Bone-ablation mechanism using CO2 lasers of different pulse duration and wavelength

Bone ablation using different pulse parameters and four emission lines of 9.3, 9.6, 10.3, and 10.6 m of the CO₂ laser exhibits effects which are caused by the thermal properties and the absorption spectrum of bone material. The ablation mechanism was investigated with light- and electron-microscopy at short laser-pulse durations of 0.9 and 1.8 s and a long pulse of 250 s. It is shown that different processes are responsible for the ablation mechanism either using the short or the long pulse durations. In the case of short pulse durations it is shown that, although the mineral components are the main absorber for CO₂ radiation, water is the driving force for the ablation process. The destruction of material is based on explosive evaporation of water with an ablation energy of 1.3 kJ/cm³. Histological examination revealed a minimal zone of 10–15 m of thermally altered material at the bottom of the laser drilled hole. Within the investigated spectral range we found that the ablation threshold at 9.3 and 9.6 m is lower than at 10.3 and 10.6 m. In comparison the ablation with a long pulse duration is determined by two processes. On the one side, the heat lost by heat conduction leads to carbonization of a surface layer, and the absorption of the CO₂ radiation in this carbonized layer is the driving force of the ablation process. On the other side, it is shown that up to 60% of the pulse energy is absorbed in the ablation plume. Therefore, a long pulse duration results in an eight-times higher specific ablation energy of 10 kJ/cm³.     

New structural motifs in the aggregation of neutral gold(I) complexes: structures and luminescence from (alkyl isocyanide)AuCN

The preparation and X-ray crystal structures of (CyNC)Au(I)CN, (n-BuNC)Au(I)CN, and (i-PrNC)Au(I)CN.0.5CH(2)Cl(2) are reported and compared with those of (MeNC)Au(I)CN and (t-BuNC)Au(I)CN, which were previously described. These linear molecules are all organized through aurophilic interactions into three structural classes: simple chains ((CyNC)Au(I)CN and (t-BuNC)Au(I)CN), side-by-side chains in which two strands make Au...Au contact with each other ((n-BuNC)Au(I)CN), and nets in which multiple aurophilic interactions produce layers of gold(I) centers ((i-PrNC)Au(I)CN and (MeNC)Au(I)CN). All of these five solids dissolve to produce colorless, nonluminescent solutions with similar UV/vis spectra. However, each of the solids displays a unique luminescence with emission maxima occurring in the range 371-430 nm.     

Stereospecific uncatalyzed alpha-O-glycosylation and alpha-C-glycosidation by means of a new D-Gulal-derived alpha vinyl oxirane

The reaction of alpha vinyl oxirane 5, prepared through a new route to the d-gulal system, with O-nucleophiles (alcohols and di-O-isopropylidene-alpha-d-monosaccharides) and C-nucleophiles (lithium alkyls) affords, in a completely stereoselective way, the corresponding 2-unsaturated alpha O- and C-glycosides having the same configuration as the starting epoxide.     

Anodic oxidation mechanism of 3-arylaminoindoles at a rotating and at a (new type of) pulsing platinum electrode in dipolar aprotic solvents

The oxidation of 2-phenyl-3-arylaminoindoles (1a–e) has been studied in acetonitrile, dimethylformamide and propylene carbonate at a new type of platinum electrode with periodical renewal of the diffusion layer and at a rotating platinum electrode by cyclic voltammetry, d.c. and a.c. voltammetry, controlled potential coulometry and spectrometry (e.s.r. and u.v.). In the examined aprotic solvents and in the presence of a base (like water or diphenylguanidine) amines 1a–e undergo a two-electron oxidation to the corresponding imines. In a protophobic medium (acetonitrile or propylene carbonate) 1a–e are oxidized in two one-electron steps, the first of which leads to the formation of a cation radical (identified by e.s.r. spectrum), oxidizable at more positive potentials (second step). The second electron transfer, however, must be simultaneous with, or be immediately followed by, a very fast deprotonation reaction. The same e.e.c. sequence explains the observed bielectronic step of 1a–e in a protophilic aprotic solvent, like dimethylformamide.     

Ab initio molecular modeling of 13C NMR chemical shifts of polymers. 1. Ethylene–norbornene copolymers

Cycloolefin copolymers (COC) have recently raised much interest because of their excellent thermal and optical properties, largely determined by the chain composition and stereochemistry. Previous force‐field computations allowed us to define the main conformational characteristics of ethylene–norbornene (E–N) copolymers and to contribute to the elucidation of their microstructure on the basis of empirical relationships between conformation and ¹³C nuclear magnetic resonance (NMR) chemical shifts. A thorough test of ab initio ¹³C chemical shifts computations [gauge‐invariant atomic orbitals (GIAO)] on known cases shows that the agreement with experimental data is quite good, especially with the MPW1PW91 density functional theory (DFT), using the 6‐311+G(2d,p) basis set on properly energy‐minimized structures. We applied this method on proper model compounds to confirm the signal assignment of the spectra of E–N copolymers in the presence of norbornene microblocks, where strong effects arising from ring distortions are expected to occur. The results nicely confirm the latest assignment of norbornene signals belonging to ENNE sequences. This shows the great potentialities of GIAO/DFT computations with regard to complex spectra interpretation and polymer microstructural investigations. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

