Indirect processes in the electron-impact ionization of Li-like ions

Detailed measurements of electron-impact ionization cross sections have been made in the vicinity of the excitation-autoionization thresholds of Li-like B²⁺, C³⁺, N⁴⁺, O⁵⁺ and F⁶⁺ ions. With an energy spread of 0.4 to 1% of the electron energy and statistical uncertainties as low as 0.1% we could clearly resolve thresholds for excitation of individual terms in 1s2s2l configurations. Numerous resonance features were found which are due to dielectronic capture of the incident electron with subsequent two-electron emission. In particular, dielectronic capture processes involving Δn=2 excitations of a 1s electron provide the dominant resonance contributions to the measured cross sections. Rydberg series 1s2snln′l′ of resonances withn=3 (Δn=2) andn=4 (Δn=3) are resolved up to principle quantum numbersn′ equal to 6 or 7.     

Thermal studies and spectral characterization of the chelate of bis-(η5-cyclopentadienyl titanium(IV) with salicylidene-4-methylaniline

A new chelate (η⁵-C₅H₅)₂Ti(SB)₂, whereSB=O, N donor Schiff base salicylidene-4-methylaniline, was synthesized. The course of thermal degradation of the chelate was studied by thermogravimetric (TG) and differential thermal analysis (DTA) under dynamic conditions of temperature. The order of the thermal decomposition reaction and energy of activation was calculated from TG curve while from DTA curve the change in enthalpy was calculated. Evaluation of the kinetic parameters was performed by Coats-Redfern as well as Piloyan-Novikova methods which gaven=1, ΔH=1.114 kJ·mol^?1, ΔE=27.01 kJ·mol^?1, ΔS=?340.12 kJ·mol^?1·K^?1 andn=1, ΔH=1.114 kJ·mol^?1, ΔE=20.01 kJ·mol^?1, ΔS=?342.60 kJ·mol^?1·K^?1, respectively. The chelate was also characterized on the basis of different spectral studies viz. conductance, molecular weight, IR, UV-visible and¹H NMR, which enabled to propose an octahedral structure to the chelate.     

Kinetic investigation of some steroids by thermogravimetry

The kinetic values of thermal degradation of some steroids were calculated by using TG and DTG curves and the Freeman-Carroll and the Jeres methods. Then andE _a values calculated by the Jeres method are more reasonable. The kinetic thermal stabilities of the simple functional groups of the steroids were compared by using theE _a values of Jeres, and the following sequences were found: 17β-OH>17-octy 1>17-Ac-CHO>17-keto; 3β-OH> 3-keto>3a-OH; and 5a-H>5β-H>Δ⁵⁽⁶⁾>Δ⁴. The k,Z, ΔH^*, ΔS^* and ΔG^* values were calculated at the maximum decomposition rate temperatures by using the Jeres values. The ΔS^* values are negative and suggest a high ordering of the transition state. The ΔH^* and ΔG^* values are positive, as expected.     

Synthesis,spectroscopic characterization and DFT study of dinuclear ruthenium sawhorse-type complexes derived from the reaction of trinuclear aggregates and (Z)-5-arylidenerhodanines

The synthesis of dinuclear ruthenium sawhorse-type complexes [Ru₂(μ-ArCH:Rhod)₂(CO)₄]_n 12a–e and [Ru₂(ArCH:Rhod)₂(μ-ArCH:Rhod)₂(CO)₄] 13a–e through reaction of [Ru₃(CO)₁₀(NCMe)₂] and [Ru₃(CO)₁₂] and the corresponding (Z)-5-arylidenerhodanines (ArCH:Rhod) 10a–e, respectively, are reported. These complexes are arranged in a sawhorse structure in which two bridged (Z)-5-arylidenerhodanines coordinate to the metals using sulfur and nitrogen of the rhodanine ring. A Density Functional Theory method was used to gain insight into the polymerization process by calculating dimerization Gibbs energies (ΔG_dim). Values between ?10.7 and ?5.3 kcal mol^?1 indicate that dimerization is a spontaneous process. A reaction pathway for formation of the sawhorse compounds [Ru₂(μ-ArCH:Rhod)₂(CO)₄] was calculated and the rate-determining step for the mechanism is coordination of a second (Z)-5-arylidenerhodanine ligand with activation energies between 41.1 and 47.8 kcal mol^?1. In order to understand the apparent thermodynamic favorability of the fragmentation step, we calculated the fragmentation energy (ΔE_Frag) for the key intermediate and its energetic contributors, the interaction energy, ΔE_int and the reorganization energy, ΔE_reorg. Low values of ΔE_Frag imply that the fragmentation is thermodynamically facile. Large values of ΔE_int are countered by opposite and large values of ΔE_reorg which indicate that the cleavage of the trimetallic intermediate aggregate is determined by the nature of the ligand and the balance between its interaction with the metal and the extent of structural reorganization.     

