Bond rotation energies

MAECIS also contains a molecular conformation analysis system (4). This system allows the user to generate all possible conformations of the current molecule over a series of single bond rotations. Energy contour maps can be obtained for the various conformations and this allows for the selection of low energy conformations for further manipulation or calculations. 

For PI-2545 the unstructured band has an activation energy of 7.3 kcal/mole and the structured band has an activation energy of 12.4 kcal/mole. These activation energies are relatively small, on the order of bond rotation energies. The lower activation energies for the BTDA-ODA polyimide are consistent with the increased flexibility of the chain due to the addition of the carbonyl linkage in the pyromellitimide moiety. 

S.Gjjind calculated as the sum of partitioning, hydrogen bond, 71-71 stacking, and loss of bond rotational energies, correlated highly (0.978) with an experimental... 

For real elastomers, however, the internal energy term (dU jdl )t cannot be exactly zero, since chain uncoiling would require that the bond rotational energy barriers are overcome. 

The most flexible chains are those where the lowest amount of energy is required for one bond to rotate with respect to its neighbour, i.e. there should be low bond rotational energies. Such low... 

Nucleophile catalysis. Bond rotation energy barriers. 

Rosenbluth algorithm can also be used as the basis for a more efficient way to perform ite Carlo sampling for fully flexible chain molecules [Siepmann and Frenkel 1992], ch, as we have seen, is difficult to do as bond rotations often give rise to high energy rlaps with the rest of the system. 

The progression of sections leads the reader from the principles of quantum mechanics and several model problems which illustrate these principles and relate to chemical phenomena, through atomic and molecular orbitals, N-electron configurations, states, and term symbols, vibrational and rotational energy levels, photon-induced transitions among various levels, and eventually to computational techniques for treating chemical bonding and reactivity. 

FIGURE 3 4 Potential energy diagram for rotation about the carbon-carbon bond in ethane Two of the hydrogens are shown in red and four in green so as to indicate more clearly the bond rotation... 

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... 

For molecules and ions having more than one atom, the extra energy can make the component bonds rotate and vibrate faster (rovibrational energy). Isolated atoms, having no bonds, cannot be excited in this way. 

Question. Using Equation (1.62) calculate, to four significant figures, the rotational energy levels, in joules, for J= 0, 1 and 2 for Then convert these to units of cm. [Use a bond... 

Before considering the special case of rotation about bonds in polymers it is useful to consider such rotations in simple molecules. Although reference is often made to the free rotation about a single bond, in fact rotational energies of the order of 2kcal/mole are required to overcome certain energy barriers in such simple hydrocarbons as ethane. During rotation of one part of a molecule about... 

Figure 4.2. Rotational-energy barriers as a function of substitution. Tbe small barrier ( 2kcal) in ethane (a) is lowered even further ( O.Skcal) if three bonds are tied back by replacing three hydrogen atoms of a methyl group by a triple-bonded carbon, as in methylacetylene (b). The barrier is raised 4.2 kcal) when methyl groups replace the smaller hydrogen atoms, as in neopentane (c). Dipole forces raise the barrier further ( 15 kcal) in methylsuccinic acid (d) (cf. Figure 4.3). Steric hindrance is responsible for the high barrier (> 15 kcal) in the diphenyl derivative (e). (After...

Figure 4.3. Energy versus bond rotation in methylsuccinic acid (schematic). The diagram shows the greater stability of staggered as compared with eclipsed forms, and the effect of size and dipole moment of substituents on the barriers. The slope of the curve at any point represents the force opposing rotation there. ( = energy of activation of rotation.) (After Gordon )...

It has been common practice to blend plasticisers with certain polymers since the early days of the plastics industry when Alexander Parkes introduced Parkesine. When they were first used their function was primarily to act as spacers between the polymer molecules. Less energy was therefore required for molecular bond rotation and polymers became capable of flow at temperatures below their decomposition temperature. It was subsequently found that plasticisers could serve two additional purposes, to lower the melt viscosity and to change physical properties of the product such as to increase softness and flexibility and decrease the cold flex temperature (a measure of the temperature below which the polymer compound loses its flexibility). 

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