分子动力学模拟飞秒激光对C60光裂解反应机理的影响

基于半经典分子动力学方法模拟超快激光诱导的C60光裂解反应.选择能量为2.0eV,半峰全宽(FWHM)分别为40和500fs的激光作用于C60分子,调节脉冲强度使其发生裂解反应,比较长短脉宽飞秒激光对C60裂解反应机理的影响.通过分析产物分布、原子平衡指数、温度以及吸收能量(包括动能、势能和电子能量),证实飞秒激光脉冲下C60的光裂解主要由电子激发态控制,非热力学效应在该反应中起重要作用.激光场的作用时间和强度均影响光裂解反应过程,而电子吸收能量饱和后光场强度的作用则变得不明显.     

Thermal and Photofragmentation of N-Benzoylhydrazone Derivatives

Two selected benzoylhydrazones I and II were subjected to thermolysis by reflux at 200 °C. Benzil, benzoic acid, biphenyl, benzanilide together with the corresponding ketones, nitriles and imines were isolated. Similar treatment of the third hydrazone III at 250 °C afforded, in addition to the previous products, bibenzyl, stilbene, and 2-phenylindole. Photolysis of the same hydrazones I-III in acetonitrile gave the previously reported products but in different ratios along with azine derivatives and substituted methanes. A free radical mechanism involving homolysis of the N-N and C-N bonds is suggested, substantiated by trapping of phenyl radical with isoquinoline, to account for the formation of the identified products.     

Photofragmentation in selected tautomers of protonated adenine

The photofragmentation by UV excitation of selectively prepared 1(+) and 3(+) tautomers of protonated adenine is studied after excitation at a 266 and 263 nm wavelengths with two different experimental set-ups located in Seoul and Orsay. While the production of 1(+) tautomers with an electrospray ion source is now well accepted, calculations were used to ascribe the preparation of 3(+) tautomers from cold adenine dimers. The fragmentation patterns are rather similar for both tautomers, suggesting similar mechanisms as a statistical fragmentation in the ground electronic state after internal conversion.     

Photofragmentation of mass resolved carbon cluster ions

The results of a detailed study of the photodissociation of carbon cluster ions, C ₃ ⁺ to C ₂₀ ⁺ , are presented and discussed. The experiments were performed using internally cold cluster ions derived from pulsed laser evaporation of a graphite target rod in a helium buffer gas followed by supersonic expansion. The mass selected clusters were photodissociated using 248 nm and 351 nm light from an excimer laser. Photofragment branching ratios, photodissociation cross sections and data on the laser fluence dependence of photodissociation are reported. For almost all initial clusters, C _n ⁺ , the dominant photodissociation pathway was observed to be loss of a C₃ unit to give a C _n?3 ⁺ ion. This observation is interpreted as indicating that dissociation occurs by a statistical unimolecular process rather than by direct photodissociation. The photodissociation was found to be linear with laser fluence forn>5 with 248 nm and 351 nm light; quadratic forn=5 for 248 nm and 351 nm; and linear forn=4 at 248 nm. Dissociation energies for the carbon cluster ions implied by these results are discussed. The photodissociation cross sections were found to change dramatically with cluster size and with the wavelength of the photodissociating light.     

Radioiodination of C60 Derivative C60(OH)xOy

In this paper water-soluble fullerene derivative C₆₀(OH)_xO_y was radioiodinated with the iodogen method. The labeling yield was determined by radio-TLC. The effects of pH value, reaction time, temperature and amount of the iodogen on the labeling yield were studied. The labeled product was purified by Sephadex G-25 column chromatography and then the stability of ¹²⁵I-C₆₀(OH)_xO_y was examined . The results showed that the radiochemical purity of ¹²⁵I-C₆₀(OH)_xO_y solution with benzylalcohol remained 82.7% after 43 hours.     

Positron annihilation in C60 and K6C60

The positron density distributions in C₆₀ and K₆C₆₀ have been evaluated using the positron lifetime and Doppler-broadening spectroscopy for the annihilation radiation.In C₆₀, positrons are distributed in the interstitial sites between the C₆₀ molecules,which has been demonstrated by measurements of the temperature dependence of the Doppler-broadening of the annihilation radiation. On the other hand, the positron density distribution must be greatly changed in K₆C₆₀, because positrons are repelled by Coulomb interactions by the positively charged K atoms. It has been observed that there is an extremely short lifetime and a small Doppler-broadening component for the positron annihilation in K₆C₆₀. This component is considered to reflect the positron annihilation inside a C₆₀ molecule.     

Band structure studies on crystalline C60, Ca3C60, and Ca5C60

The three-dimensional EHMO crystal orbital calculations for crystalline C₆₀, Ca₃C₆₀, and Ca₅C₆₀ are reported. The ground states of both undoped solid C₆₀ and partially doped Ca₃C₆₀ are found to be insulating with an indirect energy gap of 1.2 and 0.5 eV, respectively. In contrast, Ca₅C₆₀ forms a metallic conducting phase with a set of three half-filled bands crossing the Fermi level, which is found to be located close to a peak of the density of state. The character of crystal orbitals near the Fermi level for both Ca₃C₆₀ and Ca₅C₆₀ is completely carbonlike. In both cases, the Ca atoms are almost fully ionized and C₆₀ molecules form a stable negative charge state with six to 10 additional electrons. © 1995 John Wiley & Sons, Inc.     

