Hybridization cations

Wodzki R, Swiatkowski M, Pretula J, and Kaluzynski K. Poly[poly(ox3qjropylene) phosphate] macroionophores for transport and separation of cations in a hybrid Cation-exchange polymer and hquid membrane system. J. Appl Polym Sci, 2004 93(3) 1436-1445. 

The first step of addition to 2,4-hcxadiene yields, then, not cation 1, but the hybrid cation VI in which the charge is spread over two carbons ... 

CP-29 is a cecropin/mellitin hybrid cationic peptide (Hancock, Gough, and Fanner, unpublished data). cationic peptide (Falla, T., Hancock, R. E. W., unpublished dau). 

Baran P S, Monaco R R, Khan A U, Schuster D I and Wilson S R 1997 Synthesis and cation-mediated electronic interactions of two novel classes of porphyrin-fullerene hybrids J. Am. Chem. See. 119 8363-4... 

Dissolve ca. 0 2 g. of product (I) in cold ethanol, and add with shaking 1-2 drops of dilute sulphuric acid. A deep purple coloration appears at once. This shows that salt formation has occurred on the quinoline nitrogen atom to form the cation (Ha), which will form a resonance hybrid with the quinonoid form tils). [Note that the forms (IIa) and (11b) differ only in electron position, and they are not therefore tautomeric.] If, hoAvever, salt formation had occurred on the dimethylaniino group to give the cation (III), thrs charge separiition could not occur, and the deep colour would be absent. 

The properties of tert butyl cation can be understood by focusing on its structure which IS shown m Figure 4 9 With only six valence electrons which are distributed among three coplanar ct bonds the positively charged carbon is sp hybridized The unhybridized 2p orbital that remains on the positively charged carbon contains no elec Irons Its axis is perpendicular to the plane of the bonds connecting that carbon to the three methyl groups... 

FIGURE 4 9 tert Butyl cation (a) The positively charged carbon is sp hybridized Each methyl group IS attached to the positively charged carbon by a cr bond and these three bonds he in the same plane (b) The sp hybridized car bon has an empty 2p orbital the axis of which is perpen dicular to the plane of the carbon atoms... 

An alkyl radical is neutral and has one more electron than the corresponding carbocation Thus bonding m methyl radical may be approximated by simply adding an electron to the vacant 2p orbital of sp hybridized carbon m methyl cation (Figure 4 19a) Alternatively we could assume that carbon is sp hybridized and place the unpaired elec tron m an sp orbital (Figure 4 9b)... 

Depending on the reaction conditions, the product can be isolated in either the lactoid form A [2321-07-5] (2) or the quinonoid form B [56503-30-1] (3). These 9-phenylxanthenes are closely related stmcturaHy to the triphenyl methane dyes (4) and, like them, are cationic resonance hybrids. 

Vinyl ethers can also be formulated with acryHc and unsaturated polyesters containing maleate or fumarate functionaHty. Because of their abiHty to form alternating copolymers by a free-radical polymeri2ation mechanism, such formulations can be cured using free-radical photoinitiators. With acryHc monomers and oligomers, a hybrid approach has been taken using both simultaneous cationic and free-radical initiation. A summary of these approaches can be found in Table 9. 

AH anthocyanidins have the 2-phenylben2opyr5lium or flavyhum cation stmcture (86,87) (23), a resonance hybrid of oxonium forms and carbenium... 

The hydrogen-deuterium exchange rates for 1,2-dimethylpyrazolium cation (protons 3 and 5 exchange faster than proton 4 Section 4.04.2.1.7(iii)) have been examined theoretically within the framework of the CNDO/2 approximation (73T3469). The final conclusion is that the relative reactivities of isomeric positions in the pyrazolium series are determined essentially by inductive and hybridization effects. 

