DOCSLIB.ORG

Diastereoisomers As Probes for Solvent Reorganizational Effects On

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Diastereoisomers As Probes for Solvent Reorganizational Effects On Chemical Physics 324 (2006) 8–25 www.elsevier.com/locate/chemphys Diastereoisomers as probes for solvent reorganizational effects on IVCT in dinuclear ruthenium complexes Deanna M. DÕAlessandro, F. Richard Keene * Department of Chemistry, School of Pharmacy and Molecular Sciences, James Cook University, Townsville, Qld. 4811, Australia Received 19 June 2005; accepted 1 September 2005 Available online 3 October 2005 Abstract 5+ 0 0 IVCT solvatochromism studies on the meso and rac diastereoisomers of [{Ru(bpy)2}2(l-bpm)] (bpy = 2,2 -bipyridine; bpm = 2,2 - bipyrimidine) in a homologous series of nitrile solvents revealed that stereochemically directed specific solvent effects in the first solvation shell dominated the outer sphere contribution to the reorganizational energy for intramolecular electron transfer. Further, solvent pro- portion experiments in acetonitrile/propionitrile mixtures indicated that the magnitude and direction of the specific effect was dependent on the relative abilities of discrete solvent molecules to penetrate the clefts between the planes of the terminal polypyridyl ligands. In particular, the specific effects were dependent on the dimensionality of the clefts, and the number, size, orientation and location of the solvent dipoles within the interior and exterior clefts. 5+ 5+ IVCT solvatochromism studies on the diastereoisomeric forms of [{Ru(bpy)2}2(l-dbneil)] and [{Ru(pp)2}2(l-bpm)] {dbneil = dibenzoeilatin; pp = substituted derivatives of 2,20-bipyridine and 1,10-phenanthroline} revealed that the subtle and systematic changes in the nature of the clefts by the variation of the bridging ligand, and the judicious positioning of substituents on the terminal ligands, profoundly influenced the magnitude of the reorganizational energy contribution to the electron transfer barrier. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Solvatochromism; Intervalence charge transfer; Dinuclear; Stereochemistry; Ruthenium; Solvent reorganization 1. Introduction The particular appeal of mixed-valence complexes of the II III 5+ form [{LnM }(l-BL){M Ln}] (M = metal centers, Dinuclear ligand-bridged mixed-valence complexes have L = terminal ligands and BL = bridging ligand) is the played a pivotal role in the assessment of activation barri- observation of an absorption band in the near-infrared ers to intramolecular electron transfer since the disclosure (NIR) region of the electronic spectrum which is identified of the Creutz–Taube ion, [{Ru(NH3)5}(l-pyz){Ru- as the optically induced intervalence charge transfer 5+ (NH3)5}] (pyz = pyrazine), in 1973 [1]. Systems of this (IVCT) transition. IVCT measurements provide a sensitive genre have provided important experimental insights into and powerful probe to elucidate aspects of intramolecular the roles of solvent dynamics [2–15], ion-pairing [16–21], electron transfer processes as the energy (mmax), intensity encapsulation [22,23], temperature [24–28], and redox (e) and bandwidth (Dm1/2) of these transitions can be quan- asymmetry [29,30], and they have been used as model sys- titatively related to the factors which influence the activa- tems to verify the salient predictions of several important tion barrier to electron transfer [31,32]. theoretical models that describe the activation barrier to For symmetrical, valence-localized mixed-valence sys- electron transfer [31–35]. tems, Hush [31,32] proposed the relationship 0 mmax ¼ ki þ ko þ DE ; ð1Þ 0 * Corresponding author. Tel.: +61 0 7 47814433; fax: +61 0 7 47816078. where DE represents the energy contributions due to E-mail address: [email protected] (F.R. Keene). spin–orbit coupling and/or ligand field asymmetry, and 0301-0104/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2005.09.016 D.M. DÕAlessandro, F.R. Keene / Chemical Physics 324 (2006) 8–25 9 ki and ko are the inner- and outer-sphere reorganizational The elucidation of the relative contributions of contin- (Franck–Condon) parameters, respectively: ki corresponds uum and non-continuum effects is the subject of consider- to the energy required for reorganization of the metal–li- able experimental interest in the attempt to develop more gand and intra-ligand bond lengths and angles, and ko is sophisticated theoretical models for solvent reorganiza- the energy required for reorganization of the surrounding tional contributions to the electron transfer barrier [2,40]. solvent medium. The solvent contribution is generally Dielectric continuum theory obscures the ‘‘molecularity’’ modelled as a one-dimensional classical mode due to the of the solvent by neglecting the properties of individual sol- low frequencies of the coupled vibrations [31–34]. The vent molecules, and this underpins the recent theoretical spherical cavity dielectric continuum model given by Eq. interest directed towards understanding the molecular basis (2) provides a framework for the calculation of the sol- of reorganizational effects [2,40]. vent reorganizational contribution