A few questions from doing the worksheet

[irisc23]

I tried doing some of the worksheet today, and I came across a few questions:

1) Chem3D uses degrees/Angstrom to measure the bond angles. Why do they do that, when in my opinion it should just be degrees?
2) I tried doing the energy minimization for cyclobutane. The molecule came out to be non-planar as I expected, but the CCC bond angles (which are sp³ hybridized, and ideally tetrahedral) were 88.298 º/A. This did not make sense to me, because it seems like even if the molecule were planar the bond angles would be 90º, which is closer to the tetrahedral optimum of 109.5º. Why does cyclobutane then prefer having smaller bond angles?
3) Also for cyclobutane: when I took the CCH angles, I got two different values, around 2 degrees/Angstrom apart. Why would the CCH angles be different, even though the molecule is quite symmetrical?
4) One of the exercises says to set the C1-C2-C3-C4 dihedral angle to 5 degrees. What does the dihedral angle represent, and why is it determined by 4 atoms (while bond angles are generally determined by 3)? How do you get dihedral angles in Chem3D?

Sorry for the long list of questions. Thank you very much!

[Chem_Mod]

1) Angles should be in degrees. Check with one of the TAs during office hrs this week in SLC.

2) Cyclobutane is slightly non-planar to relieve torsional strain. This will be discussed in class.

3) These are measured values for individual bonds and angles. One of the objectives of using ChemBio3D is to show all textbooks give averages for bond lengths and angles. You are calculating specific values.

4) This will be discussed in Wed class.

See Announcements for discussion sections this week. They are all in the SLC.

[Ikeda2M]

Hi, I'm getting degrees/Angstrom as well.

This is the link if the other doesn't work: http://tinypic.com/r/20p7v6h/8

[Chem_Mod]

Correct, your table shows bond length in Angstrom, and bond angle in degrees.