J. G. Izquierdo

Bio: J. G. Izquierdo is an academic researcher from Complutense University of Madrid. The author has contributed to research in topics: Femtosecond & Laser. The author has an hindex of 13, co-authored 31 publications receiving 599 citations. Previous affiliations of J. G. Izquierdo include Spanish National Research Council.

A detailed experimental and theoretical study of the femtosecond A-band photodissociation of CH(3)I.

Abstract: The real time photodissociation dynamics of CH(3)I from the A band has been studied experimentally and theoretically. Femtosecond pump-probe experiments in combination with velocity map imaging have been carried out to measure the reaction times (clocking) of the different (nonadiabatic) channels of this photodissociation reaction yielding ground and spin-orbit excited states of the I fragment and vibrationless and vibrationally excited (symmetric stretch and umbrella modes) CH(3) fragments. The measured reaction times have been rationalized by means of a wave packet calculation on the available ab initio potential energy surfaces for the system using a reduced dimensionality model. A 40 fs delay time has been found experimentally between the channels yielding vibrationless CH(3)(nu=0) and I((2)P(32)) and I(*)((2)P(12)) that is well reproduced by the calculations. However, the observed reduction in delay time between the I and I(*) channels when the CH(3) fragment appears with one or two quanta of vibrational excitation in the umbrella mode is not well accounted for by the theoretical model.

Femtosecond multichannel photodissociation dynamics of CH3I from the A band by velocity map imaging

Abstract: The reaction times of several well-defined channels of the C-I bond rupture of methyl iodide from the A band, which involves nonadiabatic dynamics yielding ground state I(2P3/2) and spin-orbit excited I*(2P1/2) and ground and vibrationally excited CH3 fragments, have been measured by a combination of a femtosecond laser pump-probe scheme and velocity map imaging techniques using resonant detection of ground state CH3 fragments. The reaction times found for the different channels studied are directly related with the nonadiabatic nature of this multidimensional photodissociation reaction.

Slice imaging of the photodissociation of acetaldehyde at 248 nm. Evidence of a roaming mechanism

Abstract: The photodissociation of acetaldehyde in the molecular channel yielding CO and CH4 at 248 nm has been studied, probing different rotational states of the CO(ν = 0) fragment by slice ion imaging using a 2+1 REMPI scheme at around 230 nm From the slice images, clear evidence of the co-existence of two different mechanisms has been obtained One of the mechanisms is consistent with the well-studied conventional transition state in which CO products appear rotationally excited, and the second is consistent with a roaming mechanism This roaming mechanism is characterized by a low rotational energy disposal into the CO fragment as well as by a very low kinetic energy release, corresponding to a high internal energy in the CH4 counter-fragment

