DOCSLIB.ORG

Testing a Mechanism: Kinetic Isotope Effects

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Testing a Mechanism: Kinetic Isotope Effects Testing a Mechanism: ! Kinetic Isotope Effects Primary Isotope Effect - rate change due to isotopic substitution at a site of bond breaking or bond making in the rate determining step of a mechanism. Secondary Isotope Effect - rate change due to isotopic substitution at other than a site of bond breaking or bond making in the rate determining step of a mechanism. Primary Kinetic Isotope Effect:! Typical Values o Nuclide klight k heavy (at 25 C) C-H/C-D 6 - 8 C-H/C-T 15 - 16 12 13 C/ C 1.04 12 14 C/ C 1.07 14 15 N/ N 1.03 16 18 O/ O 1.02 32 34 S/ S 1.01 35 37 Cl/ Cl 1.01 EXAMPLES OF KINETIC ISOTOPE EFFECTS Consider the following dehydrohalogenation reactions: NaOC H H3C 2 5 C CH CH3CH2CH2Br 2 k / k C2H5OH H H D = 6.7 Case I: o NaOC H H3C 1 KIE for C-H/C-D 2 5 C CH CH3CD2CH2Br 2 C2H5OH D CH3 H2O H3C CH3 CH3CH2 C Br C C H CH3 CH3 k / k Case II: H D = 1.4 o CH3 2 KIE for C-H/C-D H2O H3C CH3 CH3CD2 C Br C C D CH3 CH3 Rationale: Transition State for Case I (E-2) Transition State for Case II (E-1) δ- C2H5O + H CH3 CH3 H(D) H H(D) δ H(D) C C CH3 C CH3 + C C rds C - - δ (D)H Br δ (D)H Br (D)H Br- H3C H3C H3C More Examples of Kinetic Isotope Effects Consider the following decomposition of an azo compound: k k R N N R R N N R 2 R + N2 14 15 = 1.02 N N Rationale: Although the above rate enhancement is small in absolute terms, for the nitrogen nuclides indicated, the enhancement is indicative of a primary kinetic isotope effect. Consider the nitration of benzene shown below: HNO /H SO 3 2 4 rate C6H6 C6H5NO 2 H HNO /H SO 3 2 4 rate C6D6 C6D5NO 2 D Observation: rate ~ rate H = D Conclusion to be drawn: A C-H (C-D) bond is not being broken in the rate determining step of electrophilic aromatic nitration. Secondary Kinetic Isotope Effects Differences in steric demand! Hyperconjugative effects! Differences in inductive effect! Examples of Secondary Kinetic Isotope Effects Differences in Steric demand! Example I: CH Cl + H O CH OH + HCl k 3 2 3 H CD3 Cl + H2O CD3 OH + HCl kD Observation: kH / kD = 0.97 H In order to initiate bond making, the incoming nucleophile must be able to Rationale: H2O C approach the substrate from the rear. H Cl H The shorter C-D bonds (relative to C-H bonds) permit a closer approach by H2O Example of an inverse and bond making can begin sooner. α-isotope effect Example II: (CH ) CHOTs + H O (CH3)2CHOH + TsOH k 3 2 2 H (CH ) CDOH + TsOH (CH3)2CDOTs + H2O 3 2 kD Observation: kH / kD = 1.13 H Rationale: C+ In the transition state leading to the H3C CH3 isopropyl carbocation, there is a greater relief of strain for C-H relative to C-D. Secondary Kinetic Isotope Effects Example III: + CH3I N N+ I- CH3 Observations: Pyridine Substrate k D / kH CD3 1.001 N CD3 1.009 N 1.030 N CD3 1.095 D3C N CD3 Secondary Kinetic Isotope Effects Hyperconjugative effects! Example: β -Isotope effect (CH ) C OH + HCl k (CH3)3C Cl + H2O 3 3 H (CD3)3C Cl + H2O (CD3)3C OH + HCl k D k = Observation: H / k D 1.21 CH + Rationale: 3 CH2 H C+ C H3C CH3 H3C CH3 Whereas a C-D bond is slightly stronger than a C-H bond, hyperconjugation as shown above is not quite as effective at stabilizing the carbocationic intermediate for the reaction using deuterated reactant relative to that using unlabeled reactant. Another view: Whereas C-H bonds at a given temperature are slightly longer than corresponding C-D bonds, formation of a carbocationic intermediate provides more relief from steric strain for unlabeled reactant compared with that experienced by deuterated reactant. SECONDARY KINETIC ISOTOPE EFFECTS Inductive effects! - + Example: CH3COOH CH3COO + H KH - + H+ K CD3COOH CD3COO D K 1.06 Observation: H /KD = Rationale: Hydrogen is slightly more electronegative than deuterium. Related Question: Which one of the following is more basic? CH2NH2 vs. CD2NH2.