CS(lav) and computational study of degeneracy averaged differerential cross sections and Δm-integral cross sections: Hez.sbndCO,HDz.sbnd

CC and l-average CS calculations of degeneracy averaged differential cross sections and Δm-integral cross sections have been performed for Hez.sbndCO at E = 60 cm^?1 and E = 80 cm^?1, for HDz.sbndNe at E = 254 cm^?1, and for Hez.sbndH₂ at E = 1520 cm^?1. The l_avz.sbndCS degeneracy averaged differential cross sections are generally in good agreement with the CC cross sections. The previously observed shifts in the diffraction oscillations for odd rotationally inelastic transitions for Hez.sbndCO and HDz.sbndNe do not occur due to proper phase choice and l? = l_av choice rather than l? = 1 or l′. The l_avz.sbndCS approximation gives reliable results for most Δm-integral cross sections except for those σ_cs(jm, jm′) cross sections for which the CC cross sections σ(jm;jm′) and σ(jm′;jm) differ by a large amount.     

Experimental and theoretical study of QED corrections in the ground state of heliumlike iron

The energy of the Lyβ and Lyγ lines of heliumlike iron has been measured with a precision of 25 ppm. The study of these transitions involving levels of high principal quantum number (n=3 andn=4) provides us with informations on the ground state 1¹ S ₀ of the ion. The experimental values are compared to MCDF calculations including QED two-body corrections.     

Positional isomerization of quinine and quinidine via rhodium on alumina catalysis: practical one-step synthesis of Δ-isoquinine and Δ-isoquinidine

The synthesis of Δ^3,10-isoquinine (5Z,5E) and Δ^3,10-isoquinidine (6Z,6E) was achieved in one-step through positional isomerization of the terminal alkene in the parent cinchona alkaloids using catalytic amounts of 5% Rh/Al₂O₃ and excess hydrochloric acid in refluxing 50% aqueous EtOH. The products were obtained in good yields as a mixture of E and Z geometric isomers and fully characterized using spectroscopic methods.     

Excess molar quantities of (a halogenated n-alkane + an n-alkane) A comparative study of mixtures containing either 1-chlorobutane or 1,4-dichlorobutane