Electron solvation by highly polar molecules: density functional theory study of atomic sodium interaction with water, ammonia, and methanol

This study further extends the scope of a previous paper [Y. Ferro and A. Allouche, J. Chem. Phys. 118, 10461 (2003)] on the reactivity of atomic Na with water to some other highly polar molecules known for their solvation properties connected to efficient hydrogen bonding. The solvation mechanisms of ammonia and methanol are compared to the hydration mechanism. It is shown that in the case of ammonia, the stability of the solvated system is only ensured by electrostatic interactions, whereas the methanol action is more similar to that of water. More specific attention is given to the solvation process of the valence 3s Na electron. The consequences on the chemical reactivity are analyzed: Whereas ammonia is nonreactive when interacting with atomic sodium, two chemical reactions are proposed for methanol. The first process is dehydrogenation and yields methoxy species and hydrogen. The other one is dehydration and the final products are methoxy species, but also methyl radical and water. The respective roles of electron solvation and hydrogen bonds network are analyzed in detail in view of the density of states of the reactive systems.     

Vapour pressures and sublimation enthalpies of thymine and cytosine

The vapour pressures of cytosine and thymine were measured using the torsion-effusion technique. The sublimation processes of cytosine and thymine were investigated over the temperature ranges 480–553 K and 420–503 K, respectively. The following pressure—temperature equations were derived by least-squares treatment of the vapour pressure data

The standard sublimation enthalpies were obtained by second-and third-law treatment of the experimental data and the values ΔH⁰₂₉₈ = 167 ± 10 kJ mole^?1 and ΔH⁰₂₉₈ = 138 ± 10 kJ mole^?1 were derived for cytosine and thymine, respectively. IR and Raman spectra were recorded in the gas phase in order to evaluate the thermodynamic functions of gaseous cytosine and thymine.     

Remarkable variations in the luminescence of frozen solutions of [Au[C(NHMe)(2)](2)](PF(6)) x 0.5(acetone). Structural and spectroscopic studies of the effects of anions and solvents on Gold(I) carbene complexes

The unusual luminescence behavior of the two-coordinate gold(I) carbene complex, [Au[C(NHMe)(2)](2)](PF(6)) x 0.5(acetone), is reported. Upon freezing in a liquid N(2) bath, the colorless, nonluminescent solutions of [Au[C(NHMe)(2)](2)](PF(6)) x 0.5(acetone) become intensely luminescent. Strikingly, the colors of the emission differ in different solvents and appear only after the solvent has frozen. Solid [Au[C(NHMe)(2)](2)](PF(6)) x 0.5(acetone) is also luminescent, and the luminescence is attributed to the formation of extended chains of gold(I) centers that are connected through aurophilic attractions. Crystallographic studies of [Au[C(NHMe)(2)](2)](PF(6)) x 0.5(acetone) and [Au[C(NHMe)(2)](2)](BF(4)), which is also luminescent, reveal that both involve extended chains of cations and that the anions are hydrogen bonded to the cations through cation N-H groups. However, these chains differ in the Au...Au separations in each and in the carbene ligand orientations. In contrast, [Au[C(NMe(2))(NHMe)](2)](PF(6)) forms a colorless, nonluminescent solid, and in that solid there are no Au...Au interactions, a factor which supports the contention that aggregated species are responsible for the luminescence of [Au[C(NHMe)(2)](2)](PF(6)) x 0.5(acetone) in the solid state and in frozen solutions.     

Determination of heats of formation of praseodymium antimonides

A systematic investigation of the heats of formation in the Pr-Sb system has been performed using a small-furnace, isoperibolic calorimeter. The compositions and the equilibrium states of the samples have been checked by metallographic and X-ray analyses. The following values have been found for the various compounds (kJ/g atom±2): Pr₂Sb:-99; Pr₅Sb₃:-112; Pr₄Sb₃:-120; PrSb:-126; PrSb₂:-97.The data available (both the results obtained in this work and those already known) are discussed briefly and related to the possible shape of the phase diagram.

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Zusammenfassung Es wurde eine systematische Untersuchung der Bildungswärmen im Pr/Sb-System unter Verwendung eines Kleinofen-Isoperibolkalorimeters ausgeführt. Die Zusammensetzung und der Gleichgewichtszustand der Proben wurden durch metallographische und röntgenographische Analyse ermittelt. Für die einzelnen Verbindungen wurden folgende Werte (kJ·mol^–1 ±2.0) gefunden: Pr₂Sb:-99; Pr₅Sb₃:-112; Pr₄Sb₃:-120;PrSb:-126; PrSb₂:-97. Die als Ergebnis dieser Arbeit erhaltenen und aus der Literatur bereits bekannte Daten werden kurz diskutiert und mit einem möglichen Phasendiagramm in Verbindung gebracht.

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-Sb, . . ( (-)^–1 ±2.0): Pr₂Sb-99; Pr₅Sb₃-112; PrSb₃-120; PrSb-126; PrSb₂-97. .