Load more

Recommended publications
  • Kinetic Isotope Effects and Hydrogen Tunnelling in PCET Oxidations of Ascorbate: New Insights Into Aqueous Chemistry?
    molecules Review Kinetic Isotope Effects and Hydrogen Tunnelling in PCET Oxidations of Ascorbate: New Insights into Aqueous Chemistry? Ana Karkovi´cMarkovi´c,Cvijeta Jakobuši´cBrala * , Viktor Pilepi´cand Stanko Urši´c* Faculty of Pharmacy and Biochemistry, University of Zagreb, A. Kovaˇci´ca1, 10 000 Zagreb, Croatia; [email protected] (A.K.M.); [email protected] (V.P.) * Correspondence: [email protected] (C.J.B.); [email protected] (S.U.); Tel.: +385-01-4870-267 (C.J.B.) Academic Editor: Poul Erik Hansen Received: 17 February 2020; Accepted: 21 March 2020; Published: 23 March 2020 Abstract: Recent experimental studies of kinetic isotope effects (KIE-s) and hydrogen tunnelling comprising three proton-coupled electron transfer (PCET) oxidations of ascorbate monoanion, (a) in aqueous reaction solutions, (b) in the mixed water-organic cosolvent systems, (c) in aqueous solutions of various salts and (d) in fairly diluted aqueous solutions of the various partial hydrophobes are reviewed. A number of new insights into the wealth of the kinetic isotope phenomena in the PCET reactions have been obtained. The modulation of KIE-s and hydrogen tunnelling observed when partially hydrophobic solutes are added into water reaction solution, in the case of fairly diluted solutions is revealed as the strong linear correlation of the isotopic ratios of the Arrhenius prefactors Ah/Ad and the isotopic differences in activation energies DEa (D,H). The observation has been proposed to be a signature of the involvement of the collective intermolecular excitonic vibrational dynamics of water in activation processes and aqueous chemistry. Keywords: kinetic isotope effects; ascorbate PCET reactions; hydrogen tunnelling; water vibrational dynamics; activation processes 1.
    [Show full text]
  • Kinetic Isotope Effect Lecture.Key
    Kinetic Isotope Effect Chapter 8, sections 8.1.1-8.1.3 Isotope Effects • Traditional kinetics studies do not provide information as to what bonds are broken/formed and changes in hybridization that occur during the rate- limiting step of a reaction. • Isotope effects can provide this information. • Substituting one iosotope for another at or near an atom at which bonds are breaking or rehybridizing typically leads to a change in the rate of the reaction. • The effect of replacing H with D often is relatively large when the bonds being broken or formed involve H. This effect can be measured routinely. • The effects of isotope replacement for other elements have been studied, but the effects are typically small and sometimes difficult to quantify. Isotope Effects: Experimental • An isotope effect is measured to determine if the bond at which the isotopic substitution has been made changes in some manner during the rate-limiting step. • The isotope effect is expressed as a ratio of rate constants: the rate constant for the reaction with the natural abundance isotope over the rate constant for the reaction with the altered isotope. [ for H/D substitutions: kH/kD] • For a hydrogen isotope effect, typically a first order (or pseudo-first order) rate constant for the reaction of interest is determined. The natural abundance of deuterium (D) and tritium (T) [0.015% and 1x10-4% respectively] is so low that their contribution to the observed rate is negligible. • The magnitude of the isotope effect (deviation of kH/kD from unity) provides information about the reaction mechanism.
