Methyl transitional modes

In the more complex dissociation of ethane into two methyl radicals, there are 12 conserved and 6 transitional modes. The conserved modes are associated with the CH stretch and bending motions of the two methyl fragments. The fate of the conserved and disappearing vibrational modes is shown in table 7.4. The 6 transitional modes for... 

The methyl radical has a significantly greater polarizability than the H atom, but still has no dipole. Furthermore, there are a large number of transitional modes that transform from rotations to vibrations. The radial dependence of similar direct MP2/cc-pvdz VRC-TST calculations of the microcanonical number of states for the CH3 + C3HJ —> C4H3 reaction are plotted in Fig. 4.13. The qualitative behavior is similar to that for the... 

B. The first complementary gronp freqnency in the hydrocarbons is the symmetric methyl bending mode, which gives rise to a medinm-to-strong band in the IR bnt is nearly absent in the Raman effect. It is therefore somewhat snrprising that a weak overtone of this fnndamental is observed near 2740 cm on the low-wavennmber side of the Raman-active C H stretching fundamentals. Why this transition is observed at all will be described later along with a few examples of systems where the fnndamental is also observed in Raman spectra. 

The mode of the diastcrcofacial selectivity is completely reversed in the case of reactions with A -methyl A-acyliminium precursors 4176. Now the nitrogen atom of the A-acyliminium ion is not able to chelate with the tin atom and the lower diastereoselectivity is explained by the less rigid nonchelation-controlled transition state 5. An electronic effect, such as n-iz attraction between the electron-deficient carbonyl group of the acyliminium ion and the electron-rich phenyl group of the phcnylthio substituent R, may account for the somewhat higher diastereoselectivity in the case of arylthio substituents R. 

A resident stereocenter in the enone part can control the formation of two new stereocenters in one step, guided by a synclinal transition state and an axial cyclization mode. The major product on cyclization of 4-methyl-3-[6-(trimethylsilyl)-4-(Z)-hexenyl]-2-cyclohexenone was formed in a ratio of 7.5 13S. 

On the basis of the X-ray structural data as well as the mode of polymerization, Yasuda et al. [3a] proposed a coordination anionic mechanism involving an eight membered transition state for the organolanthanide-initiated polymerization of MM A (Fig. 6). The steric control of the polymerization reaction may be ascribed to the intermolecular repulsion between C(7) and C(9) (or the polymer chain), since completely atactic polymerization occurred when the monomer was methyl or ethyl acrylate. 

The mismatched R/S pairing could lead to the anti,syn adduct through transition state C and the syn,anti adduct via D (Scheme 9.30). The former pathway entails non-Felkin-Anh addition but anti disposed methyl and aldehyde substituents. Transition state D proceeds through the Felkin-Anh mode of carbonyl addition but requires eclipsing of the methyl and aldehyde substituents. This interaction is the more costly one and thus disfavors the syn,anti adduct. 

Figure 6.8 Proposed modes of action of hydrogen-bonding catalyst 16 Bidentate hydrogen bonding coordination of the zwitterion derived from Michael-type DABCO attack to methyl acrylate (1) and Zimmerman-Traxler transition state for the reaction of methyl acrylate with benzaldehyde (2).

Oxa-l -silabicyclo[ . 1,0 alkanes (n = 3 111 n = 4 113) were the only products isolated from the photochemical, thermal or transition-metal catalyzed decomposition of (alkenyloxysilyl)diazoacetates 110 and 112, respectively (equation 28)62. The results indicate that intramolecular cyclopropanation is possible via both a carbene and a carbenoid pathway. The efficiency of this transformation depends on the particular system and on the mode of decomposition, but the copper triflate catalyzed reaction is always more efficient than the photochemical route. For the thermally induced cyclopropanation 112 —> 113, a two-step noncarbene pathway at the high reaction temperature appears as an alternative, namely intramolecular cycloaddition of the diazo dipole to the olefinic bond followed by extrusion of N2 from the pyrazoline intermediate. A direct hint to this reaction mode is the formation of 3-methoxycarbonyl-4-methyl-l-oxa-2-sila-3-cyclopentenes instead of cyclopropanes 111 in the thermolysis of 110. 

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