, 4-trifluoromethoxyaniline, or 3-fluoro-4-methylaniline. The structure of the Schiff base ligand has also been determined for the ligand derived from 4-trifluoromethoxyaniline. Comparisons are made between metal–nitrogen bond distances, and carbon–nitrogen and carbon–carbon bond distances in the chelate rings, for these and a number of related molybdenum(0)–tetracarbonyl-diimine compounds. Comparisons are also made between ligand geometry in the free and coordinated states.     

Synthesis of α-Aminophosphonic Acids Sy Pd(0)Alkylation of Diethyl Aminomethylphosphonate Schiff Bases

Abstract

Aminoalkyl phosphonic compounds have many interesting biological properties: they are used as herbicides, antibiotics and enzyme inhibitors. Several approaches to the preparation of α-aminophosphonic acids have been reported. One efficient method using the schiff base of diethyl amino methylphosphonate 1 has been developped, allowing alkylation at the α carbon after deprotonation using strong bases such as LDA. In our continuing program dealing with the synthesis of unusual α-aminoacids which relies on C-C bond formation by transition metal catalyzed reactions (l), we report here a method based on the palladium-promoted alkylation of diethyl aminophosphonate schiff bases 2 and 3 with n³-allyl species 5 generated in situ from the allylic derivative 4:     

Zinc (0)-Mediated Formation of Carbon-Carbon Bond from γ-Bromo Crotonate

The coupling reaction of allylic halides with diffferent organometallic reagents has been widely utilized for synthetic purpose.¹ Unfortunately, with the exception of primary halides which give mostly α attack and of hindered substrates which prefer γ attack, the lack of regioselectivity is a great handicap for the generalization of the method.     

A homobimetallic complex of chromium(0) with a σ-borane component

The synthesis, characterization, and reactivity of a chromium(0) complex bearing an amine-borane moiety (η(6)-C(6)H(5)CH(2)NMe(2)·BH(3))Cr(CO)(3) (2) is reported. Photolysis of complex 2 results in the elimination of a CO ligand followed by the formation of an intramolecular σ-borane complex (η(1)-(η(6)-C(6)H(5)CH(2)NMe(2)·BH(2)-H))Cr(CO)(2) (3). This species was characterized in solution by NMR spectroscopy. Reaction of complex 2 with photochemically generated (OC)(5)Cr(THF) affords a novel homobimetallic σ-borane complex (OC)(3)Cr(η(6)-C(6)H(5)CH(2)NMe(2)·BH(2)-H-Cr(CO)(5)) (4), wherein one of the BH moieties is bound to the chromium center in an η(1)-fashion. The σ-borane complex 4 was isolated in moderate to good yield (72%). The BH(3) fragment in the complexes 3 and 4 are highly dynamic involving exchange of the BH hydrogen bound to the metal with the terminal BH hydrogen atoms. The dynamics has been studied using variable-temperature NMR spectroscopy. Complexes 2 and 4 have been characterized by X-ray crystallography.     

Syntheses, structures and electrochemistry of some 1-(η-allylamino)-1-ferrocenylcarbene complexes of chromium(0), molybdenum(0) and tungsten(0) : Part 13: The chemistry of metallacyclic alkenylcarbene complexes

The metallacyclic complexes (OC)₄MC(η²-NHCH₂CHCHX)Fc (4; X = H) and (5; X = CH₂OH) [M = Cr: a; Mo: b; W: c; Fc = ferrocenyl = CpFe(C₅H₄)] were obtained in good yields upon photo-decarbonylation of the bimetallic allylaminocarbene complexes (OC)₅MC(NHCH₂CHCHX)Fc (2; X = H)/(3; X = CH₂OH). At room temperature complexes 2/3 exist as mixtures of E- and predominantly Z-isomers with regard to the C-N bond. The molecular structures of 4b and 4c were determined by X-ray diffraction analyses. The intermetallic communicative effects and the interplay of Fc and η²-alkene moieties of 4a and 4b were assessed by cyclovoltammetry. All complexes were also characterized in solution by one- and two-dimensional NMR spectroscopy (¹H, ¹³C, ¹H NOE, ¹H/¹H COSY, ¹³C/¹H HETCOR).     

Use of a bis-lithium amide base in asymmetric metallation of tricarbonyl(η6-1,3-dihydroisobenzothiophene)chromium(0)

Enantiomerically enriched tricarbonyl(η⁶-arene)chromium complexes derived from tricarbonyl(η⁶-1,3-dihydroisobenzothiophene)chromium(0) can be obtained in up to 95% ee by means of an enantioselective metallation reaction using the bis-lithium amide base 8.