Lone-Pair / Lone-Pair Repulsive Effects
Methodology
Molecular Mechanics (Force Field)
MNDO/AM1/PM3 (Semiempirical MO)
Techniques Used
Building, Optimization, Analysis of
Bonding Parameters, Output Analysis
Abstract . You will optimize structures for molecules
that have lone-pair bearing heteroatoms directly connected. You will then
compare the bond lengths and conformations for these molecules, to show
the steric effects of lone pair/lone pair interactions.
Procedure. Optimize structures for hydrazine, hydrogen
peroxide, dihydrogen disulfide, glyoxal, and diimide. Record the strain
energy and/or enthalpy of formation for each conformer. Can you discern
any trends concerning the preferred conformation of neighboring lone pairs
relative to one another? This is a good demonstation of the fact that a
lone pair is a substituent with a definite steric/stereoelectronic
bulk.
Results.
|
Estimated Enthalpy |
Computed Enthalpy |
| NH2-NH2 |
|
|
| HO-OH |
|
|
| HS-SH |
|
|
| H-(C=O)-(C=O)-H |
|
|
| HN=NH |
|
|
Copyright by Paul M. Lahti 1995.