同位素效应简介 - 360文档中心

同位素效应(Deuterium Kinetic Isotope Effects)

Subscriber access provided by NATIONAL CHUNG CHENG UNIVJournal of the American Chemical Society is published by the American ChemicalSociety. 1155 Sixteenth Street N.W., Washington, DC 20036 CommunicationDeuterium Kinetic Isotope Effects in Gas-Phase S2 andE2 Reactions: Comparison of Experiment and TheoryStephanie M. Villano, Shuji Kato, and Veronica M. BierbaumJ. Am. Chem. Soc., 2006, 128 (3), 736-737 • DOI: 10.1021/ja057491dDownloaded from on January 17, 2009More About This ArticleAdditional resources and features associated with this article are available within the HTML version:•Supporting Information•Links to the 1 articles that cite this article, as of the time of this article download•Access to high resolution figures•Links to articles and content related to this article•Copyright permission to reproduce figures and/or text from this articleDeuterium Kinetic Isotope Effects in Gas-Phase S N 2and E2Reactions:Comparison of Experiment and TheoryStephanie M.Villano,Shuji Kato,and Veronica M.Bierbaum*Uni V ersity of Colorado,Department of Chemistry and Biochemistry,215UCB,Boulder,Colorado 80309-0215Received November 2,2005;E-mail:Veronica.Bierbaum@The competition between nucleophilic substitution and base-induced elimination has been well-documented in the condensed phase;however,few studies have addressed this competition in the gas-phase.Studying S N 2and E2reactions in the gas-phase allows one to gain insight into intrinsic features that affect this competition without interference from solvent effects.However,these experiments are inherently difficult since these mechanisms generate the same ionic product and detection of neutral products presents serious analytical challenges.Several groups have investigated S N 2and E2reactions theoreti-cally;however due to the lack of experimental data,less work has addressed the competition between these two reactions.Hu and Truhlar 1have evaluated the reaction rate constants and deuterium kinetic isotope effects (KIE )k H /k D )for both the S N 2and E2pathways of the reaction of ClO -with C 2H 5Cl,shown in eq 1.Their work has provided us with the unique opportunity to compare theory and experiment.In this account we report experimental,overall reaction rate constants and deuterium KIEs for the reactions of RCl +ClO -(R )methyl,ethyl,isopropyl,and tert -butyl).This systematic approach indicates that,for the reaction of ClO -with C 2H 5Cl,both the S N 2and E2channels occur,as predicted by Hu and Truhlar;however the experimental KIE differs drastically from the computational predictions.There is extensive literature on gas-phase reactions that occur exclusively by S N 2or by E2mechanisms.(See refs 2-6and citations therein.)For reactions that can occur by both pathways,Gronert 7has provided an excellent summary of experimental approaches,8-13including his novel utilization of dianionic nucleo-philes that yield diagnostic product ions.14,15Our research has utilized reactivity trends 16as well as kinetic isotope effects 17to explore these processes.Kinetic isotope effects can be used to distinguish between S N 2and E2mechanisms if the reaction proceeds at a rate slower than the collision rate.18For E2reactions normal KIEs (k H /k D >1)are observed,while for S N 2reactions inverse KIEs (k H /k D <1)are observed.The origin of these effects is primarily a result of changes in vibrational frequencies as the reaction proceeds from the reactants to the transition state structure.19Deuteration lowers the vibrational frequencies in both the reactants and transition states relative to the undeuterated compounds.An E2reaction proceeds through a transition state where the C -H bond is partially broken thus lowering these frequencies relative to reactants.Since the frequen-cies for the undeuterated species are lowered more substantially than those for the deuterated species in the transition state,E H q <E D q .An S N 2reaction proceeds through a trigonal planar transition state where the C R -H bonds are tightened,thus raising these fre-quencies relative to reactants.Since the frequencies for the undeu-terated species are raised more substantially than those for the deu-terated species in the transition state,E H q >E D q .When both reaction pathways are viable an overall KIE is measured which provides qualitative insight into the competition between these mechanisms.Hu and Truhlar 1have quantitatively evaluated the competition between S N 2and E2pathways for ClO -with C 2H 5Cl and with C 2D 5Cl using dual-level generalized transition state theory and statistical calculations based on high-level,correlated electronic structure calculations using extended basis sets (MP2/ADZP).1Their calculations predict an S N 2reaction efficiency of 2%with a deuterium KIE of 0.60and an E2reaction efficiency of 26%with a deuterium KIE of 3.1at 300K.They predict an overall reaction efficiency of 28%and a KIE of 2.4.We have measured the overall reaction rate constants and KIE for the reactions of RCl +ClO -(R )methyl,ethyl,isopropyl,and tert -butyl)using a tandem flowing afterglow-selected ion flow tube instrument,FA-SIFT.20,21ClO -is an ideal choice for a nucleophile because the rate constants for the entire neutral series,methyl through tert -butyl,are within a measurable window of 10-9-10-13cm 3molecule -1s -1and are all below the calculated collision rate so that KIEs are evident.The reactant ion was formed in the source flow tube from electron impact on N 2O to produce O -which was then allowed to react with CCl 4to form ClO -.ClO -ions were mass-selected and injected into the reaction flow tube where they were thermalized to 304((2)K by collisions with He buffer gas (0.5Torr,∼104cm s -1).Neutral reactant flow rates were measured by monitoring the pressure change versus time in a calibrated volume system.Reaction rate constants were determined by introducing the neutral reagents at varying distances along the reaction flow tube,thereby varying the reaction distance and time.The 35ClO -intensity was monitored with a quadrupole mass filter coupled to an electron multiplier.Rate constants for the perprotio and perdeuterio reactions were measured sequentially on the same day under the same conditions.The error reported for the reaction rate constants is one standard deviation of three consecutive measurements.The uncertainty due to systematic error is (20%;however the systematic errors cancel in the rate ratio,and therefore,the KIEs are more accurately determined.Helium buffer gas (99.995%)was purified by passage through a molecular sieve trap at 77K.Neutral reagents were purchased from commercial sources and used without further purification;however,it was experimen-tally demonstrated that HCl,which would complicate the results,was not a significant contaminant.22Measured reaction rate constants and deuterium KIEs are reported in Table 1.The reaction of ClO -with methyl chloride can undergo only an S N 2reaction,and an inverse KIE of 0.85((0.01)was measured.This result is consistent with previous measurementsofPublished on Web 12/22/20057369J.AM.CHEM.SOC.2006,128,736-73710.1021/ja057491d CCC:$33.50©2006American Chemical Societyinverse KIEs for S N2reactions of methyl halides.23,24The reaction of ClO-with tert-butyl chloride presumably proceeds primarily by E2.16A KIE of2.31((0.12)was measured which is consistent with previously measured normal KIEs for E2reactions.5,25As