Li Fragments Research Articles

A TD-DFT study of the excited dissociative electronic states of the DNA nucleobases bound to Li+

In this work, the first and second dissociative potential curves of adenineLi+ (ADLi+), guanineLi+ (GUALi+), cytosineLi+(CYTLi+), and thymineLi+ (THYLi+) complexes, related to the dissociation of their LiO and LiN bonds, have been calculated in the gas phase and water, separately. For this purpose, the fifteen excited potential curves, in order of increasing energy, were calculated for each complex and the dissociative potential curves were distinguished from them considering the conical intersection points. The time-dependent density functional theory (TD-DFT) method employing the M06-2X functional was used for the calculations. It was observed that the electron transfer from the DNA base to the Li+ took place during the dissociation of complexes in the gas phase. The electrostatic field of water blocked this charge transfer and led to the excited DNA base and Li+ in its ground state. The vertical excitation energy for the desorption of the Li fragment as a neutral and cation species from the DNA bases was determined. The effect of the interaction site of Li+ on the dissociative potential curves was also investigated.

Study of11Li and10,11Be nuclei through elastic scattering and breakup reactions

The hybrid model of the microscopic optical potential (OP) is applied to calculate the 11 Li+p , 10,11 Be+p , and 10,11 Be+12 C elastic scattering cross sections at energies E Li+p elastic scattering, the microscopic large-scale shell model (LSSM) density of 11 Li is used, while the density distributions of 10,11 Be nuclei obtained within the quantum Monte Carlo (QMC) model and the generator coordinate method (GCM) are utilized to calculate the microscopic OPs and cross sections of elastic scattering of these nuclei on protons and 12 C. The depths of the real and imaginary parts of OP are fitted to the elastic scattering data, being simultaneously adjusted to reproduce the true energy dependence of the corresponding volume integrals. Also, the cluster models, in which 11 Li consists of 2n -halo and the 9 Li core having its own LSSM form of density and 11 Be consists of a n -halo and the 10 Be core, are adopted. Within the latter, we give predictions for the longitudinal momentum distributions of 9 Li fragments produced in the breakup of 11 Li at 62 MeV/nucleon on a proton target. It is shown that our results for the diffraction and stripping reaction cross sections in 11 Be scattering on 9 Be, 93 Nb, 181 Ta, and 238 U targets at 63 MeV/nucleon are in a good agreement with the available experimental data.

Complete and incomplete fusion of weakly bound nuclei

Existing quantum mechanics methods to study fusion reactions with weakly bound nuclei cannot evaluate separately the complete fusion and the incomplete fusion cross sections. We develop a semiclassical procedure that can calculate these cross sections, apply it to $^{6,7}\mathrm{Li}+^{209}\mathrm{Bi}$ collisions at energies just above the barrier, and show that its predictions for the different fusion cross sections are in good agreement with the data. We find that the contribution from the sequential fusion of the Li fragments to the complete fusion cross section is substantial in the case of $^{6}\mathrm{Li}$, reaching almost 40% of that from the direct process, which illustrates the importance of calculating correctly the different components of the fusion cross section.

Interaction of11Li with208Pb

Background: ${}^{11}$Li is one of the most studied halo nuclei. The fusion of ${}^{11}$Li with ${}^{208}$Pb is the subject of a number of theoretical studies with widely differing predictions, ranging over four orders of magnitude, for the fusion excitation function.Purpose: The purpose was to measure the excitation function for the ${}^{11}$Li $+$ ${}^{208}$Pb reaction.Methods: A stacked foil and degrader assembly of ${}^{208}$Pb targets was irradiated with a ${}^{11}$Li beam producing center-of-target beam energies from above-barrier to near-barrier energies (40--29 MeV). The intensity of the ${}^{11}$Li beam (chopped) was 1250 particles/s and the beam on-target time was 34 h. The $\ensuremath{\alpha}$ decay of the stopped evaporation residues (EVRs) was detected in an $\ensuremath{\alpha}$-detector array at each beam energy in the beam-off period (the beam was on for $\ensuremath{\le}$5 ns and then off for 170 ns).Results: The observed nuclidic yields of ${}^{212/215}$At and ${}^{214}$At are consistent with being produced in the complete fusion of ${}^{11}$Li with ${}^{208}$Pb. The observed yields of ${}^{213}$At appear to be the result of the breakup of ${}^{11}$Li into ${}^{9}\text{Li}+2n$, with the ${}^{9}$Li fusing with ${}^{208}$Pb. The magnitudes of the total fusion cross sections are substantially less than most theoretical predictions.Conclusions: It is possible to measure the EVR production cross sections resulting from the interaction of ${}^{11}$Li with ${}^{208}$Pb using current-generation radioactive beam facilities. Both complete fusion and breakup fusion processes occur in the interaction of ${}^{11}$Li with ${}^{208}$Pb. An important breakup process leads to the fusion of the ${}^{9}$Li fragment with ${}^{208}$Pb.

