Molecular Geometry and Electronic Structure

Molecular Geometry and Electronic Structure: A Molecular Modeling Experiment

Objective: To help recall Lewis Structures, VSEPR Theory, Electronic structure of atoms and molecules, covalent bonding in molecules and energetic consequences of geometric change at the molecular level while learning to use the Spartan '06 molecular modeling software package.

Background: The three most fundamental molecular structure motifs we encounter in organic chemistry are demonstrated by three molecules: Ethane, Ethene, and Ethyne. It is essential to understand molecular structures.

Results: To determine the bond lengths for ethane, ethene and ethyne, Spartan 06 was used to build each molecule.

The steps to carry out the Geometry Optimization:

1. Click on the New Document icon on the toolbar. Build the molecule using the interface on the right side of the screen.

2. Click the Setup tab and scroll down to calculations.

3. In the calculate box choose Equilibrium Geometry at the Ground State with Density Functional theory using the B3LYP functional and the 6-31G* basis set.

4. Be sure to check the Total Charge (Neutral for a total charge of 0, cation for +1, diction for +2, etc. and anion for -1 and dianion for -2, etc.) and the Multiplicity (singlet for most cases).

5. Click Submit

6. Save the molecule and wait for the program to do the calculations.

After calculating, the bond lengths are as follows:

Ethane: C-C = 1.531 C-H = 1.096 sp3 Hybridization

Ethene: C-C = 1.331 C-H = 1.087 sp2 Hybridization

Ethyne: C-C = 1.205 C-H = 1.067 sp Hybridization

A "twisted" Ethene was also calculated to determine how much of an effect breaking one of the C-C bonds would have on the bond lengths while still maintaining the same number of atoms.

"Twisted" Ethene: C-C = 1.450 C-H = 1.090

Conclusion: After processing the molecular structures through the Spartan software, it is determined that the C-C and C-H bond lengths for Ethane is the...