Introduction

The different spatial arrangements that a molecule can adopt due to rotation about ? bonds are called conformations and therefore,

conformational isomers or conformers are made.

The study of the energy changes that occur during these rotations is called conformational analysis. This is important because the structure of

a molecule can have a significant influence on the molecular properties, including dictating the outcome of a reaction. For butane the C2-C3

bond is the important rotational bond.

Purpose

The purpose is to study how these rotations impact the total steric energy.

Method

Using the Chem3D ultra, the structure of n-butane was established (CH3CH2CH2CH3). Then the all the dihedral angles of the compound was

formed. An MM2 Energy Minimization was performed and C1-C2-C3-C4 dihedral angle was selected for further study. The C1-C2-C3-C4 dihedral

angle was rotated by 30-degree increments and its steric energy after each rotation was calculated until it has been rotated through 360

degrees using Rotation Dial option from the Rotation Tool. The data was transferred to Excel. Two charts were made using excel.

Results

It can be seen, there are no changes between the energy values of 180, 0 and -180 degrees, as they show the same difference angle between

the methyl groups of our butane (C1 and C4). But as the distance between these carbons decreases, the total steric energy increases, due to

the change in 1, 4-van der Waals bonds. But non 1,4-van der Waals energies decrease, because the distance between them increases,

but this decrease is very small compare to the increase..

Discussion

In butane it is the rotation about the C2-C3 bond that is of most interest since the relative position of the two methyl groups is important.

A staggered conformation with the Me groups at 180o with respect to each other. This is the most stable conformation since the Me groups

are as far apart as possible. a staggered conformation with the Me groups at 60o with respect to each other.

The 1,4-van der Waals has major contribution to the total energy as the interactions between the two methyl groups can change a lot of things,

for example the molecule stability. As it can be seen in each rotation only three components change, the Torsion, 1,4-van der Waals and

non-1,4 van der Waals. Torsional strain happens due to electrostatic repulsion of the electrons in the bonds. Basically these changes in these

terms are due to different conformations and angles of two methyl groups compare to each other.