A force field has been developed to permit molecular mechanics calculations on dialkyl disulfides. Application of the method to a determination of the structure of SH, 8H-dibenzo[d,f][ 1,2 dithiocin shows that the chair conformation is more stable than the tub form for the isolated molecule. The interactions leading to this conclusion are discussed. Calculations on the 1,2,4,5-tetrathiane ring system are presented, and the conformational equilibria and barriers are discussed. Comparison with experiment is made in all cases where data exist, and predictions are made in other cases. Previous papers' have described a molecular mechanics or force field method for the calculation of the structures and energies of a variety of types of compounds, including simple organic molecules containing sulfur in the form of a thiol or thioether. The disulfides constitute another large class of sulfur-containing molecules which is of special importance because of its widespread occurrence in nature. From the structural point of view, these disulfides can be looked upon as the second-row analogue of the organic peroxides.The general methods and most of the parameters used in the present work have been described previously.'q2 Only a few parameters are needed to extend the previous work to disulfides and polysulfides; these quantities are listed in Table I.The S-S bond stretching force constant and bending force constants for S-S-S and Csp3-S-S were taken directly from the l i t e r a t~r e .~.~ We were not able to find experimental values for the deformation of the Csp2-CSp3-S angle, and hence the Csp3-CSp3-S bending force constant was used as an approximation. Torsional parameters for CSp+S-S-S, C,,2-CSp3-S-S, and S-C,,3-CSp3-S were similarly unavailable, so there were approximated by the previously determined values' for s-S-S-S, Csp3-Csp3-S-Csp3, and CSp3-Csp3-Csp3-S, respectively.The exact nature of the barrier for Csq3-S-S-CsP3 torsion is uncertain, although it has been extensively discussed. Experimental estimatesS of the barrier to rotation for R-S-S-R vary somewhat depending on the experimental method used to obtain the data. Also, in most cases the cis and trans barriers were not individually identified although it is commonly believed a gauche form is the most stable. From calculated and observed data for the entropy and heat capacity of dimethyl disulfide, the barrier has been estimated to be 6.8 kcal/mol.sa A similar barrier height, 7.29 kcal/mol, has been obtained from infrared data.sb From Raman data the following somewhat larger barriers have been obtained: dimethyl disulfide,sc 9.5 kcal/mol; diethyl disulfide,sd 13.2 kcal/mol. An even larger barrier (cis barrier), 12.3 kcal/ mol, has been estimated from N M R data.se A studysf of the millimeter-wave rotational spectrum of H2S2 led to the conclusion that the barrier is very high, much higher than in H202 (cis barrier, 7.0 kcal/mol; trans barrier, 1.1 kcal/ mol).7a We speculated that the cis barrier height should be somewhere around 10 kcal/mol, and the trans would be...