Measurements of total and partial charge-changing cross sections for 200- to 400-MeV/nucleonC12on water and polycarbonate

We have studied charged nuclear fragments produced by 200- to 400-MeV/nucleon carbon ions, interacting with water and polycarbonate, using a newly developed emulsion detector. Total and partial charge-changing cross sections for the production of B, Be, and Li fragments were measured and compared with both previously published measurements and model predictions. This study is of importance for validating and improving carbon-ion therapy treatment planning systems and for estimating the radiological risks for personnel on space missions, because carbon is a significant component of galactic cosmic rays.

Selective Syntheses of Homo- and Hetero-dimetal Complexes with the Tetramethyltetraazaannulene Ligand of the Type [(ML,M‘L‘)(TMTAA)], Where M and M‘ Are Rh(I) or Ir(I) and L and L‘ Are COD or (CO)2

The tetraazamacrocycle, tmtaaH2, reacts with [M2(COD)2(μ-OH)2], M = Rh or Ir, to give [M(tmtaaH)(COD)], which give the dicarbonyl derivatives, [M(tmtaaH)(CO)2], on exposure to CO. The COD and dicarbonyl derivatives, M = Rh, are methylated with MeOTf at the β-carbon site of the imidinate ring, giving [Rh(tmtaaHMe)(L2)]+, where L2 = COD or (CO)2. The crystal structure of [Rh(tmtaaHMe)(CO)2][OTf] shows that the site of methylation is the imidinate ring that contains the Rh(CO)2 fragment. Protonation by HOTf also occurs at the β-carbon site of the imidinate ring, assumed to be the ring that contains the Rh(L2) fragment. The dicarbonyls are deprotonated by LiN(SiMe3)2 in thf, giving [M(CO)2(tmtaa)Li(thf)]. A crystal structure of M = Rh shows an intramolecular Li−Rh distance of 2.635(10) Å and an intermolecular Rh···Rh contact distance between two molecular units of 3.198(1) Å that align along their molecular z-axis. Addition of MeI to [M(CO)2(tmtaa)Li(thf)] yields [M(tmtaaMe)(CO)2], where the methyl group is attached to the imidinate ring that contains the Li(thf) fragment, as shown by X-ray crystallography. The [M(tmtaaH)(CO)2] reacts with half an equivalent of [M2(COD)2(μ-OH)2] to give the mixed dimetal complexes [M(CO)2(tmtaa)M‘(COD)], where M,M‘ is either Rh,Rh or Rh,Ir, which react with CO to give [MM‘(tmtaa)(CO)4].

B Cells Need Cleaved Li to Differentiate

The li chain is well characterized as an invariant subunit of the major histocompatibility complex (MHC) II in antigen-presenting cells. Matza et al. expand on earlier results to propose a mechanism for how li participates in B cell maturation. Matza et al. transfected human embryonic kidney 293 cells with green fluorescent protein (GFP)-tagged or epitope-tagged li proteins or protein fragments and showed that the localization of the tag varied depending on whether the tag was fused to the cytosolic NH 2 -terminus or to the intralumenal COOH-terminus. NH 2 -tagged proteins showed a cytosolic distribution for the tag, whereas COOH-tagged proteins appeared membrane-bound and associated with the endosomes. All of the fusion proteins had transmembrane domains and could be found in the membrane fraction. Western blot analysis with an antibody against the li cytosolic domain showed that the cytosolic domain was cleaved. This cleavage product was also detectable in primary B lymphocytes. Mutation of a previously identified cleavage site in the transmembrane domain of li prevented cleavage; mutation of a "d box" consensus degradation sequence stabilized the cleaved li fragment. By using an NF-κB reporter construct, expression of the li cleavage product (either as just the fragment predicted to be produced by cleavage or as a transmembrane-containing cleavable fragment) stimulated reporter gene expression; whereas expression of the mutant that could not be cleaved did not stimulate reporter gene expression. Finally, in a B cell maturation model, expression of the cleavable or the cleavable and stabilized (mutated d box) forms of li stimulated B cell maturation; whereas the noncleavable mutant did not stimulate B cell maturation. Thus, B cell maturation appears to involve the intramembrane cleavage of li to produce a fragment capable of stimulating transcription through NF-κB. D. Matza, A. Kerem, H. Medvedovsky, F. Lantner, I. Shachar, Invariant chain-induced B cell differentiation requires intramembrane proteolytic release of the cytosolic domain. Immunity 17 , 549-560 (2002). [Online Journal]