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

Abstract: 抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

Thiyl Radicals in Organic Synthesis

Abstract: s a hydrogen atom from the thiol to give hydroperoxide 96 and a thiyl radical, which propagates the chain. Hydroperoxide 96 is reduced in the presence of triphenyl phosphine to give the corresponding alcohol 91. The preference for the formation of cis-3,5-disubstituted 1,2-dioxolanes is in agreement with the Beckwith−Houk transition state model for 5-exo-trig cyclizations. Similarly, the addition of thiophenol onto 5methylhepta-1,3,6-triene 97 under an atmosphere of oxygen led to 1,2-dioxolane 98, isolated in 49% as a single diastereoisomer after treatment with triphenyl phosphine, together with minor amounts of linear alcohols 99 and 100 (Scheme 49, eq b). This reaction is remarkable for a number of reasons. First, the addition of the thiyl radical occurs exclusively at the terminal position of the conjugated diene system and not at the terminal alkene, thus highlighting the higher reactivity of conjugated dienes as compared to isolated alkenes. Second, intermolecular trapping of the resulting allyl radical is reversible and regioselective under these reaction conditions. Because of the reversibility of the reaction between the allyl radical and molecular oxygen, both ratios 1,4/1,2-addition and 1,2dioxolane/linear alcohols strongly depend upon the initial concentration in thiol. Accordingly, the 1,2-dioxolanes were obtained in good yields only in highly diluted solutions. Finally, the 5-exo-trig cyclization occurs in a completely stereoselective manner, with only one of the two diastereomeric peroxyl radical intermediates (101) undergoing cyclization, while the other one (102) either leads to linear alcohol 99 or fragments back the allyl radical (Scheme 49, eq b). The reversible reaction of allyl radicals with molecular oxygen was also demonstrated for carotenoid-derived carbon-centered radical generated by Scheme 47 Scheme 48. Application to the Preparation of Functionalized 1,2,4-Trioxanes Chemical Reviews Review dx.doi.org/10.1021/cr400441m | Chem. Rev. XXXX, XXX, XXX−XXX X addition of a thiyl radical to the conjugated polyene carotene. This process has been extended to include the more challenging 1,5-dienes, from which six-membered ring endoperoxides can be obtained. Bachi and co-workers applied the thiol−olefin cooxidation process to the total synthesis of antimalarial agent yingzhaosu A (Scheme 50) and its C14epimer, as well as the preparation of a series of active analogues, from readily available limonene 103. The overall process is extremely challenging in this case due to the particular structure of the diene, with the 6-exo-cyclization process being in competition with intermolecular hydrogen atom abstraction from the thiol, and also potentially with intramolecular hydrogen abstraction from the activated allylic position by the reactive oxygen-centered radical. As previously observed, addition of the thiyl radical takes place at the less hindered position, and due to the lack of stereocontrol during the trapping of the resulting carbon-centered radical, peroxyl radical 105 is formed as a 1:1 mixture of diastereoisomers (Scheme 50). The latter undergoes 6-exo-trig cyclization to give carbon-centered radical 106. Unlike the initial trapping with molecular oxygen, the 2,3-dioxabicyclo[3.3.1]nonane system of 106 allows a highly diastereoselective reaction for the second trapping with molecular oxygen from the less hindered face to give 107. Alcohol 104 is then obtained following hydrogen abstraction from the thiol by peroxyl radical 107 and reduction of the resulting hydroperoxide with triphenylphosphine. The yields of endoperoxides remain relatively low (ca. 20−30%, calculated on the diene); however, considering the accessibility and the cost of the reactants (thiophenol, limonene, and oxygen), this approach represents a very attractive access to these structurally complex endoperoxides, some of which exhibit very promising activity for the treatment of malaria. 3.3.2. Intramolecular Trapping of the Carbon-Centered Radical. 3.3.2.a. Fragmentation Reaction: RingOpening of Vinyl Cyclopropanes. The carbon-centered radicals generated by addition of a thiyl radical onto the C C bond of vinylcypropanes have been shown to undergo cyclopropane ring-opening. The resulting radical species can then be trapped by hydrogen abstraction from the thiol. This fragmentation is a very fast process with rate constants in the range 10−10 s−1 (310 K) for most of the cyclopropylcarbinyl radicals, which allows for the fragmentation process to compete favorably with intermolecular reactions, as well as with most intramolecular processes. Alternatively, the carboncentered radical resulting from the β-fragmentation of the cyclopropylmethyl radical can engage further in carbon−carbon bond-forming processes. The allylsulfide moiety allows for the addition of radicals with concomitant release of a thiyl radical, and very elegant processes using only substoichiometric amounts of a source of thiyl radicals have been developed for the rearrangement of vinylcyclopropanes (see section 5.2.1.e). In particular, under nonreducing conditions and in the presence of an external olefin, efficient annulation reactions have been achieved, giving access to polycyclic compounds. The carboncentered radicals generated by the thiol-mediated ring-opening Scheme 49 Scheme 50 Chemical Reviews Review dx.doi.org/10.1021/cr400441m | Chem. Rev. XXXX, XXX, XXX−XXX Y of vinylcyclopropanes could also be trapped to form a new carbon−heteroatom bond. Here again, annulations taking advantage of the allylsulfide moiety have been developed (see section 5.2.1.e). Landais, Renaud, and co-workers used vinyl cyclopentenes such as 108, easily prepared by monocyclopropanation of silylcyclopentadienes, as radical acceptors for photogenerated thiyl radicals. The reversible addition of the thiyl radical onto the CC bond of 108 leads eventually to cyclopropylcarbinyl radical 110, which undergoes fragmentation to give carboncentered radical 111, stabilized by the neighboring ester group. Hydrogen atom abstraction from the thiol then furnishes cyclopentene 109 and regenerates a thiyl radical that propagates the chain (Scheme 51, eq a). The addition of the thiyl radical at the β-carbon center takes place in a highly stereoselective manner, opposite to the bulky silyl group. The fate of the stabilized carbon-centered radical resulting from the fragmentation process depends upon the reaction conditions. For instance, Naito and co-workers reported the use of vinylcylopropyl oxime ethers such as 112 in domino reactions promoted by a thiol or a disulfide in the presence of triethylborane. The ring-opening of the cyclopropyl moiety is initiated by addition of a thiyl radical onto the terminal position of vinylcyclopropyl oxime ether 112. The stabilized carboncentered radical resulting from the fragmentation process reacts with triethylborane to form a boryl enamine 115 (Scheme 51, eq b). Depending on the reaction conditions, the latter can engage further in a radical oxygenation process, leading eventually to α-hydroxy oxime ether 113 after reduction of peroxyl radical 116 by the thiol (Scheme 51, eq b). Alternatively, 113 can react with aldehydes in an ionic aldol process to give β-hydroxy oxime ethers in a stereoselective manner, as illustrated by the preparation of 117 from 112 (Scheme 51, eq c). In the aforementioned reactions, the allylsulfide moieties generated upon addition of a thiyl radical onto the vinylcyclopropane unit remain intact at the end of the reaction. However, radical reactions taking advantage of the fragmentation of allyl sulfides upon addition of radical species are also well documented. Some examples of intermolecular additions, as well as cyclization and annulation processes, will be described in section 5.2.1.e. 3.3.2.b. Rearrangement and Cyclization of Nonconjugated Dienes. In the addition of thiyl radicals onto nonconjugated dienes, the CC bonds can either react independently or lead to rearrangements through intramolecular trapping of the carbon-centered radical generated in the initial addition step. In many cyclic dienes, addition occurs selectively at the more strained double bond, and products resulting from rearrangements are often observed. For example, the addition of thiophenol to 5-methylene-norbornene led to the exo addition products 118 and 119, together with tricyclic adduct 120. The latter results from the rearrangement of homoallyl radical intermediate 121 into cyclopropylcarbinyl radical 122 (Scheme 52). Similar rearrangements have been observed in norbornadiene derivatives where substitution at C-7 can influence facial selectivity, while substitution of the methylene bridge in 7,7-dimethylnorbornene proved to have no effect in directing the addition of thiophenol. The formation of cyclopropylcarbinyl radical intermediates in norbornadiene derivatives can also lead to other skeletal rearrangements, as illustrated by the addition of thiophenol to hexachloronorbornadiene 123, which results in the formation of 125, beside the expected 1:1 addition product 124 (Scheme 53, eq a). Following addition of the thiyl radical, presumably from the less hindered endo-face, and subsequent 3-exo-trig cyclization onto the neighboring CC bond, cyclopropylcarbinyl radical 126 undergoes fragmentation to give the more stable α-chlorosubScheme 51 Scheme 52 Chemical Reviews Review dx.doi.org/10.1021/cr400441m | Chem. Rev. XXXX, XXX, XXX−XXX Z stituted carbon-centered radical 127. The latter then abstracts a hydrogen atom from the thiol to give 125 (relative configuration not established) and a thiyl radical, which goes on to propagate the chain. Similar rearrangement was observed i n t h e add i t i on o f t BuSH on to 1 , 2 , 3 , 4 , 7 , 7 hexamethylbicyclo[2.2.1]heptadiene. Likewise, Hodgson and co-workers have observed complete skeletal rearrangements in the addition of thiophenol to 7-azabicyclo[2.2.1]heptadienes such as 128 (Scheme 53, eq b). Transa

