Molecular Modeling Procedure - Detailed Study Notes

What is Avogadro?
Avogadro is a free cross-platform molecular editor designed for building, visualizing, and performing calculations on molecular structures.
If Avogadro is not pre-installed, it can be downloaded from the official website.

To minimize the energy of a molecule using several different initial conformations.
To determine the dihedral angle for the carbon atoms in 3-methyloctane.

Draw 3-methyloctane in various conformations with different dihedral angles across the carbon backbone.
Minimize each structure to find the lowest energy conformation accessible from each starting point.

Start a New File
- Navigate: File ➔ New
- A blank black screen will display with "Tool Settings" and "Display Settings" toolbars.
Set Up Drawing Tools
- In "Tool Settings", select the pencil icon.
- Set element to "carbon" and bond type to "single".
Construct the Molecule
- Click on the black screen to place a sp3 carbon atom (methane).
- Change one H atom to create C-CH3, forming ethane.
- Continue this process to draw pentane in a zig-zag structure.
- Add methyl groups to C(2).
- Optimize Geometry: Go to Extensions ➔ Optimize Geometry to adjust bond lengths slightly.
Measure Dihedral Angle
- Select the "Click to Measure" icon.
- Click in order on C(1), C(2), C(3), and C(4) to display the dihedral angle in the lower left corner.

Changing Conformations
- Adjust the conformation of 3-methyloctane so that the dihedral angle between C(1) and C(4) is between 2° and 5°.
- Use the "Bond Centric Manipulation Tool" to rotate the C(2)-C(3) bond appropriately.
- Use the "Navigation Tool" (star icon) to view the structure from various angles.
Minimize Structure
- Select Force Field:
  - Navigate: Extensions ➔ Molecular Mechanics ➔ Set Up for Field…
  - Choose MMF94 and retain default settings.
- Optimize Geometry again: Extensions ➔ Optimize Geometry
  - Repeat until the dihedral angle is stable (typically 3-4 iterations).
- Confirm minimized structure by calculating energy:
  - Navigate: Extensions ➔ Molecular Mechanics ➔ Calculate Energy
- When energy is minimized, it will remain constant.
Reporting Results
- Record the dihedral angle and export a high-quality image of the minimized structure:
  - File ➔ Export ➔ Graphics ➔ .png
- Crop the image to fit the worksheet.
Repeat Process
- Follow the above steps for other dihedral angles from the worksheet.
- Select the correct dihedral angle, minimized energy, and choose the descriptor (eclipsed, gauche, anti).

Minimize cyclobutane and cyclopentane conformations.
Measure bond lengths, bond angles, and evaluate potential transannular steric interactions.

Building Cyclobutane
- Create a new window: File ➔ New
- Draw butane with the pencil tool and connect terminal C atoms to form the ring, or insert a preconstructed cyclobutane ring from:
  - Insert ➔ Fragment ➔ Cyclic Alkanes ➔ cyclobutane.cml ➔ Insert
- Deselect atoms following insertion: Select ➔ None
Minimize Structure
- Navigation: Extensions ➔ Molecular Mechanics ➔ Conformer Search
- Select "Systematic rotor search" in the dialog.
- Verify that results match individual minimization if needed.
Measure Bonds and Angles
- Use the "Click to Measure" icon for:
  - C-H and C-C bond lengths
  - Specific bond angles
  - Through-space distances directly.
Report Results
- Export image of the minimized cyclobutane structure:
  - Paste the image into the worksheet with the same perspective as the line structure.
- Record calculated angles, lengths, and distances in the worksheet.
Repeat for Cyclopentane
- Follow similar steps for cyclopentane and complete worksheet questions accordingly.

Construct cis-1,3-dimethylcyclohexane
- Create new build window and draw the desired structure.
- Minimize the chair conformation of cyclohexane derivatives:
  - Use inserted structures or draw directly.
  - Change hydrogens to methyl groups as applicable.
Calculate Energy
- Ensure it is the global minimum by performing chair flips.
- Record total energies for each minimized structure.
Measure Through-Space Distances
- Use the "Click to Measure" tool for specified distances.

Additional Isomers
- Create and minimize necessary structures such as trans-1,3-dimethylcyclohexane and evaluate their energies similarly.
1,3-Dimethylcyclopentane
- Follow similar steps for the cyclopentane derivative.
Document Results
- Copy .png images of minimized structures into worksheets and record energies and measurements.

Draw Axial and Equatorial Forms
- Structures include:
  - Fluorocyclohexane
  - Cyclopropylcyclohexane
  - Trichloromethylcyclohexane
- Calculate minimized energies for each structure.
Calculate A Values
- Use axial and equatorial energy values to calculate A values:
  $A = - ext{ΔG}$
- Ensure consistency in units and dimensional analysis for ΔG in kcal/mol.
Record Findings
- Note down all calculated A values and minimized structures in the worksheet.

Through-Space Distance Measurement
- Use Click to Measure tool to find distance between an axial H and the closest H on the t-butyl group.
Record Bond Angle
- Calculate the C-C-C bond angle using the tool and document this in the worksheet.

Structural Representation
- Create chair conformations for cis-decalin and trans-decalin using ChemDraw with specific settings (ACS 1996).
Modeling in Avogadro
- Structure perspectives must match between ChemDraw and Avogadro.
- Export 3D .png images of structures for worksheet placement.

Identify Low-Energy Conformation
- Perform rotation around the C(2)-C(3) bond.
- Document the Newman projection closely aligned with minimized structure from Avogadro.

Each part consists of building structures in Avogadro, minimizing them through defined techniques, measuring required angles and distances, and accurately reporting findings in digital worksheets.