Excess molar volumes V_m^E at 298.15 K were obtained, as a function of mole fraction x, for series I: {x1-C₄H₉Cl + (1 ? x)n-C_lH_{2l + 2}}, and II: {x1,4-C₄H₈Cl₂ + (1 ? x)n-C_lH_{2l + 2}}, for l = 7, 10, and 14. 10, and 14. The instrument used was a vibrating-tube densimeter. For the same mixtures at the same temperature, a Picker flow calorimeter was used to measure excess molar heat capacities C_{p, m}^E at constant pressure. V_m^E is positive for all mixtures in series I: at x = 0.5, V_m^E/(cm³ · mol^?1) is 0.277 for l = 7, 0.388 for l = 10, and 0.411 for l = 14. For series II, V_m^E of {x1,4-C₄H₈Cl₂ + (1 ? x)n-C₇H₁₆} is small and S-shaped, the maximum being situated at x_max = 0.178 with V_m^E(x_max)/(cm³ · mvl^?1) = 0.095, and the minimum is at x_min = 0.772 with V_m^E(x_min)/(cm³ · mol^?1) = ?0.087. The excess volumes of the other mixtures are all positive and fairly large: at x = 0.5, V_m^E/(cm³ · mol^?1) is 0.458 for l = 10, and 0.771 for l = 14. The C_{p, m}^Es of series I are all negative and |C_{p, m}^E| increases with increasing l: at x = 0.5, C_{p, m}^E/(J · K^?1 · mol^?1) is ?0.56 for l = 7, ?1.39 for l = 10, and ?3.12 for l = 14. Two minima are observed for C_{p, m}^E of {x1,4-C₄H₈Cl₂ + (1 ? x)n-C₇H₁₆}. The more prominent minimum is situated at x′_min = 0.184 with C_{p, m}^E(x′_min)/(J · K^?1 · mol^?1) = ?0.62, and the less prominent at x″_min = 0.703 with C_{p, m}^E(x″_min)/(J · K^?1 · mol^?1) = ?0.29. Each of the remaining two mixtures (l = 10 and 14) has a pronounced minimum at low mole fraction (x_min = 0.222 and 0.312, respectively) and a broad shoulder around x = 0.7.     

CEPA calculations on open-shell molecules

Ab initio calculations at SCF and CEPA levels using large Gaussian basis sets have been performed for the two lowest electronic states,X ² Σ⁺ andA ² Π, of HeAr⁺. Spin-orbit coupling (SOC) effects have been added using a semiempirical treatment. The resulting potential curves for the three statesX,A ₁, andA ₂ have been used to evaluate molecular constants such as vibrational intervals ΔG(v + 1/2) and rotational constantsB _v as well as — by means of a Dunham expansion — equilibrium constants such asR _e , ω _e ,B _e etc. Comparison with the experimental data from UV emission spectroscopy shows that the calculated potential curves are slightly too shallow and have too large equilibrium distances:D _e = 242 cm^?1 andR _e = 2.66 Å compared to the experimental values of 262 cm^?1 and 2.585 Å, respectively, for theX ²Σ⁺ ground state. However, the ab initio calculations yield more bound vibrational levels than observed experimentally and allow for a more complete Dunham analysis, in particular for theA ₂ state. The experimental value of 154 cm^?1 for the dissociation energyD _e of this state is certainly too low; our best estimate is 180±5 cm^?1. For theA ₁ state our calculations are predictions since this state has not yet been observed experimentally.     

3-Ferrocenyl-3-(1-naphthyl)cyclopropene. Synthesis, structure, and chemical transformations

Crystalline 3-ferrocenyl-3-(1-naphthyl)cyclopropene was prepared by dehydrobromination ofZ- andE-2-bromo-1-ferrocenyl-1-(1-naphthyl)-cyclopropanes by Bu^tOK in DMSO. The resulting compound and the startingZ-monobromocyclopropane were characterized by X-ray diffraction analysis. The obtained cyclopropene reacts with 1,3-diphenylisobenzofuran to give a [4+2]-cycloadduct. The small ring opens upon treatment with HBF₄ etherate to afford isomericZ- andE-prop-1-enes and 1-ferrocenyl-3H-benzo[e]indene. Thermolysis of this cyclopropene results in the formation of 1-ferrocenyl-9bH-benzo[e]indene. In all cases, opening of the small ring is accompanied by exclusive alkylation of the naphthalene moiety. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 499–506, March, 1998.     

Generalized 1/Z expansion for heteronuclear molecules

We have first studied empirical regularities in various series of heteronuclear diatomic molecules between the energy E, the total number of electrons N, the equilibrium distance R_e and Z? = (Z₁Z₂)^1/2 where Z₁e and Z₂e are the nuclear charges in the diatom. In particular, for various alkali halide series, R_e²|E|/N^5/3 is shown to correlate rather simply with Z?R_e³. Some theoretical basis is afforded by generalizing the 1/Z expansion used early by the writers in work on homonuclear diatomics. Finally, when Z₂/Z₁ → ∞, a model is presented which predicts a finite asymptotic bond length and this prediction is confronted with available experimental data for both heteronuclear diatoms and for the polyatomic series CH₄ to SnH₄.     

