Carbocations geometry

AMI optimised carbocation geometry was used as input for single point INDO/S calculation of the carbocation optical spectra and the result discussed in light of experimental spectra. For protonation near the end of the polyene chain one single peak with longer A,max than for central protonation was predicted. 

It is important to note that changes of carbocation geometries due to hyperconjugation effects are not included in the UNICAT 4 parameter set. Comparisons of the UNICAT 4... 

A formula (equation 4) was developed, which is based on these five factors and calculates the strengths of hyperconjugation interactions in excellent agreement with hyperconjugation energies determined by NLMO/NPA analysis of MP2(full)/6-31G carbocation geometries t... 

One possible explanation is that adamantyl cation, an intermediate in the reaction, is particularly unstable because it cannot accomodate a planar carbocation center (see Chapter 1, Problem 9). Examine the geometry of adamantyl cation. Does it incorporate a planar carbocation center Compare electrostatic potential maps of adamantyl cation and 2-methyl-2-propyl cation. Which cation better delocalizes the positive charge Assuming that the more delocalized cation is also the more stable cation, would you expect adamantyl tosylate to react slower or faster than tcrf-butyl tosylate Calculate the energy of the reaction. 

HCl addition usually proceeds through a carbocation intermediate, with the dominant product resulting from the most stable cation. Compare energies for the possible cations that can be obtained from isoprene (isoprene+H+). Rank them from most to least stable. Examine the geometries of the cations, and sketch one or more Lewis stmctures for each which are consistent with its geometry. Be sure to point out any unusual features. What factors might be responsible for the ordering of cation stability ... 

Protonation and subsequent loss of water should generate a earbocation. Examine all of the carbocations derived from protonation of (3-D-glueose. Identify the most stable carboeation (this is the one that will form most readily), and draw whatever resonance eontributors are needed to describe the geometry, energy, and atomie charges in this cation. Can you explain why substitution oeeurs selectively at Ci ... 

What is the preferred geometry about the radical center in free radicals Carbocation centers are characterized by a vacant orbital and are known to be planar, while carbanion centers incorporate a nonbonded electron pair and are typically pyramidal (see Chapter 1, Problem 9). 

For El eliminations, if there is a free carbocation (25), it is free to rotate, and no matter what the geometry of the original compound, the more stable situation is the one where the larger of the D-E pair is opposite the smaller of the A-B pair and the corresponding alkene should form. If the carbocation is not completely free, then to that extent, E2 type products are formed. Similar considerations apply in ElcB eliminations. ... 

For example, let s look at the stereochemistry of SnI reactions. We already saw that Sn2 reactions proceed via inversion of configuration. But SnI reactions are very different. Recall that a carbocation is sp hybridized, so its geometry is trigonal planar. When the nucleophile attacks, there is no preference as to which side it can attack, and we get both possible configurations in equal amounts. Half of the molecules would have one configuration and the other half would have the other configuration. We learned before that this is called a racemic mixture. Notice that we can explain the stereochemical outcome of this reaction by understanding the nature of the carbocation intermediate that is formed. 

Carbocations form during the course of a reaction but are usually consumed rapidly. Describe the bonding and geometry of the carbocation (CH3)3 C ... 

Despite the successful prediction of chemical shifts for a great structural variety of carbocations some difficulties have been encountered for vinyl cations.47 The effect of electron correlation, basis sets and geometry on calculated NMR spectra of vinyl cations has been studied in some detail also for the parent vinyl cation in its linear form.48 Comparative experimental and computational NMR studies, however, have... 

The experimental 3J(HH) spin-spin coupling constants for Ha, Hp, and Hp for the -isomer 22 were quite satisfactorily reproduced (A = 0.1 - 1 Hz) by calculations, using a finite perturbation method (FPT level (26), Perdew/IGLO-III at a MP2/6-31G(d)) geometry for the model structure (E)-1 -cyclopropyl-2-(trimethylsilyl)ethyl cation. The calculations confirm the /rexperimentally observed carbocation 22 (20, 27, 28). The (E)- 1-cyclopropy 1-2-... 

Conjugative interactions can be important in influencing the geometry as well as rotational barriers of carbocations. We will first consider examples where a sigma bond conjugates with an empty carbon 2p AO. 

Although the geometric relationship suggested by Shiner and by Sunko and their co-workers clearly demonstrates that hyperconjugation is the major contributor to the secondary /3-deuterium KIE in carbocation reactions, Williams (1985) has suggested that there is a significant inductive component to these KIEs. Williams used ab initio MO methods to calculate the geometries of the substrates and the isopropyl carbocation formed in a gas-phase heterolysis (30) of series of isopropyl derivatives at the RHF/4-31G level. 

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