Hydrogen exchange studies. VII. Isotopic exchange of molecular hydrogen in dimethyl sulfoxide-water mixtures under base cataysis

1972 ◽  
Vol 94 (10) ◽  
pp. 3641-3642 ◽  
Author(s):  
E. A. Symons ◽  
E. Buncel
Keyword(s):  
Dimethyl Sulfoxide ◽  
Molecular Hydrogen ◽  
Hydrogen Exchange ◽  
Isotopic Exchange

Base-catalyzed Isotopic Exchange of Molecular Hydrogen. II. Rate Dependence on Basicity in the Dimethyl Sulfoxide–Water System

1973 ◽  
Vol 51 (11) ◽  
pp. 1673-1681 ◽  
Author(s):  
E. Allan Symons ◽  
Erwin Buncel
Keyword(s):  
Proton Transfer ◽  
Dimethyl Sulfoxide ◽  
Molecular Hydrogen ◽  
Isotopic Exchange ◽  
Water System ◽  
Medium Composition ◽  
Hydroxide Ion ◽  
Hydride Ion ◽  
Low Sensitivity ◽  
Concerted Reaction

Isotopic exchange of D2 has been determined in mixtures of dimethyl sulfoxide and water under catalysis by hydroxide ion. The rate of exchange increases by ca. 104 as the medium composition is changed from 0% DMSO to 99.6% DMSO at 65 °C. The unusually low sensitivity to medium basicity is reflected in abnormally low slope values for the plots of log kobs υs. H− or J−. When [Me4NOH] is varied at a given medium composition, the apparent order with respect to [Me4NOH] is greater than unity.These results are discussed in terms of (1) rate determining proton transfer from D2 to OH− with formation of hydride ion; (2) a concerted reaction of OH−, D2, and H2O; (3) addition of OH− to D2 to yield an intermediate adduct, [HODD]−; (4) a 6-centered process involving cation participation and electrophilic assistance by H2O in the reaction of OH− with D2.L'échange isotopique de D2 a été étudié dans des mélanges de sulfoxyde de diméthyle et d'eau sous l'effet d'une catalyse par l'ion hydroxyle. Le taux d'échange augmente d'environ 104 alors que la composition du milieu change de 0% en DMSO à 99.6% en DMSO à 65 °C. La sensitivité anormalement faible à la basicité du milieu est réflétée par des valeurs anormalement faibles de la pente dans les graphiques du log kobs υs. H− ou J−. Quand la concentration de Me4NOH est changée pour une composition donnée du milieu, l'ordre apparent en ce qui a trait à la [Me4NOH] est plus élevé que l'unité.

Theory of Hydrogen Interactions with Amorphous Silicon

1999 ◽  
Vol 557 ◽  
Author(s):  
Chris G. Van De Walle ◽  
Blair Tuttle
Keyword(s):  
Amorphous Silicon ◽  
First Principles ◽  
Molecular Hydrogen ◽  
Hydrogen Exchange ◽  
Defect Formation ◽  
Exchange Process ◽  
Current Understanding ◽  
First Principles Calculations ◽  
Microscopic Mechanism ◽  
Enhanced Stability

AbstractWe present an overview of recent results for hydrogen interactions with amorphous silicon (a-Si), based on first- principles calculations. We review the current understanding regarding molecular hydrogen, and show that H2 molecules are far less inert than previously assumed. We then discuss results for motion of hydrogen through the material, as relating to diffusion and defect formation. We present a microscopic mechanism for hydrogen-hydrogen exchange, and examine the metastable ≠ SiH2 complex formed during the exchange process. We also discuss the enhanced stability of Si-D compared to Si-H bonds, which may provide a means of suppressing light-induced defect generation.

Metal ion – biomolecule interactions. Part 16. C(2)-H isotopic exchange in Co(III)-coordinated imidazoles

1995 ◽  
Vol 73 (6) ◽  
pp. 772-780 ◽  
Author(s):  
Erwin Buncel ◽  
Fan Yang ◽  
Robert Y. Moir ◽  
Ikenna Onyido
Keyword(s):  
Metal Ions ◽  
Metal Ion ◽  
Hydrogen Exchange ◽  
Isotopic Exchange ◽  
Negative Charge ◽  
Ligand Complex ◽  
Transition State Stabilization ◽  
Anionic Species ◽  
Metal Ligand ◽  
Ligand Bond

Transition-metal-bound imidazoles are suitable models for evaluating the roles of metal ions in biomolecules having the imidazole moiety and similar heterocyclic residues as part of their structure. Such studies provide useful insights into metal–biomolecule interactions in biological systems, especially when the lability of the metal–ligand bond is substantially reduced, such that the identity of the metal–ligand complex is preserved during the course of the reaction under investigation. The present paper reports on a kinetic study of tritium exchange from the C(2) position of the imidazole moiety in the substitution-inert complex cations [Co(NH3)5[2-3H]-imidazole]3+ (1) and [Co(NH3)5-1-methyl-[2-3H]-imidazole]3+ (2). Rate–pH profiles have been determined in aqueous solution at 60 °C. Both substrates are believed to react through rate-determining attack of hydroxide ion (kM+ pathway) at C(2)-T. Dissection of the kinetic data reveals an additional pathway for 1 consequent upon deprotonation of its pyrrole-like N-H(T) to yield 3, which is then attacked by hydroxide at C(2) (kM pathway). The ratio kM+/kM = 103 that is obtained is in accord with the expected reduced reactivity of 3. Comparison of the present data with those reported for a variety of heterocyclic substrates shows that the order of reactivity, protonated [Formula: see text] metal ion coordinated [Formula: see text] neutral form of substrates, prevails. The superiority of the proton over metal ions in catalyzing isotopic hydrogen exchange is attributed to its larger ground state acidifying effect coupled with the greater transition state stabilization it affords, relative to metal ions. The exchange reaction of 3 via the kM pathway is the first example of a reactive anionic species in which the negative charge is located α to the exchanging C-H. Keywords: tritium exchange, cobalt (III)-coordinated imidazoles.

Sign in / Sign up

Export Citation Format

Share Document