Kinetics of photo-induced dissociation of Na clusters deposited on Mica

Na clusters bound to mica surfaces have been irradiated with pulsed and cw visible laser light. Kinetic energy and angular distributions of the Na atoms desorbing from the clusters have been determined using cw two-photon laser-induced fluorescence detection. In addition the dependence of the desorption rate on laser power, wavelength and polarization has been measured. The most probable kinetic energyE _kin of the photodesorbed atoms at the surface temperatureT _S =300 K was found to beE _kin=18±5 meV, independent of laser irradiance (3 µJ/cm²...20 mJ/cm²) and wavelength (450 nm, 505 nm, 658 nm). With increasing orientation angle between detection axis and surface normal (0°≦Θ≦90°)E _kin was observed to decrease slightly, while it was nearly independent of surface temperature betweenT _S =30 K andT _S =300 K. Also, with increasing radius of the Na clusters the desorbing Na atoms slowed down. The angular distribution of the Na atoms was of cos²-type with respect to the surface normal. These observations suggest that laser-induced desorption of Na from Na clusters bound to mica surfaces involves an initial rate-limiting step of direct surface plasmon excitation followed by a final step of delayed thermal desorption.     

A Rapid and Simple UPLC-MS–MS based Simultaneous Determination of the Medicinally Important E- and Z-Guggulsterone from Oleogum-Resins of Naturally Occurring Commiphora wightii Plants

A fast and simple method for simultaneous detection and quantification of the medicinally important E- and Z-guggulsterone from raw oleogum-resin of Commiphora wightii by UPLC was developed. Both E- and Z-guggulsterone were extracted from naturally occurring resin samples using ethyl acetate and methanol. Chromatographic separation of the analytes and their respective standards were performed on an Acquity UPLC BEH C₁₈ column followed by UV as well as a triple quadrupole detector in positive ionization mode. A linear gradient elution profile followed; mobile phase consisted of acetonitrile and 2 mM ammonium acetate in water. The method was validated over a range of 6.25 to 100 ng mL^?1 for both the guggulsterones. The calibration curves were linear with correlation coefficients of 0.9998 for E-guggulsterone and 0.9999 for Z-guggulsterone. The LOD and LOQ were 1.65 and 5.02 ng mL^?1 for E-guggulsterone and 2.57 and 7.79 ng mL^?1 for Z-guggulsterone respectively. The average recovery of E-guggulsterone (104.63%) and Z-guggulsterone (104.33%) achieved from spiked samples were consistent and reproducible. The intra- and inter-day assay precision of the analytes over the entire concentration range was less than 2%. The developed method required only 6 min to complete a run including 1 min to equilibrate the system and hence suitable for high throughput applications. Efficiency, reliability and accuracy of the developed method were evaluated by analyzing resin sample from different C. wightii populations. The result of this study offers improvement in terms of speed and sensitivity as compared to previously reported methods.     

Molecular beam study of iodine atom abstraction by oxygen atoms

Reactive scattering of O atoms with CF₃I molecules has been studied at an initial translational energy E = 32.1 kJ mol^?1 using a He seeded O atom beam and at E = 13.9 kJ mol^?1 using a Ne seeded O atom beam. Reactive scattering of IO product favours the backward hemisphere at low energy but becomes almost isotropic at high energy. The product translational energy distribution at low energy indicates substantial energy transfer with internal modes of the collision complex but at high energy the excess energy is disposed into product translation.     

Evaluation of metal hydride thermograms by a differential-correction technique

A least-squares linear-Taylor differential-correction technique has been used for the rapid evaluation of thermogravimetric curves obtained during the decomposition of magnesium hydride, iron—titanium hydride and lanthanum—nickel hydride. For magnesium hydride and iron—titanium hydride the Avrami—Erofe'ev equation fits the experimental data, thus indicating that nucleation is the rate-determining step under thermogravimetric conditions. For lanthanum—nickel hydride a combination of the Avrami—Erofe'ev equation and the phase boundary movement equation fits the data up to a fractional decomposition of 0.8. For magnesium hydride decomposition the activation energy E and the pre-exponential factor Z are dependent on the hydrogen pressure (E = 101.2 kJ mole^?1 and Z = 8.96 × 10⁷ at 0.30 MPa, while E = 66.3 kJ mole^?1 and Z = 4.77 × 10⁷ at 0.11 MPa). For iron—titanium hydride (E = 28.4 kJ mole^?1) and lanthanum—nickel hydride (E = 13.4 kJ mole^?1) the values are independent of pressure.     