in which the electron The experimental strategy for extracting information at donor and acceptor are modelled as two non-interpene- the molecular level using IVCT solvatochromism studies trating spheres, embedded in the dielectric continuum involves probing the first solvation shell separately from [31,32]. the bulk solution. Dinuclear ruthenium complexes incorpo- 1 1 1 1 rating ammine and cyano ligands have been extensively k ¼ e2 À À . ð2Þ o a d D D investigated in this regard because of the existence of op s strong directional H-bonding and donor–acceptor interac- The parameters a and d define the molecular radii and dis- tions between the chromophore ligands and individual sol- tance between the donor and acceptor, e is the unit elec- vent molecules [6,11–15,41]. These specific solvent tronic charge, and Ds and Dop are the macroscopic static interactions coexist with, and often dominate dielectric and optical dielectric constants of the solvent, respectively. continuum effects [2]. Correlations have been found be- In accordance with Eqs. (1) and (2), mmax should vary line- tween the IVCT solvent shifts and empirical solvent param- arly with the solvent dielectric function (1/Dop À 1/Ds), eters such as the Gutmann donor and acceptor numbers 2 0 with slope e (1/a À 1/d) and intercept ki + DE at (1/ [42]. In studies of dinuclear ruthenium mixed-valence com- Dop À 1/Ds) = 0, and when the length of the bridging li- plexes based on –Ru(NH3)5,–trans-Ru(NH3)4(py) and 0 gand is varied (at fixed a) mmax should vary linearly with –Ru(bpy)(NH3)3 with pyz, 4-cyanopyridine and 4,4 -bpy 2 1/d, with slope e (1/Dop À 1/Ds) in a given solvent. bridging ligands [19,43,44], the IVCT energies correlate lin- While the predictions of the dielectric continuum model early with the Gutmann solvent donor number (DN) due have been consistent with the results from a number of to specific H-bonding interactions between the ammine li- IVCT solvatochromism studies of mixed-valence dinuclear gands and the solvent molecules. In each case, the magni- ruthenium complexes [2,36,37], the model breaks down tude of the specific interaction increased with the donor when the underlying assumptions of the classical model number of the solvent, and the number of NH3 ligands in are invalidated, or in the presence of specific solvent–solute the chromophore. interactions or dielectric saturation effects [2]. Theoreti- IVCT solvatochromism studies in solvent mixtures have cally, the dielectric continuum model is formulated in terms demonstrated that the solvent reorganizational process oc- of a one-dimensional classical mode for the solvent, but it curs predominantly within the first solvation layer, and is inadequate for systems which exhibit coupled high-fre- may be profoundly influenced by the systematic replace- quency quantum modes which must be explicitly treated ment of individual solvent molecules in the immediate through a quantum mechanical approach [15]. Eq. (2) also vicinity of the mixed-valence chromophore neglects the volume occupied by the donor and acceptor [2,5,7,13,14,19,45]. (the ‘‘excluded volume’’) and is valid only when the dis- There is clearly a need for experimental studies of IVCT tance between the redox centers exceeds the sum of their ra- solvatochromism which provide insights into the micro- dii (d 2a). The corrections due to non-spherical fields scopic solvent reorganizational contributions to the intra- around the metal centers become increasingly important molecular electron transfer barrier. as the distance between the metal centers is decreased. Experimentally, the analysis of IVCT solvatochromism 1.1. Scope and objectives of the present study data according to the spherical (and ellipsoidal [13,15,38,39]) dielectric continuum models has often been The majority of experimental IVCT studies have been severely confounded by non-continuum effects. These is- conducted by varying global features of the complexes, sues have been addressed in an extensive review of medium such as the identities of the bridging and terminal ligands, effects on the IVCT properties of mixed-valence complexes and the constituent metal centers. In addition, the theoret- by Chen and Meyer [2], and include specific solvent–solute ical implications of the results have often been complicated interactions and dielectric saturation effects, in addition to by ion-pairing, and ambiguities in the geometries of the ion-pairing contributions from the counter and electrolyte complexes due to a lack of structural rigidity and/or stereo- ions, the concentration of the chromophore and the chem- isomeric purity [46,47]. ical oxidant used for the generation of the mixed-valence In the present study, the investigation of the IVCT sol- complex. vatochromism in the mixed-valence diastereoisomeric 10 D.M. DÕAlessandro, F.R. Keene / Chemical Physics 324 (2006) 8–25 5+ 0 forms of [{Ru(pp)2}2(l-bpm)] (pp = 2,2 -bipyridine and uents on the terminal polypyridyl ligands – while maintain- its derivatives; bpm = 2,20-bipyrimidine) provides a new ing the identity and coordination environments of the experimental approach to probe the microscopic origins component metal centers.
