Heats of Formation for Hydrocarbons
Methodology
Molecular Mechanics (Force Field)
MNDO/AM1/PM3 (Semiempirical)
Techniques Used
Building, Optimization
Abstract. Benchmarks are crucial to gain and build
trust in any computational methodology that is aimed at prediction and reproduction
of experiment. The values of a variety of standard hydrocarbon enthalpies
of formation are given below in kcal/mol, along with values computed
by Allinger’s MM2 force field method. It is worth noting that MM2 is
specifically parameterized to be able to reproduce the enthalpies of formation
of the types of systems shown below, so the agreement between MM2 and experiment
is not surprising.
Procedure. Use any of the force field or semiempirical
MO methodologies available to you and compute the enthalpies of formation
of the molecules shown below. Use one type of method at a time, so as to
compare trend across a variety of systems.
Results.
| Compound |
DH MM2 (calc’d) |
DH Exptl |
|
|
|
| CH2=CH2 |
12.40 |
12.45 |
|
|
|
| CH2=CH-CH=CH2 |
26.19 |
26.11 |
|
|
|
| Cis-1,3-pentadiene |
19.77 |
19.5 |
|
|
|
| Trans-1,3-pentadiene |
18.35 |
18.12 |
|
|
|
| Benzene |
20.01 |
19.81 |
|
|
|
| Naphthalene |
35.66 |
35.85 |
|
|
|
| Anthracene |
55.29 |
55.44 |
|
|
|
| Toluene (Methylbenzene) |
11.95 |
11.99 |
|
|
|
| 2-Ethyltoluene |
1.00 |
0.39 |
|
|
|
| 3-Ethyltoluene |
-0.55 |
-0.43 |
|
|
|
| 4-Ethyltoluene |
-0.65 |
-0.76 |
|
|
|
| Pyrene |
54.76 |
51.59 |
|
|
|
| Chrysene |
64.84 |
62.8 |
|
|
|
