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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 18, Number 2
BKCSDE 18(2)
February 20, 1997 

Relationship between Conformational Preferences and Torsional Parameters in Molecular Mechanics (MM3) Calculations
Son Gyeong Cho, Gyoosoon Park
We have investigated a relationship between conformational preferences of various substituents in monosubstituted cyclohexanes and pertinent torsional parameter values in molecular mechanics calculations. We have manipulated torsional parameters to supply a certain energy difference between gauche and anti conformers, and applied those parameters to monosubstituted cyclohexanes. After investigating 6 different substituents, namely Me, SiH3, F, Cl, Br, and I, MM3 calculations show that (1) the MM3 calculated A values with the current torsional parameters reproduce the available experimental values well, (2) the conformational energy difference between axial and equatorial conformations (the A value) correlates perfectly with the gauche/anti energy differences of the corresponding butane-like fragment (correlation coefficient=l.000), and (3) the A values are essentially twice as the gauche/anti energy differences (slopes=1.86-2.00). On the basis of our analysis, the A values as well as the gauche/anti energy differences are easily calibrated by an adjustment of the relevant torsional parameter. Thus, our technique for tuning the torsional parameters may be of great use in updating molecular mechanics results about conformational preferences whenever a further refinement is necessary.
143 - 149
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