Load more

Recommended publications
  • Solvent Effects on Structure and Reaction Mechanism: a Theoretical Study of [2 + 2] Polar Cycloaddition Between Ketene and Imine
    J. Phys. Chem. B 1998, 102, 7877-7881 7877 Solvent Effects on Structure and Reaction Mechanism: A Theoretical Study of [2 + 2] Polar Cycloaddition between Ketene and Imine Thanh N. Truong Henry Eyring Center for Theoretical Chemistry, Department of Chemistry, UniVersity of Utah, Salt Lake City, Utah 84112 ReceiVed: March 25, 1998; In Final Form: July 17, 1998 The effects of aqueous solvent on structures and mechanism of the [2 + 2] cycloaddition between ketene and imine were investigated by using correlated MP2 and MP4 levels of ab initio molecular orbital theory in conjunction with the dielectric continuum Generalized Conductor-like Screening Model (GCOSMO) for solvation. We found that reactions in the gas phase and in aqueous solution have very different topology on the free energy surfaces but have similar characteristic motion along the reaction coordinate. First, it involves formation of a planar trans-conformation zwitterionic complex, then a rotation of the two moieties to form the cycloaddition product. Aqueous solvent significantly stabilizes the zwitterionic complex, consequently changing the topology of the free energy surface from a gas-phase single barrier (one-step) process to a double barrier (two-step) one with a stable intermediate. Electrostatic solvent-solute interaction was found to be the dominant factor in lowering the activation energy by 4.5 kcal/mol. The present calculated results are consistent with previous experimental data. Introduction Lim and Jorgensen have also carried out free energy perturbation (FEP) theory simulations with accurate force field that includes + [2 2] cycloaddition reactions are useful synthetic routes to solute polarization effects and found even much larger solvent - formation of four-membered rings.
    [Show full text]
  • Solvents and Solvent E¤Ects in Organic Chemistry
    Contents 1 Introduction ......................................................... 1 2 Solute-Solvent Interactions ........................................... 7 2.1 Solutions . .......................................................... 7 2.2 Intermolecular Forces . ............................................ 12 2.2.1 Ion-Dipole Forces . ................................................. 13 2.2.2 Dipole-Dipole Forces................................................ 14 2.2.3 Dipole-Induced Dipole Forces ....................................... 15 2.2.4 Instantaneous Dipole-Induced Dipole Forces . ...................... 16 2.2.5 Hydrogen Bonding . ................................................. 17 2.2.6 Electron-Pair Donor/Electron-Pair Acceptor Interactions (EPD/EPA Interactions) . ..................................................... 23 2.2.7 Solvophobic Interactions ............................................ 31 2.3 Solvation . .......................................................... 34 2.4 Preferential Solvation................................................ 43 2.5 Micellar Solvation (Solubilization) ................................... 48 2.6 Ionization and Dissociation . ........................................ 52 3 Classification of Solvents ............................................. 65 3.1 Classification of Solvents according to Chemical Constitution ......... 65 3.2 Classification of Solvents using Physical Constants . .................. 75 3.3 Classification of Solvents in Terms of Acid-Base Behaviour. ......... 88 3.3.1 Brønsted-Lowry
    [Show full text]
  • S.T.E.T.Women's College, Mannargudi Semester Iii Ii M
