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Orientation rotation barriers

Table 3.23 summarizes the rotation barriers and leading vicinal cr-cr interactions for methyl rotors CH3—X(X = CH3, NH2, OH) as well as higher group 14 congeners H3M—MH3(M = Si, Ge). Figure 3.59 shows orbital contour diagrams for syn and anti orientations of selected vicinal donor-acceptor NBOs in these species. We now discuss some qualitative trends of torsion barrier potentials in terms of these examples. [Pg.234]

Table 3.23. Rotation barriers (Ecc — Estg) and leading a-cr donor acceptor stabilizations (—A Eaa ) in anti and syn orientations for methyl rotors (CH3—X, X = CH3, NH2, OH) and higher ethane-like congeners... Table 3.23. Rotation barriers (Ecc — Estg) and leading a-cr donor acceptor stabilizations (—A Eaa ) in anti and syn orientations for methyl rotors (CH3—X, X = CH3, NH2, OH) and higher ethane-like congeners...
The perpendicular orientation of the alkene in such complexes is favored because it maximizes the overlap of the bond with the LUMO (dx2 — y2, Figure 13.7) and minimizes 4e repulsive interactions with the HOMO (ndz2). The in-plane orientation is not expected to be strongly disfavored, however, because of the secondary interaction between the orbital and the dxy orbital. The rotational barrier of ethylene in Zeise s anion was theoretically estimated to be 55 kj/mol [282], within the range 42-63 kj/mol measured by NMR for related complexes [286]. [Pg.189]

The reverse reaction corresponds to intramolecular C—H bond formation. It requires that the alkene rotate from its equilibrium perpendicular orientation toward the less favored parallel orientation. The barrier hindering the rotation of the alkene may be partially or totally offset by the incipient agostic interaction. If the alkene is unsym-metrical, the question of regioselectivity of hydride transfer arises. In the case of an unsymmetrical alkene with an X or C substituent, the donor orbital is polarized away from the substituent (Figure 13.9) and the metal lies closer to that end. Conversely,... [Pg.191]

The symmetry and height of the rotational barriers and hence the tunnel splittings depend strongly on the orientation of the molecule and its adsorption site. The results of these measurements (a higher resolution experiment is planned) when combined with model calculations based on empirical atom-atom potentials (see below) may be able to provide corroborative evidence for the orientation of an adsorbed molecule as well as details of the molecule-substrate interaction. The principal obstacle to a wider application of the technique may simply be the small number of adsorbed systems in which these tunnel splittings can be observed. [Pg.255]

There has been considerable interest in the factors that control the stereoselectivity of cyclobutanol formation. Three main factors were identified quite early pre-existing conformational preferences due to steric effects or to internal hydrogen bonding solvation of the OH group and variable rotational barriers for cyclization. More recently Griesbeck has proposed that orbital orientation favoring soc produces another form of conformational preference in triplet biradicals [55], These factors have different importance depending upon the molecule. [Pg.31]

FIGURE 3. Rotational barriers (in kcal mol-1) of vinylamine calculated with use of the 6-31G basis set from values presented in Table 9. is the angle between the orientation of the electron lone pair on N and the vinylic 7c-system... [Pg.31]

The pseudo-double bond character of amides is much more pronounced than for esters due to the conjugation of the H-N-C=0 moiety and is correlated to the ability of distorted amides to be hydrolyzed to bases [19]. For this reason, the barrier to interconversion is significantly higher that for the ester series, with AGl typically ranging from 16 to 22 kcal mol-1 [17]. However, the rotational barrier is not solely due to conjugation and also partly arises from the orientation of the nitrogen lone pair which is perpendicular to the amide plane [20]. Therefore, the rates of isomerization are considerably slower than for esters. This means that both isomers can be observed by simple techniques, for example at room temperature by H and 13C NMR spectrometry and UV spectrophotometry [21]. [Pg.145]


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Rotation barrier

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