Energies for rotation

Figure 4-18 The Potential Energy for Rotation of n-Butane About its Central Bond Axis. The anti conformer in the center is slightly lower in energy than the two gauche conformers.

In principle cis 2 butene and trans 2 butene may be mterconverted by rotation about the C 2=C 3 double bond However unlike rotation about the C 2—C 3 single bond in butane which is quite fast mterconversion of the stereoisomeric 2 butenes does not occur under normal circumstances It is sometimes said that rotation about a carbon-carbon double bond is restricted but this is an understatement Conventional lab oratory sources of heat do not provide enough energy for rotation about the double bond m alkenes As shown m Figure 5 2 rotation about a double bond requires the p orbitals of C 2 and C 3 to be twisted from their stable parallel alignment—m effect the tt com ponent of the double bond must be broken at the transition state... 

Shape of potential energy diagram is identical with that for ethane (Figure 3 4) Activation energy for rotation about the C—C bond is higher than that of ethane lower than that of butane... 

The activation energy for rotation about a typical carbon-carbon double bond is very high—on the order of 250 kj/mol (about 60 kcal/ mol). This quantity may be taken as a measure of the tt bond contribution to the total C=C bond strength of 605 kJ/mol (144.5 kcal/mol) in ethylene and compares closely with the value estimated by manipulation of thermochemical data on page 191. 

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. 

Anharmonicity and Nonrigid Rotator Corrections With the rigid rotator and harmonic oscillator approximations, the combined energy for rotation and... 

K. S. Pitzer, Potential Energies for Rotation about Single Bonds . Discuss. Faradav Soc., 10,66-73 (1951). 

The transition between crystalline and amorphous polymers is characterized by the so-called glass transition temperature, Tg. This important quantity is defined as the temperature above which the polymer chains have acquired sufficient thermal energy for rotational or torsional oscillations to occur about the majority of bonds in the chain. Below 7"g, the polymer chain has a more or less fixed conformation. On heating through the temperature Tg, there is an abrupt change of the coefficient of thermal expansion (or), compressibility, specific heat, diffusion coefficient, solubility of gases, refractive index, and many other properties including the chemical reactivity. 

Looney and associates (74) examined the NMR spectra of /V-benzyl-N-meth-ylnitrosamine and obtained an Arrhenius activation energy for rotation of 23... 

Nakamura and Oki (96) isolated the rotamers of 9-(2-bromomethyl-6-meth-ylphenyl)fluorene (56), and found that the Arrhenius activation energy for rotation was 27.1 kcal/mol for the sp — ap process, log A being 10.8. For the reverse process, the values were 27.1 kcal/mol and 11.4, respectively. This is direct proof that the energy barrier obtained by the dynamic NMR technique is useful for diagnosing the possibility of isolating atropisomers, since the barrier... 

It may be argued that direct comparison of the barrier of 9-(2-methyl-l-naphthyl)fluorene with that of 9-(2-bromomethyl-6-methylphenyl)fluorene is not fair, because the latter carries a bromine atom. However, the discussion just presented is valid because Saito and Oki (98) found the Arrhenius activation energy for rotation of 9-(2-bromomethyl-l-naphthyl)fluorene (57) for the process ap — sp... 

The activation energy for rotation about the terf-alkyl-to-triptycyl bond was 36.6 kcal/mol for 101, and the frequency factor was 10 7 sec 1. It should be noted that the barrier to rotation is raised by ca. 4 kcal/mol by going from a bis(methoxycarbonyl)etheno bridge to a benzeno bridge. The equilibrium constant ( sc/ap) was 2.0, as expected on statistical grounds. 

Contemporaneously with these studies, Iwamura (145) described the separation of the C2 and C2v isomers of 9,10-bis(l-cyano-l-methylethyl)triptycene (102). The Arrhenius activation energy for rotation obtained was 37.7 kcal/mol,... 

Gibbs energy for rotation rotational barrier (kcalmoP )... 

Figure 2.3 Fourier decomposition of the torsional energy for rotation about the C-O bond of fluoromethanol (bold black curve, energetics approximate). The Fourier sum (A) is composed of the onefold (o), twofold (o), and threefold ( ) periodic terms, respectively. In the Newman projection of the molecule, the oxygen atom lies behind the carbon atom al center...

Redfield limit, and the values for the CH2 protons of his- N,N-diethyldithiocarbamato)iron(iii) iodide, Fe(dtc)2l, a compound for which Te r- When z, rotational reorientation dominates the nuclear relaxation and the Redfield theory can account for the experimental results. When Te Ti values do not increase with Bq as current theory predicts, and non-Redfield relaxation theory (33) has to be employed. By assuming that the spacings of the electron-nuclear spin energy levels are not dominated by Bq but depend on the value of the zero-field splitting parameter, the frequency dependence of the Tj values can be explained. Doddrell et al. (35) have examined the variable temperature and variable field nuclear spin-lattice relaxation times for the protons in Cu(acac)2 and Ru(acac)3. These complexes were chosen since, in the former complex, rotational reorientation appears to be the dominant time-dependent process (36) whereas in the latter complex other time-dependent effects, possibly dynamic Jahn-Teller effects, may be operative. Again current theory will account for the observed Ty values when rotational reorientation dominates the electron and nuclear spin relaxation processes but is inadequate in other situations. More recent studies (37) on the temperature dependence of Ty values of protons of metal acetylacetonate complexes have led to somewhat different conclusions. If rotational reorientation dominates the nuclear and/or electron spin relaxation processes, then a plot of ln( Ty ) against T should be linear with slope Er/R, where r is the activation energy for rotational reorientation. This was found to be the case for Cu, Cr, and Fe complexes with Er 9-2kJ mol" However, for V, Mn, and... 

Figure 13. Map of conformational energy for rotations about the Si-Si skeletal bonds in polysilane, [SiHz-Jn- The energies (in kilocalories per mole) relative to the minima (designated by the plus signs) are shown as contour lines. (Reproduced from reference 60. Copyright 1986 American Chemical Society.)...

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