    S.T.E.T.WOMEN’S COLLEGE, MANNARGUDI SEMESTER III II M.Sc., CHEMISTRY ORGANIC CHEMISTRY - II – P16CH31 UNIT I Aliphatic nucleophilic substitution – mechanisms – SN1, SN2, SNi – ion-pair in SN1 mechanisms – neighbouring group participation, non-classical carbocations – substitutions at allylic and vinylic carbons. Reactivity – effect of structure, nucleophile, leaving group and stereochemical factors – correlation of structure with reactivity – solvent effects – rearrangements involving carbocations – Wagner-Meerwein and dienone-phenol rearrangements. Aromatic nucleophilic substitutions – SN1, SNAr, Benzyne mechanism – reactivity orientation – Ullmann, Sandmeyer and Chichibabin reaction – rearrangements involving nucleophilic substitution – Stevens – Sommelet Hauser and von-Richter rearrangements. NUCLEOPHILIC SUBSTITUTION Mechanism of Aliphatic Nucleophilic Substitution. Aliphatic nucleophilic substitution clearly involves the donation of a lone pair from the nucleophile to the tetrahedral, electrophilic carbon bonded to a halogen. For that reason, it attracts to nucleophile In organic chemistry and inorganic chemistry, nucleophilic substitution is a fundamental class of reactions in which a leaving group(nucleophile) is replaced by an electron rich compound(nucleophile). The whole molecular entity of which the electrophile and the leaving group are part is usually called the substrate. The nucleophile essentially attempts to replace the leaving group as the primary substituent in the reaction itself, as a part of another molecule. The most general form of the reaction may be given as the following: Nuc: + R-LG → R-Nuc + LG: The electron pair (:) from the nucleophile(Nuc) attacks the substrate (R-LG) forming a new 1 bond, while the leaving group (LG) departs with an electron pair. The principal product in this case is R-Nuc. The nucleophile may be electrically neutral or negatively charged, whereas the substrate is typically neutral or positively charged.
    [Show full text]
  • Nucleophilic Substitution and Elimination Reactions
    8 NUCLEOPHILIC SUBSTITUTION AND ELIMINATION REACTIONS substitution reactions involve the replacement of one atom or group (X) by another (Y): We already have described one very important type of substitution reaction, the halogenation of alkanes (Section 4-4), in which a hydrogen atom is re- placed by a halogen atom (X = H, Y = halogen). The chlorination of 2,2- dimethylpropane is an example: CH3 CH3 I I CH3-C-CH3 + C12 light > CH3-C-CH2Cl + HCI I I Reactions of this type proceed by radical-chain mechanisms in which the bonds are broken and formed by atoms or radicals as reactive intermediates. This 8-1 Classification of Reagents as Electrophiles and Nucleophiles. Acids and Bases mode of bond-breaking, in which one electron goes with R and the other with X, is called homolytic bond cleavage: R 'i: X + Y. - X . + R : Y a homolytic substitution reaction There are a large number of reactions, usually occurring in solution, that do not involve atoms or radicals but rather involve ions. They occur by heterolytic cleavage as opposed to homolytic cleavage of el~ctron-pairbonds. In heterolytic bond cleavage, the electron pair can be considered to go with one or the other of the groups R and X when the bond is broken. As one ex- ample, Y is a group such that it has an unshared electron pair and also is a negative ion. A heterolytic substitution reaction in which the R:X bonding pair goes with X would lead to RY and :X? R~:X+ :YO --' :x@+ R :Y a heterolytic substitution reaction A specific substitution reaction of this type is that of chloromethane with hydroxide ion to form methanol: In this chapter, we shall discuss substitution reactions that proceed by ionic or polar mechanisms' in which the bonds cleave heterolytically.