Large-Z and -N dependence of atomic energies from renormalization of the large-dimension limit

By combining Hartree–Fock results for nonrelativistic ground-state energies of N-electron atoms with analytic expressions for the large-dimension limit, we have obtained a simple renormalization procedure. For neutral atoms, this yields energies typically threefold more accurate than the Hartree–Fock approximation. Here, we examine the dependence on Z and N of the renormalized energies E(N, Z) for atoms and cations over the range Z, N = 2 → 290. We find that this gives for large Z = N an expansion of the same form as the Thomas–Fermi statistical model, E → Z^7/2(C₀ + C₁Z^?1/3 + C₂Z^?2/3 + C₃Z^?3/3 + ?), with similar values of the coefficients for the three leading terms. Use of the renormalized large-D limit enables us to derive three further terms. This provides an analogous expansion for the correlation energy of the form δE δZ^4/3(δC₃ + δC₅Z^?2/3 + δC₆Z^?3/3 + ?); comparison with accurate values of δE available for the range Z ? 36 indicates the mean error is only about 10%. Oscillatory terms in E and δE are also evaluated. © 1994 John Wiley & Sons, Inc.     

Chlorures vinyliques : I. Lithiation directe; radicaux vinyliques

Reaction of E- and Z-4-chlorohept-3-enes (R_ZCl and R_ECl with lithium gives reduction products in addition to the organolithium compound. This reduction occurs with partial inversion of the double bond while retention of configuration takes place when the n-butyllithium exchange reaction is used.A partial inversion also occurs when _Z- and _E-4-bromohept-3-enes (R_ZBr and R_EBr) are formed from exchange with the corresponding vinylic organolithium and dibromoethane.A vinylic radical is postulated to explain the lack of stereospecificity during the formation of the

Li bond in the reaction with lithium.     

Synthesis and structure of β-aryl-α-nitroacrylates

The method for preparation of ethyl α-nitrocinnamates by nitroacetic acid ester alkenylation with aromatic aldehydes in the presence of acetic acid and β-alanine has been modified. Structures of the prepared compounds have been proved by electronic, IR, ¹H, and ¹³C-{¹H} NMR spectroscopy (including heteronuclear correlation experiments ¹H-¹³C HMQC and ¹H-¹³C HMBC). In solution these compounds exist in the form of Z-isomer; the Z?E isomerization is observed in the case of the compound containing strong electron-donor group [N(CH₃)₂] at benzene ring.     

Structural studies of carbanionic species α to a phosphoryl group; anion from ketophosphonate comparison with related enoxysilanes and p-ylid

The structure of the carbanionic species 4 formed from diethyl(2-oxopropyl)phosphonate and t-BuOK has been studied by ¹H, ¹³C and ³¹P NMR. In an associating medium (pyridine), a single chelated (C) carbanionic species 4Z (C) is observed. Two slowly interconverting species 4Z and 4E co-exist when the anion-cation interaction is loose, i.e. in Me₂SO and when the gegenion K⁺ is complexed to (2,2,2)cryptand in Me₂SO. The anionic carbon of 4Z and 4E is planar, and they have a strong enolate character as shown by the barrier to rotation around the C-1? C-2 bond (ΔG_c?～92 KJ mol^?1). Their structures are compared to those of the related E and Z 1-diethylphosphonato-2-trimethylsilyloxypropenes and to (acetylmethylene)triphenylphosphorane.     

The electron correlation cusp

The Kais function is an exact solution of the Schrödinger equation for a pair of electrons trapped in a parabolic potential well with r ₁₂ ^?1 electron-electron interaction. Partial wave analysis (PWA) of the Kais function yields E _L = E + C₁(L + \-C ^?1 ₂)^?3 + O(L ^?5) where E is the exact energy and E _L the energy of a renormalized finite sum of partial waves omitting all waves with angular momentum ? > L. Slight rearrangement of an earlier result by Hill shows that the corresponding full CI energy differs from E _L only by terms of order O(L ^?5) with FCI values of C ₁ and \-C ^?1 ₂ identical to PWA values. The dimensionless \-C ₂ parameter is weakly dependent upon the size of the physical system. Its value is 0.788 for the Kais function, and 0.893 for the less diffuse helium atom, and approaches \-C ₂→ 1 in the limit of an infinitely compact charge distribution. The ?th energy increment satisfies an approximate virial theorem which becomes exact in the high ? limit. This analysis, formulated to facilitate use of the Maple system for symbolic computing, lays the mathematical ground work for subsequent studies of the electron correlation cusp problem. The direction of future papers in this series is outlined.     