    [Show full text]
  • Kinetic Isotope Effect in the Hydrogenation and Deuteration of Graphene
    Kinetic isotope effect in the hydrogenation GR-ThP3 and deuteration of graphene A. Nefedov, A. Paris, N. Verbitsky, Y. Wang, A. V. Fedorov, D. Haberer, M. Oehzelt, L. Petaccia, D. Usachov, D.V. Vyalikh, H. Sachdev, Ch. Wöll, M. Knupfer, B. Büchner, L. Calliari, L. V. Yashina, S. Irle, and A. Grüneis Motivation Sample preparation and layout of the experiment Experimental facilities: BESSY II and Elletra Kinetic isotope effects (KIEs), i.e. a different reaction constant for isotopes, are important phenomena in physical chemistry. They Ni(111),10 nm Graphene Experimental stations @ BESSY II ARPES @ Elletra have been studied for a long time in their relation to the activation and rate of chemical reactions. A significant contribution to KIE are vibrational zero-point energy (ZPE) effects that upshift the energy of a chemical bond by half the phonon frequency of its W(110) W(110) W(110) constituents. Hence, KIEs are easy to observe in the hydrogen isotopes (deuterium and tritium) due to their large relative mass Ni difference and were studied, even at the single-molecule level for hydrogen transfer in organic reactions including acid and base Sputtering, Flashing up to 1700°C, Ni evaporation, CVD from C3H6 catalysis, enzyme reactions and catalytic decomposition. Most of these reactions were carried out with H and D in a molecular Annealing in O2 at 1000°C Thickness control: QCM Characterization: LEED; XPS configuration and it was found that the reaction constant for the D compounds is lower than for the corresponding H compounds. Characterization: LEED, XPS Characterization: LEED These results are explained by the larger ZPE for H bonds, which results in a lower potential energy barrier to be overcome for H- bonds breaking as compared to D bonds.
    [Show full text]
  • Haghnegahdar M.A.*, Schauble E.A., and Young E.D. (2017)
    PUBLICATIONS Global Biogeochemical Cycles 12 13 RESEARCH ARTICLE A model for CH2D2 and CH3D as complementary 10.1002/2017GB005655 tracers for the budget of atmospheric CH4 Key Points: 1 1 1 13 Mojhgan A. Haghnegahdar , Edwin A. Schauble , and Edward D. Young • CH3D is expected to trace atmospheric methane sources 1 12 Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, California, USA • CH2D2 is expected to trace both atmospheric methane sources and sinks 13 12 • The reactions of methane with •OH Abstract We present a theoretical model to investigate the potential of CH3D and CH2D2, the doubly and Cl• generate a distinct signature of substituted mass-18 isotopologues of methane, as tools for tracking atmospheric methane sources and sinks. Δ12 higher CH2D2 relative to the source We use electronic structure methods to estimate kinetic isotope fractionations associated with the major composition sink reactions of methane in air (reactions with OH and Cl radicals) and combine literature data with 13 12 reconnaissance measurements of the relative abundances of CH3D and CH2D2 to estimate the Supporting Information: compositions of the largest atmospheric sources. This model atmospheric budget is investigated with a • Supporting Information S1 simplified box model in which we explore both steady state and dynamical (nonsteady state) conditions fl Correspondence to: triggered by changes in emission or sink uxes. The steady state model predicts that sink reactions will 12 M. A. Haghnegahdar, generate a marked (>100‰) clumped isotope excess in atmospheric Δ CH2D2 relative to the net source [email protected] 12 composition. CH2D2 measurements may thus be useful for tracing both atmospheric source and sink fluxes.