Photofragmentation of Isoleucine by Vacuum Ultraviolet Photoionization

Vacuum ultraviolet photon-induced ionization and dissociation of isoleucine are investi-gated with synchrotron radiation photoionization mass spectroscopy and theoretical cal-culations. The main fragment ions at m/z=86, 75, 74, 69, 57, 46, 45, 44, 41, 30, 28, and 18 from isoleucine are observed in the mass spectrum at the photon energy of 13 eV. From the photoionization e±ciency curves, appearance energies for the principal fragment ions C₅H₁₂N⁺ (m/z=86)、C₂H₅NO₄⁺ (m/z=75)、C₅H₉⁺ (m/z=69)、C₄H₉⁺(m/z=57), and CH₄N⁺(m/z=30) are determined to be 8.84±0.07, 9.25±0.06, 10.20±0.12, 9.25±0.10, and 11.05±0.07 eV, respectively, and possible formation pathways are established in detail by the calculations at the B3LYP/6-31++G(d, p) levels. These proposed channels include simple bond cleavage reactions as well as reactions involving intermediates and transition structures. The experimental and computational appearance energies or barriers are in good agreement.     

Contrasting photodynamics between C60-dithiapyrene and C60-pyrene dyads

The photodynamics of a C60-dithiapyrene donor-acceptor conjugate were compared with the corresponding C60-pyrene conjugate. The photoinduced charge separation and subsequent charge recombination processes were examined by time-resolved fluorescence measurements on the picosecond timescale and transient absorption measurements on the picosecond and microsecond timescales with detection in the visible and near-infrared regions. We have observed quite long lifetimes (i.e., up to 1.01 ns) for the photogenerated charge-separated state in a C60-dithiapyrene dyad without the need for i) a long spacer between the two moieties, or ii) a gain in aromaticity in the radical ion pair.     

Photofragmentation of nitro-based explosives with chemiluminescence detection

A simple, fast, reliable, sensitive and potentially portable explosive detection device was developed employing laser photofragmentation (PF) followed by heterogeneous chemiluminescence (CL) detection. The PF process involves the release of NO_{x(x = 1,2)} moieties from explosive compounds such as TNT, RDX, and PETN through a stepwise excitation–dissociation process using a 193 nm ArF laser. The NO_{x(x = 1,2)} produced upon PF is subsequently detected by its CL reaction with basic luminol solution. The intensity of the CL signal was detected by a thermoelectrically cooled photomultiplier tube with high quantum efficiency and negligible dark current counts. The system was able to detect trace amounts of explosives in various forms in real time under ambient conditions. Detection limits of 3 ppbv for PETN, 2 ppbv for RDX, and 34 ppbv for TNT were obtained. It was also demonstrated that the presence of PETN residue within the range of 61 to 186 ng/cm² can be detected at a given signal-to-background ratio of 10 using a few microjoules of laser energy. The technique also demonstrated its potential for the direct analysis of trace explosive in soil. An LOD range of 0.5–4.3 ppm for PETN was established, which is comparable to currently available techniques. Figure Photofragmentation–chemiluminescence detector     

Crystal solvate of C60 with tricloroethylene,C60 · C2HCl3

Extraction of fullerenes from carbon soot by trichloroethylene has been studied. We have found that C₆₀ forms a solvate with trichloroethylene (C₆₀ · C₂HCl₃:a=31.31(1);b= 10.156(4);c=10.146(4) Å;V=3228.6 ų,Z=4,d _calc=1.752 g cm^–3, orthorhombic symmetry). Its thermal stability has been studied using TG and DSC. A phase transition of the first order at 167 K has been detected.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1248–1250, July, 1994.The authors are grateful to V. P. Bubnov and I. S. Krainskii for providing them with the samples of fulle-rene-containing carbon soot, and to M. G. Kaplunov and A. V. Zvarykina for assistance in the work.This work was carried out with the financial support of the Russian Foundation for Basic Research, Project Nos. 93-03-18705 and 93-03-5650.     

Synthesis of C60H2 by rhodium-catalyzed hydrogenation of C60.

Reduction of C60 with rhodium(0) on alumina and hydrogen in deuterated benzene (C6D6) at ambient temperature and pressure yields a mixture of hydrogenated compounds; C60H2 has been characterized as the major product in 14% yield based on 1H NMR.     