In this work hybrid method is suggested to determine anionic surfactants in waters. It is based on preconcentration of anionic surfactants as their ion associates with cationic dyes on the membrane filter and measurement of colour intensity by solid-phase spectrophotometry method. Effect of different basic dyes, nature and hydrophobicity of anionic surfactants, size of membrane filter pores, filtration rate on sensitivity of their determination was studied. Various cationic dyes, such as Methylene Blue, Crystal Violet, Malachite Green, Rhodamine 6G, Safranin T, Acridine Yellow were used as counter ions. The difference in reflection between the blank and the sample was significant when Crystal Violet or Rhodamine 6G or Acridine Yellow were used. 

In this work hybrid method is suggested to determine cationic surfactants in water. It is based on preconcentration of cationic surfactants in the some of ion associates with acidic dyes on the paper filter and measurement of color intensity by solid-phase specdophotomenic method or visual comparison. 

Further examination of Table 1.10 reveals that the lowest unoccupied molecular orbital, i/ 4, is a pure p orbital, localized on carbon, since the coefficients are zero for all but the 2p orbital. The MO picture is in agreement with the usual qualitative hybridization picture for the methyl cation. 

Up to this point in our discussion, we have considered only carbocations in which the cationic carbon can be 5p -hybridized and planar. When this hybridization cannot be achieved, die carbocations are of higher energy. In a classic experiment, Bartlett and Knox demonstrated that the tertiary chloride 1-chloroapocamphane was inert to nucleophilic substitution. Starting material was recovered unchanged even after refluxing for 48 h in ethanolic silver nitrate. The umeactivity of this compound is attributed to the structure of... 

In other sections in this chapter, we have referred to a variety of macropolycyclic structures which are more elaborate than the simple three-stranded bicyclic cryptands. This includes bridged double-macrocycles " , in-out bicyclic amines and the macrotricyclic quaternary ammonium salts of Schmidtchen. In addition to these, there are two other types of compounds which deserve special note. The first of these is a stacked twin-ring cryptand, but it is a hybrid molecule rather than a double-cryptand . The species shown below as 20 is a crowned porphyrin, and was designed to provide a pair of metal cation binding sites similar to those which might be available in natural biological systems . 

Compare atomic charges and electrostatic potential maps for the three cations. For each, is the charge localized or delocalized Is it associated with an empty a-type or Tt-type orbital Examine the lowest-unoccupied molecular orbital (LUMO) of each cation. Draw all of the resonance contributors needed for a complete description of each cation. Assign the hybridization of the C" atom, and describe how each orbital on this atom is utilized (o bond, n bond, empty). How do you explain the benzene ring effects that you observe ... 

When 1-hydroxymelatonin (19) is treated with acid, removal of its 1-hydroxy group leaves an indolyl cation (a hybrid of resonance structures 254,168, and so on) as shown in Scheme 37. If there is a subsequent intramolecular nucleophilic attack by the Ab-nitrogen atom on the side chain or if an intermolecular attack by suitable nucleophiles occurs on this intermediate cation, the birth of a new type of product can be expected. 

Figure 8.2 The structure of a secondary vinylic carbocation. The cationic carbon atom is sp-hybridized and has a vacant p orbital perpendicular to the plane of the tt bond orbitals. Only one R group is attached to the positively charged carbon rather than two, as in a secondary alkyl carbocation. The electrostatic potential map shows that the most positive (blue) regions coincide with lobes of the vacant p orbital and are perpendicular to the most negative (red) regions associated with the ir bond.

To understand why a racemic product results from the reaction of T120 wjtl 1-butene, think about the reaction mechanism. 1-Butene is first protonaled tc yield an intermediate secondary (2°) carbocation. Since the trivalent carbon i sp2-hybridized and planar, the cation has no chirality centers, has a plane o symmetry, and is achiral. As a result, it can react with H20 equally well fron either the top or the bottom. Reaction from the top leads to (S)-2-butano through transition state 1 (TS 1) in Figure 9.15, and reaction from the bottorr leads to R product through TS 2. The two transition states are mirror images. The] therefore have identical energies, form at identical rates, and are equally likeb to occur. 

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