    [Show full text]
  • Observation of Kinetic Isotope Effect in Electrocatalysis with Fully
    Observation of Kinetic Isotope Effect in Electrocatalysis with Fully Deuterated Ultrapure Electrolytes Ken Sakaushi,1 1Center for Green Research on Energy and Environmental Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba, 305-0044 Ibaraki, Japan Corresponding to K.S.: [email protected] Abstract Kinetic isotope effect (KIE) is a common physicochemical effect to elucidate complicated microscopic reaction mechanism in biological, chemical and physical systems. Especially, the exchange of hydrogen to deuterium is a standard approach to investigate kinetics and pathways of a wide spectrum of key reactions involving proton transfer. However, KIE in electrocatalysis is still challenge. One main reason is owing to the high sensitivity to impurities in electrochemical systems. Aiming to establish an appropriate approach to observe KIE in electrocatalysis, we investigated KIE in electrocatalysis by using fully deuterated ultrapure electrolytes. With these electrolytes, we studied oxygen reduction reaction with platinum catalyst, which is well-known to be sensitive to impurity, as the model systems. In conclusion, the electrode processes in these systems can be strongly influenced by a purity of a selected deuterated electrolyte, especially in case of alkaline conditions. Therefore a highly pure deuterated electrolyte is indispensable to study microscopic electrode processes of electrocatalysis by analyzing KIE. This work shows a key criterion and methods to observe a reliable KIE in electrocatalytic systems, and therefore,
    [Show full text]
  • Kinetic Isotope Effect and Its Use for Mechanism Elucidation
    ISIC - LSPN Kinetic Isotope Effect: Principles and its use in mechanism investigation Group Seminar 05.03.2020 Alexandre Leclair 1 Important literature Modern Physical Organic Chemistry by Eric. V. Anslyn and Dennis A. Dougherty - University Science Books: Sausalito, CA, 2006 → Chapter 8 – Experiments Related to Thermodynamics and Kinetics – 421-441. Annual Reports on NMR Spectroscopy, Chapter 3 – Application of NMR spectroscopy in Isotope Effects Studies by Stefan Jankowski - (Ed.: G.A. Webb), Academic Press, 2009, 149–191. E. M. Simmons, J. F. Hartwig, Angew. Chem. Int. Ed. 2012, 51, 3066-3072. M. Gomez-Gallego, M. A. Sierra, Chem. Rev. 2011, 111, 4857-4963 2 Table of contents I. Origins of the Kinetic Isotope Effect II. Types of Kinetic Isotope Effect III.Classical experiments IV.KIE measured at natural-abundance V. Conclusion and Outlook 3 Table of contents I. Origins of the Kinetic Isotope Effect II. Types of Kinetic Isotope Effect III.Classical experiments IV.KIE measured at natural-abundance V. Conclusion and Outlook 4 Origins of the Kinetic Isotope Effect (KIE) Origin of isotope effects: • Difference in frequencies of various vibrational modes of a molecule → No important difference in the potential energy of the system → However, difference in vibrational states, quantified by the formula: en = (n + ½)hν, where n= 0,1,2,… • The vibrational modes for bond stretches → dominated by n=0, with e0=1/2hν e0 = zero-point energy (ZPE) Morse potential and vibrational energy levels: M. Gomez-Gallego, M. A. Sierra, Chem. Rev. 2011, 111, 4857-4963 5 Origins of the Kinetic Isotope Effect (KIE) Importance of the reduced mass and force constant: en = (n + ½)hν, where n= 0,1,2,… 1 퐤 1 Vibration frequency ν : Proportional to ν= 퐦 2π 퐦퐫 퐫 With mr: reduced mass and k: force constant m m 1 2 m1: “Heavy” atom (C, N, O, …) 퐦퐫 = m1 + m2 m2: “Light” atom (H, D, …) Implications in KIE: H and D different mass (1.00797 / 2.01410) So for C-H vs C-D: mr(C-H) < mr(C-D) → νC-H > νC-D ZPE(C-H) > ZPE(C-D) Scheme: A.