Correlation of singlet energies of aromatic hydrocarbons with the rates of protonation of their anion radicals

Formation and protonation of aromatic anion radicals in 2-propanol were studied by kinetic spectrophotometric pulse radiolysis. All polycyclic hydrocarbons studied were found to react very rapidly with e^?_solv. Those with relatively high electron affinity were also reduced by (CH₃)₂CO^?. The anion radicals formed undergo protonation by direct reaction with the alcohol molecule. The rate constants for this protonation vary from ≈ 6 × 10⁵ s^?1 for cis-stilbene and naphthalene down to 20 s^?1 for perylene. The variations in rates are discussed in terms of changes in singlet energy separation (ΔE_{S1 ← S0}). The logarithm of the protonation rate constant for alternant hydrocarbons is linearly dependent on ΔE_{S1 ← S0}.     

Low energye ± — Li2 scattering using LFCC method

The scattering of slow electrons and positrons by lithium metal dimer has been studied in the laboratory frame close coupling method. The effect of polarization is included through the parameter free model correlation potential given by Perdew and Zunger. In the case of electron, the exchange kernel is replaced by a local model exchange potential as used by Sur and Ghosh. To have a convergent result seven rotational states (j=0, 2, 4, 6, 8, 10, 12) are retained in the coupling scheme. The results for elastic, rotational and total cross sections fore ^? — Li₂ ande ⁺ — Li₂ are reported. The electron results are in fair agreement with measured values and existing theoretical predictions.     

Theoretical study of spin–orbit coupling and kinetics in spin-forbidden reaction between Ta(NH2)3 and N2O

The activation mechanism of the nitrous oxide (N₂O) with the Ta(NH₂)₃ complex on the singlet and triplet potential energy surfaces has been investigated using the hybrid exchange correlation functional B3LYP. The minimum energy crossing point (MECP) is located by using the methods of Harvey et al. The rate-determining step of the N–O activation reaction is the intersystem crossing from ¹ 2 to ³ 2. The reacting system will change its spin multiplicities from the singlet state to the triplet state near MECP-1, which takes place with a spin crossing barrier of 32.5 kcal mol^?1, and then move on the triplet potential energy surface as the reaction proceeds. Analysis of spin–orbit coupling (SOC) using localized orbitals shows that MECP-1 will produce the significant SOC matrix element, the value of SOC is 272.46 cm^?1, due to the electron shift between two perpendicular π orbitals with the same rotation direction and the contribution from heavy atom Ta. The rate coefficients are calculated using Non-adiabatic Rice-Ramsperger-Kassel-Marcus (RRKM). Results indicate that the coefficients, k(E), are exceedingly high, k(E) > 10¹² s^?1, for energies above the intersystem crossing barrier (32.5 kcal mol^?1); however, in the lower temperature range of 200–600 K, the intersystem crossing is very slow, k(T) < 10^?6 s^?1.     

A relationship between correlation energies and sizes: The helium-like ions

The correlation energy (E _corr) is found to be inversely proportional to the product of the electron pair size (2>) and the square of the effective nuclear charge (Z _eff) for the helium-like ions.     

Raman analysis of SF6 molecular beams excited by a cw CO2 laser

In a molecular beam the effects of vibrational pumping of SF₆ (ν₃ = 948 cm^?1) are studied, using a line-tunable cw CO₂ laser. Intracavity spontaneous Raman scattering is used for analysis. For excitation in the collision regime (x_E/D ≤ 1), a thermal redistribution of the ν₃ excitation over all vibrational modes is found, together with an average absorption up to six photons per molecule. The infrared absorption profile shows a red-shift of 6 cm^?1. For excitation in the relatively rare collision regime (x_E/D ? 4), a structured non-thermal ν₁ Raman spectrum is observed, especially in the case of seeded molecular beams (10% in He). The observed hot-band peaks can be explained in terms of single-photon absorptions and collision-induced near-resonant V-V energy transfer, leading to single, double and triple excitations of the ν₃ mode. The value of T_rot in the beam is found to influence sensitively the non-resonant energy-transfer rate [e.g. hν₃(948 cm^?1)+ΔE_rot → h(ν₄ + ν₆)(962 cm^?1) relative to the near-resonant transfer rate (hν₃ + hν₃ → 2hν₃ + 3.5 cm^?1)].     