    [Show full text]
  • Chapter 6 Ionic Reactions-Nucleophilic Substitution
    Introduction ÎThe polarity of a carbon-halogen bond leads to the carbon having a partial positive charge + In alkyl halides this polarity causes the carbon to become activated to substitution reactions with nucleophiles Chapter 6 Ionic Reactions-Nucleophilic ÎCarbon-halogen bonds get less polar, longer and weaker in going Substitution and Elimination Reactions from fluorine to iodine of Alkyl Halides Chapter 6 2 Alkyl Halides and Nucleophilic Substitution Alkyl Halides and Nucleophilic Substitution General Features of Nucleophilic Substitution • Three components are necessary in any substitution reaction. The Polar Carbon-Halogen Bond Chapter 6 3 Chapter 6 4 1 Alkyl Halides and Nucleophilic Substitution Alkyl Halides and Nucleophilic Substitution General Features of Nucleophilic Substitution General Features of Nucleophilic Substitution • Negatively charged nucleophiles like HO¯ and HS¯ are used as • Furthermore, when the substitution product bears a positive salts with Li+, Na+, or K+ counterions to balance the charge. charge and also contains a proton bonded to O or N, the initially Since the identity of the counterion is usually inconsequential, it formed substitution product readily loses a proton in a is often omitted from the chemical equation. BrØnsted-Lowry acid-base reaction, forming a neutral product. • When a neutral nucleophile is used, the substitution product bears a positive charge. Chapter 6 5 Chapter 6 6 Leaving Groups The Nucleophile A leaving group is a substituent that can leave as a relatively A nucleophile may be any molecule with an unshared electron pair stable entity It can either leave as an anion…. …or as a neutral molecule We’ll discuss later what makes a nucleophile strong or weak (good or bad) Chapter 6 7 Chapter 6 8 2 Alkyl Halides and Nucleophilic Substitution Alkyl Halides and Nucleophilic Substitution The Leaving Group The Leaving Group • In a nucleophilic substitution reaction of R—X, the C—X bond is heterolytically cleaved, and the leaving group departs with the electron pair in that bond, forming X:¯.
    [Show full text]
  • Investigation of Factors Influencing the Stereochemistry of the Wittig Reaction
    THE INVESTIGATION OF FACTORS INFLUENCING THE STEREOCHEMISTRY OF THE WITTIG REACTION By JEROME THOMAS KRESSE A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA December, 1965 ACKNOWLEDGMENTS The author wishes to express his appreciation to his research director, Dr. George B. Butler, for his guidance and encouragement during the execution of this work. The author also expresses his gratitude to his fellow graduate students and associates for their helpful suggestions and criticisms. Particular thanks are due Mrs. Frances Kost and Mrs. Thyra Johnston for their conscientious typing of this dissertation. The author also thanks his wife for her patience, encouragement and under- standing. Without her cooperation this work would not have been possible. The financial support of the Petroleum Research Fund is also gratefully acknowledged. TABLE OF CONTENTS PAGE ACKNOWLEDGMENTS ii LIST OF TABLES v LIST OF FIGURES vi CHAPTER I INTRODUCTION 1 Historical Background 1 Stereochemistry 9 Statement of the Problem 19 Method of Attack 20 H DISCUSSION AND RESULTS 22 Solvents 22 Temperature Effects 27 Reaction Times 33 Concentration Effects 34 Substituent Effects 37 Anion Effects 49 1, 4 -Addition and Isomerization 51 TABLE OF CONTEXTS (Continued) CHAPTER PAGE EI EXPERIMENTAL 54 Equipment and Data 54 Source and Purification of Materials 54 Solvents 55 Aldehydes " 57 Miscellaneous Chemicals 57 Preparation and Purification of Phosphonium
    [Show full text]
  • Kinetic Solvent Effects on 1,3-Dipolar Cycloadditions of Benzonitrile Oxide
    CORE Metadata, citation and similar papers at core.ac.uk JOURNAL OF PHYSICAL ORGANIC CHEMISTRY Provided by UniversityJ. Phys. of Org. Groningen Chem. Digital2005; Archive18: 908–917 Published online 13 April 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.917 Kinetic solvent effects on 1,3-dipolar cycloadditions of benzonitrile oxide Theo Rispens and Jan B. F. N. Engberts* Physical Organic Chemistry Unit, Stratingh Institute, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands Received 21 December 2004; revised 7 February 2005; accepted 7 February 2005 ABSTRACT: The kinetics of 1,3-dipolar cycloadditions of benzonitrile oxide with a series of N-substituted maleimides and with cyclopentene are reported for water, a wide range of organic solvents and binary solvent mixtures. The results indicate the importance of both solvent polarity and specific hydrogen-bond interactions in governing the rates of the reactions. The aforementioned reactions are examples for which these factors often counteract, leading to a complex dependence of rate constants on the nature of the solvent. For the reactions of N- ethylmaleimide and N-n-butylmaleimide with benzonitrile oxide, isobaric activation parameters have been deter- mined in several organic solvents, water, and water–1-propanol mixtures. Interestingly, the activation parameters reveal significant differences in solvation in different solvents that are not clearly reflected in the rate constants. In highly aqueous mixtures, enforced hydrophobic interactions lead to an increase in rate constant, relative to organic solvents. However, the overall rate enhancement in water is modest, if present at all, because the solvent polarity diminishes the rate constant.
