Both tetrahydrofuran(THF) and 2-methyltetrahydrofuran(MTHF) are studied systematically at desired temperatures using molecular dynamics simulations.The results show that the calculated densities are well consistent with experiment.Their glass transition temperatures are obtained:115K ～ 130K for THF and 131K ～ 142K for MTHF.The calculated results from the dipolar orientational time correlation functions indicate that the "long time" behavior is often associated with a glass transition.From the radial and spatial distributions,we also find that the methyl has a direct impact on the structural symmetry of molecules,which leads to the differences of physical properties between THF and MTHF.
The isospin effects of projectile fragmentation at intermediate energies are investigated using an isospin-dependent Boltzmann-Langevin model.The collisions of mass-symmetric reactions including 58Fe,58Ni+58Fe,and Ni at intermediate energies,in the 30 to 100 MeV/A range,are studied for different symmetry energies.Yield ratios of the isotopic,isobaric,and isotonic pairs of fragments from the intermediate-mass region using three symmetry energies are extracted as functions of the N/Z ratio of the composite systems in the entrance channel and the incident energies.It is found that the yield ratios are sensitive to symmetry energies,especially for neutron-rich systems,and the calculations using soft symmetry energy are closer to the experimental data.The isospin effect is stronger for the soft symmetry energy,owing to the competition of the repulsive Coulomb force and the symmetry energy attractive force on the proton.For the first time,the splits are presented,revealing a transition from the isospin equilibrium at lower energies to translucency at intermediate energies.The results show a degree of transparency in that intermediate mass fragments undergo a transition from dependence on the composite systems in the entrance channel to reliance on the projectile and target nuclei.
Optical and electrical properties of composites,prepared by filling of high density polyethylene(HDPE) with two kinds of multi-walled carbon nanotubes(MWNTs) differing in diameters,were explored by terahertz time-domain spectroscopy(THz-TDS) in the frequency range from 0.2 to 1.6 THz.It is found that composite with larger-diameter MWNTs possesses larger absorption coefficient and conductivity at the same concentration.The real part of the ac conductivity of the composites follows a power law dependence on frequency.And the power index is around 0.75 regardless of the MWNT concentration and diameter.The experimental data were analyzed with Cole-Cole equation under the assumption that the conductive clusters dispersed in the polymer matrix behave like dipoles and contribute mainly to the dielectric loss.It is found that both of the composites have similar values of relaxation time and distribution parameter.With increase of the MWNTs concentration,the relaxation time increases and tends to saturate at 0.7 ps after passing through the percolation threshold.
We propose an alternative way to constrain the density dependence of the symmetry energy from the neutron skin thickness of nuclei which shows a linear relation to both the isospin asymmetry and the nuclear charge with a form of Z2/3. The relation of the neutron skin thickness to the nuclear charge and isospin asymmetry is systematically studied with the data from antiprotonic atom measurement, and with the extended Thomas-Fermi approach incorporating the Skyrme energy density functional. An obviously linear relationship between the slope parameter L of the nuclear symmetry energy and the isospin asymmetry dependent parameter of the neutron skin thickness can be found, by adopting 70 Skyrme interactions in the calculations. Combining the available experimental data, the constraint of -20 MeV L 82 MeV on the slope parameter of the symmetry energy is obtained. The Skyrme interactions satisfying the constraint are selected.
The adenine–thymine base pair was studied in the presence of hydroxyl radicals in order to probe the hydrogen bond effect.The results show that the hydrogen bonds have little effect on the hydroxylation and dehydrogenation happened at the sites,which are not involved in a hydrogen bond,while at the sites involved in hydrogen bond formation in the base pair,the reaction becomes more difficult,both in view of the free energy barrier and the exothermicity.With a 6-311++G(d,p)level of description,both B3LYP and MP2 methods confirm that the C8 site of isolated adenine has the highest possibility to form covalent bond with the hydroxyl radicals,though with different energetics:B3LYP predicts a barrierless pathway,while MP2 finds a transition state with an energy of 106.1 kJ/mol.For the dehydrogenation reactions,B3LYP method predicts that the free energy barrier increases in the order of HN9
Molecular dynamics simulations are performed to investigate the polymorphism and flexibility of DNA in water,ethylene glycol(EG)and ethanol(EA)solutions.DNA in EG resembles the structure of DNA in water exhibiting B-DNA.In contrast,the DNA is an A-DNA state in the EA.We demonstrate that one important cause of these A$\leftrightarrow$B state changes is the competition between hydration and direct cation coupling to the phosphate groups on DNA backbones.To DNA structural polymorphism,it is caused by competition between hydration and cation coupling to the base pairs on grooves.Unlike flexible DNA in water and EA,DNA is immobilized around the canonical structure in EG solution,eliminating the potential biological effects of less common non-canonical DNA sub-states.