the extent of substitution in the neutral reactants increases,the KIE effects become increasingly more normal.These results indicate that the E2pathway becomes the dominant channel as the neutral reagent becomes more sterically hindered.The experimental reaction efficiencies(10%for ClO-with C2H5Cl)26and KIE reported here for the reaction of ClO-with ethyl chloride differ from the theoretical values,suggesting that the S N2channel is more prominent than calculations predict.It is difficult to account for the discrepancies between experiment and theory;however nonstatistical dynamics27or errors in the calculation of the individual KIE or in the branching ratios of the two channels could account for the discrepancy.Hu and Truhlar’s treatment of the reaction between ClO-and ethyl chloride assumes that the reaction is statistical.Statistical theories are based on the assumption that energy is randomized among different modes as the reaction proceeds from reactants to the transition state.Nonstatistical effects have been observed for S N2reactions of monatomic nucleophiles with methyl halides,and these effects have been extensively documented in the liter-ature.23,24,28-32However the use of statistical theories has been successful for slightly larger systems.33-35Furthermore,Hu and Truhlar have previously calculated the KIE for the S N2reaction of solvated fluoride with methyl chloride using a statistical approach.36 Their results show excellent agreement with our published experi-mental values.37Thus the assumption that the ClO-+C2H5Cl reaction behaves statistically is reasonable.The individual KIE values,which Hu and Truhlar evaluated, appear quite reasonable.It can be argued that the errors in the branching ratio calculations are the primary cause for the large deviation from the experimental results.38If the predicted branching ratios were correct,the KIE for the E2channel would need to be 1.0or smaller or the KIE for the S N2channel would need to be 0.13or smaller to be consistent with the measured overall KIE. These values are not reasonable based on previous results.On the other hand assuming that the individual KIE are correct and that the discrepancy lies in the calculation of the branching ratios,we can use the calculated individual KIE along with the experimental overall rate constant and KIE to predict the rate constants for the S N2and E2channels to be1.10×10-10cm3molecule-1s-1and 1.15×10-10cm3molecule-1s-1,respectively,for ClO-+C2H5Cl. This analysis suggests that the calculated results overestimate the E2rate constant by a factor of∼6and underestimate the S N2rate constant by a factor of∼2.5.This magnitude of errors would occur if the E2barrier height was underestimated by only1kcal mol-1, while the S N2barrier height was overestimated by only0.6kcal mol-1.These small deviations in the activation enthalpy can have a large effect on the branching ratios.Determining activation enthalpies with an accuracy of0.5kcal mol-1or better is a difficult computational task.39These results illuminate the need for additional studies to describe nucleophilic substitution and elimination reactions more accurately. We hope that the discrepancy between experiment and theory will spark interest for further exploration.Acknowledgment.The authors thank Professors Charles DePuy and Barry Carpenter for valuable discussions and Professor Gustavo Davico for the initiation of the first experiment.This research was supported by the National Science Foundation(Grant No.CHE-0349937).References(1)Hu,W.P.;Truhlar,D.G.J.Am.Chem.Soc.1996,118,860.(2)Olmstead,W.N.;Brauman,J.I.J.Am.Chem.Soc.1977,99,4219.(3)Laerdahl,J.K.;Uggerud,E.Int.J.Mass Spectrom.2002,214,277.(4)Hase,W.L.Encyc.Mass Spectrom.2005,4,504.(5)de Koning,L.J.;Nibbering,N.M.M.J.Am.Chem.Soc.1987,109,1715.(6)Gronert,S.;Kass,.Chem.1997,62,7991.(7)Gronert,S.Acc.Chem.Res.2003,36,848.(8)Lieder,C.A.;Brauman,J.I.Int.J.Mass Spectrom.Ion Phys.1975,16,307.(9)Noest,A.J.;Nibbering,N.M.M.Ad V.Mass Spectrom.1980,8,227.(10)Bartmess,J.E.;Hays,R.L.;Kahatri,H.N.;Misra,R.N.;Wilson,S.R.J.Am.Chem.Soc.1981,103,4746.(11)Lum,R.C.;Grabowski,J.J.J.Am.Chem.Soc.1988,110,8568.(12)Jonas,M.E.;Ellison,G.B.J.Am.Chem.Soc.1989,111,1645.(13)Wladkowski,B.D.;Brauman,J.I.J.Am.Chem.Soc.1992,114,10643.(14)Gronert,S.;Pratt,L.M.;Mogali,S.J.Am.Chem.Soc.2001,123,3081.(15)Gronert,S.;Fagin,A.E.;Okamoto,K.;Mogali,S.;Pratt,L.M.J.Am.Chem.Soc.2004,126,12977.(16)DePuy,C.H.;Gronert,S.;Mullin,A.;Bierbaum,V.M.J.Am.Chem.Soc.1990,112,8650.(17)Gronert,S.;DePuy,C.H.;Bierbaum,V.M.J.Am.Chem.Soc.1991,113,4009.(18)Melander,L.;Saunders,J.H.,Jr.Reaction Rates of Isotopic Molecules;John Wiley and Sons:New York,1980.(19)Poirier,R.A.;Youliang,W.;Westaway,K.C.J.Am.Chem.Soc.1994,116,2526.(20)Van Doren,J.M.;Barlow,S.E.;DePuy,C.H.;Bierbaum,V.M.Int.J.Mass Spectrom.Ion Proc.1987,81,85.(21)Bierbaum,V.M.Encyc.Mass Spectrom.2003,1,98.(22)(CH3Cl99.9%,CD3Cl99.9%,C2H5Cl>99%,C2D5Cl98%,i-C3H7Cl99%,i-C3D7Cl99.4%,t-C4H9Cl99%,t-C4D9Cl99.1%).An HCl impurity would complicate the rate measurements due to a rapid proton-transfer pathway, which could not be distinguished from the S N2and E2channels.Therefore, 37Cl-was mass selected and allowed to react with the neutral reagents.We have previously shown[Van Doren,J.M.;DePuy,C.H.;Bierbaum, V.M.J.Phys.Chem.1989,93,1130]that the rates of these reactions are below the detection limits of our instrument;however,the Cl-+HCl exchange reaction proceeds at approximately half of the collision rate.The absence of35Cl-as a product ion indicates that HCl is not a significant contaminant.(23)Viggiano,A.A.;Morris,R.A.;Paschkewitz,J.S.;Paulsen,J.F.J.Am.Chem.Soc.1992,114,10477.(24)Kato,S.;Davico,G.E.;Lee,H.S.,DePuy,C.H.;Bierbaum,V.M.Int.J.Mass Spectrom.2001,210/211,223.(25)Bierbaum,V.M.;Filley,J.;DePuy,C.H.;Jarrold,M.F.;Bowers,M.T.J.Am.Chem.Soc.1985,107,2818.(26)Based on the collision rate of2.09×10-9cm3molecule-1s-1,calculatedusing the parametrized trajectory collision theory.[Su,T.;Chesnavich, W.J.J.Chem.Phys.1982,76,5183.](27)Baer,T.;Hase,W.L.Unimolecular Reaction Dynamics;Theory andExperiment;Oxford University Press:New York,1996.(28)Graul,S.T.;Bowers,M.T.J.Am.Chem.Soc.1994,116,3875.(29)Tonner,D.S.;McMahon,T.B.J.Am.Chem.Soc.2000,122,8783.(30)Sun,L.;Song,K.;Hase,W.L.J.Am.Chem.Soc.2001,123,5753.(31)Sun,L.;Song,K.;Hase,W.L.Science2002,296,875.(32)Angel,L.A.;Ervin,K.M.J.Am.Chem.Soc.2003,125,1014.(33)Chabinyc,M.L.;Craig,S.L.;Regan,C.K.;Brauman,J.I.Science1998,279,1882.(34)Morris,R.A.;Viggiano,A.A.J.Phys.Chem.1994,98,3740.(35)Midey,A.J.;Viggiano,A.A.J.Phys.Chem.A2000,104,6786.(36)Hu,W.P.;Truhlar,D.G.J.Am.Chem.Soc.1994,116,7797.(37)O’Hair,R.A.J.;Davico,G.E.;Hacaloglu,J.;Dang,T.T.;DePuy,C.H.;Bierbaum,V.M.J.Am.Chem.Soc.1994,116,3609.(38)Errors in the branching ratio would not affect the individual KIE becausethe contributions from the barrier height,E q,cancel out in the rate ratio.(39)Zhao,Y.;Gonzalez-Garcia,N.;Truhlar,D.G.J.Phys.Chem.A2005,109,2012.JA057491DTable1.Experimental Reaction Rate Constants(10-10cm3molecule-1s-1)and Deuterium Kinetic Isotope Effects for RCl+ClO-f ProductsRCl k KIECH3Cl 2.01(0.010.85(0.01CD3Cl 2.36(0.01C2H5Cl 2.25(0.01a0.99(0.01aC2D5Cl 2.27(0.01ai-C3H7Cl 1.74(0.03 1.72(0.05i-C3D7Cl 1.01(0.02t-C4H9Cl 2.33(0.03 2.31(0.12t-C4D9Cl 1.01(0.05a Computational results predict k)6.7×10-10and k)2.8×10-10cm3molecule-1s-1for ClO-with C2H5Cl and C2D5Cl,respectively,anda KIE of2.4.1C O M M U N I C A T I O N SJ.AM.CHEM.SOC.9VOL.128,NO.3,2006737。