Zur konformation geschützter aminosäuren—I: NMR-untersuchungen von t-BOC-glycin

Z, E-Isomerism of the urethane bond of t-BOC-glycine was observed by ¹H- and ¹³C-NMR spectroscopy at various temperatures in several solvents. The special stabilization of the Z isomer at low temperatures in CDCl₃ has been explained by intra- and intermolecular H-bond forming a 7-membered ring. Thermodynamic data have been determined for the ground state (AH°= ?7 kcal/mol, Δ° = ?25 Clausius) as well asfor the barrier of interconversion (ΔG° = 15·4 kcal/mole for the deuterated title compound) in CDCl₃. The equilibrium between the Z and the E conformation is shifted towards the E conformation in more polar solvents (acetonitril-d₃, acetone-d₆, DMSO-d₆), in which cyclization of the Z conformation is not important.     

Stereoselective epoxidation of 2-arylidene-1-indanones and 2-arylidene-1-benzosuberones

Summary Oxidation of the (E) and (Z) isomers of 2-arylidene-1-indanones (1) and 2-arylidene-1-benzosuberones (4) by alkaline hydrogen peroxide (methodi) afforded the spiroepoxidestrans-2a–g andtrans-5a–g from both isomers as sole products in high yields. On the other hand, dimethyldioxirane epoxidation(methodii) of the (E) isomers1a–g and4a–g gave the correspondingtrans spiroepoxides in good yields, whereas the (Z) isomers1a,c,e and4a,c,e led to thecis spiroepoxides in moderate yields. Dimethyldioxirane oxidation (methodii) of (Z)-1c and (Z)-4c,e gave diones3c and6c,e as by-products as well. Epoxidation of (Z)-1a,c,e and (Z)-4a,c,e bym-chloroperoxybenzoic acid (methodiii) resulted inca. 6:1 mixtures ofcis-2a,c,e andtrans-2a,c,e orcis-5a,c,e andtrans-5a,c,e spiroepoxides.Dedicated to Prof.W. Fleischhacker on the occasion of his 65^th birthday     

Two pentadehydropeptides with different configurations of the ΔPhe residues

Comparison of the crystal structures of two pentadehydropeptides containing ΔPhe residues, namely (Z,Z)‐N‐(tert‐butoxycarbonyl)glycyl‐α,β‐phenylalanylglycyl‐α,β‐phenylalanylglycine (or Boc⁰–Gly¹–Δ^ZPhe²–Gly³–Δ^ZPhe⁴–Gly⁵–OH) methanol solvate, C₂₉H₃₃N₅O₈·CH₄O, (I), and (E,E)‐N‐(tert‐butoxycarbonyl)glycyl‐α,β‐phenylalanylglycyl‐α,β‐phenylalanylglycine (or Boc⁰–Gly¹–Δ^EPhe²–Gly³–Δ^EPhe⁴–Gly⁵–OH), C₂₉H₃₃N₅O₈, (II), indicates that the Δ^ZPhe residue is a more effective inducer of folded structures than the Δ^EPhe residue. The values of the torsion angles ϕ and ψ show the presence of two type‐III′β‐turns at the Δ^ZPhe residues and one type‐II β‐turn at the Δ^EPhe residue. All amino acids are linked trans to each other in both peptides. β‐Turns present in the peptides are stabilized by intramolecular 4→1 hydrogen bonds. Molecules in both structures form two‐dimensional hydrogen‐bond networks parallel to the (100) plane.     

