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Chem 380.37 Lecture Notes 6

1569 00185 02954 00476 00071 00007 16598 06387 00528

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Unformatted text preview: 0.0007 1.6598 0.6387 0.0528 0.0000 2.1719 0.7055 Gauche conformer MM+ Amber 0.1669 0.0386 0.6050 0.3964 0.4507 0.1142 1.7393 0.7342 0.0750 0.0000 3.0369 1.2833 In this example, it is again clear that the energy of the anti form of n-butane is totally different when MM+ is used as the force field compared to when Amber is used. Similarly, the MM+ and Amber energies of the gauche form of n-butane are completely different, once again emphasizing that results from one force field cannot be compared to results from another force field. Comparing relative energies of the anti and gauche conformers is reasonable. The gauche-anti energy difference is 0.87 kcal/mol using the MM+ force field, and it is 0.58 kcal/mol using the Amber force field. While there is some variation in these values, their magnitudes are comparable. Finally, both force fields suggest qualitatively that some of the difference in energy between the gauche and anti conformers occurs in the energy due to torsions. However, in the MM+ force field, 0.44 kcal/mol out of the total difference of 0.87 kcal/mol is attributed to a difference in the torsional energy (the major difference). In the Amber force field, only 0.11 kcal/mol out of the total difference of 0.58 kcal/mol is attributed to torsional energy. In the Amber force field, about 0.35 kcal/mol is additionally attributed to a difference in the energy of the bond angles (the major difference), while a difference of 0.31 kcal/mol is due to this term in the MM+ force field. These differences among force fields further illustrate that individual terms in the force field should not be used to quantitatively evaluate reasons for conformational energy differences. 2 3 Common Molecular Mechanics Force Fields – References Force F ield MM2/MM3/ MMX/MM+ References N.L. Allinger, J. Am. Chem. Soc. 99, 8127 (1977)....
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