Rotational energy barriers alkenes

RNA, mRNA, rRNA, and tRNA. See Ribonucleic acid Roberts, John D., 928 Robinson, Sir Robert, 4, 402, 724 Robinson annulation, 724, 728 Rotamer, 90. See also Conformation Rotational energy barrier alkenes, 172—173 amides, 779 butane, 94—95 conjugated dienes, 31(r-371 ethane, 93—94... 

Explain the dramatic difference in rotational energy barriers of the following three alkenes. Hint Consider what the transition states must look like.)... 

An alkene complexed to platinum(II) is only slightly modified on coordination, but complexation to platinum(O) causes major changes. Platinum(O) alkene complexes show both weakening and lengthening of the carbon-carbon bond, as well as distortion of the plane of the double bond away from the platinum. In platinum(ll) alkene complexes the double bond lies approximately perpendicular to the square plane of platinum(II), but in platinum(O) complexes there is only a small dihedral angle between the platinum and alkenic planes. For platinum(II) the energy barrier to free rotation of the alkene about the platinum(D)-alkene bond is only about 40-65 kJ mol-1, whereas no rotation is observed with platinum(O) alkene complexes. Alkenes bonded to platinum(ll) exert a large trans effect but only have a small trans influence. 

The preferred conformations of carbonyl compounds, like 1-alkenes, are eclipsed rather than bisected, as shown below for ethanal and propanal. The barrier for methyl group rotation in ethanal is 1.17kcal/mol. Detailed analysis has indicated that small adjustments in molecular geometry, including a-bond lengthening, must be taken into account to quantitatively analyze the barrier. The total barrier can be dissected into nuclear-nuclear, electron-electron, nuclear-electron, and kinetic energy (At), as described in Topic 1.3 for ethane. MP2/6-311+G (Mf,2p) calculations lead to the contributions tabulated below. The total barrier found by this computational approach is very close to the experimental value. Contributions to the ethanal energy barrier in kcal/mol are shown below. 

In monomeric aminoboranes, rotation about the double B N bond is hindered. This can be demonstrated readily by the n.m.r. spectra of compounds Mc2BNRR or XYBNMe2, in which the methyl groups differ, with energy barriers to rotation of ca. 14-18 kcal/mole (cf. ca. 60 kcal/mole for rotation about the C=C bond of comparable alkenes). Moreover, derivatives RR BNRR can exist in cis and treats forms, although interaction between the substituents may make the trans isomer more stable. Cis- trans -isomerism may also occur in dimeric aminoboranes (X2BNHR)2 (3.16) or aldiminoboranes, e.g. (MeCH N BMe2)2 (3.17 and... 

Isomeric alkenes may be either constitutional isomers or stereoisomers There is a sizable barrier to rotation about a carbon-carbon double bond which corresponds to the energy required to break the rr component of the double bond Stereoisomeric alkenes are configurationally stable under normal conditions The configurations of stereoisomeric alkenes are described according to two notational systems One system adds the prefix CIS to the name of the alkene when similar substituents are on the same side of the double bond and the prefix trans when they are on opposite sides The other ranks substituents according to a system of rules based on atomic number The prefix Z is used for alkenes that have higher ranked substituents on the same side of the double bond the prefix E is used when higher ranked substituents are on opposite sides... 

Absorption of a photon by an alkene produces a (tt,Jt ) vertical (Franck-Condon) excited state in which the geometry of the ground state from which it was formed is retained. Since the (it,it ) state has no net n bonding, there is little barrier to free rotation about the former double bond. Thus, relaxation takes place rapidly, giving a nonvertical (it,it ) state with a lower energy and different geometry to the vertical excited state. 

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