同位素富集效应

同位素富集效应同位素富集效应（Isotope Fractionation）是指在自然界中，同一化学元素的不同同位素之间，在化学或生物过程中发生的分馏现象。

同位素富集效应是地球科学、环境科学、生物科学等领域中的一个重要概念，对于研究地质过程、生物地球化学循环和环境变化等具有重要意义。

在本文中，将详细介绍同位素富集效应的原理、影响因素以及其在不同领域的应用。

一、同位素富集效应的原理同位素富集效应的原理可以通过同位素分馏的概念来解释。

同位素分馏是指在化学或生物过程中，不同同位素的分布比例发生变化的现象。

同位素分馏主要受到以下两个因素的影响：1. 质量效应（Mass Effect）质量效应是指由于同位素的质量差异，导致在分子或原子运动中发生分馏的现象。

根据经典物理学的原子运动理论，质量较轻的同位素由于具有更高的平均速度，更容易被排除或分离出来，从而导致质量较重的同位素富集。

这种质量效应在气体分子运动、挥发性物质的蒸发、地球大气中的气体交换等过程中起着重要作用。

2. 反应动力学效应（Kinetic Effect）反应动力学效应是指在化学反应或生物过程中，由于反应速率的不同，导致同位素的分馏现象。

在某些反应中，由于同位素反应速率的差异，一个同位素的转化速率可能比另一个同位素更快或更慢，从而导致同位素富集或贫化。

这种反应动力学效应在生物体内的代谢过程、化学反应中的催化作用等方面具有重要意义。

二、同位素富集效应的影响因素同位素富集效应的大小和方向可以受到多种因素的影响。

以下是一些常见的影响因素：1. 温度温度是影响同位素富集效应的重要因素之一。

在很多地质和生物过程中，温度的变化会引起同位素富集效应的变化。

例如，氧同位素在水分子中的分馏程度随温度的升高而增大，这是因为在较高温度下，氧同位素的振动速度增加，导致分子运动更加剧烈，进而增加了同位素分馏的程度。

2. 压力压力也可以影响同位素富集效应。

在地质过程中，高压环境下的同位素分馏效应通常比低压环境下更加显著。

同位素比——精选推荐

同位素⽐油⽓的碳、氢稳定同位素研究是油⽓地球化学的重要组成部分。

按油⽓的元素组成，稳定同位素相应包括碳、氢稳定同位素；硫、氮、氧稳定同位素；微量—痕量元素稳定同位素和稀有⽓体稳定同位素4部分。

本节将着重介绍碳、氢稳定同位素。

⼀、有关同位素的基本概念（⼀）同位素效应在参与⽣物、化学、物理的作⽤过程中，元素的⼀种同位素被另⼀种同位素所取代，从⽽引起物理、化学性质上的变化．称为同位素效应。

同位素效应可以明显地影响分⼦扩散迁移速率，化学反应速率等。

同位素效应的根本原因是同位素质量上的差异，这种差异越⼤，则同位素效应越显著。

（⼆）同位素分馏作⽤在各种地质作⽤过程中，由于各种同位素效应的存在，同⼀种物质或物相中，其同位素丰度仍保持相对的稳定性；⽽不同物质或物相间，其同位素的丰度则出现差异，引起上述现象的同位素转移和分配作⽤过程称为同位素分馏作⽤。

⼆、碳、氢稳定同位素在⾃然界的丰度、⽐值、标准和δ值（⼀）碳、氢稳定同位素在⾃然界的丰度、⽐值和标准原油中常见的碳、氢的稳定同位素在⾃然界的分布、⽐值符号和标准，如表1-17所⽰。

表1-17 ⾃然界中碳、氢稳定同位素的符号、丰度、同位素⽐值及标准简表（摘⾃Stark，Walleace，1975）⾃然界不同的含碳、氢物质的同位素丰度有着明显的区别，同位素的⽐值：D/H，13C/12C也各不相同。