    [Show full text]
  • Reactions in Organic Compounds HOMO LUMO Reaction Energy Gain
    Reactions in Organic Compounds As we first learned with acid/base reactions with Lewis definition, any reaction can be considered as a nucleophile reacting with an electrophile electrophile The closer in energy LUMO the HOMO is to the reaction LUMO means there will be a greater Energy gain energy gain HOMO nucleophile product All reactions thus involve a filled molecular orbital (called HOMO, representing the nucleophile) reacting with an empty molecular orbital (called LUMO, representing the electrophile) electrophile nucleophile product Studying an Organic Reaction How do we know if a reaction can occur? And – if a reaction can occur what do we know about the reaction? CH3ONa CH3Cl CH3OCH3 NaCl Information we want to know: How much heat is generated? (What is the THERMODYNAMICS of the reaction?) How fast is the reaction? (What is the KINETICS of the reaction?) Are any intermediates generated? (What is the reaction mechanism?) All of this information is included in an Energy Diagram Possible Mechanism 1 Possible Mechanism 2 Transition Transition states states Potential Potential Intermediate energy energy Starting material Starting Products material Products Reaction Coordinate Reaction Coordinate If we know the “shape” of the reaction coordinate, then all questions about the mechanism can be answered (thermodynamics and kinetics) Equilibrium Constants Equilibrium constants (Keq) indicate thermodynamically whether the reaction is favored in the forward or reverse direction and the magnitude of this preference A B C D ΔG Keq = ([C][D]) /
    [Show full text]
  • Kinetic and Mechanistic Studies of Free Radical Reactions in the 21St Century
    Pure & Appl. Chem., Vol. 69, No. 2, pp. 241-243, 1997. Printed in Great Britain. (0 1997 IUPAC Kinetic and mechanistic studies of free radical reactions in the 21st century K.U. lngold Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, Ontario, KIA OR6, Canada Biography: Keith U. Ingold received his B. Sc. degree from University College London and his D. Phil. from Oxford. He holds the position of Distinguished Research Scientist in the Steacie Institute for Molecular Sciences, National Research Council of Canada, and his research has concentrated on the chemistry of free radicals in solution. The past 50 years has seen an incredible growth in our understanding of free radical reactions in homogene- ous gaseous and liquid systems. Fifty years ago, so far as the vast majority of chemists were concerned, radicals were overly reactive species of no practical value or interest since all radical-mediated reactions were presumed to give gunk and tars. This negative view of radical reactions persisted even long after the successful commercialization of the free radical polymerization of vinyl monomers. Half a century ago the prevailing view of an organic or physical organic chemist about any of their colleagues who expressed an interest in research into free radical reactions can best be summarized by a joking remark once made by the father of physical organic chemistry: "Homolysis, even between consenting adults, is grounds for instant dismissal from this Department" C. K. Ingold, ca. 1955 The idea that radical chain reactions might actually be useful in organic synthesis was regarded with scorn, and the idea that radical chemistry might be important in living organisms was dismissed as utter nonsense.
    [Show full text]
  • A Model for the Calculation of Solvent Effects on Reaction Rates for Process Design Purposes
    A Model For the Calculation of Solvent Effects on Reaction Rates for Process Design Purposes by Josio Paulo Dantas Aumond M.S. in Chemical Engineering Practice, MIT (1995) Engenheiro Quimico, Universidade de Sdo Paulo (1992) Submitted to the Department of Chemical Engineering in partial fulfillment of the requirement for the degree of DOCTOR OF PHILOSOPHY in Chemical Engineering at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY June 1999 @ Massachusetts Institute of Technology. All rights reserved. Signature of Author 'Department of Chemical Engineering April 5th, 1999 Certified by Cte< y ge Stephagopoulos Arthur D. LittI Profess r of Chemical Engineering T)esis Supervisor Accepted by Robert E. Cohen St. Laurent Professor of Chemical Engineering Chairman, Committee for Graduate Students LIBRAR iU A Model For the Calculation of Solvent Effects on Reaction Rates for Process Design Purposes by Joio Paulo Dantas Aumond Submitted to the Department of Chemical Engineering on March 3rd 1999 in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemical Engineering Abstract Solvents are used in chemical processes as separation and cleaning agents as well as reaction media. Their selection has a significant impact on the environmental impact, efficiency and profitability of a process. For this reason it would be desirable that solvents be chosen with respect not only to their effectiveness in their respective process tasks but also for process-wide requirements such as their ease of recovery, low toxicity and environmental impact and possible applicability to other process tasks. Although there are models for the evaluation of solvents as separation agents, there is a need for models for the assessment of solvents as reaction media that are fast, and therefore able to screen large numbers of molecules in a reasonable time, as well as applicable to a wide range of solvents and reactions and able to provide results that are accurate for process design needs.
