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21.
Mahmoud Mirmehrabi Sohrab Rohani Michael C. Jennings 《Acta Crystallographica. Section C, Structural Chemistry》2005,61(12):o695-o698
The crystal structure of the title compound (systematic name: 2′,3′‐didehydro‐2′,3′‐deoxythymidine), C10H12N2O4, consists of two molecules in the asymmetric unit bound together by hydrogen bonds. The conformational geometry differentiates this form of stavudine from its two previously published polymorphs. In addition, a different hydrogen‐bonding scheme is observed compared with the previous two structures. This polymorph is the thermodynamically most stable form of the antiviral drug, as evidenced by differential scanning calorimetry (DSC) and IR data. 相似文献
22.
Sohrab Behnia Amin Jafari Farzan Ghalichi Ashkan Bonabi 《Central European Journal of Physics》2008,6(2):211-222
In this paper, the finite-element method (FEM) simulation of ultrasound brain surgery is presented. The overheating problem
of the post-target bone, which is one of the limiting factors for a successful ultrasound brain surgery, is considered. In
order to decrease bone heating, precise choices of frequency, focal pressure, and scanning path are needed. The effect of
variations in the mentioned scanning parameters is studied by means of the FEM. The resulting pressure and temperature distributions
of a transdural ultrasound brain surgery are simulated by employing the FEM for solving the Helmholtz and bioheat equations
in the context of a two-dimensional MRI-based brain model. Our results show that for a suitable value of the frequency, an
increase in focal pressure leads to a decrease in the required duration of the treatment and is associated with less heating
of the surrounding normal tissue. In addition, it is shown that at a threshold focal pressure, the target temperature reaches
toxic levels whereas the temperature rise in the bone is minimal. Wave reflections from sinus cavities, which result in constructive
interference with the incoming waves, are one of the reasons for overheating of the bone and can be avoided by choosing a
suitable scanning path.
相似文献
23.
A series of graft copolymers were synthesized based on ethylene‐co‐m,p‐methylstyrene (EMS) (backbone copolymer), ethylene‐1‐hexene‐m,p‐methylstyrene (EHMS) (backbone terpolymer), and polyethylene glycol monomethyl ethers (PEGM) (grafts) in this study. The PEGMs with molecular weights of 750 and 2000 were used. The chemical composition of the graft copolymers was analyzed by NMR and DSC measurements. The graft copolymers exhibited a phase‐separated morphology with the backbone and the methoxy polyethylene glycol (MPEG) grafts forming separate crystalline phases. The MPEG phase had a melting temperature lower than the corresponding MPEG homopolymer, as determined by DSC. The melting point of the crystalline phase formed by the EMS and EHMS main chains was lower than that of pure polymer backbone. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
24.
In this article, we investigate globally exponentially attractive sets and chaos synchronization for a hyperchaotic system, namely, Lorenz–Stenflo system. For this system, two ellipsoidal globally exponentially attractive sets are derived based on generalized Lyapunov function theory and the extremum principle of function. Furthermore, we propose linear feedback control with a one, two, three, and four inputs to realize globally exponential synchronization of two four‐dimesional hyperchaotic systems using inequality techniques. Numerical simulations are presented to show the effectiveness of the proposed synchronization scheme. © 2014 Wiley Periodicals, Inc. Complexity 20: 30–44, 2015 相似文献
25.
The spin of a polarized neutron beam subjected to a partial projection in another direction, traces a geodesic arc in the
2-sphere ray space. We delineate the geometric phase resulting from two successive partial projections on a general quantal
state and derive the direction and strength of the third partial projection that would close the geodesic triangle. The constraint
for the three successive partial projections to be identically equivalent to a net spin rotation regardless of the initial
state, is derived.
相似文献
26.
Sohrab Behnia Amin Jafari Sojahrood Wiria Soltanpoor Okhtay Jahanbakhsh 《Ultrasonics sonochemistry》2009,16(4):502-511
Nonlinear dynamics of a spherical cavitation bubble was studied. A method based on applying a periodic perturbation to suppress chaotic oscillations is introduced. The relation between this method and dual frequency ultrasonic irradiation is correlated to prove its applicability in applications involving cavitation phenomena. Results indicated its strong impact on reducing the chaotic oscillations to regular ones. The governing parameters are the secondary frequency value and the phase difference between the secondary frequency and the fundamental one. In the end, the possible application of this method in high intensity focused ultrasound tumor ablation as an instance, is discussed accounting for both free bubbles and microbubbles. 相似文献
27.
Despite considerable work in the field, the precise mechanism for the photoisomerization of azobenzene, C(12)H(10)N(2), is still an open issue. Early theoretical studies of the problem indicated that isomerization occurs through an in-plane inversion path, and this has been used to explain recent time-resolved UV-visible spectroscopy measurements. On the other hand, a number of recent theoretical studies have concluded that a torsion of the N-N bond ("rotation path") is probably the most favorable mechanism for photoisomerization involving the first excited state. We have performed first-principles calculations using constrained density-functional theory (DFT) and time-dependent DFT in the local-density approximation, with results that also favor the rotation path mechanism. Our results are compared with other analyses, primarily based on configuration interaction. 相似文献
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30.
In this paper, epsilon and Ritz methods are applied for solving a general class of fractional constrained optimization problems. The goal is to minimize a functional subject to a number of constraints. The functional and constraints can have multiple dependent variables, multiorder fractional derivatives, and a group of initial and boundary conditions. The fractional derivative in the problem is in the Caputo sense. The constrained optimization problems include isoperimetric fractional variational problems (IFVPs) and fractional optimal control problems (FOCPs). In the presented approach, first by implementing epsilon method, we transform the given constrained optimization problem into an unconstrained problem, then by applying Ritz method and polynomial basis functions, we reduce the optimization problem to the problem of optimizing a real value function. The choice of polynomial basis functions provides the method with such a flexibility that initial and boundary conditions can be easily imposed. The convergence of the method is analytically studied and some illustrative examples including IFVPs and FOCPs are presented to demonstrate validity and applicability of the new technique. 相似文献