Transition structures for rotation

Several attempts have been made to analyse the captodative effect through rotational barriers in free radicals. This approach seems to be well suited as it is concerned directly with the radical, i.e. peculiarities associated with bond-breaking processes do not apply. However, in these cases also one has to be aware that any influence of a substituent on the barrier height for rotation is the result of its action in the ground state of the molecule and in the transition structure for rotation. Stabilization as well as destabilization of the two states could be involved. Each case has to be looked at individually and it is clear that this will provide a trend analysis rather than an absolute determination of the magnitude of substituent effects. In this respect the analysis of rotational barriers bears similar drawbacks to all of the other methods. 

Stabilized allyl radical will be stabilized further if substituents are introduced. This stabilization occurs to different degrees in the ground state and the transition structure for rotation. In the ground state the substituent acts on a delocalized radical. Its influence on this state should be smaller than in the transition structure, where it acts on a localized radical. In the transition state the double bond and the atom with the unpaired electron are decoupled, i.e. in the simple Hiickel molecular orbital picture, the electron is localized in an orbital perpendicular to the jt(- c bond. 

The rotational barrier about the C—O bond in the cyanomethoxymethyl radical, [35]/[36], constitutes a similar case, although the situation is somewhat more complicated (Beckwith and Brumby, 1987). As oxygen carries two lone pairs of electrons, the transition structure for rotation about the C—O bond can still be stabilized by conjugation. Compared to the methoxy-methyl radical, the barrier in the captodative-substituted radical is 1-2 kcal mol higher. 

One 11 Bond. It hardly needs saying that a n bond is not usually free to rotate. The n energy 2En that we saw in Fig 1.26 (280 kJ mol ) would be lost at the transition structure for rotation about the C—C bond, which would have the two p orbitals orthogonal. This value is higher than the energy normally available for a chemical reaction. For rotation about a n bond to become easy in the ground state, either the transition structures like diradical 2.101 or the zwitterion 2.102 must be stabilised or the planar structure 2.100 must be destabilised. 

C, and the cation 2.111 into the W-shaped cation 2.110 with the same half-life at 35 °C. These correspond to enthalpies of activation of 74 and 101 kJ mol 1 (18 and 24 kcal mol-1), respectively. This measurement only sets lower limits to the rotation barrier of an allyl cation, because it is not known whether rotation takes place in the cations themselves or in the corresponding allyl chlorides with which they could be in equilibrium.141 The barrier in cations is also much affected by solvation and by the degree of substitution at the termini, since the transition structure for rotation draws on such stabilisation more strongly than the delocalised allyl cation does. 

The frequency job on the middle structure produces one imaginary frequency, indicating that this conformation is a transition structure and not a minimum. But what two minima does it connect Is it the transition structure for the cis-to-trans conversion reaction (i.e. rotation about the C=C bond) ... 

In order to understand the rates of racemization of biphenyls and bihetero-cyclics, an accurate knowledge of their geometric structure is essential. Such knowledge makes it possible to estimate the amount of interference caused by substituents in the vicinity of the pivot bond in an assumed coplanar transition state for rotation. A study of the crystal structure of 1 (X = Se) found that the selenophene rings have a small but significant deviation from planarity and are nearly perpendicular to each other.17 The deviation from 9(T is such that the carboxyl groups are in transoid positions. 

JThese are conformations different from the lowest energy conformation in the absence of the restraints imposed by the crystal lattice, in solution or in the gas phase. The two rings of biphenyl, for example, are coplanar in the crystal, though not in solution. The solid state structure thus formally represents the transition state for rotation about the central C-C bond. 

Cs s) conformation. The latter is the transition state for rotation of the H2 moiety of 3c-2e bond. These structures resemble a complex between CH3+ and a hydrogen molecule, resulting in the formation of a 3c-2e bond. Of the possible structures, Olah, Klopman, and Schlosberg751 suggested preference for the Cs front-side protonated form. 

The transition state for rotation about the Group 13-N bond shows that the pi bond energy is significant, but not nearly as large as for ethene (except in the case of H2BNH2, which has a pi bond energy of 30 kcal/mol, ethene has a 65 kcal/mol pi energy). The transition-state structures are all of the same type... 

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