Methyl cation: geometry

The 13C, 15N and 170 NMR chemical shifts of some substituted methyl cations and the corresponding protonated dications were calculated by the GIAO-MP2 method for MP2/6-31G(d) optimized geometries.129... 

Fig. 6. Ab initio calculated (MP2/6-31G(d)) geometries of the 1-silylbicyclobutoniura ion 6 and the (I -silylcyclo-propyl)methyl cation 7 selected bond lengths in ppm and relative energies in kcal/mol (ZPE included).

The interaction of the methyl cation with disilazanes was modeled by quantum chemical methods. The equilibrium geometry and electronic structure of the methyl cation-disilazane adduct were obtained by the Hartree-Fock and MP2/6-31G methods. It was shown that the interaction between the highest occupied molecular orbital (HOMO) of the H3SiNHSiH3 molecule with the lowest unoccupied molecular orbital (LUMO) of the methyl cation is prohibited by symmetry, but the interaction between the next to the highest occupied molecular orbital of the silazane molecule (with the lower energy) and the methyl cation LUMO is possible. So, the gap between these molecular orbitals increases and the overlap probability decreases. Thus, the condensation channel for the HsSiNHSiHs molecule becomes less realizable and as a consequence the probability of the proton transfer channel increases. In the case of the aminosilane molecule the same situation is not true because of the absence of symmetry prohibition. 

FIGURE 7.2 (a) MP2 geometry for the methyl radical plus ethylene transition state (b) MP2 geometry for the ethylene plus bis(cyclopentadienyl)zirconium methyl cation transition state (c) schematic of the correlation between the 1,2 propylene insertion transition state and corresponding 1,2 and 2,1 methyl radical transition states. 

The geometries for acetylene, methyl cation, and methane correspond to the bond angles for the different hybridization states sp, sp, and sp respectively. Again, most organic molecules display measurable deviations from these ideals, but we still loosely refer to the atoms as sp, sp, or sp hybridized, even though we don t expect angles of exactly 180°, 120° or 109.5°. 

PROBLEM 3.52 In this chapter, we developed a picture of the methyl cation ( CH3), a planar species in which carbon is hybridized. In Chapter 2, we briefly saw compounds formed from several rings, bicychc compounds. Explain why it is possible to form carbocation (a) in the bicychc compound shown below, but very difficult to form carbocation (b). It may be helpful to use models to see the geometries of these molecules. 

The first solvation shell of the methyl cation in water and HF can satisfactorily be represented by five and that of the fluorine anion—by six solvent molecules, i.e., the total number of the solvent molecules included in the supermolecular approximation is 11. The C—X distance was taken as the reaction coordinate and all other geometry parameters, includingg those of the solute environment, were optimized. The most important result of the calculations of Eq. (5.7) by the CNDO/2 and ab initio (STO-3G basis set) methods is the detection of three minima along the MERP. The first of these characterized by the distance r( p= 1.388 A corresponds to the hydrated undissociated molecule CH3F. The second minimum corresponds to rcp = 3.480 A. There are no solvent molecules between the ions CHj and F , hence they are located in one cage and may be structurally described as a contact ion pair of type IV. The third minimum corresponds to a completely dissociated system (r< p = 5.463 A), i.e., the solvent-separated ion pair V with each ion surrounded by its own solvation shell with n = 11, j = 5, and k = 6 in Eq. (5.7). 

We first present some recent results on small hydrocarbons methane, acetylene, and the methyl cation CH. Benchmark calculations have been performed on the determination of the equilibrium geometries of these species in their ground electronic state. These systems identified in numerous planetary atmospheres and interstellar medium present a renewed astrophysical interest [66, 102] and are extensively studied in high resolution laboratory experiments [14, 51]. 

Dioxolan-2-ylium cation, 2-methyl-INDO optimized geometry, 6, 750 total charge density, 6, 750... 

Draw a Lewis structure (or series of Lewis structures) foi 2-norbornyl cation which adequately describes its geometry, charge distribution and bond density surface, Relate this structure to your description of 3-methyl-1-butyl cation. 

Examine the geometry of 3-methyl-3-hexyl bromide, and assign the configuration (R or S) to the chiral atom. Examine the geometry of 3-methyl-3-hexyl cation. Is it chiral ... 

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. 

Examine the geometries (in particular, CN bond distances) of methyl diazonium, tert-butyl diazonium and phenyl diazonium ions. Which, if any, of these ions is best described as a weak complex between a cation and N2 Which is furthest away from this description Is your result consistent with the observed reactivity patterns Explain. 

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