Fragmentation of 200 and 244 MeV/u carbon beams in thick tissue-like absorbers

Stacks consisting of thin CR-39 sheets sandwiched between thick lucite and water absorbers were perpendicularly bombarded by 12C ions at 200 and 244 MeV/u. Track radius distributions representing the charge composition of the fragmented beams were automatically measured by a particle track analysis system. After analysis of the nuclear charge distributions, the total charge removal cross-sections and elemental production cross-sections of fragments with atomic numbers from 5 to 3, were obtained down to the lower energies (∼50 and 100 MeV/u, respectively). It has been found that the measured total charge removal cross-section agrees with theoretical predictions within ∼10% and very well with previous experiments in corresponding energy regions. Two model calculations for production of B fragment are in good agreement with our measured data while a third model overestimates it by ∼12%. Theoretical cross-sections for Be and Li fragments differ strongly among the different models and from measured values.

Core-breakup reactions of the halo nuclei 11Be and 11Li: momentum distributions and shadow effects

The halo nuclei 11Be and 11Li have been studied in core-breakup reactions where the halo neutrons are expected to be released without a major distortion due to the reaction. The widths of the halo-neutron momentum distributions have been extracted in coincidence with He fragments, Γ = 32 ± 4 MeV/ c, and Li fragments, Γ = 42 ± 4 MeV/ c for 11Be and with He fragments, Γ = 42 ± 6 MeV/ c for 11Li. The 11Be breakup gives a very low neutron multiplicity of 0.38±0.09 which is a manifestation of the shadowing of the neutron in the core-breakup reaction. This value can be understood from a simple theoretical calculation, which also accounts for the observed transverse momentum widths at small angles.

Which sort of particles was emitted earlier during decay of highly excited nuclei?

Emission orders of light charged particles and Li fragments from highly excited fissioning nuclei in the reaction of40Ar +197Au atEbeam/A = 25 MeV have been studied by measuring difference velocity distributions of two correlated particles at small relative angles in coincidence with two fission fragments. By comparing the data with three-body trajectory calculations, it is found that high-velocity light particles (tritons and He particles) are emitted prior to high-velocity Li fragments but low-velocity light particles are emitted after low-velocity Li fragments. On the other hand, no preferential emission was observed among light particles, such as deuterons and tritons.

Emission order of light particles and light fragments from hot nuclei followed by binary fission

Emission orders of light charged particles and Li fragments from highly excited fissioning nuclei in the reaction of 40Ar+197Au at E beam /A=25 MeV have been studied by measuring difference velocity distributions of two correlated particles at small relative angles in coincidence with two fission fragments. By comparing the data with three-body trajectory calculations, we found that high velocity deuterons are emitted prior to high velocity Li fragments but low velocity deuterons are emitted after low velocity Li fragments. On the other hand, no preferential emission was observed among light particles, such as protons and deuterons. Furthermore, the emission orders are found to depend only weakly on the mass asymmetry of fission fragments.

Momentum distributions of 9Li fragments from the breakup of 11Li and the neutron halo.

The inclusive parallel momentum distributions of {sup 9}Li fragments from the breakup of a secondary {sup 11}Li beam have been measured at 66 MeV/nucleon over a wide range of targets ({sup 9}Be to {sup 238}U). The measurements were performed using a zero-degree fragment separator as an energy-loss spectrometer operating in a dispersion matched mode. Earlier measurements have been extended here to the case of breakup by {sup 238}U where a distribution with a width of FWHM=38.1{plus_minus}1.9 MeV/{ital c} was observed. Together with a remeasurement of breakup on {sup 93}Nb (FWHM)=42.8{plus_minus}2.6 MeV/({ital c}) a weak dependence of distribution width on target nucleus was observed. A discussion of the nature of the momentum distributions and relationship to the structure of the halo is presented in the light of recent calculations. It is concluded that the {sup 9}Li fragment parallel momentum distributions are relatively insensitive to the reaction and reflect the extended neutron distribution of the halo.