利⽤这⼀特性可确定油⽓的成因，进⾏油⽓源对⽐和油⽓运移等研究。

（⼆）δ值的定义及不同标准δ值的换算同位素⽐值的测量和对⽐单位⼀般是⽤⼲分数来表⽰的。

δ值的定义如下：式中的R s为样品的同位素⽐值，对于碳、氢稳定同位素。

分别以D/H、13C/12C；Rr为标准的稳定同位素⽐值(氢为SMOW的D/H，碳为PDB的13C/12C)。

同⼀种元素的同位素在不同⽂献中所采⽤的标准有时不同。

如碳同位素。

不仅国外有多种标准，国内也有不同标准。

常⽤的碳同位素标准的13C/12C值及国际常⽤标准(PDB)的δ13C值，如表1-18所⽰。

第6章+同位素效应

15N14N14C13C12C14C13C 12C密闭体系的热力学分馏影响因素热力学同位素分馏系数的理论计算同位素交换反应动力学常见的动力学同位素分馏过程14C13C12C14C13C12C14C13C12C如In如果更喜欢吃白色的，那么？14C13C12C14C13C12C14C13C 12C、光合补充较914C 13C 12C底泥附近：呼吸无分馏深层：夜间呼吸强表层：光合强14C13C 12C14C13C 12C ⎟⎟⎠⎞t ,w H 14C13C 12C14C13C 12C14C13C 12C14C13C12C检验模型光合与呼吸相等光合光合。

设定时间间隔：天Gain 14C13C12C14C13C12C模型的误差来源步长过长14C 13C12C4014C13C12Cδ的近似分别计算14C13C12C14C13C12C2214C13C12Cz 如果排泄没有分馏，那么？14C13C12C如果吸收100，损失14C13C12C如果吸收15，损失1514C13C12C如果吸收，损失14C13C12C400d 后，饲料氮-2‰，80，损失14C 13C12C14C13C 12C密闭体系的热力学分馏影响因素热力学同位素分馏系数的理论计算同位素交换反应动力学常见的动力学同位素分馏过程14C13C 12C14C13C12C14C13C 12C14C13C 12C14C 13C 12CεA/B ≈10‰A = 剩余水B = 瞬时水蒸气C = ∑水蒸气D,E: 密闭体系14C13C12C14C13C12Cα的温度依赖关系13C (HCO 3-/CO 2)18O (液相/气相)2H (液相/气相)14C13C12C14C13C12C 14C13C12C14C13C 12C摩尔数14C13C12C14C13C12C14C13C12C14C13C12C14C13C12C)014C13C12C二级动力学反应中反应物和产物浓度的变化曲线14C13C 12C，而在测定交换速率是希望交换分数介于14C13C 12C若干试验体系中同位素交换的一级反应速率14C13C12C14C13C 12C14C13C 12C14C13C12C5214C13C 12Cz D 0：活化能，E 0：零点能，D e ：谷深14C13C12CE a ：活化能，与同位素有中间复合物的能态不受同位素取代影响14C13C12C可分解为：22CO O C →+单向化学反应：14C13C12C14C13C12C14C13C 12C例如：密闭容器内水发生气液分离时，14C13C12C14C 13C12C14C13C 12C的两个产物的总量相等，并且两个产物的量也保持= P/(P + Q)对同位素也有相同的关系：+ (1-f P )δQ14C 13C12C14C13C 12CδInput = δOutputδOutput = δBody + ΔO/B98.0( (//−==ΔB O B O α14C 13C 12CBody = δOutput = δ-Δ14C13C 12CδInput = δOutput =δStore storebody14C13C 12CInput = δA (1-f B )+δB f B=δStore ; δA = δBody; δStore = δBody + ΔS/B 14C13C12C14C 13C 12C14C13C 12C14C13C12C14C13C12C14‰14C13C12C开放系统趋向于稳态的过程中同位素组成的变化14C13C12Cz 简单模型A14C13C 12C14C13C 12C14C 13C 12C14C13C 12C14C 13C 12C14C13C 12C14C 13C12C14C13C 12C14C13C 12C14C13C 12C3b φ14C13C 12C)3a −14C13C12C14C13C12C14C13C 12C一个特定反应的同位素分馏效应不是固定不变的14C13C12C硫循环的例子8814C13C12C14C13C12C14C13C12C9114C13C12C14C13C12C 14C13C12C14C13C12C 14C13C12C14C13C12C14C 13C12C海水蒸发雨滴凝结晶体析出岩浆去气等14C13C 12C14C13C 12C14C13C12C14C13C12C14C13C12C水从水汽中凝聚的瑞利分馏14C13C 12C14C13C12C14C13C 12C14C13C12C，反应过程中某时14C13C12C14C13C12C14C 13C12C14C13C 12C14C13C12C14C13C12C14C13C 12C14C13C12C14C13C12C11614C13C12Cz 混合：keeling plot14C 13C12C14C13C 12C。

同位素磁效应-概述说明以及解释

同位素磁效应-概述说明以及解释1.引言1.1 概述概述:同位素磁效应是指同位素在外加磁场作用下，其磁化率与外磁场的关系。

它是由同位素核子的自旋和核磁矩与外磁场相互作用引起的。

同位素磁效应是一种重要的物理现象，具有广泛的应用领域，对于理解和研究材料的磁性质具有重要的意义。

同位素磁效应的研究起源于20世纪初。

当时，科学家们发现一些同位素在外磁场作用下会表现出与其化学成分和结构无关的磁性行为。

后来的研究表明，同位素磁效应与同位素的核素自旋、核磁矩以及电子态密切相关。

同位素磁效应在多个领域都有重要应用。

首先，在地球科学领域，利用同位素磁效应的原理可以对岩石和矿物的地磁学特性进行研究，从而揭示地球的演化历史和地壳运动过程。

其次，在材料科学领域，同位素磁效应可以用于研究材料的磁性质和磁性杂质，为新材料的设计和开发提供理论指导。

此外，在生物医学领域，同位素磁效应也被应用于核磁共振成像（MRI）技术中，帮助医生进行疾病的诊断和治疗。

同位素磁效应的研究对于科学的发展和人类社会的进步具有重要的意义。

通过深入研究同位素磁效应的原理和机制，可以为相关领域的科学家们提供更多关于材料、岩石和生物体等方面的信息，推动科学的发展和技术的进步。

同时，同位素磁效应的未来发展也是一个有待探索的领域，科学家们可以进一步研究同位素磁效应的新应用，开发更加高效和精确的磁性测量技术，为人类社会的各个领域做出更大的贡献。

