Propane, rotational barrier

Pentadienyl radical, 240 Perturbation theory, 11, 46 Propane, 16, 165 n-Propyi anion conformation, 34 n-Propyl cation, 48, 163 rotational barrier, 34 Propylene, 16, 139 Protonated methane, 72 Pyrazine, 266 orbital ordering, 30 through-bond interactions, 27 Pyridine, 263 Pyrrole, 231... 

The rotational barrier is now slightly higher than for ethane 14 kJ mol-1 as compared to 12 kJ mol-1. This again reflects the greater repulsion of electrons in the coplanar bonds in the eclipsed conformation rather than any steric interactions. The energy graph for bond rotation in propane would look exactly the same as that for ethane except that the barrier is now 14 kJ 1110I-1. 

In practical evaluation of force-fields parameters the H... H and C... C non-bonded potentials were chosen first. Calculations of staggered and eclipsed rotamers of ethane, propane and n-butane then revealed the contribution of the non-bonded interactions to the total rotation barrier. For example, the van der Waals parameters proposed by Alhnger et al. in their first paper 40) on calculation of conformations were such that the repulsion between hydrogens on adjacent carbon atoms accounted for about 31% of the barrier in ethane. The remainder was accoimted for as a quantity which was added by considering the torsion interaction to be zero for all... 

Chandross and Depster (16) studied the temperature dependence of the rate constant of excimer formation for l,3-bls(a-naphtyl) propane and for l,3-bls(3-naphtyl)-propane. They found an activation energy of 4 kcal/mol, comparable to the energies related to the rotation barrier in a methylene chain. Recently Avourls, Kordas, and El-Bayouml studied the time dependence of the intramolecular excimer fluorescence of 1,3-dlnaphtylpropane. They determined rate constants kg and k j at different viscosities (37). 

Wiberg studied rotational barriers in formaldehyde, propanal and acetone coordinated to Lewis acids such as BFj or AICI3, where all complexes were found to prefer bent geometries. For formaldehyde complexes, linear structures are 6.10kcal/mol higher in energy and out-of-plane structure (Tt-complexes) even higher [10]. 

The rotational barrier in propane has three-fold periodicity. 

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. 

Thermodynamic contributions from the internal rotation of several symmetric tops may be readily calculated by appropriate summation of terms in Table 4. Few reliable calculations, however, have been reported. Thermodynamic properties of propane and several methyl-substituted benzenes have been reported, for example, but subsequent more accurate work has shown the necessity for considering that the internal rotation may be restricted. " Although the subsequent calculations for m-xylene and p-xylene used 6-fold internal rotation barriers of 2.1 to 3.1 kJ mol", more recent statistical calculations for toluene employing the presence of free rotation suggest that internal rotation in the two xylenes may be effectively unrestricted. 

Propane, the next higher member in the alkane series., also has a torsional barrier that results in hindered rotation around the carbon-carbon bonds. The barrier is slightly higher in propane than in ethane—a total of 14 kj/mol (3.4 kcal/mol) versus 12 kj/mol. 

A difference between cyclopropane and propane is also seen in the barrier to rotation about the bonds to the substituents. Isopropyl alcohol and amine have normal threefold rota-... 

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