Stability in Cis- vs. Trans Olefins
Methodology
Molecular Mechanics (Force Field)
MNDO/AM1/PM3 (Semiempirical MO)
Techniques Used
Building, Optimization, Analysis of
Bonding Parameters, Output Analysis
Abstract. You will build pairs of isomeric molecules having cis
or trans C=C bonds. You will then compare the strain energies and/or
enthalpies of formation in each pair, to evaluate trends in preference for
cis- vs. trans C=C as a function of substituent or ring size.
Procedure Build and optimize the following molecules.
In the results table, record the method used, strain energies, enthalpy
of formation (if available), C=C bond length, and torsional X-C=C-X bond
angle formed between the substituents on the double bond. Evaluate the trends
in preference for geometry of a C=C (cis or trans) as ring
size increases in a cycloolefin. Evaluate the trends in torsional angle
as a function of substituent size vs. cis/trans placement.
Benchmarks. Known enthalpy data is shown in the
large table where available. Some benchmark torsional angle data are also
noted below:
| Molecule |
C-C=C-C Torsion |
| E-Stilbene |
zero (planar) |
| Z-Stilbene |
9° |
| di(fluorenylidene) |
32° |
Copyright by Paul M. Lahti, 1995. All rights reserved.