中英文摘要格式

HPGe γ谱
HPGe γ谱摘
要 给出了HPGe γ 谱仪系统死时间校正的一种实用方法。

用强源干扰找到了HPGe γ
从而解决了放射性定量测量中的一些困难。

Experimental study on the method of deadtime
correction For HPGe γ spectrometer system
A practical method of deadtime correction for HPGe γ spectrometer system was given.The relation
between the percent deadtime and the correction of counting rate losses for HPGe γ
spectrometer system was found by using a strong γ source. Therefore ,the activity measurement can be finished at very high count
rate,with this γ
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α-取代甲苯定向硝化的理论研究
摘要
运用密度泛函理论(DFT), 研究了吸电子氟基和供电子羟基在取代甲苯的α-H 以后, 其邻、间、对各位次进行硝化反应的速控步骤, 在B3LYP/6-311G**水平上, 计算了该速控步骤基元反应各反应驻点(反应物、过渡态和中间体)的优化几何、电子结构和能量性质, 并首次给出了目标硝化反应速控步骤的IR 谱学的动态特征及解析, 从微观层面上验证了反应坐标C—N的形成和C—H的断裂是非协同的, 从而无一级动力学同位素效应的实验事实. 通过对目标硝化反应速控步骤的微观动态计算, 验证了氟基对甲基定位的影响. 氟基的电负性大, 吸电子能力强, 取代甲苯的α-H 以后对
硝酰阳离子的进攻有抑制作用, 活化能较取代前高, 但比较苄基氟各位次硝化活化能的相对大小得知, －CH2F 仍为邻、对位定向基团. 而供电子羟基取代甲苯的α-H 以后, 则对硝酰阳离子的进攻有促进作用, 因而各反应驻点络合物的稳定化能较α-H 取代前甲苯的有所增大, 且邻、对位硝化的活化能较间位低, 故－CH2OH 为邻、对位定位基. 但对位因硝化活化能低, 反应放热多, 空间位阻小, 为亲电试剂NO＋最有利的进攻位；而邻位则因羟基取代甲苯α-H 后多了2一个氧原子, 增大了邻位进攻的空间位阻, 使得其络合物的能量比相应对位的高.
关键词：苄基氟，苄醇，α-取代，取代基效应，速控步骤
Theoretical Study on Selectivity for Nitration of α-Substituted Toluene with Nitronium
Abstract
The rate-determining steps of isomeric ortho, meta and para nitrations of benzyl alcohol and benzyl fluoride have been theoretically investigated at B3LYP/6-311G** level. Stationary points of the step involving reactant, transition state and intermediate complexes were successfully located and characterized without any restriction on the internal coordinates. Their molecular geometries, electronic structures, IR spectra, and the FMO symmetries of two initial aromatic compounds and the nitronium ion
NO+have
2
been studied. The vibrational shifts of C—N and C—H stretches from TS to INT at the rate-determining step of target aromatic nitrations show up that the formation of C—N and the cleavage of C—H are not con-cerned but stepwise to provide an explanation, on a microscopic scale, for the experimental fact of the absence of kinetic isotopic effect in the nitration process for the first time. Also, the influence of introduced fluorine on the orientation effect was figured by calculating the microscopic and kinetic properties of the rate-determining step of concerned aromatic nitrations. The
NO+attack to the benzyl
2
fluoride was deactivated because of high electronegativity and strong electron withdrawal of fluorine atom. The activation energy af ter α-substitution of methyl group by a fluorine atom became higher. Yet the CH2F group was still an ortho-para directive, since the activation energies of ortho and para nitrations were relatively lower than that of meta nitration. Contrarily, the introduction of electron-donating substituent OH onto the methyl accelerated the
NO+attack. Consequently the stabilization energies of complexes were droped even 2
lower than before the α-substitution occurred. The activation energies of ortho and para nitrations, however, were also quite lower than the energy of meta nitration. Therefore, the CH2OH is an ortho-para directing group. Compared to the ortho, the para position is the most favorable due to the
NO+to approach, for its low steric hindrance effect and high
2
exothermicity. Attack at ortho, however, became even more crowd since one more O atom has been introduced after the α-substitution onto the methyl group. Hence, the energy of ortho nitrobenzyl alcohol complex is higher than the energy of para one.
Key words：benzyl fluoride; benzyl alcohol，α-substitution，substituent effect，rate-determining step。