Ethylene rotation

Restricted Rotation. A little reflection will reveal that maximum overlap in a 7r bond is produced when the p orbitals are parallel to each other, and any rotation of these orbitals about the bond direction would decrease overlap. Since the directions of the p orbitals are fixed at right angles with respect to the cr bond planes, the most stable configuration of ethylene with maximum tt overlap is achieved when the cr bond planes coincide in other words, all the atoms in the molecule should be in one plane. This is indeed true in ethylene, and experiments indicate that rotation of the two CH2 halves about the C=C bond results in an increase in potential energy. There is restricted rotation in ethylene. Rotation about pure cr bonds is not restricted in this way. 

Long wavelength absorption bands for ethylene (n = 2) and benzene (n = 3) are observed at 176 nm and 260 nm respectively, solving for r = 1.64A and 2.36 A respectively. This result is to be compared with r = 1.65A calculated from the observed barrier to ethylene rotation (section 6.2.2). 

The staggered conformation observed for / is interesting when compared with RuCl2(CO)(C2H4)(PMe2Ph)2 [mutually cw-chlorines, mutually fra/ts-phosphines] in which the ethylene eclipses the phosphines. The origin of the ethylene rotational barrier in complexes of this type has been discussed by Hoffmann and others and has been shown to be due primarily to repulsions between the filled ethylene n orbital and the filled metal d, orbital. Obviously this type of interaction is much less important for V -S02. 

Harman has developed an analog of the CpRe(NO)(L)+ systems using hydrido(tris)pyrazolylborate, Tp, complexes. The [TpRe(CO)(PMc3)] moiety is more electron rich than its counterpart and will even bind naphthalene in an jj -fashion. Ethylene rotation in TpRe(CO)(L)(ene) has a barrier of 40.0 kJmol as determined by SST experiments. Substantial variation of barriers correlated with basicity were noted with different L groups L = t-BuNC, 32 kJmol estimated from extensive... 

Figure IS Two paths for energy flow from an electronically excited ethylene molecule to the water solvent, (a) Multistep energy transfer where the ethylene rotation acts as a mediator between the ethylene vibration and a single water solvent molecule, (b) Direct energy transfer from the ethylene vibration to a single water molecule. Adapted from ref. 119.

Abstract We investigate the dependence of multiconfigurational perturbation theory framework on the choice of the Fermi-vacuum. A new formulation, based on a posteriori averaging is suggested. The averaged theory is invariant with respect to Fermi-vacuum choice but enhances the intruder effect. The performance of the averaged formulation is illustrated on the ethylene rotational potential curve. 

Cramer et al. observed ethylene rotation in M( n -CjH5XC2H4)2 (M Rh, Ir) complexes with the following structure ... 

In contrast to the rotation that occurs around the C—C bond of ethane, no rotation occurs around the C=C bond of ethylene. Rotation around the C=C intemuclear axis would not disrupt the sp -sp O bond. However, this motion would destroy the overlap of the two 2p orbitals and break the tt bond. A large amount of energy, approximately 250 kj mole, is required to break the tc bond. 

Iridium.—The barriers to ethylene rotation in [Ir(C2H4)4Cl] (58) are solvent-dependent. The low-temperature limiting n.m.r. spectrum for both equatorial and axial olefin ligands was not reached in chloroform at 213 K. In... 

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