The program is smart enough to figure out that any unassigned bonds should be to hydrogen. That means we need to constrain the geometry of the molecule to a particular dihedral angle.
To do this, either click [pic] or use the dropdown menu Geometry ( Constrain Dihedral. At the bottom right of the screen the word “Constrain” followed by “(, , ,) = ” should appear.
You should be able to see that the H’s are very close to one another in the eclipsed conformation. Click OK and wait for the message box that tells you that the calculation is complete. To change the dihedral angle and obtain an existing constraint so that it is easier to select.) Do not add a new constraint.
To set up your calculations and obtain the energy for dropdown menu Setup ( Calculations. For calculate, choose “Equilibrium Geometry” with “Hartree-Fock” “3-21G(*).” Start from Initial Geometry. constraints” box because you have constrained the dihedral angle, and you don’t want the program to change that angle. The constraint initially appears as a double pink triangle.
Notice that we are looking for programs like Spartan is ensuring that the models are sufficiently accurate to give meaningful results but simple enough that the calculations can be performed in a reasonable amount of time. For ethane, you will need to have Spartan vary the H-C-C-H dihedral angle from 0 to 120 degrees in 15( increments (i.e. Even setting up a calculation at a higher level will delete the lower level data. Usually used to get an initial geometry for a more complex calculation. Can be used to approximate geometries, but not good for calculating relative energies of compounds or conformations.
It is recommended that you copy any needed energy information from the output into Excel as it is generated. Uses parameters for typical bond lengths, bond angles, dihedral angles, nonbonding interactions, etc. Combines the parameters of molecular mechanics with the more mathematically correct quantum mechanics calculations to approximate the location of valence electrons. Approximations must be introduced to solve multi-electron systems.
Click on any one of the yellow open bond sites to add a second sp3 hybridized carbon.
You should have a gray bond between the two carbon atoms and six yellow lines/open valences in the molecule. Get a rough idea of the conformation of the molecule by program tell us the total energy of the molecule at the angle we designate.
The challenge is to describe this wavelike behavior in enough mathematical detail that we can accurately predict important features of the molecule such as bond lengths, bond energies, and force constants. For larger atoms (and molecules), the problem is so complicated that it cannot be solved. Years of research have shown that we need to retain the approximate 3-dimensional shapes of the orbitals, but that we can simplify considerably how we describe the wavelike behavior of the electron with respect to distance from the nucleus. An atom’s core electrons are closely held, while the atom’s valence electrons are shared among atoms.
We will concentrate on calculating the total energy of the molecule. If any calculations are being carried out, they will appear in the top half of the window.