Laser and radiofrequency spectroscopy of the 5d 4 6s 6 D multiplet in Ta I

The hyperfine structure (hfs) of¹⁸¹Ta has been investigated using laser radio-frequency double resonance and high resolution laser spectroscopy on collimated atomic beams. The magnetic dipole and electric quadrupole hyperfine structure coupling constants of the 5d ^{4 6} s 6D _{3/2, 5/2, 7/2, 9/2} metastable states have been determined using radio-frequency spectroscopy. In the 5d ⁴ 6s ⁶ D _1/2 metastable state and the excited 5d ³ 6s 6p ⁴ D _3/2,⁶ D _{5/2, 9/2} as well as the unidentified 28 182.6 cm^–1 and 30 021.2 cm^–1 states, hfs constants have been obtained from high resolution laser spectroscopy. A radio-frequency converter has been developed in order to reach the frequency region 2.7–10 GHz.     

Conformational preferences of 6-furfuryl amino purine and 6-benzyl amino purine

Conformational preferences of N⁶-furfurylamino purine (kinetin) and N⁶-benzyl amino purine (BAP ) have been investigated theoretically by the quantum chemical perturbative configuration interaction using localized orbitals method. The predicted most stable conformations for these molecules are quite similar. The N⁶ substituents in both these molecules are oriented toward N(1) and away from the imidazole moiety of the purine. The furfuryl ring in kinetin as well as the aromatic benzene ring in BAP are not coplanar with the purine ring. Comparison of these results with the preferred conformation of another compound N⁶-(Δ²-isopentenyl) adenine reveals striking similarity in the orientations of the N⁶ substituents in these cytokinin-active plant-growth-stimulating substances.     

Thermophysical Properties of CeB6 and PrB6 at subambient temperatures

Equilibrium adiabatic heat-capacity measurements have been made on zone refined samples of CeB₆ and PrB₆. Companion measurements made on LaB₆, NdB₆, and GdB₆ have been reported elsewhere. These show cooperative lambda-type anomalies associated with antiferro-magnetic ordering. Except for lanthanum hexaboride, Schottky internal crystal field levels result in significant contributions to the thermodynamic functions. The gross thermodynamic properties at 298.15 K heat capacity (C_p/R), entropy increment (Δ^T _0,m S ⁰/R), and Gibbs energy function are correlated with the nature of the lanthanide. For LaB₆, CeB₆, PrB₆, NdB₆, and GdB₆ the three properties are, respectively: {11.654, 12.014, 11.997, 11.916, 11.695} Cp/R; {10.001, 11.803, 12.430, 12.558, 13.982} S⁰/R, and finally {4.379, 5.912, 6.232, 6.451, 7.905}Φ⁰ _m/R. This revised version was published online in August 2006 with corrections to the Cover Date.     

Thermochemistry of copper complex of 6-benzylaminopurine

The copper(II) complex of 6-benzylaminopurine (6-BAP) has been prepared with dihydrated cupric chloride and 6-benzylaminopurine. Infrared spectrum and thermal stabilities of the solid complex have been discussed. The constant-volume combustion energy, Δ_c U, has been determined as −12566.92±6.44 kJ mol⁻¹ by a precise rotating-bomb calorimeter at 298.15 K. From the results and other auxiliary quantities, the standard molar enthalpy of combustion, Δ_c H _m ^θ, and the standard molar of formation of the complex, Δ_f H _m ^θ, were calculated as −12558.24±6.44 and −842.50±6.47 kJ mol⁻¹, respectively.     

Electronic and vibrational spectra of 2,4,6-trichloro- and 6-chloro-2,4-dimethoxypyrimidine

This paper presents the investigations of the electronic and vibrational spectra of 2,4,6-trichloropyrimidine and 6-chloro 2,4-dimethoxypyrimidine. The electronic spectra have been recorded in the liquid and vapour states in the region 50000-28000 cm⁻¹. Then-π and π-π transitions are classified in each case. Infrared absorption spectra of 2,4,6-trichloropyrimidine in nujol have been recorded in the region 4000-400 cm⁻¹. It is concluded that substituents like Cl or OCH₃ do not have much influence on thesp ² electron of the nitrogen atom of the ring. From infrared studies the mass-dependent modes and the modes insensitive to substitution have been pointed out. The assignments are tentative and are based on the group frequency approach and data available for similar molecules.     

