Conformational analysis butane

In this chapter we 11 examine the conformations of various alkanes and cyclo alkanes focusing most of our attention on three of them ethane butane and cyclo hexane A detailed study of even these three will take us a long way toward understanding the mam ideas of conformational analysis... 

A significant issue widi modem force fields is that it can be difficult to simultaneously address both generality and suitability for use in condensed-phase simulations. For example, the MMFF94 force field is reasonably robust for gas-phase conformational analysis over a broad range of chemical functional groups, but erroneously fails to predict a periodic box of n-butane to be a liquid at —0.5 °C (Kaminski and Jorgensen 1996). The OPLS force field, on the other hand, is very accurate for condensed-phase simulations of molecules over which it is defined, but it is an example of a force field whose parameterization is limited primarily to functionality of particular relevance to biomolecules, so it is not obvious how to include arbitrary solutes in the modeling endeavor. 

Conformational analysis about the C-2—C-3 bond of butane is more complex than previous examples because each carbon has a methyl and two hydrogens bonded to it Several different interactions occur in the conformations. 

Conformational analysis is the study of the energetics of different conformations. Many reactions depend on a molecule s ability to twist into a particular conformation conformational analysis can help to predict which conformations are favored and which reactions are more likely to take place. We will apply conformational analysis to propane and butane first, and later to some interesting cycloalkanes. 

Conformation and Conformational Analysis Conformation of Ethane Conformation of Propane Conformation of Butane Eclipsed and Staggered Eorms Ring Strains in Cycloalkanes Principles of Conformation 165... 

Conformation analysis is particularly important in aliphatic systems. Taking butane as an example, there is obviously an infinite number of possible dihedral angles that the two methyl groups can adopt with respect to each other. However, two of the conformers are of particular interest in that they represent the states of lowest and highest interaction between the hydrogens on the adjacent methyl groups these are the staggered and eclipsed conformers respectively. 

The increment of energy due to the skew-butane or gauche interaction is variously assigned values of about 0.7 to 1.2 kcal/mole. The value used throughout this contribution is 1.0 kcal. This is a very important parameter because, as will be seen, it permits conformational analysis not only of the cyclohexanes but of the decalins and methyldecalins as well. 

Unlike conformational analysis, in which relative energies are compared by assigning fixed amounts of strain to specific interactions (such as butane gauche interactions), molecular mechanics determines the energy of a conformation by computing the value of a mathematical function. An equation such as equation 3.3 can be used to calculate the total steric energy of the molecule as the sum of a number of different kinds of interactions. Most molecular mechanics methods include as a... 

The difference in steric energy between the 65° gauche conformer and the anti conformer of butane is thus computed to be 0.86kcal/mol, very close to the value used in our discussion of conformational analysis. 

---