    [Show full text]
  • Kinetic Isotope Effects of [Superscript 12]CH[Subscript 3]D##+#OH and [Superscript 13]CH[Subscript 3]D##+#OH from 278 to 313#K
    Kinetic isotope effects of [superscript 12]CH[subscript 3]D##+#OH and [superscript 13]CH[subscript 3]D##+#OH from 278 to 313#K The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Joelsson, L. M. T. et al. “Kinetic Isotope Effects of 12CH3D + OH and 13CH3D + OH from 278 to 313 K.” Atmospheric Chemistry and Physics 16.7 (2016): 4439–4449. As Published http://dx.doi.org/10.5194/acp-16-4439-2016 Publisher Copernicus GmbH Version Final published version Citable link http://hdl.handle.net/1721.1/110743 Terms of Use Creative Commons Attribution Detailed Terms http://creativecommons.org/licenses/by/3.0/ Atmos. Chem. Phys., 16, 4439–4449, 2016 www.atmos-chem-phys.net/16/4439/2016/ doi:10.5194/acp-16-4439-2016 © Author(s) 2016. CC Attribution 3.0 License. 12 13 Kinetic isotope effects of CH3D C OH and CH3D C OH from 278 to 313 K L. M. T. Joelsson1,3, J. A. Schmidt1, E. J. K. Nilsson2, T. Blunier3, D. W. T. Griffith4, S. Ono5, and M. S. Johnson1 1Department of Chemistry, University of Copenhagen, Copenhagen, Denmark 2Division of Combustion Physics, Department of Physics, Lund University, Lund, Sweden 3Centre for Ice and Climate (CIC), Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark 4University of Wollongong, Department of Chemistry, Wollongong, Australia 5Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA Correspondence to: M. S. Johnson ([email protected]) Received: 16 September 2015 – Published in Atmos. Chem.
    [Show full text]
  • Kinetic Isotope Effects of 12CH3D+OH and 13CH3D+OH from 278 to 313 K
    Atmos. Chem. Phys., 16, 4439–4449, 2016 www.atmos-chem-phys.net/16/4439/2016/ doi:10.5194/acp-16-4439-2016 © Author(s) 2016. CC Attribution 3.0 License. 12 13 Kinetic isotope effects of CH3D C OH and CH3D C OH from 278 to 313 K L. M. T. Joelsson1,3, J. A. Schmidt1, E. J. K. Nilsson2, T. Blunier3, D. W. T. Griffith4, S. Ono5, and M. S. Johnson1 1Department of Chemistry, University of Copenhagen, Copenhagen, Denmark 2Division of Combustion Physics, Department of Physics, Lund University, Lund, Sweden 3Centre for Ice and Climate (CIC), Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark 4University of Wollongong, Department of Chemistry, Wollongong, Australia 5Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA Correspondence to: M. S. Johnson ([email protected]) Received: 16 September 2015 – Published in Atmos. Chem. Phys. Discuss.: 15 October 2015 Revised: 2 March 2016 – Accepted: 16 March 2016 – Published: 11 April 2016 Abstract. Methane is the second most important long-lived source signal with relatively small adjustment due to the sink greenhouse gas and plays a central role in the chemistry signal (i.e., mainly OH oxidation). of the Earth’s atmosphere. Nonetheless there are signifi- cant uncertainties in its source budget. Analysis of the iso- topic composition of atmospheric methane, including the 13 doubly substituted species CH3D, offers new insight into 1 Introduction the methane budget as the sources and sinks have dis- tinct isotopic signatures. The most important sink of at- Atmospheric methane is the subject of increasing interest mospheric methane is oxidation by OH in the troposphere, from both the climate research community and the pub- which accounts for around 84 % of all methane removal.