Plasma etching of refractory metals (W,Mo, Ta) and silicon in SF6 and SF6-O2. An analysis of the reaction products

The etching rates and reaction products of refractory metals (W, Mo, and Ta) and silicon have been studied in a SF₆-O₂ r.f. plasma at 0.2 torr. The relative concentrations of WF₆ and WOF₄ and the intensities of the WF _n ⁺ (n=3–5), WOF _m ⁺ (m=1–3), MoF _n ⁺ , and MoF _m ⁺ ions have been measured by mass spectroscopy. An analysis of the neutral composition of the plasma during etching of these metals and a comparison with the results obtained for silicon show that at least two species are involved for W and Mo etching: fluorine and oxygen atoms. A reaction scheme is proposed.     

Peptide conformations. Part 30. Assignment of the 1H-, 13C-, and 15N-NMR spectra of cyclosporin A in CDCl3 and C6D6 by a combination of homo- and heteronuclear two-dimensional techniques

The ¹H-, ¹³C-, and ¹⁵N-NMR spectra of the immunosuppressive cyclic undecapeptide cyclosporin A ( 1 ) have been analyzed at 300 MHz in CDCl₃, C₆D₆, and mixtures of these solvents. A combination of different homo- and heteronuclear two-dimensional NMR techniques enable complete assignment of all H-, C- and 4 N-signals. Recognition of the proton spin systems has been achieved via ¹H,¹H–COSY and double-quantum-¹H-NMR spectroscopy. NOESY spectra yield some sequence assignments, but two techniques using coupling across amide bonds have been applied to get independent assignments of all amino acids in the sequence: (i) An ¹H,¹H-COSY spectrum optimized for small coupling constants enables the detection of long-range couplings from N-methyl groups to both α-protons attached to that amide bond. (ii) An ¹H, ¹³C-COSY spectrum optimized for C,H-long-range couplings (J = 5 to 10 Hz) to the eleven CO groups again yields coupling to both α-protons attached to that amide bond. Additionally these two experiments yield the assignment of N-methyl protons and carbonyl C-atoms. Normal and relayed ¹H,¹³C-COSY in both solvents have been applied to assign all C-atoms via their directly attached and remote protons. An ¹H,¹³C-COLOC spectrum at 500 MHz in CDCl₃, which uses H,C-long-range couplings confirms the assignment of all proton spin systems as well as the C-signals of each individual amino acid. Ambiguities in the assignment of the C(δ)'s of MeLeu have thus been removed. An ¹H,¹⁵N-COSY spectrum enables the assignment of the 4 NH N-atoms.     

Radiative lifetimes of the 6p 2 P 1 2/0 and 6p 2 P 3 2/0 levels in Yb II

Radiative lifetimes have been determined for the 6² P ₁ ^2/0 and 6² P ₃ ^2/0 levels in Yb II using the method of laser-induced fluorescence from sputtered metal vapour. The results, 8.0(2)ns (6² P ₁ ^2/0 ) and 6.3(3)ns (6² P ₃ ^2/0 ), are compared with lifetimes obtained from ab initio manybody perturbation calculations.     

The thermodynamics of mixing C6 and C7 compounds with a bicyclic compound at infinite dilution

The activity coefficients at infinite dilution have been measured at 25°C for cyclohexane, cyclohexene, 1,3-cyclohexadiene, 1,4-cyclohexadiene, benzene, n-hexane, 1-hexene, 1-hexyne, n-heptane, 1-heptene and 1-heptyne in decahydronaphthalene, bicyclohexyl, 1,2,3,4-tetrahydronaphthalene and cyclohexylbenzene. These results, together with previously determined H _m ^E and V _m ^E have been used to calculate the partial molar excess thermodynamic properties of mixing at infinite dilution.     