Li-Li azimuthal angular correlations: A test for emission from a rotating source versus instantaneous multifragmentation.

Azimuthal angle correlations have been measured for Li-Li pairs from $^{40}\mathrm{Ar}$ + $^{197}\mathrm{Au}$, $^{\mathrm{nat}}\mathrm{Ag}$, $^{\mathrm{nat}}\mathrm{Cu}$, $^{27}\mathrm{Al}$ (17A, 27A, and 34A MeV). Many of these correlations exhibit enhancements at \ensuremath{\Delta}cphi of 0\ifmmode^\circ\else\textdegree\fi{} and 180\ifmmode^\circ\else\textdegree\fi{}, the classical pattern for evaporation from a hot, high-spin source. A very different pattern is predicted by a simple multifragmentation model, i.e., a peak at \ensuremath{\Delta}cphi\ensuremath{\approxeq}60\ifmmode^\circ\else\textdegree\fi{}. This peak is driven by the rapid Coulomb explosion of a nonrotating nucleus. The latter pattern is not observed experimentally, however, if collective rotation is included in the multifragmentation model, its predictions are more consistent with the observations. Such comparisons can give a promising test for sequential emission from a rotating source versus instantaneous explosive multifragmentation, but one needs a very good selection of collision centrality to reduce the role of the collective rotation. For most of these data the dominant driving forces seem to be rotational motion perturbed by final-state Coulomb repulsions for time delays of the order of ${10}^{\mathrm{\ensuremath{-}}22}$ s between successive emissions of Li fragments.

Transverse momentum correlations between particle-particle pairs as a probe of effective emitter mass: 40Ar+natAg (E/A=7, 17, 27, and 34 MeV).

Average transverse momenta have been carefully measured for \ensuremath{\alpha}-particle pairs at laboratory angles of 21\ifmmode^\circ\else\textdegree\fi{}, 31\ifmmode^\circ\else\textdegree\fi{}, 47\ifmmode^\circ\else\textdegree\fi{}, and 67\ifmmode^\circ\else\textdegree\fi{}. They exhibit small but distinct shifts for different relative azimuthal angles \ensuremath{\Delta}cphi. Detection on the same side of the beam (\ensuremath{\Delta}cphi\ensuremath{\approxeq}24\ifmmode^\circ\else\textdegree\fi{}) yields a smaller average tranverse momentum or energy than on the opposite side (\ensuremath{\Delta}cphi\ensuremath{\approxeq}168\ifmmode^\circ\else\textdegree\fi{}). For \ensuremath{\alpha}-\ensuremath{\alpha} pairs detected at \ensuremath{\approxeq}67\ifmmode^\circ\else\textdegree\fi{} to the beam these small shifts are analyzed with a recoil model to extract the average emitter mass for the first \ensuremath{\alpha} particle from the coincident pairs. The results are A=130\ifmmode\pm\else\textpm\fi{}10 to 115\ifmmode\pm\else\textpm\fi{}10 for reactions of $^{107,109}\mathrm{Ag}$ with $^{40}\mathrm{Ar}$ for 7A and 34A MeV, respectively. The particle pairs emitted at 21\ifmmode^\circ\else\textdegree\fi{} are produced from more complex source mixtures. There is evidence from p-Li and \ensuremath{\alpha}-Li pairs that Li fragments are emitted after protons or \ensuremath{\alpha} particles in \ensuremath{\approxeq}1/2 of the events. The overall pattern is consistent with incomplete fusion leading to the formation of a hot, essentially thermalized composite nucleus and its associated statistical decay.

