2005 Vol. 18, No. 1

Letter
Catalytically-grown carbon nanofibers of two different conformations,fishbone and parallel types of the arrangement of carbon layers,were employed as the support of Pd-Pt metal catalysts for the hydrogenation of naphthalene to tetralin. The sulfur tolerance of the catalyst system was investigated with the addition of 0. 05% thiophene to the reactant of naphthalene in the process. The dispersion of Pd-Pt metal particles on the support was observed with a HREM and a pulsed hydrogen chemisorption method. The hydrogenation reaction of naphthalene was carried out in a CSTR at 250℃ and with the hydrogen pressure of 6 MPa. The results showed that the Pd-Pt catalyst supported on the carbon nanofibers was active in the process. The Pd-Pt metal catalyst supported on the parallel carbon nanofibers showed a higher sulfur tolerance than that on the fishbone carbon nanofibers. The reason may be attributed to their different conformations of the carbon layers,which leads to the different interaction of carbon layers with the supported metal particles.
Article
The pump-probe experiments for 1,3-dichlorobenzene in gas phase have been performed by femtosecond multiphoton ionization mass spectra at a pump pulse of 270 nm and a probe pulse of 810 nm by using our homebuilt femtosecond laser system. The models for the exponential process and quantum beat are presented to extract the photodissociation dynamics information from the data. A fitting and analyzing software which is programmed by graphical language LabVIEW has been developed for the experimental data. The Convolution, Levenberg Marquardt,and Deconvolution mathematics VIs of LabVIEW are used in this program to make the program much simpler than using other programming languages. The experimental data have been well fitted with this program. The results provide the lifetime of excited-state S1 and the frequency of quantum beat.
Predissociation lifetimes of totally 13 vibrational levels in the 1B2 state of jet-cooled CS2 have been determined by simulating the rotational band contours of the PHOFEX spectrum in the range of 209. 5 ~ 216 nm, where the PHOFEX spectrum was obtained by monitoring the laser-induced fluorescence of CS fragment via the Q band head excitation of(A 1Π,v' = 0)&(X 1Σ + ,v" = 0)transition. In order to investigate the influence of vibrational angular momentum quantum number K of the excited 1B2 ( 1Σ + U )state,the nozzle was heated to increase the hot-band transition intensities. The results indicate that,for levels with the same vibrational quantum number but different K numbers in the 1B2 ( 1Σ + u )state of CS2 ,the lifetime decreases with the increasing of quantum number K, suggesting that the dissociation could be accelerated by K number.
The variable cross feedback(VCF)method is proposed to control the nonlinear chaos system. The control principle is introduced,and the Peroxidase-Oxidase(PO)system is taken as a typical example to demonstrate the effectiveness of this method. The dynamical behaviors of the PO system controlled using the VCF method are given by computer numerical simulation. The results show that the return map of the system can be changed into the finite isolated points from the assemblage of discrete points having regular distribution and structure by appropriately
The effect of random long-range connections(shortcuts)on Ca2 + signal propagation in the coupled cell chain is investigated by using the Euler method. It is found when the first cell is subjected to the external stimuli,the firing activity is triggered and then its neighboring cells excited,then the Ca2 + signal may propagate along the chain helped by the shortcuts,indicating the shortcuts can enhance intercellular Ca2 + signal propagations significantly. In addition,it is also found that there is an optimal level of randomness where the shortcuts can enhance the regularity of Ca2 + oscillations in the whole system. These results suggest that shortcuts among the cells may play a constructive role in helping the cell information propagating.
The geometries,electronic states and energies of Alm Pn ( m + n = 2 ~ 6)neutral and anionic clusters have been investigated using the density functional theory(DFT)method of Becke’s three-parameter hybrid exchange functional with the nonlocal correlation of B3LYP. Structural optimization and frequency analyses are performed with the basis of 6-311G( d). The calculations predict the existence of a number of previously unknown isomers(i. e. ,Al3P_ and AlmPn ( m + n.5)). The calculations have also predicted that small AlP and(AlP)2 clusters adopt two-and three-dimensional structures characteristic of Si2 and Si4 clusters,while the structures of the larger AlP clusters are different completely from those of Sin clusters with the same electrons. The results show that the structures with the singlet have higher symmetries,while those with the doublet have lower symmetries. The vertical detachment energy of AlmPn ( m + n = 2 ~ 6)are also discused and the adiabatic electron affinities of AlmPn(m + n = 2 ~ 6)and also discussed at the same level. The results agree satisfactorily with the anion photoelectron spectroscopy of aluminum phosphide clusters reported recently by Gomez et al.
The structures of 1,3-substituted squaraine derivatives Sq1 ~ Sq12 were fully optimized by ab initio HF method with 6-31G* basis set level,and the electronic structures of Sq1 ~ Sq12 were also calculated. Based on the optimized structures,the electronic spectra were obtained by the CIS / 6-31G* method,which suggested that the max absorption wavelength mainly resulted from the electronic transition from HOMO to LUMO. The second nonlinear optical coefficients(β0)were calculated using ab initio CPHF method at 6-31G* basis set level and FF / AM1,FF / PM3,FF / MNDO,FF / MINDO3 methods. A systematic comparison between the results was carried out. It indicated that the second nonlinear optical coefficients were affected dramatically by the properties of five-membered hetero-rings. β0 can be enhanced by introducing pyrrole,thiazole and oxazole. The position of five-membered rings containing two heteroatoms which were connected with four-membered squaraine rings also affected β0 .