A CNDO-MO calculation of VCl4

The electronic structure of the tetrahedral molecule VCL₄ is investigated within the CNDO-MO approximations. The metal and ligand valence orbitals, 3d, 4s, 4p; and 3s, 3p; respectively, have been systematically varied in an attempt to minimize the total energy; “optimum” V 4s(χ₄ = 1.10) and 4p(d ³ p ²) orbitals have been established, but V 3d(d ⁿ ) and Cl(-δ) valence orbitals are only seen to favor lower energy for expanded orbitals. Since determining the one-electron molecular orbital level which is occupied by the vanadium lone electron is a major aspect of this investigation, all calculations have been performed in triplicate: calculations assuming the unpaired electron occupies the 3a ₁, 2 _e and 4t ₂ molecular orbital (ground state electronic configurations² A ₁,² E, and² T ₂, respectively). The Hartree-Fock equations have been solved by Roothaan's SCF method for open shells, but off-diagonal multipliers between filled and partly filled molecular orbitals of the same symmetry have been neglected. As a qualitative estimate of the error introduced by this simplification, the pertinent overlap integrals between the eigenfunctions from calculations for the three possible configurations,² A ₁,² E, and² T ₂, are investigated as functions of the component 3d(d ⁿ ) and Cl(-δ) valence orbitals. The overlap integrals from the relevant² A ₁ and² T ₂ calculations are reasonably small, but the neglect of off-diagonal multipliers in calculations on the² E state is found to be a poor approximation. An ordering of the non-filled molecular orbitals in VCl₄ of 4t ₂ < 3a ₁ < 2e < 5t ₂ seems most consistent with the numerous calculations. This suggested ground state electronic configuration of² T ₂ introduces new aspects to the consideration of a (dynamic) Jahn-Teller effect in VCl₄. Experimental data pertinent to the electronic structure of VCl₄ has been briefly summarized, but unfortunately it is inadequate to confirm or deny the present calculations.     

Kinetic Parameters of Alkyl,Alkoxy, and Peroxy Radical Isomerization

The parabolic model of radical abstraction reactions is used to analyze experimental data on monomolecular hydrogen-atom transfer in the reactionsRC^.H(CH₂) _n CH₂R¹ RCH₂(CH₂) _n C^.HR¹(n= 2, 3, 4)RCH(O^.)(CH₂)₂CH₂R¹ RCH(OH)(CH₂)₂C^.HR¹ RCH(OO^.)(CH₂) _n CH₂R¹ RCH(OOH)(CH₂) _n C^.HR¹(n= 1, 2).The activation energies and rate constants that specify each class of these reactions are calculated. Alkyl radical isomerization is characterized by the following activation energies of a thermally neutral reaction depending on the cycle size in the transition state (nis the number of atoms in a cycle): E _{e , 0}(kJ/mol) = 46.6 (n= 6), 59.4 (n= 5), and 57.1 (n= 7). Alkoxy radicals isomerize with E _{e , 0}(kJ/mol) = 53.4 (n= 6), whereas peroxy radicals isomerize with E _{e , 0}(kJ/mol) = 53.2 (n= 6) and E _{e , 0}(kJ/mol) = 54.8 (n= 7). The E _{e , 0}value varies with changes in the cycle size and the strain energy in cycloparaffin C _n H_2n in the same manner. The activation energies E _{e , 0}for the intra- and intermolecular H-atom abstractions are compared. It is found that E _{e , 0}(isomerization) < E _{e , 0}(R^.+ R¹H) for alkyl radicals and that E _{e , 0}(isomerization) E _{e , 0}(RO^.(RO^.) + R¹H) for alkoxy and peroxy radicals.     

Macrocycle complexation chemistry. 38. Crystallographic and ultraviolet/visible characterization of nitrobenzo-15-crown-5, dinitrobenzo-15-crown-5, and dinitrodibenzo-18-crown-6·2CH3CN

The crystal structures of three nitrated benzocrown ethers have been determined. Nitrobenzo-15-crown-5 crystallizes in the orthorhombic space group,Pca2₁, witha = 15.367(2),b = 4.8499(8),c = 19.963(5)Å, andD _calc = 1.40 g cm^–3 forZ = 4. Dinitrobenzo-15-crown-5 crystallizes in the monoclinic space group,P2₁/n, witha = 11.716(2),b = 8.495(3),c = 17.441(5)Å, = 108.40(2)° andD _calc = 1.44 g cm^–3 forZ = 4.Dinitrodibenzo-18-crown-6·2CH₃CN ismonoclinic,P2₁/n,witha = 8.138(2),b = 20.435(9),c = 15.953(9)Å, = 100.55(4)° andD _calc = 1.36 g cm^–3 forZ = 4. The nitro substituents are in the plane of the benzo ring except in the sterically congested dinitrobenzo-15crown-5. The observed crown ether conformations are similar to their substituted analogs.For part 37, see reference [1].     