    [Show full text]
  • Mechanisms for Solvolytic Elimination and Substitution Reactions Involving Short-Lived Carbocation Intermediates
    ! " # $ % & # % '( )%*+",- .",- ** ,%/"$%*% 0*1%",% 00*1* 2332 ACTA UNIVERSITATIS UPSALIENSIS Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 743 Distributor: Uppsala University Library, Box 510, SE-751 20 Uppsala, Sweden Xiaofeng Zeng Mechanisms for Solvolytic Elimination and Substitution Reactions Involving Short-Lived Carbocation Intermediates Dissertation in Organic Chemistry to be publicly examined in The Svedberg Lecture Hall at the Institute of Chemistry, Uppsala University, on September 25, 2002 at 10.15 a.m., for the Degree of Doctor of Philosophy. The examination will be conducted in English. ABSTRACT Zeng X. 2002. Mechanisms for Solvolytic Elimination and Substitution Reactions Involving Short-Lived Carbocation Intermediates. Acta Univ. Ups., Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 743. 45 pp. Uppsala. ISBN 91-554-5389-9. Solvolysis reactions of a range of tertiary substrates in largely aqueous solvents were studied in such respects as E-deuterium kinetic isotope effects, linear free energy relationships and stereochemistry. Solvolysis of the fluorene derivatives 9-methyl–9-(2´-X-2´-propyl)fluorene (1-X, X = Cl, Br, OOCCF3) involves a very short-lived carbocation intermediate. The fraction of alkene is increased by addition of general bases, which can be expressed by a BrInsted parameter ȕ = 0.07. The kinetic deuterium isotope effects vary with solvent composition in a way which is not consistent with a common carbocation intermediate which has time to choose between dehydronation and addition of a solvent water molecule. In the absence of bases, the reaction of 4-chloro-4-(4´-nitrophenyl)pentan-2-one (2-Cl) proceeds through a short-lived carbocation intermediate yielding 4-(4´-nitrophenyl)-2-oxopent-4-ene (2-t-ne) as the main elimination product.
    [Show full text]
  • Effect of Solvent on 1,3-Dipolar Cycloaddition Transition States
    Loyola University Chicago Loyola eCommons Dissertations Theses and Dissertations 1993 Effect of solvent on 1,3-dipolar cycloaddition transition states Sheila Louise Celsor Loyola University Chicago Follow this and additional works at: https://ecommons.luc.edu/luc_diss Part of the Chemistry Commons Recommended Citation Celsor, Sheila Louise, "Effect of solvent on 1,3-dipolar cycloaddition transition states" (1993). Dissertations. 3013. https://ecommons.luc.edu/luc_diss/3013 This Dissertation is brought to you for free and open access by the Theses and Dissertations at Loyola eCommons. It has been accepted for inclusion in Dissertations by an authorized administrator of Loyola eCommons. For more information, please contact [email protected]. This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 License. Copyright © 1993 Sheila Louise Celsor LOYOLA UNIVERSITY OF ClllCAGO EFFECT OF SOLVENT ON 1,3-DIPOLAR CYCWADDITION TRANSmON STATES A DISSERTATION SUBMITTED TO THE FACULTY OF THE GRADUATE SCHOOL IN CANDIDACY FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CHEMISTRY BY SHEILA LOUISE CELSOR CHICAGO, ILLINOIS MAY 1993 Copyright by Sheila L. Celsor, 1993 All rights reserved TABLE OF CONTENTS LIST OF ILLUSTRATIONS . iv LIST OF TABLES . vi LIST OF ABBREVIATIONS ..................................... ix CHAPTER I . 1 INTRODUCTION . 1 CHAPTER II . 7 HISTORY . 7 CHAPTER III 37 RESULTS . 37 1. Overall yields of mono-pyrazoline adducts. 37 2. Relative yields of mono-pyrazoline adducts and product ratios. 37 3. Rate of formation of monoadducts in n-hexane. 42 4. Rate of formation of monoadducts in acetonitrile. 53 5. Rate of disappearance of diphenyldiazomethane in hexane. 58 6.
    [Show full text]