Geometries and combination energies are predicated at B3LYP / 6-31G(d)and MP2 / 6-31G(d)level for thymine-BH3 complexes and 5 geometries have been obtained. Then single point energy calculations using larger basis sets(6-311 + G(2df)and aug-cc-pVDZ)and vibrational analysis and natural bond orbital analysis are carried out on the 5 optimized conformers. The outcome indicates that the conformers with the boron atom combined with O directly are relatively stable ones,(a)and(b),with the combination energies of 90. 4 and 88. 0 kJ / mol (B3LYP / 6-31G(d),BSSE corrected). The fact is that the nitrogen atom offers electron to the empty atomic orbital of boron which produces the conformers(c)and(d). Only one conformer is found which is formed because two carbon atoms offer π electron to the empty orbital of boron. The charge transference exists in all the conformers. The combination energies have a good line relation with their charge transference. The calculated results show that when the complex forms their IR spectrum moved to the red side and the frequency shifts are relative to the stabilities of the complexes.
The buckling behavior of single-wall carbon nanotubes(SWCNTs)under compression is simulated by using the molecular dynamics method with Tersoff-Brenner potential to describe the interactions between atoms in SWCNT. The results show that the Young's modulus of SWCNTs decreases as the radius of SWCNTs increases,and critical stress and critical strain when the buckling of SWCNTs occurs are related to the slender ratio of SWCNTs. The difference of slender ratio determines two different buckling modes. The global buckling first happens for SWCNTs with the smaller slender ratio,while the local buckling first occurs for those with the larger slender ratio. The critical stress in the global buckling is proportional to the inverse of length of SWCNTs,while the critical stress in the local buckling is inversely proportional to the radius and the square of length of SWCNTs,which shows that the buckling theory of circular cylindrical shell in continuum mechanics can not be directly applied to the buckling of SWCNTs.
The static polarizabilities and polarizability anisotropies of Cun,Agn and Aun (n≤9)clusters have been calculated by the B3LYP density functional method,which is a three parameter mixture of density functional and"exact" HartreeFock exchange. The calculated results are compared with experimental polarizabilities of sodium clusters. It is shown that the size dependency of the static polarizabilities per atom of Cun,and Agn clusters possesses the same trend as that observed in sodium clusters exception of the Aunclusters while the polarizability of Au atom is much smaller than these of Cu and Ag.The(α-)of Au atom is the smallest and the(α-)per atom of Au approach to the values of Cu from the dimmer to the hexamer. It indicates that in Au clusters the electrons are more strongly attracted by the nuclei because of the more electrons. However,the absolute polarizabilities of the noble mental clusters are considerably smaller than those of the sodium clusters and the electronic structures of the noblemental are much more compact.
Molecular dynamics(MD)was performed to simulate and calculate the combination energy and static mechanical properties(i. e. elastic coefficient,modulus and poisson's ratio)of composite material,1,3,5-triamino-2,4,6-trinitrobenzene coated with polychlorotrifluoethylene(TATB / PCTFE). It is found that the intermolecular interaction especially H-bond is quite strong. The results show that the elastic properties of Fluorine-Polymer Bonded Explosive(PBX)have changed much compared to those of pure TATB. Its tensile modulus,bulk modulus and shear modulus are reduced evidently. The rigidity of PBX is lowered while the elasticity is increased,which manifests the mechanical property of PBX is improved greatly.
Thiophene polymers(C4H4S)n( n = 1 ~ 8)have been studied on B3LYP / 6-31G theory level. Vibrational frequencies,total energy(ET),zero point energy(ZPE),capacity(C0P),entropy(S0 )and energy gap(ΔE)was calculated based on the optimized equilibrium structures. By means of frequency analysis,the equilibrium structures were confirmed. The relationships between ET,ZPE,C0P,S0,ΔE and n were studied. The formation enthalpy of thiophene polymers was calculated with the total energies. The polymerized process and relative stability of the title compounds was determined according to the data of the formation enthalpy. The calculated results indicated that the thiophene molecular which has a planar structure exists in the stable chain polymers and the chain structures(C4H4S)n( n > 1)present different stabilities when n is an even or odd number. ZPE,C0P,and S0 show a linear increasing trend with n. The energy gaps ΔE show a direct proportion to n too,which indicated that the conduct property increases with n. The results also illustrated that the stepsize polymerized process is the primary way in the polymerized reactions.
The ground states of dimethyl siloxane under different intense electric fields ranging from - 0. 04 to 0. 04 a. u. are optimized using density functional theory DFT / B3P86 at 6-311 ++ G(d,p)level. The excitation energies and oscillator strengths under the same intense applied electric fields are calculated employing the revised hybrid CIS-DFT method. The result shows that the electronic state,molecular geometry,total energy,dipole moment and excitation energy are strongly dependent on the field strength and behave asymmetry to the direction of the applied electric field. As the electric field changes from - 0. 04 to 0. 04 a. u. ,the bond length of Si-O increases whereas the bond length of Si-C decreases because of the charge transfer induced by the applied electric field. The dipole moment of the ground state decreases linearly with the applied field strength. However,the dipole moment of molecule changes from positive to negative as the inverse electric field increase to - 0. 03 a. u. Further increase of the inverse electric field results in an increase of the total energy of the molecule. The dependence of the calculated excitation energies on the applied electric field strength is fitting well to the relationship proposed by Grozema. The excitation energies of the first five excited states of dimethyl siloxane decrease as the applied electric filed increases because the energy gap between the HOMO and LUMO become close with the field,which shows that the molecule is easy to be excited under electric field and hence can be easily dissociated.