1.2文章结构文章结构部分的内容可以写成这样：1.2 文章结构本文将按照以下结构进行论述同位素磁效应的定义、原理以及其在不同领域的应用。

首先，在引言部分概述同位素磁效应的基本概念和背景。

接着，正文部分将详细介绍同位素磁效应的定义和原理，包括其在磁学和物理学中的意义和作用机制。

然后，我们将讨论同位素磁效应在各个领域中的应用，如地球科学、生物医学、材料科学等。

结尾部分将总结同位素磁效应的重要性，并展望其未来的发展前景。

通过这样的结构安排，读者可以全面了解同位素磁效应的基本知识和应用领域，为进一步研究和应用提供基础和启示。

第6章+同位素效应

15N14N14C13C12C14C13C 12C密闭体系的热力学分馏影响因素热力学同位素分馏系数的理论计算同位素交换反应动力学常见的动力学同位素分馏过程14C13C12C14C13C12C14C13C12C如In如果更喜欢吃白色的，那么？14C13C12C14C13C12C14C13C 12C、光合补充较914C 13C 12C底泥附近：呼吸无分馏深层：夜间呼吸强表层：光合强14C13C 12C14C13C 12C ⎟⎟⎠⎞t ,w H 14C13C 12C14C13C 12C14C13C 12C14C13C12C检验模型光合与呼吸相等光合光合。

设定时间间隔：天Gain 14C13C12C14C13C12C模型的误差来源步长过长14C 13C12C4014C13C12Cδ的近似分别计算14C13C12C14C13C12C2214C13C12Cz 如果排泄没有分馏，那么？14C13C12C如果吸收100，损失14C13C12C如果吸收15，损失1514C13C12C如果吸收，损失14C13C12C400d 后，饲料氮-2‰，80，损失14C 13C12C14C13C 12C密闭体系的热力学分馏影响因素热力学同位素分馏系数的理论计算同位素交换反应动力学常见的动力学同位素分馏过程14C13C 12C14C13C12C14C13C 12C14C13C 12C14C 13C 12CεA/B ≈10‰A = 剩余水B = 瞬时水蒸气C = ∑水蒸气D,E: 密闭体系14C13C12C14C13C12Cα的温度依赖关系13C (HCO 3-/CO 2)18O (液相/气相)2H (液相/气相)14C13C12C14C13C12C 14C13C12C14C13C 12C摩尔数14C13C12C14C13C12C14C13C12C14C13C12C14C13C12C)014C13C12C二级动力学反应中反应物和产物浓度的变化曲线14C13C 12C，而在测定交换速率是希望交换分数介于14C13C 12C若干试验体系中同位素交换的一级反应速率14C13C12C14C13C 12C14C13C 12C14C13C12C5214C13C 12Cz D 0：活化能，E 0：零点能，D e ：谷深14C13C12CE a ：活化能，与同位素有中间复合物的能态不受同位素取代影响14C13C12C可分解为：22CO O C →+单向化学反应：14C13C12C14C13C12C14C13C 12C例如：密闭容器内水发生气液分离时，14C13C12C14C 13C12C14C13C 12C的两个产物的总量相等，并且两个产物的量也保持= P/(P + Q)对同位素也有相同的关系：+ (1-f P )δQ14C 13C12C14C13C 12CδInput = δOutputδOutput = δBody + ΔO/B98.0( (//−==ΔB O B O α14C 13C 12CBody = δOutput = δ-Δ14C13C 12CδInput = δOutput =δStore storebody14C13C 12CInput = δA (1-f B )+δB f B=δStore ; δA = δBody; δStore = δBody + ΔS/B 14C13C12C14C 13C 12C14C13C 12C14C13C12C14C13C12C14‰14C13C12C开放系统趋向于稳态的过程中同位素组成的变化14C13C12Cz 简单模型A14C13C 12C14C13C 12C14C 13C 12C14C13C 12C14C 13C 12C14C13C 12C14C 13C12C14C13C 12C14C13C 12C14C13C 12C3b φ14C13C 12C)3a −14C13C12C14C13C12C14C13C 12C一个特定反应的同位素分馏效应不是固定不变的14C13C12C硫循环的例子8814C13C12C14C13C12C14C13C12C9114C13C12C14C13C12C 14C13C12C14C13C12C 14C13C12C14C13C12C14C 13C12C海水蒸发雨滴凝结晶体析出岩浆去气等14C13C 12C14C13C 12C14C13C12C14C13C12C14C13C12C水从水汽中凝聚的瑞利分馏14C13C 12C14C13C12C14C13C 12C14C13C12C，反应过程中某时14C13C12C14C13C12C14C 13C12C14C13C 12C14C13C12C14C13C12C14C13C 12C14C13C12C14C13C12C11614C13C12Cz 混合：keeling plot14C 13C12C14C13C 12C。

同位素效应(Deuterium Kinetic Isotope Effects)