    [Show full text]
  • Use of Isotopes for Studying Reaction Mechanisms 2
    View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Publications of the IAS Fellows SERIES ⎜ ARTICLE Use of Isotopes for Studying Reaction Mechanisms 2. Primary Kinetic Isotope Effect Uday Maitra and J Chandrasekhar The effects of isotopic substitution on equilibria and reaction Uday Maitra and rates are described. In particular, the mechanistic details J Chandrasekhar are which can be obtained by quantifying the primary kinetic members of the Organic isotope effect are illustrated. Chemistry faculty in Indian Institute of Science at Bangalore. Introduction The previous article of this In the first part of this series we discussed how isotopes can be series was: used as markers to determine the nature of intermediates in 1. Isotopes as markers, May chemical reactions. The focus in such studies is primarily the 1997. location and distribution of the label in the product(s). There is a more subtle effect of isotopes which when studied quantitatively can give even greater details of chemical processes. Isotopic substitution can influence an equilibrium, sometimes dramatically. For example, the equilibria are shifted significantly to the right for the two cases given below. º HT + H2O H2 + HTO Keq = 6.3 (1) + º + 2D3O + 3 H2O 2H3O +3 D2OKeq = 8.2 (2) Isotopes also affect reaction rates. The magnitude of this kinetic isotope effect (KIE) would depend on the location of the isotope with respect to the ‘scene of action’! Therefore the following types of effects are commonly encountered: (a) primary KIE, (b) secondary KIE (a and b types) and (c) steric KIE.
    [Show full text]
  • Isotope Effects As Probes for Enzyme Catalyzed Hydrogen-Transfer Reactions
    Molecules 2013, 18, 5543-5567; doi:10.3390/molecules18055543 OPEN ACCESS molecules ISSN 1420-3049 www.mdpi.com/journal/molecules Review Isotope Effects as Probes for Enzyme Catalyzed Hydrogen-Transfer Reactions Daniel Roston †, Zahidul Islam † and Amnon Kohen * Department of Chemistry, The University of Iowa, Iowa City, IA 52242, USA † These authors contributed equally to this work. * Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel.: +1-319-335-0234. Received: 11 April 2013; in revised form: 30 April 2013 / Accepted: 3 May 2013 / Published: 14 May 2013 Abstract: Kinetic Isotope effects (KIEs) have long served as a probe for the mechanisms of both enzymatic and solution reactions. Here, we discuss various models for the physical sources of KIEs, how experimentalists can use those models to interpret their data, and how the focus of traditional models has grown to a model that includes motion of the enzyme and quantum mechanical nuclear tunneling. We then present two case studies of enzymes, thymidylate synthase and alcohol dehydrogenase, and discuss how KIEs have shed light on the C-H bond cleavages those enzymes catalyze. We will show how the combination of both experimental and computational studies has changed our notion of how these enzymes exert their catalytic powers. Keywords: kinetic isotope effects; Marcus-like models; alcohol dehydrogenase; thymidylate synthase; hydrogen tunneling 1. Introduction One of the most powerful tools in the chemist’s arsenal for studying reaction mechanisms and transition state structures is to measure kinetic isotope effects (KIEs). KIEs have become a pivotal source of information on enzymatic reactions, and a number of recent reviews have described the successes of KIEs in enzymology, from determining molecular mechanisms and transition state Molecules 2013, 18 5544 structures that are useful for drug design [1], to answering questions on the roles of nuclear quantum tunneling [2,3], electrostatics [4], and dynamic motions [5–8] in enzyme catalyzed reactions.
