Ethane conformational energy

The actual Fock-matrix elements between orthogonalized ctch and ctc H orbitals are too small to account for appreciable conformational energy differences in the actual geometry of ethane (as can be seen from the small numerical values of /fsteric in Fig. 3.57). 

The conformational energy E(op) associated with gauche states about CH2-CH2 bonds in polyethylene sulfide) (PSE) is estimated from the RIS analysis of experimental dipole moments and their temperature coefficients corresponding to the alternating copolymer of pentamethylene sulfide and ethylene sulfide (PXS) as well as to 1,2-bis(butylthio)ethane. 

A Pairwise Additivity Scheme for Conformational Energies of Substituted Ethanes. 

The rotational profile of n-butane can be understood as a superimposition of van der Waals repulsion on the ethane rotational energy profile. The two gauche conformations are raised in energy relative to the anti by an energy increment resulting from the van der Waals repulsion between the two methyl groups of 0.6 kcal/mol. The... 

Figure 1.2. Conformational energies of ethane, butane and 1,2-dichloroethane. [in part, reprinted with permission from Hine, J. Physical Organic Chemistry, 2nd Ed., McGraw-Hill, New York, 1962, p 36. Copyright 1962, McGraw-Hill.]...

The ethane conformers are thus separated by a relatively low potential barrier, which can be overcome by thermal energy supplied by the collision of molecules. On the average, thermal energies of about 0.5RT per degree of freedom are transferred during such a collision. Because this is less than... 

Figure 16.7 Potential energy for ethane conformations as a fnnction of dihedral angle.

Figure 2.2 Conformational energy of ethane as a function of torsion angle.

Matsubara T, Sieber S, Morokuma K (1996) A test of the new integrated MO + MM (IMOMM) method for the conformational energy of ethane and n-butane. Int J Quantum Chem 60 1101-1109... 

Figure 1 Conformational energy profiles of ethane and butane...

Concerning the conformational energy barriers studied in Section 15.9, they are all well reproduced by calculations. For H2O. we obtain an electronic barrier to linearity of 132.2 kJ/mol, compared with the experimental barrier of 131.2 kJ/mol. For the inversion of NH3, the experimental and calculated electronic barriers are 21.2 and 21.1 kJ/mol, respectively. Finally, for the rotation of ethane, the experimental and calculated electronic barriers are 11.4 and 11.5 kJ/mol, respectively. Clearly, the theoretical calculations compare well with the measurements, again providing a useful alternative to experiment. 

Energy is required to convert from the staggered conformation to the eclipsed conformation, and for ethane, the energy required is about 12.0 kJ moP. Because of this energy requirement, there is a barrier to internal rotation, and thus the —CH3 groups in ethane do not rotate entirely freely about the C—C bond. However, the barrier to internal rotation is small enough that, at room... 

The origin of a torsional barrier can be studied best in simple cases like ethane. Here, rotation about the central carbon-carbon bond results in three staggered and three eclipsed stationary points on the potential energy surface, at least when symmetry considerations are not taken into account. Quantum mechanically, the barrier of rotation is explained by anti-bonding interactions between the hydrogens attached to different carbon atoms. These interactions are small when the conformation of ethane is staggered, and reach a maximum value when the molecule approaches an eclipsed geometry. 

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