Retardation (Casimir) energy shifts for Rydberg helium-like low-Z ions

The development of storage rings and electromagnetic traps for heavy charged particles is opening up new regimes of atomic physics, including, in particular, spectroscopic studies of Rydberg helium-like ions — with nuclear chargeZ, one electron in the 1s state, and one electron in a near-hydrogenic state of highn andl <n, withn andl the principal and orbital quantum numbers, respectively. We consider the possibility of detecting energy shifts due to retardation, ΔE _ret (n,l), Casimir-like effects. These are quantum electrodynamic (QED) retardation effects associated with the finite speed of light. (As opposed to basically kinematic and dynamic QED effects for small quantum numbersn andl, the appropriate expansion parameter forn andl large for retardation QED corrections is notZ(e ²/?c) but [(Z ? 1)/n ² Z ²](?c/e ²).) We wish to provide some orientation to those planning experiments in the area, with regard to the choices ofn,l, andZ most likely to be able to generate a high-precision confirmation of a retarded interaction. To do so, we provide extensive tables of estimates, for 1s,nl states, of ΔE _ret(n,l), of radiative widths, and ofE, the spin-independent (“electric” fine structure) energy in the absence of retardation shifts, for (nuclear spin zero) ions withZ=2, 6, 8, 16 and 20. These ions might be experimentally accessible in storage rings, and theZ's are low enough that virtual pair production effects may not yet be significant. There is also a brief survey of possible experimental techniques.     

Thiophene‐S,S‐dioxidized Indophenine: A Quinoid‐Type Building Block with High Electron Affinity for Constructing n‐Type Polymer Semiconductors with Narrow Band Gaps

Three thiophene‐S,S‐dioxidized indophenine (IDTO) isomers, 3 a (E,E,E), 3 b (Z,E,E), and 3 c (Z,E,Z), were synthesized by oxidation of an indophenine compound. 3 b and 3 c could be converted into the most‐stable 3 a by heating at 110 °C. An IDTO‐containing conjugated polymer, PIDTOTT, was prepared using 3 a as a comonomer through a Stille coupling reaction, and it possesses a narrow band gap and low energy levels. In organic field effect transistors (OFETs), PIDTOTT exhibited unipolar n‐type semiconductor characteristics with unexpectedly high electron mobility (up to 0.14 cm² V^?1 s^?1), despite its rather disordered chain packing.     

Stereospecific preparation of symmetrical (1Z, 3Z)- and (1E, 3E)-2,3-difluoro-1,4-disubstituted-buta-1,3-dienes from 1-bromo-1-fluoroalkenes

A straightforward method to prepare symmetrical (1Z, 3Z)- and (1E, 3E)-2,3-difluoro-1,4-disubstituted-buta-1,3-dienes is described. High E/Z ratio 1-bromo-1-fluoroalkenes, prepared by isomerization from the E/Z ≈ 1:1 isomeric mixtures, reacted with Bu₃SnSnBu₃ and Pd(PPh₃)₄ to afford (1Z, 3Z)-2,3-difluoro-1,4-disubstituted-buta-1,3-dienes in good yield. (Z)-1-Bromo-1-fluoroalkenes, which were prepared by kinetic reduction from 1-bromo-1-fluoroalkenes (E/Z ≈ 1:1), can undergo similar reaction with Bu₃SnSnBu₃ and Pd(PPh₃)₄/CuI to prepare (1E, 3E)-2,3-difluoro-1,4-disubstituted-buta-1,3-dienes.     

Palladium-catalyzed isomerization of (Z)-1-functionalized-4-acetoxy-2-butenes: Solvent and substituent effects

The Pd(PPh₃)₄-catalyzed isomerization of (Z)-1,4-diacetoxy-2-butene, (Z)-1-(t-butyldimethylsilyloxy)-4-acetoxy-2-butene and (Z)-1-(t-butyldiphenylsilyloxy)-4-acetoxy-2-butene affords the corresponding (E)-isomers and 1,2-difunctionalized-3-butenes. In THF, the formation of the (E)-isomers is mainly due to reaction from an η¹-allylpalladium intermediate while an η³-allylpalladium is the main key intermediate in DMF. The time to reach equilibrium between the products and their respective concentrations depend on the nature of the substituents and the solvent.     

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.