Theoretical Prediction of Activation Free Energies of Various Hydride Self-Exchange Reactions in Acetonitrile at 298 K

Hydride transfer reactions are very important chemical reactions in organic chemistry. It has been a chemist’s dream to predict the rate constants of hydride transfer reactions by only using the physical parameters of the reactants. To realize this dream, we have developed a kinetic equation (Zhu equation) in our previous papers to predict the activation free energies of various chemical reactions using the activation free energies of the corresponding self-exchange reactions and the related bond dissociation energies or redox potentials of the reactants. Because the activation free energy of the hydride self-exchange reaction is difficult to measure using the experimental method, in this study, the activation free energies of 159 hydride self-exchange reactions in acetonitrile at 298 K were systematically computed using an accurately benchmarked density functional theory method with a precision of 1.1 kcal mol^–1. The results show that the range of the activation free energies of the 159 hydride self-exchange reactions is from 16.1 to 46.6 kcal mol^–1. The activation free energies of 25 122 hydride transfer reactions in acetonitrile at 298 K can be estimated using the activation free energies of the 159 hydride self-exchange reactions and the corresponding heterolytic bond dissociation free energies of the reactants. The effects of the heteroatom, substituent, and aromaticity on the activation free energies of hydride self-exchange reactions were examined. The results show that heteroatoms, substituents at the reaction center, and the aromaticity of reactants, all have remarkable effects on the activation free energy of hydride self-exchange reactions. All kinetic information provided in this work on the hydride self-exchange reactions in acetonitrile at 298 K should be very useful in chemical labs and chemical industry

Determination and Comparison of Thermodynamic Driving Forces of Elementary Steps for the Reductions of Alkynes and the Corresponding Alkenes in Acetonitrile

In this work, five substituted 1-phenyl-2-benzensulfonyl ethynes and the corresponding five substituted 1-phenyl-2-benzensulfonyl ethenes were designed and synthesized as representatives of the polar alkynes and the polar alkenes. Thermodynamic driving forces of eight elementary steps for reductions of the substituted ethynes and ethenes to the corresponding alkenes and alkanes in acetonitrile were determined. The differences of chemical properties between the alkynes and the alkenes as well as their various derived reaction intermediates were quantitatively examined or compared according to the determined thermodynamic driving forces of the eight elementary steps. The relative C–C π-bond heterolytic and homolytic dissociation energies of the alkynes and alkenes in acetonitrile were estimated according to the difference of the hydride affinities and hydrogen atom affinities of the related chemical species. The relative effective charges on the active center atom of the alkynes and the alkenes as well as their derived various reaction intermediates, which can be used to quantitatively measure the polarity of the corresponding chemical species, were estimated according to the Hammett substituent effects using the Hammett-type linear free energy relationships. Molecule ID Cards of the alkynes and the alkenes in acetonitrile were constructed from the determined thermodynamic driving forces of the eight elementary steps. The thermodynamic tendencies and detailed mechanisms for the reductions of the alkynes and alkenes by Hantzsch ester in acetonitrile were diagnosed according to the thermodynamic analytic platforms that were made of the Molecule ID Cards of the related reactants. It is clear that the results of this work are not only to provide good guidance for synthetic chemists to safely choose a suitable reducing agent for selective reductions of alkynes and alkenes and to rationally examine the reaction mechanisms but also to facilitate theoretical chemists to develop novel calculation methods to examine the chemistry of alkynes and alkenes

Unusual Topological RNA Architecture with an Eight-Stranded Helical Fragment Containing A‑, G‑, and U‑Tetrads

Human telomeric RNA performs various cellular functions such as telomere length regulation, heterochromatin formation, and chromosome end protection. Using a combination of nuclear magnetic resonance, circular dichroism, and gel electrophoresis, we observed an unusual topological structure formed by human telomere RNA r­(GUUAGGGU). Our results showed that every set of four strands formed a parallel G-quadruplex as symmetry-related units containing four G-tetrads, two U-tetrads, and one A-tetrad. An eight-stranded helical fragment containing A-, G-, and U-tetrads provided a central intercalated scaffold that connected two G-quadruplex units in an alternating antiparallel arrangement, giving rise to a novel RNA architecture. This higher order RNA structure is so stable that it would be surprising if similar structures do not occur in nature. Our findings provide a new insight into the behavior of human telomeric RNA molecules

Determination of Thermodynamic Affinities of Various Polar Olefins as Hydride, Hydrogen Atom, and Electron Acceptors in Acetonitrile

A series of 69 polar olefins with various typical structures (<b>X</b>) were synthesized and the thermodynamic affinities (defined in terms of the molar enthalpy changes or the standard redox potentials in this work) of the polar olefins obtaining hydride anions, hydrogen atoms, and electrons, the thermodynamic affinities of the radical anions of the polar olefins (<b>X^•–</b>) obtaining protons and hydrogen atoms, and the thermodynamic affinities of the hydrogen adducts of the polar olefins (<b>XH^•</b>) obtaining electrons in acetonitrile were determined using titration calorimetry and electrochemical methods. The pure CC π-bond heterolytic and homolytic dissociation energies of the polar olefins (<b>X</b>) in acetonitrile and the pure CC π-bond homolytic dissociation energies of the radical anions of the polar olefins (<b>X^•–</b>) in acetonitrile were estimated. The remote substituent effects on the six thermodynamic affinities of the polar olefins and their related reaction intermediates were examined using the Hammett linear free-energy relationships; the results show that the Hammett linear free-energy relationships all hold in the six chemical and electrochemical processes. The information disclosed in this work could not only supply a gap of the chemical thermodynamics of olefins as one class of very important organic unsaturated compounds but also strongly promote the fast development of the chemistry and applications of olefins

Prediction of Kinetic Isotope Effects for Various Hydride Transfer Reactions Using a New Kinetic Model

In this work, kinetic isotope effect (KIE_self) values of 68 hydride self-exchange reactions, XH­(D) + X⁺ → X⁺ + XH­(D), in acetonitrile at 298 K were determined using a new experimental method. KIE values of 4556 hydride cross transfer reactions, XH­(D) + Y⁺ → X⁺ + YH­(D), in acetonitrile were estimated from the 68 determined KIE_self values of hydride self-exchange reactions using a new KIE relation formula derived from Zhu’s kinetic equation and the reliability of the estimations was verified using different experimental methods. A new KIE kinetic model to explain and predict KIE values was developed according to Zhu’s kinetic model using two different Morse free energy curves instead of one Morse free energy curve in the traditional KIE theories to describe the free energy changes of X–H bond and X–D bond dissociation in chemical reactions. The most significant contribution of this paper to KIE theory is to build a new KIE kinetic model, which can be used to not only uniformly explain the various (normal, enormous and inverse) KIE values but also safely prodict KIE values of various chemical reactions