Expt. 11- Conformational Isomerism in n-Butane
Comparison of Conformer Energies (kcal/mol) Calculated by Different Methods
|
C-C-C-C Angle (Degr.) |
AM1 (kcal/mol) |
AM1 Relative (kcal/mol) |
SYBYL (kcal/mol) |
SYBYL Relative (kcal/mol) |
MMFF94 (kcal/mol) |
MMFF94 Relative (kcal/mol) |
3-21G (hartrees) |
3-21G Relative (hartrees) |
3-21G Relative (kcal/mol) |
|||||||||
|
0 |
-27.849 |
3.277 |
6.884 |
5.895 |
0.134 |
5.209 |
-156.41638 |
0.01609 |
10.096 |
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|
30 |
-28.984 |
2.142 |
3.500 |
3.384 |
-2.385 |
2.690 |
-156.42670 |
0.00284 |
1.782 |
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|
60 |
-30.256 |
0.870 |
1.606 |
0.617 |
-4.254 |
0.821 |
-156.43106 |
0.00141 |
0.885 |
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|
90 |
-30.197 |
0.929 |
2.708 |
1.719 |
-3.311 |
1.764 |
-156.42958 |
0.00289 |
1.813 |
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|
120 |
-29.599 |
1.527 |
5.361 |
4.372 |
-1.123 |
3952 |
-156.42673 |
0.00574 |
3.602 |
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|
150 |
-30.392 |
0.734 |
2.698 |
1.709 |
-3.401 |
1.674 |
-156.42963 |
0.00284 |
1.7821 |
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|
180 |
-31.126 |
0.0 |
0.989 |
0.0 |
-5.075 |
0.0 |
-156.43247 |
0.0 |
0.0 |
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Suggested Reading: "Quantum Chemistry Study of Conformational Energies and Rotational Energy Barriers in n-Alkanes", Smith, et al.,
J. Phys. Chem. 100, 18718-18724 (1996)
WATCH OUT: SMITH HAS INCORRECT DEFINITION of cis (c), gauche (g), gauche-trans (g-t) barrier, & trans (t)
conformations in terms of C-C-C-C torsional angles. Correct definition is given in table below. Smith's energies are
correct for c, g, g-t, & t, but his angle definitions are incorrect
Comparision of Expt . 11 Results to those of Smith, et al. (kcal/mol)
|
C-C-C-C |
AM1 relative |
SYBYL Relative |
MMFF94 Relative |
3-21G Relative |
6-31G** (Smith) |
Expt. |
|
0 cis |
3.3 |
5.9 |
5.2 |
10.1 |
6.3 |
4-6 |
|
60 gauche |
0.9 |
0.6 |
0.8 |
0.9 |
0.9 |
0.8 |
|
120 g-t barrier |
1.5 |
1.5 |
4.0 |
3.6 |
3.6 |
3.4 |