BUTANE.html

BUTANE CONFORMATIONAL ANALYSIS

Methodology
Molecular Mechanics (Force Field)
MNDO/AM1/PM3 (Semiempirical MO)

Techniques Used
Building, Optimization, Analysis of
Bonding Parameters, Output Analysis

Abstract. You will optimize a global minimum structure for n-butane and carry out conformational analysis to determine its conformational energy surface.

Procedure. Create n-butane structures with constrained C-C-C-C dihedral angles at 0 (eclipsed), 60, 120, 180 (anti), 240, 300, and 360 (eclipsed). Optimize with the dihedral constraint, and find the strain energy and/or enthalpy of formation of the structure at each dihedral angle. Plot the data to determine the most stable conformer and the computed energy barrier at the highest transition point. Alternatively, use more points at smaller incremental changes in the dihedral angle, for a more precise energy surface determination. How well does your computed energy surface compare with the experimental results shown below?

Benchmarks. The experimental energy surface for n-butane is given in the figure below.