Ethene, rotation barrier

Coupling of changes of coordination geometry with alkene rotation is possible. An ethene rotation barrier of 134 kJ mol" in [Fe(CO)4(C2H4)] is calculated if the Fe(CO)4 group is maintained rigidly C2v but coupling of Berry pseudorotation (BPR) with alkene rotation reduces this to 50 kJ mol". Scheme 1 shows... 

MO calculations (HF/STO-3G and HF/3-21G) indicate a rotational barrier that is substantially reduced relative to the corresponding barrier in ethene. The TS for the rotation is calculated to have a charge separation of the type suggested by the dipolar resonance structure. ... 

NMR spectra show the ethene molecules to undergo a propeller type rotation about the metal-alkene axis the fluxionality is removed on cooling such rotation is not observed with coordinated C2F4, indicating a higher barrier to rotation, in keeping with the stronger Rh—C bonds [66]. 

Figure 10.9 Potential energy for internal rotation (a), as a function of angle fb), for a molecule such as dimethylcadmium with a small potential barrier and (c), for a molecule such as ethene with a large potential barrier.

One more example of the CASSCF procedure will be outlined calculating the barrier to rotation around the CC double bond in ethene. Step 2, orbital localization, showed nicely localized orbitals when NBO localization was used, but the orbitals were harder to identify with Boys localization. For a CAS(2,2)/6-31G optimization the active orbitals chosen were the n and 7t MOs, and for a CAS(4,4)/6-31G optimization the n, n, cr and cr MOs. The input structures were the normal planar ethene and perpendicular (90° twisted) ethene. Optimization and frequency calculations gave a minimum for the planar and a transition state for the perpendicular structures. The energies (without ZPE, for comparison with those calculated with the GVB method by Wang and Poirier [71]) were ... 

However, it is to be expected that the rotation of the dimethylamino groups out of the plane should diminish their donor capacity. This is supported by the observation that the C=C barriers are lower in acceptor-substituted 1-dimethylamino-1-methyl-thioethenes (ketene N,5-acetals, 26) than in the l,l-bis(dimethylamino)ethenes (24) with the same acceptor combination (Table 6), in spite of the fact that dimethylamino groups in general are much better donors than methylthio groups33. However, the situation is not quite simple, since in a crystallographic study the ketene A S-acetal 26a was found to have the dimethylamino group twisted 25° out of the plane with a C1=C2 bond twist of ca 20°67. 

Moreno and coworkers published a study on the triplet carbene-ethene addition reaction. This process should involve two steps, namely the formation of a triplet trimethylene 1,3-diradical as an intermediate followed by intersystem crossing and formation of 1. The intermediate may live long enough to permit rotation at the CC bonds. In this way, the stereochemistry of the alkene will be lost and a non-stereospecific addition takes place. Moreno and coworkers calculated for the first step of the reaction C 2C i) H2C = CH2 a barrier of 11 kcalmol" and a reaction energy of -26 kcalmoF at the MP2/3-21G level. (It has to be mentioned in this connection that the 3-21G basis is far too small to lead to reliable energies in correlation calculations and therefore results are just of a qualitative nature ) Activation energies varied from 5 to 17 kcalmoF if CH2 was replaced by the triplet state of CH(CN), CH(BeH) and CHLi ... 

Experimental and theoretical studies on the solvent influence on molecular geometries and cis/trans isomerization processes of other so-called push-pull ethenes, R2N CH=CH A R2N+=CH CH=A- (with A = NO2, CHO, CN, etc.), have been collected in references [287, 288], The barriers to isomerization about the C=C bonds of these acceptor-substituted enamines are considerably smaller than those for simple ethenes such as 2-butene Ea = 259 kJ/mol [82]), owing to a significant contribution of the mesomeric zwitterionic structure to the electronic ground state. Increasing solvent polarity increases the contribution of this dipolar mesomeric structure, and hence leads to a decrease in the barrier to C=C isomerization and a simultaneous increase in the barrier to rotation about the C—N bond [288], The calculated sequence of solvent stabilization for such acceptor-substituted enamines is (activated complex for C=C rotation) E form > Z form > (activated complex for C—N rotation). Obviously, the activated complex for isomerization about the C=C bond corresponds to a full zwitterion with maximal solvent stabilization [288]. 

Another, very rare anomaly is that single reaction steps of radicals at very low temperatures may not obey the Arrhenius equation at all. Normal Arrhenius behavior arises from an activation barrier which the reaction must surmount. In many exothermic reactions of radicals with one another or with small molecules, there is no such barrier to speak of (see Section 10.4), and a hindrance from other effects, say, molecular rotation, may increase as the temperature is raised. This makes the rate decrease in a non-Arrhenius fashion with increasing temperature. Examples include reactions of CN- with 02, ethene, ethyne, and ammonia and of OH- with O , butenes, and HBr. This interesting anomaly has recently been reviewed by Sims [11], The discussion here and in other part of this book assumes no such steps are involved. 

The bond dissociation energy is indicated by the special term DH°. Recall from Section 2.3 that the barrier to rotation about the tt bond of ethene is 63 kcal/mol. In other words, it takes 63 kcal/mol to break the tt bond. 

Figure 2.22. Computed structures of the C-O bond activation transition states, and barriers, for ethene" and propene epoxidation . The putative parallel TS for ethene epoxidation is generated by rotating the computed one around the O-C bond.

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