Theoretical studies of C60/C70 fullerene derivatives: C60O and C70O

A semiempirical (AM1) calculation on the structures and stabilities of isomers of the fullerene derivatives C₆₀O and C₇₀O is carried out. The ozonolysis reaction mechanism and the thermodynamics of the compounds are studied. The two isomers of C₆₀O (56 bond and 66 bond) formed by an oxygen atom bridging across a C-C bond have an epoxide-like or an annulene-like structure. According to the ozonolysis reaction mechanism and kinetic factor analysis, the possible products of this ozonolysis reaction are C₆₀O with oxygen bridging over the 66 bond (C_2v) as an epoxide-like isomer and that with oxygen bridging over the 56 bond (C_s) as an annulene-like isomer. Further, the sixteen isomers of C₇₀O (both epoxide-like and annulene-like structures) have been studied with respect to the same reaction mechanism. The most possible product in this ozonolysis reaction contains oxygen bridging across in the upper part (66 bond in C₇₀O-2 or C₇₀O-4) as an epoxide-like structure. The other possible product is C₇₀O-8 (annulene-like structure), in which oxygen bridges across an broken equatorial CC bond in C₇₀ (D_5h). The vibrational frequency analysis and the electronic structure of the selected C₆₀O and C₇₀O isomers are generated for experimental characterisation. The experimental results indicate that C₆₀O and C₇₀O may decompose into the odd number fullerenes C₅₉ and C₆₉. We therefore studied the structures of C₅₉ and C₆₉ also.     

Rotation dynamics of C60 molecules in organic superconductor K3C60

The rotation dynamics of C₆₀ molecules in organic superconductor K₃C₆₀ has been investigated from the viewpoint of intramolecular interaction. It is determined that the rotation of C₆₀ at mom temperature has been frozen up within a small region of rotation angle (0°–50°), and pointed out that the reason for the freeze is the physical interaction rather than the geometrical hindrance. The computations of the interactions for alkali-doped compounds A_3-x A′ _x C₆₀ (x = 1, 2, 3; A, A′ = K, Rb, Cs) other than K₃C₆₀ have also been camed out. Fmm the obtained results, it is seen that the superconducting transition temperatures T, are strongly connected with the interactions in them, and this observation is consistent with the discovery of the correlation between T_c, and lattice constants a. Project supported by the National Natural Science Foundation of China.     

C60 carbon cages

Arrangements of carbon atoms in cage-like structures with no dangling bonds are considered as possible novel allotropic forms of carbon. Five different C₆₀ cages, having certain favorable structural characteristics, are identified. Quantitative resonance-theoretic calculations are made and compared to simpe Hückel results. The favored structure is found to be the so-called Buckminsterfullerene structure.     

Hydrosilylation of fullerence C60

The hydrosilylation of C₆₀ by diphenylsilane was studied. Silicon-containing derivatives C₆₀H _n (HSiPh₂) _n (n=2, 4, 6) were obtained. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1697–1699, September, 1997.     

Incorporating C2 into C60 films

The material formed by depositing C(2)(-) anions onto/into thin C(60) films (on graphite) at room temperature has been studied by means of thermal desorption mass spectroscopy, ultraviolet photoionization spectroscopy, atomic force microscopy (AFM), and surface enhanced Raman spectroscopy. As-prepared, C(2)/C(60) films manifest thermal desorption behaviour which differs significantly from pure C(60) films. Whereas the latter can be fully sublimed, we observe decomposition of C(2)/C(60) films to a high-temperature-stable material while predominantly C(60), C(62), and C(64) are desorbed in parallel. Deposition of C(2)(-) also leads to significantly modified electronic and vibrational properties. Based on DFT model calculations of the Raman spectra, we suggest that as-prepared C(2)/C(60) films contain appreciable amounts of polymeric networks comprising -C(2)-C(60)-C(2)-C(60)- chains. Detection of sublimed C(62) and C(64) upon heating implies that thermal decomposition of C(2)/C(60) films involves addition/uptake of C(2) units into individual fullerene cages. Correspondingly, annealing films up to various intermediate temperatures results in significant modifications to valence-band UP spectra as well as to surface topographies as imaged by AFM. The novel carbonaceous material obtained by heating to T > 950 K has a finite density of states at the Fermi level in contrast to as-prepared C(2)/C(60). It comprises fused fullerene cages.     

Redox-induced hydrogen transfer from hydrofullerene C60H36 to fullerene C60

The possibility of hydrogen transfer from hydrofullerene C₆₀H₃₆ to electrogenerated radical anion C₆₀ ^.− or dianion C₆₀ ²⁻ in propylene carbonate-toluence (3∶2, v/v) was demonstrated by cyclic voltammetry. The process affords C₆₀H₂ as the product. The reaction found is the typical redox-induced process. Translated fromIzvestiya Akodemii Nauk. Seriya Khimicheskaya, No. 6, pp. 1136–1139, June, 1998.     

Charge transfer collisions between fullerenes: C 60 3+ + C60

Charge transfer collisions between C ₆₀ ³⁺ and C₆₀ are studied for collision energies between 400 and 3600 eV. Single and double electron transfers are observed, both occuring under single collision conditions. Absolute charge transfer cross sections are determined as a function of collision energy. The cross section for single electron capture of approx. 300 Ų is about two times larger than that for double electron transfer. For both processes the cross section increases slightly with increasing collision energy.