Classical cation

Table 17 contains the enthalpies of activation and reaction for the three propagation steps in the gas phase and in solution. They were calculated by using classical cations possessing all-trans conformation. 

Mechanism a involves a comer-protonated cyclopropane (26) we have already seen examples of such ions in the 2-norbomyl and 7-norbomenyl cations (pp. 409, 414). Mechanism b involves an edge-protonated cyclopropane (27). Mechanism c consists of a one-step SE2-type attack by to give the classical cation 28, which... 

When Wlnsteln " was studying the possibility of generating non-classical cation (44) from ais tosylate (43), he needed both cis and trans alcohols to show there was a substantial rate difference in the reaction of the tosylates. 

The extent of retention of optical purity differentiates Corey s results from those of Berson and Remanick. Corey found similar optical purity in the products of both epimers, from which it was concluded that a common ion was an intermediate in both reactions. Since optical purity could not be retained, if the symmetrical non-classical ion was an intermediate or transition state in the reaction, he concluded that deamination led to the classical cation from both the exo- and endo-reactants. 

Table 7 contains shift information for a number of other compounds that have been used to analyse the norbomyl spectra. Included among these are data for the isopropyl ion which has been used as the principal model for the secondary ion, and for the 1,2-dimethylnorbomyl cation which has been reported to be an example of an equilibrating classical cation, the Wagner-Meerwein shift being too fast to be stopped at —140°. In the table it may also be noticed that the t-butyl cationic centre experiences a shift of about 11 p.p.m. on being transferred from neat SbFs to SbFs—SO2. 

Classically, cation (3) might be anticipated to be considerably stabilized by the neighboring oxygen atom and double bond on the other hand, its 47r-electron system would suggest anti-aromaticity. The isodesmic calculations showed it to be about 200 kJ mol-1 less stable than the model cation (19b) and about 40 kJ mol-1 less stable than the allyl cation. Thus it appears that it would have an anti-aromatic electronic structure, if it was formed (80TL1807). 

In the classical cation counterdiffusion experiment, oxygen gas is excluded from the interfaces and rM = 0 (Fig. 6-6 a). Thus the coupling condition is simply... 

Saunders, Telkowski and Vogel developed84 another powerful method for distinguishing nonclassical from classical cations using the deuterium isotopic perturbation method813. 

The unsymmetrically bridged ions 506 would be undistinguishable from the symmetrically bridged system 504 in solution NMR experiments. (However, see subsequent discussion of solid-state low-temperature 13C NMR as well as ESCA studies.) It is important to recognize that equilibrating open classical cations 505 cannot explain the NMR data and thus cannot be involved as populated species. 

An entirely different spectrum was obtained for 572 with the C(2)13C resonance at 813C 92.3, more than 200 ppm removed from the position expected for a static classical cation. Since a static structure like 572 was incompatible with the observed spectrum, a chemical shift estimate was made for the protoadamantyl cation 575. However, the discrepancy between these estimated chemical shifts and those observed was too large to explain the behavior of the l,3,5,7-tetramethyl-2-adamantyl cation within the properties of an equilibrating set of ions 575 even with the partial contribution of 572. This left the set of o-bridged ions 574 equilibrating according to the mechanism in Eq. (3.141) as the only possible structure for this ion. 

Fig. 7.9 The classical/nonclassical 2-norbornyl cation problem. Grey a pair of rapidly equilibrating classical cations with a nonclassical, bridged transition structure black the nonclassical cation as the minimum cation. The fully MP2/6-31G(d) optimised 2-norbornyl cations are depicted the nonclassical ion is 13.6 kcal mofi1 more stable at this and comparable levels of theory.

Fig. 5.17 The ethyl cation problem at various levels. At the three Hartree-Fock levels the classical cation is a minimum, but at the post-Hartree-Fock (MP2/6-31G ) level only the symmetrical bridged ion is a minimum. The HF/6-31G results are calculations by the author (ZPE ignored), the other three levels are taken from ref. [75]...

The surface characteristics of net anionic DNA-liposome complexes are distinctly different from classic cationic liposome-DNA complexes and, therefore, their interaction with biological fluids and membranes is different. In Figure 3, the transfection efficacy of both types of complexes in the presence of serum... 

In conclusion, we have designed a synthetic vesicular DNA carrier that physically and functionally mimics an enveloped virus particle. To achieve an acceptable degree of encapsulation within the vesicle, we use a process that is essentially inverse to the preparation of cationic lipid-DNA complexes. A suitable DNA condensing agent is introduced that, at a certain critical concentration, conveys a weak net cationic charge to the condensed DNA that then interacts spontaneously with a liposome containing one or more anionic components. These DNA formulations behave distinctly different from classic cationic liposome DNA complexes in vitro in as much as they have been shown to be nontoxic, to display a traditional linear dose response, and to be serum-insensitive. 

In addition to a relatively small inductive effect, two modes of stabilization of positive charge by the P-R3M substituent have been suggested. The first involves the classical cation 7, where the positive charge is stabilized by hyperconjugation (G-p conjugation) between the C-M o bonding orbital and the vacant carbocation... 

Frequently antibodies are produced by cell culture in protein-free media. Although the expression is low, their purification becomes a simple task. When classical cation-exchangers with S groups are used for the capture step, the ionic strength of the cell culture supernatant must be decreased to at least 5 mS/cm and the pH adjusted to 5.2-5.5. Note, however, that the required... 

However, the low yield of carbenium ions (< 1%) and often incomplete initiation in these classic cationic systems resulted in very low apparent rate constants of propagation (/rpapp = 1 mol L sec 1). In the past 20 years, three methods have been developed to obtain more accurate rate constants for carbocationic propagation. They are based on direct measurement of the concentration of growing species by rapid spectroscopic techniques, on systems initiated by -y-irradiation, and on systems initiated by trityl salts which are consumed slowly. The latter method follows the concentration of unreacted initiator spectroscopically. Although all three methods have some disadvantages, they provide consistent carbocationic propagation rate constants (kp 105 mol-,L-sec l at 0° C) [207]. 

Olah et al. considered several possible structures [32]-[35] and [101]-[103] for the ion. Structures [33], [35] and [101] are equilibrating classical cations and [32], [34], [102] and [103] are nonclassical ones. Correlation of the observed spectral parameters of [100] with those estimated from suitable models and theoretical calculations favoured rapidly equilibrating bicyclobutonium ions (75) as structural representation of ion [100]. Some of the arguments are summarized below. 

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