Complex ions, long lived intermediate

For example, for a bimolecular nucleophilic substitution (Sn2) reaction like Cl- + CHoBr — CICHo-l-Br, the potential energy has a double-well shape, i.e., two minima separated by a central barrier. The minima for this reaction reflects the stability (an effect that is also well known within classical electrostatics) of the ion-dipole complexes Cl- CHoBr and CICH3 Br . For other indirect (or complex mode) reactions one finds two saddle points separated by a well on the path from reactants to products. The existence of a well along the reaction path implies that the collision may be sticky , and a long-lived intermediate complex can be formed before the products show up. Examples of complex mode reactions are H + O2 — OH + O (with the intermediate H02), H+ + D2 and KC1 + NaBr. 

These reactions usually proceed by incorporation of Si into the product ions by way of long-lived intermediate complexes which allow a rearrangement of the existing bonds and the formation of new bonds. The reactions of ground state Si+ (2P) with HC1, H2O, H2S and NH3 have been studied as a function of the ion-neutral center of mass kinetic... 

Macroscopic solvent effects can be described by the dielectric constant of a medium, whereas the effects of polarization, induced dipoles, and specific solvation are examples of microscopic solvent effects. Carbenium ions are very strong electrophiles that interact reversibly with several components of the reaction mixture in addition to undergoing initiation, propagation, transfer, and termination. These interactions may be relatively weak as in dispersive interactions, which last less than it takes for a bond vibration (<10 14 sec), and are thus considered to involve "sticky collisions. Stronger interactions lead to long-lived intermediates and/or complex formation, often with a change of hybridization. For example, onium ions are formed with -donors. Even stable trityl ions react very rapidly with amines to form ammonium ions [41], and with water, alcohol, ethers, and esters to form oxonium ions. Onium ion formation is reversible, with the equilibrium constant depending on the nucleophile, cation, solvent, and temperature (cf., Section IV.C.3). 

The proposed mechanism (Scheme 11) of the asymmetric epoxidation of a-ylideneoxindoles (97) by TBHP to cis- and frani-spiro[oxirane-oxindole] derivatives (101 and 102) had TBHP react with the catalyst (5)-a,a-diphenylprolinol (95) in the initial step forming a tight ion pair (96), which attacks the Cp carbon of the substfate (97) to give a transitory intermediary complex (98) as precursor of a long-living intermediate (99) from which (101) and (102) are derived. " 0... 

Dr. Halpem But all that is necessary to explain your observation is that this intermediate be fairly long lived and then that two of these ultimately combine. There is some evidence for a similar phenomenon in the case of free radicals. J. K. Kochi and F. F. Rust (9) have shown that transition metal ions, among them Fe(II), catalyze the recombination of free radicals by what appears to be substantially this kind of mechanism—i.e., the transition metal stabilizes the radical against abstraction presumably by forming a complex with it, which then lives long enough to combine with another one. 

Recent mechanistic discussions of unimolecular decompositions of organic ions have invoked ion—molecule complexes as reaction intermediates [102, 105, 361, 634]. The complexes are proposed to be bound by long-range ion—dipole forces and to be sufficiently long-lived to allow hydrogen rearrangements to occur. The question of lifetime aside, there is more than a close similarity between the proposed ion—dipole intermediate and the assumed loose or orbiting transition state of phase space theory. 

If product formation proceeded via an intermediate long-lived ion whose dissociation were governed by competitive channeling of energy into the possible dissociation coordinates, one would expect the complex to fragment at the weakest bond—i.e., in the most thermodynamically favorable direction. This is clearly not the case. The most exothermic reaction (17) does not dominate the fate of the complex formed by collision of methanol ion with acetaldehyde. Similarly, one would expect Reaction 4 to be favored and Reactions 5, 6, and 7 to occur with about equal probability. 

Rnotolo, B.T. Hyung, S.J. Robinson, P.M. GUes, K. Bateman, R.H. Robinson, C.V. Ion mobihty-mass spectrometry reveals long-lived, unfolded intermediates in the dissociation of protein complexes. Angew. Chem. Int. Ed. 2007, 46, 8001-8004. 

The properties of unstable negative-ion states of the kind which play the intermediate complex role in associative-detachment have been the subject of a great deal of attention in recent years, primarily because of their role in electron-molecule collisions. An excellent discussion of such resonances is given by Bardsley and Mandl. " The long-lived N02 intermediate in (29), stabilized on a curve such as 3 in Fig. 8 due to rotational excitation of the NO, is again a nuclear-excited Feshbach resonance. 

In the presence of oxidizable metal ions, however, e.g. Ce or Mn ) there is always the possibility of trapping the intermediates in competition with the ligands reacting to reduce the Cr. Chromium(v) plays an important role in oxidations of this type and a long-lived Cr intermediate has been identified in the reaction with oxalic acid. Using both e.s.r. and difference absorbance spectra, it has been shown that there is a significant build-up of a complex in moderately acidic media. The data... 

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