The B3Li molecule’s electronic and geometrical structure

The geometries, electronic energies, dipole moments, and local harmonic vibrational frequencies of the singlet ground and low-energy singlet valence excited states of B3Li have been investigated using (i) correlation-consistent double-zeta plus polarization basis sets, (ii) small valence-space complete active space multiconfiguration self-consistent field electronic wave functions, and (iii) analytical energy derivative techniques combined with automated potential energy surface ‘‘walking’’ algorithms. The lowest-energy structure of B3Li is found to have C3v symmetry and to consist of a Li atom that sits above the center of an equilateral B3 moiety; the dipole moment of this species is 6.7 D and its dissociation energy (not zero point corrected) to ground state B3+Li is 60 kcal/mol. The B3H analog of this C3v structure is not a geometrically stable species for reasons that are analyzed here. A second stable structure (with dipole moment of 8.5 D and dissociation energy of 47 kcal/mol) is planar and has the Li atom σ bonded to one of the B atoms in the triangular B3 group. The B3H analog of this σ-bonded species is stable and, in fact, is the lowest-energy structure of B3H. Finally, in contrast to findings for B3H, no bridge-bonded structure was found to be stable for B3Li. Both the σ-bonded and C3v structures of B3Li are shown to possess significant charge–transfer character near their equilibrium geometries, but to dissociate to ground state neutral B3+Li fragments.

Evolution of fragment-fragment correlations in reactions of 197Au and 107,109Ag with 40Ar from 7A to 34A MeV.

In-plane and out-of-plane angular correlations have been measured between fragments of Z>3, Li fragments, $^{3,4}\mathrm{He}$, and $^{1,2,3}\mathrm{H}$. The changing patterns for $^{40}\mathrm{Ar}$ induced reactions of 7A, 17A, 27A, and 34A MeV give an overview of the decreasing importance of mass-symmetric fissionlike reactions at the expense of a broad range of more mass-asymmetric breakups. Evidence is given that these fragments come from a central collision group of reactions that have similar violence and from which many combinations of fragments and particles are ejected. Very similar azimuthal angular correlations are observed for particles with a Li fragment and for particles with a pair of heavier fragments (Z>3). This similarity suggests comparable strengths of association with the reaction plane for single Li fragments and for fragment pairs of Z>3. Azimuthal angular correlations for Li-Li pairs exhibit distinct asymmetries; their interpretation via trajectory-model calculations indicates mean delay times of \ensuremath{\approxeq}5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}22}$ s.

Longitudinal collective motion in intermediate-energy heavy-ion collisions

Light charged fragments from the reactions 40 Ar + 51 V at 35 MeV/nucleon and 12 C + 12 C at 50 MeV/nucleon have been measured with the MSU 4 π Array. The longitudinal component of the directed collective motion is extracted from both data sets. This component is observed for p, d, t, He and Li fragments and apparently decreases in strength for the two heaviest fragments, in contrast to the transverse component which has been found to increase with fragment mass. At least part of the decrease in the strength of the longitudinal component for heavier fragments is due to the low-energy thresholds of the detectors.

Inclusive angular distribution of alpha and Li fragments produced in Fe-C and Fe-Pb collisions at 1.88 GeV/nucleon.

From the measurements of laboratory system emission angles \ensuremath{\theta} of 2188\ensuremath{\alpha} and 298 Li fragments, produced inclusively by relativistic Fe-C Fe-Pb collisions, a target-independent differential frequency formula, dN=exp(a+b cot\ensuremath{\theta})d(cot\ensuremath{\theta}) is obtained with the constant b\ensuremath{\simeq}-0.026 at 1.88 GeV/nucleon, which seems also to be independent on the kinds of projectile fragments. The significance of this approximate formula is discussed in relation mainly with Kaplon's formula.

Nuclear Fragmentation of Clinical Silicon Beam

Heavy ions used for biomedical studies fragment when they pass through matter. It is known that different fragments cause different types of biological damage. In this paper, results pertaining to different kinds of secondary fragments produced by a 28Si beam at 463 MeV/nucleon in its interaction with nuclear emulsion are presented. It is observed that the production frequencies of secondary fragments are independent of the target. Partial production cross sections and fluences of the secondary fragments are also discussed. The partial production cross sections for different fragments at three lower energy ranges are practically the same within their statistical errors except for the cross section for Li fragments, which is larger by almost a factor of 1.5 at the lowest energy investigated. The dose contributions of the primary Si beam and its components at different points of the Bragg curve are presented for two different energy regions.

Phosphine complexes of main group elements: crystal structure of {Li[C(PMe2)3]-(tetrahydrofuran)}2

The X-ray structure determination of the title compound, obtained by recrystallisation of LiC(PMe2)3 from pentane–tetrahydrofuran (thf) as colourless crystals, reveals it to be dimeric with two nearly planar (Me2P)3C-moieties linked by (thf)Li fragments via chelating and additional π-type interactions.