Rotation barrier double bond

Rotational barriers for bonds which have partly double bond character are significantly too low. This is especially a problem for the rotation around the C-N bond in amides, where values of 5-10 kcal/mol are obtained. A purely ad hoc fix has been made for amides by adding a force field rotational term to the C-N bond which raises the value to 20-25 kcal/mol, and brings it in line with experimental data. Similarly, the barrier for rotation around the central bond in butadiene is calculated to be only 0.5-2.0 kcal/mol, in contrast to the experimental value of 5.9 kcal/mol. 

Some common limitations to MNDO, AMI and PM3 are l) Rotational barriers for bonds which have partly double bond character are... 

Other applications are studies of rotation about double bonds 136>, ring inversion in cyclooctatetraene 137>, the insertion of carbon into ethylene and. fraws-2-butene to give allenes 133), the barrier height to inversion of nitrogen in hydrazine and alkylamines 133>, the Cope... 

Hiickel found that, by treating only the n electrons explicitly, it is possible to reproduce theoretically many of the observed properties of unsaturated molecules such as the uniform C-C bond lengths of benzene, the high-energy barrier to internal rotation about double bonds, and the unusual chemical stability of benzene. Subsequent work by a large number of investigators has revealed many other useful correlations between experiment and this simple HMO method for n electrons. 

Figure 2-51. a) The rotational barrier in amides can only be explained by VB representation using two resonance structures, b) RAMSES accounts for the (albeit partial) conjugation between the carbonyl double bond and the lone pair on the nitrogen atom. 

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... 

This IS an unusually high rotational energy barrier for a single bond and indicates that the carbon-nitrogen bond has significant double bond character as the reso nance picture suggests... 

Geometrical Isomerism. Rotation about a carbon-carbon double bond is restricted because of interaction between the p orbitals which make up the pi bond. Isomerism due to such restricted rotation about a bond is known as geometric isomerism. Parallel overlap of the p orbitals of each carbon atom of the double bond forms the molecular orbital of the pi bond. The relatively large barrier to rotation about the pi bond is estimated to be nearly 63 kcal mol (263 kJ mol-i). 

Butadiene, the simplest conjugated diene, has been the subject of intensive theoretical and experimental studies to understand its physical and chemical properties. The conjugation of the double bonds makes it 15 kJ/mole (3.6 kcal/mol) (13) more thermodynamically stable than a molecule with two isolated single bonds. The r-trans isomer, often called the trans form, is more stable than the s-cis form at room temperature. Although there is a 20 kJ/mole (4.8 kcal/mol) rotational barrier (14,15), rapid equiUbrium allows reactions to take place with either the s-cis or r-trans form (16,17). 

In Eq. (2), the dihedral tenn includes parameters for the force constant, Ky, the periodicity or multiplicity, n and the phase, 8. The magnimde of Ky dictates the height of the barrier to rotation, such that Ky associated with a double bond would be significantly larger that that for a single bond. The periodicity, n, indicates the number of cycles per 360° rotation about the dihedral. In the case of an bond, as in ethane, n would... 

There is another mechanism for equilibration of the cation pairs A, Aj and B, Bj, namely, inversion at oxygen. However, the observed barrier represents at least the minimum for the C=0 rotational barrier and therefore demonstrates that the C-O bond has double-bond character. 

Rotation about single bonds and conformational changes can be studied. Amides constitute a classic example. Because of the partial double bond character of the carbon-nitrogen bond as a consequence of the contribution of 2 to the electronic structure, there is an energy barrier to rotation about this bond. 

The 180° trans structure is only about 2.5 kcal/mol higher in energy than the 0° conformation, a barrier which is quite a bit less than one would expect for rotation about the double bond. We note that this structure is a member of the point group. Its normal modes of vibration, therefore, will be of two types the symmetrical A and the non-symmetrical A" (point-group symmetry is maintained in the course of symmetrical vibrations). 

The predicted energy of this structure is approximately -215.76438 Hartrees, yielding a reaction barrier of about 86.6 kcal/mol. This value is more in line with expectations, although it is on the high side. Rotation of a double bond is a problem which often requires a higher level of theory than Hartree-Fock (for example, CASSCF) for accurate modeling. 

Some derivatives of triafulvene undergo rotation about the carbon-carbon double bond even at room temperature. Given that cis-trans isomerization about double bonds is normally very difficult (see Chapter 7, Problem 1), how would you rationalize this Examine the electrostatic potential map for perpendicular hexaphenyltriafulvene (the rotational transition state).Would polar solvents tend to lower or raise the rotation barrier Explain. 

The natural bond length varies between 1.503 A and 1.337 A for bond orders between 0 and 1, these are the values for pure single and double bonds between two sp -carbons. Similarly the force constant varies between the values used for isolated single and double bonds. The rotational barrier for an isolated double bond is 60kcal/mol, since there are four torsional contributions for a double bond. 

Although essentially free rotation is possible around single bonds (Section 3.6), the same is not true of double bonds. For rotation to occur around a double bond, the -rrbond must break and re-form (Figure 6.2). Thus, the barrier to double-bond rotation must be at least as great as the strength of the 7r bond itself, an estimated 350 kj/mol (84 kcal/mol). Recall that the barrier to bond rotation in ethane is only 12 kj/mol. 

For 6, the activation energy for rotation about the MSi bond has been measured as AG = 40.3 (+ 5) kJ/mol [143]. According to MO calculations, a genuine Cr = Si double bond has no rotational barrier worth mentioning. This applies also, with some restrictions, to the discussed base adducts. 

They found no evidence of (Z)/(E)-isomerism in the diazosulfones formed. This may be due to the lowering of the barrier of rotation about the NN double bond by the contributions of expanded octet structures such as 6.17 b. It is therefore likely that the observed diazosulfones are ( -compounds. 

The X-ray structure of lithium l-(dimethylamido)boratabenzene, reported in 1993, provided the first crystallographic characterization of a transition metal-free boratabenzene (Scheme 13).18a The observed bond lengths are consistent with a delocalized anion and with significant B—N double-bond character. In a separate study, the B—N rotational barrier of [C5H5B—NMeBnjLi has been determined to be 10.1 kcal/mol, and it has been shown that TT-complexation to a transition metal can increase this barrier (e.g., 17.5 kcal/mol for (C5H5B-N(i-Pr)2)Mn(CO)3).24... 

Table 1. Double bond lengths X=X [A] p-p(x) overlap integrals S ionization potentials IP [eV] of the dissociation products XH2 T-bond strengths Erot [kcal mol 1] from the barrier of rotation, calculated at the CASSCF(2,2)/6-31G + ZPE (zero point energy) level of theory.

H-NMR studies of oligocarbene Ru(II) complexes indicate a substantial barrier to rotation about the metal-carbene carbon and nitrogen-R bonds. This restricted rotation is thought to arise as a consequence of intramolecular non-bonding cis interactions of the carbene nitrogen-R substituents, and not because of any significant double bond character in ruthenium-carbene carbon (76). 

The energetical description of rotations around bonds with high torsional barriers (e.g. the C=C double bond) demands the evaluation of the influence of higher cosine terms. Rotations around single bonds with sixfold symmetric torsional potentials have very low barriers (18) they occur in alkylsubstituted aromatic compounds (e.g. toluene), in nitro-alkanes and in radicals, for example. 

There is a large energy barrier to rotation associated with groups joined by a double bond. 

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