    [Show full text]
  • Experimentally Determined Kinetic Isotope Effects in The
    GEOPHYSICAL RESEARCH LETTERS, VOL. 27, NO. 12, PAGES 1715-1718,JUNE 15, 2000 Experimentally determinedkinetic isotopeeffects in the reaction of CH4 with CI: Implicationsfor atmosphericCH4 StanleyC. Tyler, Henry O. Ajie, Andrew L. Rice, andRalph J. Cicerone Earth SystemScience Department, University of California, Irvine Ernesto C. Tuazon Air PollutionResearch Center, University of California, Riverside Abstract. We report experimental values for the carbon and agreement[Atkinson et al., 1999]. While generally considered hydrogenkinetic isotope effects (KIEs) in the reactionof CH4 only a minor loss processin the troposphere,(1) becomesmore with C1 at temperaturesbetween 273 and 350 K. Isotoperatio important in the stratospherewhere it converts C1 atoms into massspectrometry wasutilized to measure 13CH4/12CH4 and HC1. Its importancein the marine boundarylayer (MBL) is not CH3D/CH4 ratioson samplestaken from a 5870- L reaction yet established because of uncertainty in MBL C1 radical chamber.At 299K, kc12/kcl3 = 1.0621+0.0001(2a) andkh/k d = concentrations. Overall, the effect of C1 has been calculated to 1.474+0.020 (2a). For both KIEs, the ratio decreased with accountfor from 2 to 4% of all atmosphericCH 4 loss[e.g., increasing temperature over the range studied. These results Gupta et al., 1996]. agree well with experimentalstudies using tunable diode laser Both measured and theoretically calculated values for the absorptionspectroscopy and FTIR absorptionspectroscopy and kinetic isotope effects (KIEs) in (1) are much larger than for a with a recenttheoretically-calculated set of values. similarreaction involving CH 4 andOH, the principalsink for atmosphericCH 4. This makes(1) of increasedimportance to researchersusing isotopicmeasurements to study the CH4 1.
    [Show full text]
  • Carbon-13 Kinetic Isotope Effects of Catalytic CO2 Reduction by Polypyridyl Transition Metal Complexes
    University of Connecticut Masthead Logo OpenCommons@UConn Doctoral Dissertations University of Connecticut Graduate School 12-18-2018 Carbon-13 Kinetic Isotope Effects of Catalytic CO2 Reduction by Polypyridyl Transition Metal Complexes Taylor William Schneider University of Connecticut - Storrs, [email protected] Follow this and additional works at: https://opencommons.uconn.edu/dissertations Recommended Citation Schneider, Taylor William, "Carbon-13 Kinetic Isotope Effects of Catalytic CO2 Reduction by Polypyridyl Transition Metal Complexes" (2018). Doctoral Dissertations. 2142. https://opencommons.uconn.edu/dissertations/2142 Carbon-13 Kinetic Isotope Effects of Catalytic CO2 Reduction by Polypyridyl Transition Metal Complexes Taylor Schneider, Ph.D. University of Connecticut, 2019 Abstract 13 Herein, we have used carbon-13 kinetic isotope effects ( C KIEs) to probe the catalytic CO2 reduction mechanisms of two different polypyridyl transition metal complexes, Re(bpy)(CO)3Cl (1), which will convert CO2 to CO, and [Ru(tpy)(bpy)Cl]PF6 (2) (bpy = 2,2’-bipyridine; tpy = 2,2’:6’,2”- terpyridine), which will convert CO2 to a mixture of CO and formic acid. We used several sets of reaction conditions to drive CO2 reduction. These included sodium mercury (Na(Hg)) amalgam as a sacrificial reducing agent, photocatalytic conditions in which triethanolamine (TEOA) was a sacrificial electron donor, and electrocatalytic conditions in which the reaction potential was applied by a potentiostat. Under some photocatalytic reactions with 1 and all photocatalytic reactions with 2, the photosensitizer [Ru(bpy)3]Cl2 was included in the reaction mixture. Furthermore, photocatalytic CO2 reduction by 1 was performed in two different solvents, acetonitrile (ACN) and dimethylformamide (DMF), whereas all experiments by 2 were performed in ACN.
    [Show full text]