Subscriber access provided by NATIONAL CHUNG CHENG UNIVJournal of the American Chemical Society is published by the American ChemicalSociety. 1155 Sixteenth Street N.W., Washington, DC 20036 CommunicationDeuterium Kinetic Isotope Effects in Gas-Phase S2 andE2 Reactions: Comparison of Experiment and TheoryStephanie M. Villano, Shuji Kato, and Veronica M. BierbaumJ. Am. Chem. Soc., 2006, 128 (3), 736-737 • DOI: 10.1021/ja057491dDownloaded from on January 17, 2009More About This ArticleAdditional resources and features associated with this article are available within the HTML version:•Supporting Information•Links to the 1 articles that cite this article, as of the time of this article download•Access to high resolution figures•Links to articles and content related to this article•Copyright permission to reproduce figures and/or text from this articleDeuterium Kinetic Isotope Effects in Gas-Phase S N 2and E2Reactions:Comparison of Experiment and TheoryStephanie M.Villano,Shuji Kato,and Veronica M.Bierbaum*Uni V ersity of Colorado,Department of Chemistry and Biochemistry,215UCB,Boulder,Colorado 80309-0215Received November 2,2005;E-mail:Veronica.Bierbaum@The competition between nucleophilic substitution and base-induced elimination has been well-documented in the condensed phase;however,few studies have addressed this competition in the gas-phase.Studying S N 2and E2reactions in the gas-phase allows one to gain insight into intrinsic features that affect this competition without interference from solvent effects.However,these experiments are inherently difficult since these mechanisms generate the same ionic product and detection of neutral products presents serious analytical challenges.Several groups have investigated S N 2and E2reactions theoreti-cally;however due to the lack of experimental data,less work has addressed the competition between these two reactions.Hu and Truhlar 1have evaluated the reaction rate constants and deuterium kinetic isotope effects (KIE )k H /k D )for both the S N 2and E2pathways of the reaction of ClO -with C 2H 5Cl,shown in eq 1.Their work has provided us with the unique opportunity to compare theory and experiment.In this account we report experimental,overall reaction rate constants and deuterium KIEs for the reactions of RCl +ClO -(R )methyl,ethyl,isopropyl,and tert -butyl).This systematic approach indicates that,for the reaction of ClO -with C 2H 5Cl,both the S N 2and E2channels occur,as predicted by Hu and Truhlar;however the experimental KIE differs drastically from the computational predictions.There is extensive literature on gas-phase reactions that occur exclusively by S N 2or by E2mechanisms.(See refs 2-6and citations therein.)For reactions that can occur by both pathways,Gronert 7has provided an excellent summary of experimental approaches,8-13including his novel utilization of dianionic nucleo-philes that yield diagnostic product ions.14,15Our research has utilized reactivity trends 16as well as kinetic isotope effects 17to explore these processes.Kinetic isotope effects can be used to distinguish between S N 2and E2mechanisms if the reaction proceeds at a rate slower than the collision rate.18For E2reactions normal KIEs (k H /k D >1)are observed,while for S N 2reactions inverse KIEs (k H /k D <1)are observed.The origin of these effects is primarily a result of changes in vibrational frequencies as the reaction proceeds from the reactants to the transition state structure.19Deuteration lowers the vibrational frequencies in both the reactants and transition states relative to the undeuterated compounds.An E2reaction proceeds through a transition state where the C -H bond is partially broken thus lowering these frequencies relative to reactants.Since the frequen-cies for the undeuterated species are lowered more substantially than those for the deuterated species in the transition state,E H q <E D q .An S N 2reaction proceeds through a trigonal planar transition state where the C R -H bonds are tightened,thus raising these fre-quencies relative to reactants.Since the frequencies for the undeu-terated species are raised more substantially than those for the deu-terated species in the transition state,E H q >E D q .When both reaction pathways are viable an overall KIE is measured which provides qualitative insight into the competition between these mechanisms.Hu and Truhlar 1have quantitatively evaluated the competition between S N 2and E2pathways for ClO -with C 2H 5Cl and with C 2D 5Cl using dual-level generalized transition state theory and statistical calculations based on high-level,correlated electronic structure calculations using extended basis sets (MP2/ADZP).1Their calculations predict an S N 2reaction efficiency of 2%with a deuterium KIE of 0.60and an E2reaction efficiency of 26%with a deuterium KIE of 3.1at 300K.They predict an overall reaction efficiency of 28%and a KIE of 2.4.We have measured the overall reaction rate constants and KIE for the reactions of RCl +ClO -(R )methyl,ethyl,isopropyl,and tert -butyl)using a tandem flowing afterglow-selected ion flow tube instrument,FA-SIFT.20,21ClO -is an ideal choice for a nucleophile because the rate constants for the entire neutral series,methyl through tert -butyl,are within a measurable window of 10-9-10-13cm 3molecule -1s -1and are all below the calculated collision rate so that KIEs are evident.The reactant ion was formed in the source flow tube from electron impact on N 2O to produce O -which was then allowed to react with CCl 4to form ClO -.ClO -ions were mass-selected and injected into the reaction flow tube where they were thermalized to 304((2)K by collisions with He buffer gas (0.5Torr,∼104cm s -1).Neutral reactant flow rates were measured by monitoring the pressure change versus time in a calibrated volume system.Reaction rate constants were determined by introducing the neutral reagents at varying distances along the reaction flow tube,thereby varying the reaction distance and time.The 35ClO -intensity was monitored with a quadrupole mass filter coupled to an electron multiplier.Rate constants for the perprotio and perdeuterio reactions were measured sequentially on the same day under the same conditions.The error reported for the reaction rate constants is one standard deviation of three consecutive measurements.The uncertainty due to systematic error is (20%;however the systematic errors cancel in the rate ratio,and therefore,the KIEs are more accurately determined.Helium buffer gas (99.995%)was purified by passage through a molecular sieve trap at 77K.Neutral reagents were purchased from commercial sources and used without further purification;however,it was experimen-tally demonstrated that HCl,which would complicate the results,was not a significant contaminant.22Measured reaction rate constants and deuterium KIEs are reported in Table 1.The reaction of ClO -with methyl chloride can undergo only an S N 2reaction,and an inverse KIE of 0.85((0.01)was measured.This result is consistent with previous measurementsofPublished on Web 12/22/20057369J.AM.CHEM.SOC.2006,128,736-73710.1021/ja057491d CCC:$33.50©2006American Chemical Societyinverse KIEs for S N2reactions of methyl halides.23,24The reaction of ClO-with tert-butyl chloride presumably proceeds primarily by E2.16A KIE of2.31((0.12)was measured which is consistent with previously measured normal KIEs for E2reactions.5,25As the extent of substitution