Ni + B2H6: Spectroscopic observations on NiB and NiH

An electronic spectrum of the nickel monoboride radical has been observed for the first time, in a reaction between a nickel plasma and diborane. Numerous bands of ⁵⁸Ni¹⁰B and ⁵⁸Ni¹¹B have been recorded between 442 and 503 nm in laser-induced fluorescence (LIF). Dispersed fluorescence experiments have also been performed. The LIF spectrum is dominated by a strong progression of bands of a [19.7]²Σ⁺–X²Σ⁺ transition. Analyses have been carried out to yield the following ⁵⁸Ni¹¹B ground state parameters: r₀ = 0.1698 nm, ω_e = 778 cm⁻¹, ω_ex_e = 4.9 cm⁻¹. Strong signals from NiH have also been observed.     

Synthesis of 6-cyanopurines and the isolation and X-ray structure of novel 2H-pyrroles

(Z)-N¹-(2-Amino-1,2-dicyanovinyl)-N²-substituted-formainidines react with dimethylformamide diethyl acetal at room temperature to give 6-cyanopurines as the major product together with novel 5-amino-2-arylimino-3,4-di[(N,N-dimethylamino)methylideneamino]-2H-pyrroles, which have been fully characterised and a single crystal X-ray analysis has been carried out on the N-phenyl derivative.     

Ion Complexation Properties of Calix[6]Arene Derivatives: I. 1,4-Calix[6]Crown-4 Derivatives

Abstract

A series of 1,4-p-tert-butyl-calix[6]crown-4 tetraesters, tetraamides, tetraacids with defined conformation have been synthesized, and their complexation properties towards metal ions and alkyl ammonium ions were investigated systematically. It was found that 1,4-p-tert-butyl-calix[6]crown-4 tetraethylester (3a) and 1,4-p-tert-butyl-calix[6]benzocrown-4 tetramethyl-ester (4b) show high selectivity towards Na⁺, Li⁺, respectively and all of them exhibit high complexation abilities towards Et₂NH₂ ⁺ cation.     

Free radical reactions of pyridoxal (vitamin B6): a pulse radiolysis study

Redox reactions of pyridoxal (P-OH) with e¯_aq, ^. OH, N ^. ₃, SO ^. ₄¯ and various organo-haloperoxyl radicals have been studied using pulse radiolysis technique. The rate constants for the reaction of P-OH or P-O¯ with the above-mentioned radicals and the transient absorption spectra have been measured. The transients formed in the reaction of hydrated electron and oxidizing radicals with pyridoxal have been assigned. An attempt has been made to find a correlation between the rate constants and Taft parameter for the reactions with the organo-haloperoxyl radicals. It has also been observed that the one-electron oxidized radical of pyridoxal is repaired by uric acid. The reduction potential for the P-OH ^.+/P-OH couple at pH 7, as measured by cyclic voltammetry, has been found to be +1.11 V vs. NHE.     

Beiträge zur Chemie des Phosphors. 168 [1]. Zur Kenntnis der Isomere von Tetraphosphan(6), P4H6

Contributions to the Chemistry of Phosphorus. 168. About the Isomers of Tetraphosphane(6), P₄H₆ Additional informations about the ³¹P-NMR parameters of d,l- and meso-n-P₄H₆ as well as of iso-P₄H₆ have been obtained by ³¹P-NMR spectroscopic investigations of mixtures of phosphanes containing 26—48 P-% of tetraphosphane(6). On the basis of the differences in the ³J(PP)-coupling constants the observed AA′BB′-spin systems have been reassigned to the various diastereomers of n-P₄H₆.     