Light-Controlled Destruction and Assembly: Switching between Two Differently Composed Cage-Type Complexes

We report on two regioisomeric, diazocine ligands 1 and 2 that can both be photoswitched between the E- and Z-configurations with violet and green light. The self-assembly of the four species ( 1 -Z, 1 -E, 2 -Z, 2 -E) with Co^II ions was investigated upon changing the coordination vectors as a function of the ligand configuration (E vs Z) and regioisomer ( 1 vs 2 ). With 1 -Z, Co₂( 1 -Z)₃ was self-assembled, while a mixture of ill-defined species (oligomers) was observed with 2 -Z. Upon photoswitching with 385 nm to the E configurations, the opposite was observed with 1 -E forming oligomers and 2 -E forming Co₂( 2 -E)₃. Light-controlled dis/assembly was demonstrated in a ligand competition experiment with sub-stoichiometric amounts of Co^II ions; alternating irradiation with violet and green light resulted in the reversible transformation between Co₂( 1 -Z)₃ and Co₂( 2 -E)₃ over multiple cycles without significant fatigue by photoswitching.     

A detailed experimental and computational study of monocarbohydrazones

The substituent and solvent effect on intramolecular charge transfer (ICT) of twelve monocarbohydrazones (mCHs) were studied using experimental and theoretical methodology. The effects of specific and non-specific solvent–solute interactions on the UV–Vis absorption maxima shifts were evaluated using linear free energy relationships (LFERs) principles, i.e. using the Kamlet-Taft and Catalán models. Linear free energy relationships in the form of single substituent parameter equation (SSP) was used to analyze substituent effect on UV–Vis, NMR and pK_a change. According to crystallographic data and quantum chemical calculations, the trans (E) form was found to be more stable. A photochromism of compounds with 2-hydroxyphenyl and 2-pyridylimino groups substituted at imine carbon atom results in E/Z isomerization due to creation of intermolecular hydrogen bond in E and Z form, respectively. Multiple stage mass spectrometry (MS-MSⁿ) analysis was applied to define main fragmentation pathways. Furthermore, the experimental findings were interpreted with the aid of ab initio MP2/6–311 G(d,p) and time-dependent density functional (TD-DFT) methods. TD-DFT calculations were performed to quantify the efficiency of intramolecular charge transfer (ICT) with the aid of the charge-transfer distance (D_CT) and the amount of transferred charge (Q_CT) calculation. It was found that both substituents and solvents influence electron density shift i.e. extent of conjugation, and affect ICT character in the course of excitation.     

Behavior ofZ- andE-s-cis-ferrocenyl-1,3-dienes in cycloaddition and dimerization reactions

Z-3-Ferrocenylmethylene-2-methylenecamphane andE-2-ferrocenylmethylene-3-methylenequinuclidine were synthesized by isomerization of the corresponding isomericE- andZ-1,3-dienes in an acidic medium. The dienes obtained form [4+2]-cycloadditionendo-adducts withN-phenylmaleimide, do not form cyclodimers upon thermal or acid-catalyzed [4+2]-cyclodimerization, and addZ-3-ferrocenylmethylene-1,2,7,7-tetramethylbicyclo[2.2.1]hept-2-ylium andE-2-ferrocenylmethylene-3-methyl-1-azoniabicyclo[2.2.1]oct-3-ylium salts, respectively, at the terminal methylene group to give linear addition products. The latter undergo fragmentation on treatment with HBF₄ to form the corresponding carbocation tetrafluroborates. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1223–1228, June, 1998.