in the neutral reactants increases,the KIE effects become increasingly more normal.These results indicate that the E2pathway becomes the dominant channel as the neutral reagent becomes more sterically hindered.The experimental reaction efficiencies(10%for ClO-with C2H5Cl)26and KIE reported here for the reaction of ClO-with ethyl chloride differ from the theoretical values,suggesting that the S N2channel is more prominent than calculations predict.It is difficult to account for the discrepancies between experiment and theory;however nonstatistical dynamics27or errors in the calculation of the individual KIE or in the branching ratios of the two channels could account for the discrepancy.Hu and Truhlar’s treatment of the reaction between ClO-and ethyl chloride assumes that the reaction is statistical.Statistical theories are based on the assumption that energy is randomized among different modes as the reaction proceeds from reactants to the transition state.Nonstatistical effects have been observed for S N2reactions of monatomic nucleophiles with methyl halides,and these effects have been extensively documented in the liter-ature.23,24,28-32However the use of statistical theories has been successful for slightly larger systems.33-35Furthermore,Hu and Truhlar have previously calculated the KIE for the S N2reaction of solvated fluoride with methyl chloride using a statistical approach.36 Their results show excellent agreement with our published experi-mental values.37Thus the assumption that the ClO-+C2H5Cl reaction behaves statistically is reasonable.The individual KIE values,which Hu and Truhlar evaluated, appear quite reasonable.It can be argued that the errors in the branching ratio calculations are the primary cause for the large deviation from the experimental results.38If the predicted branching ratios were correct,the KIE for the E2channel would need to be 1.0or smaller or the KIE for the S N2channel would need to be 0.13or smaller to be consistent with the measured overall KIE. These values are not reasonable based on previous results.On the other hand assuming that the individual KIE are correct and that the discrepancy lies in the calculation of the branching ratios,we can use the calculated individual KIE along with the experimental overall rate constant and KIE to predict the rate constants for the S N2and E2channels to be1.10×10-10cm3molecule-1s-1and 1.15×10-10cm3molecule-1s-1,respectively,for ClO-+C2H5Cl. This analysis suggests that the calculated results overestimate the E2rate constant by a factor of∼6and underestimate the S N2rate constant by a factor of∼2.5.This magnitude of errors would occur if the E2barrier height was underestimated by only1kcal mol-1, while the S N2barrier height was overestimated by only0.6kcal mol-1.These small deviations in the activation enthalpy can have a large effect on the branching ratios.Determining activation enthalpies with an accuracy of0.5kcal mol-1or better is a difficult computational task.39These results illuminate the need for additional studies to describe nucleophilic substitution and elimination reactions more accurately. We hope that the discrepancy between experiment and theory will spark interest for further exploration.Acknowledgment.The authors thank Professors Charles DePuy and Barry Carpenter for valuable discussions and Professor Gustavo Davico for the initiation of the first experiment.This research was supported by the National Science Foundation(Grant No.CHE-0349937).References(1)Hu,W.P.;Truhlar,D.G.J.Am.Chem.Soc.1996,118,860.(2)Olmstead,W.N.;Brauman,J.I.J.Am.Chem.Soc.1977,99,4219.(3)Laerdahl,J.K.;Uggerud,E.Int.J.Mass Spectrom.2002,214,277.(4)Hase,W.L.Encyc.Mass Spectrom.2005,4,504.(5)de Koning,L.J.;Nibbering,N.M.M.J.Am.Chem.Soc.1987,109,1715.(6)Gronert,S.;Kass,.Chem.1997,62,7991.(7)Gronert,S.Acc.Chem.Res.2003,36,848.(8)Lieder,C.A.;Brauman,J.I.Int.J.Mass Spectrom.Ion Phys.1975,16,307.(9)Noest,A.J.;Nibbering,N.M.M.Ad V.Mass Spectrom.1980,8,227.(10)Bartmess,J.E.;Hays,R.L.;Kahatri,H.N.;Misra,R.N.;Wilson,S.R.J.Am.Chem.Soc.1981,103,4746.(11)Lum,R.C.;Grabowski,J.J.J.Am.Chem.Soc.1988,110,8568.(12)Jonas,M.E.;Ellison,G.B.J.Am.Chem.Soc.1989,111,1645.(13)Wladkowski,B.D.;Brauman,J.I.J.Am.Chem.Soc.1992,114,10643.(14)Gronert,S.;Pratt,L.M.;Mogali,S.J.Am.Chem.Soc.2001,123,3081.(15)Gronert,S.;Fagin,A.E.;Okamoto,K.;Mogali,S.;Pratt,L.M.J.Am.Chem.Soc.2004,126,12977.(16)DePuy,C.H.;Gronert,S.;Mullin,A.;Bierbaum,V.M.J.Am.Chem.Soc.1990,112,8650.(17)Gronert,S.;DePuy,C.H.;Bierbaum,V.M.J.Am.Chem.Soc.1991,113,4009.(18)Melander,L.;Saunders,J.H.,Jr.Reaction Rates of Isotopic Molecules;John Wiley and Sons:New York,1980.(19)Poirier,R.A.;Youliang,W.;Westaway,K.C.J.Am.Chem.Soc.1994,116,2526.(20)Van Doren,J.M.;Barlow,S.E.;DePuy,C.H.;Bierbaum,V.M.Int.J.Mass Spectrom.Ion Proc.1987,81,85.(21)Bierbaum,V.M.Encyc.Mass Spectrom.2003,1,98.(22)(CH3Cl99.9%,CD3Cl99.9%,C2H5Cl>99%,C2D5Cl98%,i-C3H7Cl99%,i-C3D7Cl99.4%,t-C4H9Cl99%,t-C4D9Cl99.1%).An HCl impurity would complicate the rate measurements due to a rapid proton-transfer pathway, which could not be distinguished from the S N2and E2channels.Therefore, 37Cl-was mass selected and allowed to react with the neutral reagents.We have previously shown[Van Doren,J.M.;DePuy,C.H.;Bierbaum, V.M.J.Phys.Chem.1989,93,1130]that the rates of these reactions are below the detection limits of our instrument;however,the Cl-+HCl exchange reaction proceeds at approximately half of the collision rate.The absence of35Cl-as a product ion indicates that HCl is not a significant contaminant.(23)Viggiano,A.A.;Morris,R.A.;Paschkewitz,J.S.;Paulsen,J.F.J.Am.Chem.Soc.1992,114,10477.(24)Kato,S.;Davico,G.E.;Lee,H.S.,DePuy,C.H.;Bierbaum,V.M.Int.J.Mass Spectrom.2001,210/211,223.(25)Bierbaum,V.M.;Filley,J.;DePuy,C.H.;Jarrold,M.F.;Bowers,M.T.J.Am.Chem.Soc.1985,107,2818.(26)Based on the collision rate of2.09×10-9cm3molecule-1s-1,calculatedusing the parametrized trajectory collision theory.[Su,T.;Chesnavich, W.J.J.Chem.Phys.1982,76,5183.](27)Baer,T.;Hase,W.L.Unimolecular Reaction Dynamics;Theory andExperiment;Oxford University Press:New York,1996.(28)Graul,S.T.;Bowers,M.T.J.Am.Chem.Soc.1994,116,3875.(29)Tonner,D.S.;McMahon,T.B.J.Am.Chem.Soc.2000,122,8783.(30)Sun,L.;Song,K.;Hase,W.L.J.Am.Chem.Soc.2001,123,5753.(31)Sun,L.;Song,K.;Hase,W.L.Science2002,296,875.(32)Angel,L.A.;Ervin,K.M.J.Am.Chem.Soc.2003,125,1014.(33)Chabinyc,M.L.;Craig,S.L.;Regan,C.K.;Brauman,J.I.Science1998,279,1882.(34)Morris,R.A.;Viggiano,A.A.J.Phys.Chem.1994,98,3740.(35)Midey,A.J.;Viggiano,A.A.J.Phys.Chem.A2000,104,6786.(36)Hu,W.P.;Truhlar,D.G.J.Am.Chem.Soc.1994,116,7797.(37)O’Hair,R.A.J.;Davico,G.E.;Hacaloglu,J.;Dang,T.T.;DePuy,C.H.;Bierbaum,V.M.J.Am.Chem.Soc.1994,116,3609.(38)Errors in the branching ratio would not affect the individual KIE becausethe contributions from the barrier height,E q,cancel out in the rate ratio.(39)Zhao,Y.;Gonzalez-Garcia,N.;Truhlar,D.G.J.Phys.Chem.A2005,109,2012.JA057491DTable1.Experimental Reaction Rate Constants(10-10cm3molecule-1s-1)and Deuterium Kinetic Isotope Effects for RCl+ClO-f ProductsRCl k KIECH3Cl 2.01(0.010.85(0.01CD3Cl 2.36(0.01C2H5Cl 2.25(0.01a0.99(0.01aC2D5Cl 2.27(0.01ai-C3H7Cl 1.74(0.03 1.72(0.05i-C3D7Cl 1.01(0.02t-C4H9Cl 2.33(0.03 2.31(0.12t-C4D9Cl 1.01(0.05a Computational results predict k)6.7×10-10and k)2.8×10-10cm3molecule-1s-1for ClO-with C2H5Cl and C2D5Cl,respectively,anda KIE of2.4.1C O M M U N I C A T I O N SJ.AM.CHEM.SOC.9VOL.128,NO.3,2006737。