Hexagonal Bipyramidal [Ta2B6]−/0 Clusters: B6 Rings as Structural Motifs

It has been a long‐sought goal in cluster science to discover stable atomic clusters as building blocks for cluster‐assembled nanomaterials, as exemplified by the fullerenes and their subsequent bulk syntheses. ¹ , ² Clusters have also been considered as models to understand bulk properties, providing a bridge between molecular and solid‐state chemistry. ³ Because of its electron deficiency, boron is an interesting element with unusual polymorphism. While bulk boron is known to be dominated by the three‐dimensional (3D) B₁₂ icosahedral motifs, ⁴ new forms of elemental boron are continuing to be discovered. ⁵ In contrast to the 3D cages commonly found in bulk boron, in the gas phase two‐dimensional (2D) boron clusters are prevalent. ⁶ – ⁸ The unusual planar boron clusters have been suggested as potential new bulking blocks or ligands in chemistry. ^6a Herein we report a joint experimental and theoretical study on the [Ta₂B₆]⁻ and [Ta₂B₆] clusters. We found that the most stable structures of both the neutral and anion are D_6h bipyramidal, similar to the recently discovered MB₆M structural motif in the Ti₇Rh₄Ir₂B₈ solid compound. ⁹      

Microwave assisted synthesis of 72-membered chiral hexanuclear [6+6] macrocyclic Schiff base

A novel double thicalix[4]arene possessing two amide sites was prepared. The binding behavior with Ag⁺ has been examined by ¹H NMR titration experiment. The association constants K _ass of the amide sites are quite similar (K _ass = 2.10 × 10⁴ M⁻¹ and K _ass = 2.00 × 10⁴ M⁻¹), suggesting that the two amide sites work independently.     

Ab initio calculations ofo- andm-Cl2C6H4 molecules and their35CI NQR parameters

The MNDO andab initio (Hartree—Fock method, split 6-31G^* basis set, full optimization of geometry) calculations for theo- andm-dichlorobenzene molecules have been performed. The populations of the valent p-orbitals of the carbon and chlorine atoms have been analyzed. Estimations of the³⁵Cl NQR frequencies and the asymmetry parameters of the electric field gradient at the³⁵CI nuclei have been made. It was confirmed that these values are not determined by the contribution of the whole valent p-shell, but only by its less diffuse component.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 2180–2183, September, 1996.     

13C NMR Spectra of 5- and 7-Bromo-6-ketosteroids and Related Compounds

¹³ C NMR spectra have been studied and signals of C atoms have been assigned for 5- and 7-bromo-6-ketosteroids, which are intermediates in the synthesis of ecdysteroids.     

Löslichkeiten und Aktivitätskoeffizienten von H2S in Elektrolytmischungen

From solubility measurements on hydrogen sulfide in aqueous solutions of the composition [H⁺] = HM, [Na⁺] = (I — H) M = A M, [Cl^?] = I M at 25°C, the molar and molal activity coefficients of H₂S have been calculated. The activity coefficients of H₂S in the electrolyte mixtures have been found to be additive functions of the activity coefficients in the individual electrolyte solutions at the same ionic strength. This result is predicted by the internal pressure theory of salt effects on non-electrolyte activity coefficients, provided that the volume change upon mixing two electrolyte solutions of the same ionic strength is zero.     

O6-Alkylguanine DNA Alkyltransferase Mediated Disassembly of a DNA Tetrahedron

Tetrahedron DNA structures were formed by the assembly of three-way junction ( TWJ ) oligonucleotides containing O⁶-2′-deoxyguanosine-alkylene-O⁶-2′-deoxyguanosine (butylene and heptylene linked) intrastrand cross-links (IaCLs) lacking a phosphodiester group between the 2′-deoxyribose residues. The DNA tetrahedra containing TWJs were shown to undergo an unhooking reaction by the human DNA repair protein O⁶-alkylguanine DNA alkyltransferase (hAGT) resulting in structure disassembly. The unhooking reaction of hAGT towards the DNA tetrahedra was observed to be moderate to virtually complete depending on the protein equivalents. DNA tetrahedron structures have been explored as drug delivery platforms that release their payload in response to triggers, such as light, chemical agents or hybridization of release strands. The dismantling of DNA tetrahedron structures by a DNA repair protein contributes to the armamentarium of approaches for drug release employing DNA nanostructures.