RELATIONSHIP BETWEEN CONFORMATIONAL PREFERENCES AND TORSIONAL PARAMETERS IN MOLECULAR MECHANICS (MM3) CALCULATIONS

We have investigated a relationship between conformational preferences of various substituents in monosubstituted cyclohexanes and pertinent torsional parameter values in molecular mechanics calculations. We ha ve manipulated torsional parameters to supply a certain energy differe nce between gauche and anti conformers, and applied those parameters t o monosubstituted cyclohexanes. After investigating 6 different substi tuents, 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 conformation al energy difference between axial and equatorial conformations (the A value) correlates perfectly with the gauche/anti energy differences o f the corresponding butane-like fragment (correlation coefficient=1,00 0), and (3) the A values are essentially twice as the gauche/anti ener gy 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 calib rated by an adjustment of the relevant torsional parameter. Thus, our technique for tuning the torsional parameters may be of great use in u pdating molecular mechanics results about conformational preferences w henever a further refinement is necessary.