同位素效应

第二类

由同位素核性质上的差异引起。虽然同位素的核电荷相同，但中子数不同会产生一系列核性质上的不同，包括核半径、核自旋、结合能、磁矩和四级矩、激发态谱、核反应截面等等。
历史发展
对于氘、重水等重要的轻元素同位素及其化合物的宏观物理常数，在20世纪30年代虽已作了普遍测定，至今仍不断补充和修正。50年代测定了诸如 DO的键长、键角等微观结构数据。70年代以来，开始深入到同位素取代异构分子的研究。动力学同位素效应的研究也深入到生命过程的研究中。同位素效应可分为光谱同位素效应、热力学同位素效应、动力学同位素效应和生物学同位素效应。
动力学同位素效应分为一级同位素效应和二级同位素效应。一级同位素效应：在决定速率步骤中与同位素直接相连的键发生了断裂的反应中所观察到的同位素效应，其KH/KD通常在2或更高。二级同位素效应：在决定速率步骤中与同位素直接相连的键不发生断裂，而是分子中其他化学键发生变化所观察到的效应，其KH/KD通常在 0.7-1.5范围内。
同位素效应
科学名词
01 定义
03 第二类
目录
02 第一类 04 历史发展
05 光谱
07 动力学
目录
06 热力学 08 生物学
同位素效应是指同位素是同一元素的化学性质相同,但原子量不同的原子,因此不能用化学方法将其分开，在观察原子光谱时能发现有微小的差异,这种效应称为同位素效应。
定义
由于质量或自旋等核性质的不同而造成同一元素的同位素原子（或分子）之间物理和化学性质有差异的现象。同位素效应指的是同一元素的同位素或者含该元素不同同位素的化合物（又称同位素置换化合物）在性质上的差异。这些差异，可以表现在物理性质上，也可以表现在化学性质上，还可以是核性质上。过去说同一种元素的原子物理、化学性质相同，是不准确的。尽管核电荷决定了元素的基本的物理和化学性质，但同一元素不同同位素间的性质的差异是存在的，还可能很大。可分为第一类同位素效应（同位素质量差异所导致的）和第二类同位素效应（同位素核性质上的差异引起的）。

同位素效应

Kinetic Isotope effects（4 学时）7.1Introduction若反应物中原子被其同位素取代，将表现出不同的化学反应性，称为同位素效应，表现在二个方面：1.平衡同位素效应HT H 2O+H 2HTO +K 298 = 6.26H 12CN 13CN+H 13CN 12CN +K 298 = 1.03Types of KIE：a.一级动力学同位素效应（primary,PKIE）连接同位素原子的键在决速步中断裂b.二级动力学同位素效应（secondary,SKIE）连接同位素原子的键在决速步中不断裂c.溶剂同位素效应(solvent isotope effects)由反应介质中的不同的同位素引起速率变化7.2Theory of isotope effects:the primary effect同位素取代不影响分子的势能面，也不影响电子态的能级。

它仅影响那些与原子质量有关的量，如：振转动频率。

那么，同位素异构分子在性质上的差别必然是由于势能面上振动和转动能级的差异引起。

1.同位素的改变引起振动能的改变C H(D)伸缩振动能可由双原子谐振子的Schrödinger 方程得到：...v )(v ,,,21021 hv hcv E v v 振动量子数v 从一级振动能级另一能级跃迁的频率v 波数（1/ ）cm -1021 v hv E v 零点能~~伸缩振动：IR v (v )C H 3000cm -1(9.0 1013s -1)C D 2100cm -1(6.3 1013s -1)C H(D)的势能曲线~2. 同位素的改变引起反应速率变化a.振动能级的每一跃迁比室温下的热能（k T ）要大得多，因此，几乎所有的分子25 C 时都分布在基态的振动能级上（v=0）;b.C D 比C H 基态振动能级低；c.随v 增大，上述差别减小，直至与电离限一致；d.D D >D H ,C D 断裂比C H 需要的能量多因此，k D 具有比k H 低的速率，这就是通常的PKIE,用k H /k D 表示。

同位素效应知识

同位素效应同位素效应同位素分析和同位素分离的基础，是由于质量或自旋等核性质的不同而造成同一元素的同位素原子（或分子）之间物理和化学性质有差异的现象。

基本概念由于质量或自旋等核性质的不同而造成同一元素的同位素原子（或分子）之间物理和化学性质有差异的现象。

详细内涵同位素效应是同位素分析和同位素分离的基础。

它在化学结构基本不变的情况下引起物理、化学常数的改变，因此能更深入地揭示物质微观结构与性质之间的关系。

对于氘、重水等重要的轻元素同位素及其化合物的宏观物理常数，在20世纪30年代虽已作了普遍测定，至今仍不断补充和修正。

50年代测定了诸如D2O的键长、键角等微观结构数据。

70年代以来，开始深入到同位素取代异构分子的研究。

动力学同位素效应的研究也深入到生命过程的研究中。

同位素效应可分为光谱同位素效应、热力学同位素效应、动力学同位素效应和生物学同位素效应。

光谱同位素效应同位素核质量的不同使原子或分子的能级发生变化，引起原子光谱或分子光谱的谱线位移。

核自旋的不同，引起光谱精细结构的变化。

如果分子中某些元素一部分被不同的同位素取代，从而破坏了分子的对称性，则能引起谱线分裂，并在红外光谱和并合散射光谱的振动结构中出现新的谱线和谱带。

早期研究中曾通过分子光谱和原子光谱发现新的同位素和进行同位素分析。

后来光谱同位素效应主要用于研究分子的微观结构。

热力学同位素效应同位素质量的相对差别越大，所引起的物理和化学性质上的差别也越大。

对于轻元素同位素化合物的各种热力学性质已作过足够精密的测定。

热力学同位素效应研究中最重要的，是同位素交换反应平衡常数的研究，已在实验和理论方面进行了大量工作。

蒸气压同位素效应也很重要，已可半定量地进行理论计算。

热力学同位素效应是轻元素同位素分离的理论基础，也是稳定同位素化学的主要研究内容。

动力学同位素效应在化学反应过程中，反应物因同位素取代而改变了能态，从而引起化学反应速率的差异。

1933年G.N.路易斯等用电解水的方法获得接近纯的重水，证实同位素取代对化学反应速率确有影响。