Femtosecond-laser ablation dynamics of dielectrics: basics and applications for thin films

Abstract: Laser ablation of dielectrics by ultrashort laser pulses is reviewed. The basic interaction between ultrashort light pulses and the dielectric material is described, and different approaches to the modeling of the femtosecond ablation dynamics are reviewed. Material excitation by ultrashort laser pulses is induced by a combination of strong-field excitation (multi-photon and tunnel excitation), collisional excitation (potentially leading to an avalanche process), and absorption in the plasma consisting of the electrons excited to the conduction band. It is discussed how these excitation processes can be described by various rate-equation models in combination with different descriptions of the excited electrons. The optical properties of the highly excited dielectric undergo a rapid change during the laser pulse, which must be included in a detailed modeling of the excitations. The material ejected from the dielectric following the femtosecond-laser excitation can potentially be used for thin-film deposition. The deposition rate is typically much smaller than that for nanosecond lasers, but film production by femtosecond lasers does possess several attractive features. First, the strong-field excitation makes it possible to produce films of materials that are transparent to the laser light. Second, the highly localized excitation reduces the emission of larger material particulates. Third, lasers with ultrashort pulses are shown to be particularly useful tools for the production of nanocluster films. The important question of the film stoichiometry relative to that of the target will be thoroughly discussed in relation to the films reported in the literature.

Study on the surface energy of graphene by contact angle measurements.

Abstract: Because of the atomic thinness of graphene, its integration into a device will always involve its interaction with at least one supporting substrate, making the surface energy of graphene critical to its real-life applications. In the current paper, the contact angle of graphene synthesized by chemical vapor deposition (CVD) was monitored temporally after synthesis using water, diiodomethane, ethylene glycol, and glycerol. The surface energy was then calculated based on the contact angle data by the Fowkes, Owens–Wendt (extended Fowkes), and Neumann models. The surface energy of fresh CVD graphene grown on a copper substrate (G/Cu) immediately after synthesis was determined to be 62.2 ± 3.1 mJ/m2 (Fowkes), 53.0 ± 4.3 mJ/m2 (Owens–Wendt) and 63.8 ± 2.0 mJ/m2 (Neumann), which decreased to 45.6 ± 3.9, 37.5 ± 2.3, and 57.4 ± 2.1 mJ/m2, respectively, after 24 h of air exposure. The ellipsometry characterization indicates that the surface energy of G/Cu is affected by airborne hydrocarbon contamination. G/Cu ex...